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A 135-141 (server), SGW, SPR AIFS, 47 802. 1x (mechanism) AKA, 117, 118, 120, 122, 143, protocols, see EAP, EAPOL, 144, 146 RADIUS A-MPDU, 63, 64, 68, 69, 74, 86 setting up keys, see four-way A-MSDU, 63, 64, 69 handshake, group key handshake ANDI, 184, 185 ANQP, 192-195, 197-199 A, B, C ASA, 17-19 ASR, 17-19 ASSOCIATION, see DIAMETER, 144, 155, 157, ASSOCIATION REQUEST, 163, 164 ASSOCIATION RESPONSE, server, 2, 5, 8, 13-19, 118-123, DISASSOCIATION, 143, 144, 146, 153-155, REASSOCIATION REQUEST, 157, 162-164 REASSOCIATION RESPONSE AAR, 17-19, 143, 144, 153, 155, ASSOCIATION REQUEST, 29, 31, 157, 163 39, 47, 65 ASSOCIATION RESPONSE, 29, 32, control frame, 30, 33-35, 70, 71, 39, 47, 65, 66 Authentication, TCP, 126, 137, 217-219,221, mechanisms, see, AKA, EAP, 224-234 EAP-AKA', OSA, SKA ADD_ADDR, 226, 233 seal, see AUTN, RES, ICV, MAC, Aggregation (MPTCP), see MPTCP AUTN, 117, 119-121, 143, 144 Backoff, 32, 33, 47, 214 Wi-Fi Integration to the 4G Mobile Network, First Edition. André Perez. C ISTE Ltd 2018. Published by ISTE Ltd and John Wiley & Sons, Inc. Wi-Fi Integration to the 4G Mobile Network Bandwidth, 57, 61, 64, 65, 70, 74, D, E, F 82-85, 87-90, 93-96, 98-104 BEACON, 26, 27, 29, 30, 36, 37, 39, Data link layer, 46, 47, 65, 66, 191-196 LTE interface, see MAC, PDCP, Beamforming, 65, 67, 75, 76, 81, 86, 93 Wi-Fi interface, see LLC, MAC Binding Data protection Acknowledgement, 177 mechanism (IP), see IPSec Error, 168, 177 mechanism (Wi-Fi), see WEP, Refresh Request, 167 Update, 148, 167, 168, 171-177, protocols (IP), see AH, ESP, IKE 179-181 protocols (Wi-Fi), see CCMP BlockAck, 70-72 TKIP, WEP BlockAckReq, 70, 71 Data transfer, see backoff, DCF, BPSK, 49, 54, 75, 81-84, 102-104 EDCA, DCF BSS, 87, 195 DCF, 30, 32, 46 BSSID, 25, 26, 30, 195 DEA, 17-19, 118, 120 Carrier aggregation, see LAA, LWA, DEAUTHENTICATION, 29, 30 LWIP, CCA DER, 17-19, 118-120, 122 CCA, 21, 34, 152-155, 157, 162, Destination extension, 130, 131, 167, 163, 213, 214 168, 170, 173 CCMP, 39, 44, 45, 114, 115 DFS, 57, 58 CCR, 21, 153, 155, 157, 1

63 DIAMETER (messages) Cell (Wi-Fi) S6b, STa, SWa, SWm interfaces, identifier, see BSSID see AAA, AAR, ASA, ASR, DEA, name, see BSS DER, RAA, RAR, STA, STR CN, 159, 161, 166 SWx interface, see MAA, MAR, coding rate, 49, 51, 54, 82-84, PPA, PPR, RTA, RTR, SAA, SAR 102-104 Gx, Gxa, Gxb interfaces, see CCA, Congestion Avoidance, 221-224 CCR, RAA, RAR Control frames (Wi-Fi), see ACK, DIFS, 30, 33, 34 BlockAck, BlockAckReq, CTS DISASSOCIATION, 29, 30 RTS, PS-POLL DSMIPv6, 13, 14, 16, 18, 19, 177, 179-181 CoT, 167, 176, 177 CoTI, 167, 175 DSS, 226, 229-231 CREATE_CHILD_SA, 133, DSSS, 27, 58-60, 64 140, 141 EAP, 17-19, 106, 107, 109-111, 113, CSMA/CA, 32 117-120, 122, 133, 137, 143, 144, CTS, 26, 30, 32-35, 66, 89, 90 146, 158, 196, 205, 206 EAP-AKA, 17-19, 117, 118, 143, 205, 206 EAP-AKA', 118, 143 Index EAPOL, 106-110, 112, 115, HSS, 2, 5, 15-17, 117, 119, 120, 116, 118 143, 144, 153, 155, 157, 163, EDCA, 46, 47 164, 178-180, 205 EHSP, 191 HT, 64-66, 72-75, 77-79, 86, 87, EIFS, 30, 33 90-92 eNB, 191, 192, 201-213, 217 IARP, 189, 192 EPC, 1, 186-188, 201, 202 ICV, 40, 41, 44, 128, 129 ePDG, 5-7, 12-15, 19-21, 142-146, IFOM, 180, 181, 186, 187, 192 154, 155, 158, 180, 190 IKE (procedures), see ERP, 58-60 CREATE_CHILD_SA, ESP, 12, 125-131, 133, 135-141 IKE AUTH, IKE SA INIT ESS, 27 IKE AUTH, 133, 136, 139, 143-146 E-UTRAN, 1, 201 IKE SA INIT, 133, 137-140, 142, 144-146 Recovery, 222, 223, 231 IKEv2, 12, 125, 126, 131, 132, 136, Retransmit, 222, 223, 231 137, 142, 143, 146 FBE, 213 Interfaces, 212 FIN, 219, 220, 226, 231, 232 S2a, 1-4, 8-11, 16, 17, 122, 147, Four-way handshake, 114-116, 205 152, 155-158, 162, 190 Frequency band (Wi-Fi), see ISM, S2b, 4-6, 12, 13, 16, 17, 144, 145, U-NII 147, 154, 155, 158, 190 Frequency multiplexing, see OFDM S2c, 7, 13, 14, 16, 17, 165, 178 Frequency selection, see DFS SWu, 5-7, 12-14, 19, 118, 125, G, H, I 142, 143, 145, 146, 154, 155, GI, 51, 52, 55, 64, 74, 76, 82-85, 93, IPSec, 12, 14, 18, 125-127, 142, 150, 102-104,231 179, 180, 206, 211, 212 Group key handshake, 114, 116 ISM, 58,

61, 78 GTP-U, 11, 155, 156, 158, 205, 206 ISMP, 185, 186 GTPv2, 11, 12, 16, 18, 19, 155, 157 ISRP, 186, 189, 192 GTPv2-C, 11, 155, 156 Guard interval, see GI K, L, M HA, 122, 123, 148, 159, 162, 166 HESSID, 194, 195 Kind, 226 Home Agent Address Discovery (options), see DD_ADDR, DSS, Reply, 169, 171, 172 MP_CAPABLE, MP_FAIL, Request, 169, 171 MP FASTCLOSE, MP_JOIN, HoT, 167, 175-177 MP PRIO, REMOVE_ADDR HoTI, 167, 175 LAA, 207, 212 HR, 58-60 LBE, 213, 214 LBT, 213, 214 LLC, 23, 40, 204 Wi-Fi Integration to the 4G Mobile Network LMA, 148-151, 154 Advertisement, Mobile Prefix LWA, 202-204, 207, 215, 217 Solicitation (procedures), see WT Addition, Mobility managed by the mobile, see WT Modification, WT Release interface S2c LWIP, 205, 206, 211, 212, 217 Mobility managed by the network, MAA, 16 see interface S2A and interface S2b Mobility extension, see Binding LTE interface, 191, 204-207 Acknowledgement, Binding Error, seal, 120, 122 Binding Refresh Request, Binding Wi-Fi interface, 2, 3, 184, 185, Update, CoT, CoTI, HoT, HoTI, 187, 188, 191, 192, 206, 207, PBA, PBU 212, 216, 236 Modulation, see BPSK, QAM, QPSK MAG, 148-151, 154 MP_CAPABLE, 226, 227 Management frames (Wi-Fi), see MP FAIL, 226, 234 ASSOCIATION REQUEST, MP FASTCLOSE, 226, 227, ASSOCIATION RESPONSE, 232, 233 AUTHENTICATION, BEACON, MP_JOIN, 226-229, 233 DEAUTHENTICATION, MP_PRIO, 226, 229 DISASSOCIATION, PROBE MPTCP, 217, 226-234 REQUEST, PROBE RESPONSE, Multiple access, see CSMA/CD, REASSOCIATION REQUEST, FBE, LBT, LBE REASSOCIATION RESPONSE MU-MIMO, 87, 91-93, 101 MAPCON, 3, 187, 188, 192 MAR, 16 N, o, P MIC, 41, 42, 44-46, 114, 116, 117 MIMO, 75, 79, 80, 82, 87, 92, Network discovery 94, 102 3GPP mechanism, see ANDI MIPv4 FA, 9, 10, 16, 122, 158, 162 IEEE/WFA mechanism, see ANQP Network selection MME, 202, 204, 205 MN, 122, 123, 147, 148, 150, 151, 3GPP mechanism, see EHSP, 153-155, 161, 163-165 PSPL, WLANSP Mobile Prefix IEEE/WFA mechanism, see ANQP Advertisement, 170 NSWO, 3, 188, 189, 192 Solicitation, 170 OFDM, 27, 49, 51-56, 58-61, 78, Mobility

(entities), see CN, HA, 79, 82-84, 93, 94, 102-104 LMA, MAG, MN OSA, 28, 31 Mobility (extensions), see PBA, 148-151, 154, 155, 158 Destination extension, Mobility PBU, 148-151, 153, 155, 158 extension, Routing extension PCRF, 2, 3, 5, 20, 21, 153-155, 157, Mobility (ICMPv6 messages), see 163, 179, 180 Home Agent Address Discovery PDCP, 203, 204, 214-216 Reply, Home Agent Address PGW, 2-21, 122, 123, 143, 145, Discovery Request, Mobile Prefix 151-158, 162-164, 177-180 Index Physical layer (Wi-Fi), see DSSS, SA, 25, 125, 133, 135-137, 140, ERP, HR, HT, VHT 141, 143, 145 PLCP, 30, 49-51, 58-61, 64, 72, SAA, 16, 144, 153, 155, 157, 73, 92 163, 164 PMD, 51, 61, 75, 94 SAR, 16, 144, 153, 155, 157, PMIPv6, 8, 9, 11, 12, 16, 18, 19, 147, 163, 164 148, 152, 154, 158 Scan, see BEACON, PROBE Power control, see TPC REQUEST, PROBE RESPONSE PPA, 16 Security association, see SA PPR, 16 SGW, 202, 205 PROBE SIFS, 30, 34-37, 48, 60 REQUEST, 27, 28, 30, 65 SKA, 28, 31 RESPONSE, 28, 31, 39, 46, 47, 65, Spatial multiplexing, see MIMO, 66, 192, 193, 195, 196 MU-MIMO, SU-MIMO PSPL, 191 SPR, 3, 15, 153, 163, 179, 180 PS-POLL, 26, 35, 37 SSID, 27, 28, 30, 31, 184, 194 STA, 17-19, 65, 66, 87 Q, R, S STBC, 64, 66, 74-76, 80, 83, 85, 86, 91, 93 QAM, 54, 81-83, 94, 102-104 STR, 17-19 QPSK, 54, 75, 81-83, 102-104 SU-MIMO, 92, 101 RAA, 17-19, 21 SYN, 219, 220, 225, 227, 228, 233, RADIUS, 106, 111-113 RAR, 17-19, 21 Rate (Wi-Fi interfaces), see T, U, V, W Bandwidth, Coding rate, Guard interval, Modulation, Spatial multiplexing anticipation window, see REASSOCIATION Congestion Avoidance, Fast REQUEST, 29, 32, 65 Recovery, Fast Retransmit, RESPONSE, 29, 32, 47, 65, 66 Slow Start REMOVE_ADDR, 226, 233 connection, see ACK, FIN, SYN RES, 117, 119-121, 143, 144 Timers, see AIFS, DIFS, EIFS, RIFS, Return routability, see CoT, CoTI, HoT, HoTI TKIP, 38, 41-44, 63, 114, 115 RIFS, 63, 65 TPC, 57, 58 RLC, 203, 204 Transmission diversity, see STBC Routing extension, 169, 170 Trusted access, see interface S2a and Routing rules, see IARP, ISRP, intreface S2c

S2b interface, see GTPv2, PMIPv6 Wi-Fi Integration to the 4G Mobile Network S2c interface, see DSMIPv6 SWu interface, see IPSec Tunneling (protocols) GTPv2 mechanism, see GTPv2-C, GTP-U U-NII, 56, 57,78, 99, 207, 213 Untrusted access, see interface S2b and interface S2c VHT, 85-88, 90-96 WEP, 28, 38-44 Wi-Fi network identifier, see SSIF, HESSID name, see ESS WLANSP, 189, 190 WPA, 38 Addition, 207, 208, 211 Modification, 208, 209 Release, 209-211 Other titles from Networks and Telecommunications ANDIA Gianfranco, DURO Yvan, TEDJINI Smail Non-linearities in Passive RFID Systems: Third Harmonic Concept and Applications BENSLAMA Malek, BENSLAMA Achour, ARIS Skander Quantum Communications in New Telecommunications Systems HILT Benoit, BERBINEAU Marion, VINEL Alexey, PIROVANO Alain Networking Simulation for Intelligent Transportation Systems: High Mobile Wireless Nodes LESAS Anne-Marie, MIRANDA Serge The Art and Science of NFC Programming (Intellectual Technologies Set - Volume 3) AL AGHA Khaldoun, PUJOLLE Guy, ALI-YAHIYA Tara Mobile and Wireless Networks (Advanced Network Set - Volume 2) BATTU Daniel Communication Networks Economy BENSLAMA Malek, BATATIA Hadj, MESSAI Abderraouf Transitions from Digital Communications to Quantum Communications: Concepts and Prospects CHIASSERINI Carla Fabiana, GRIBAUDO Marco, MANINI Daniele Analytical Modeling of Wireless Communication Systems (Stochastic Models in Computer Science and Telecommunication Networks Set - Volume 1) EL FALLAH SEGHROUCHNI Amal, ISHIKAWA Fuyuki, HÉRAULT Laurent, TOKUDA Hideyuki Enablers for Smart Cities PEREZ André VoLTE and ViLTE BENSLAMA Malek, KIAMOUCHE Wassila, BATATIA Hadj Connections Management Strategies in Satellite Cellular Networks BENSLAMA Malek, BATATIA Hadj, BOUCENNA Mohamed Lamine Ad Hoc Networks Telecommunications and Game Theory BERTHOU Pascal, BAUDOIN Cédric, GAYRAUD Thierry, GINESTE Matthieu Satellite and Terrestrial Hybrid Networks CUADRA-SANCHEZ Antonio, ARACIL Javier Traffic Anomaly Detection LE RUYET Didier, PISCHELLA Mylène Digital Comm

unications 1: Source and Channel Coding PEREZ André LTE and LTE Advanced: 4G Network Radio Interface PISCHELLA Mylène, LE RUYET Didier Digital Communications 2: Digital Modulations PUJOLLE Guy Software Networks (Advanced Network Set - Volume 1) ANJUM Bushra, PERROS Harry Bandwidth Allocation for Video under Quality of Service Constraints BATTU Daniel New Telecom Networks: Enterprises and Security BEN MAHMOUD Mohamed Slim, GUERBER Christophe, LARRIEU Nicolas, PIROVANO Alain, RADZIK José Aeronautical Air-Ground Data Link Communications BITAM Salim, MELLOUK Abdelhamid Bio-inspired Routing Protocols for Vehicular Ad-Hoc Networks CAMPISTA Miguel Elias Mitre, RUBINSTEIN Marcelo Gonçalves Advanced Routing Protocols for Wireless Networks CHETTO Maryline Real-time Systems Scheduling 1. Fundamentals Real-time Systems Scheduling 2: Focuses EXPOSITO Ernesto, DIOP Codé Smart SOA Platforms in Cloud Computing Architectures MELLOUK Abdelhamid, CUADRA-SANCHEZ Antonio Quality of Experience Engineering for Customer Added Value Services OTEAFY Sharief M.A., HASSANEIN Hossam S. Dynamic Wireless Sensor Networks PEREZ André Network Security PERRET Etienne Radio Frequency Identification and Sensors: From RFID to Chipless RFID REMY Jean-Gabriel, LETAMENDIA Charlotte LTE Standards LTE Services TANWIR Savera, PERROS Harry VBR Video Traffic Models VAN METER Rodney Quantum Networking XIONG Kaiqi Resource Optimization and Security for Cloud Services ASSING Dominique, CALÉ Stéphane Mobile Access Safety: Beyond BYOD BEN MAHMOUD Mohamed Slim, LARRIEU Nicolas, PIROVANO Alain Risk Propagation Assessment for Network Security: Application to Airport Communication Network Design BEYLOT André-Luc, LABIOD Houda Vehicular Networks: Models and Algorithms BRITO Gabriel M., VELLOSO Pedro Braconnot, MORAES Igor M. Information-Centric Networks: A New Paradigm for the Internet BERTIN Emmanuel, CRESPI Noël Architecture and Governance for Communication Services DEUFF Dominique, COSQUER Mathilde User-Centered Agile Method DUARTE Otto Carlos, PUJOLLE Guy Virtual

Networks: Pluralistic Approach for the Next Generation of Internet FOWLER Scott A., MELLOUK Abdelhamid, YAMADA Naomi LTE-Advanced DRX Mechanism for Power Saving JOBERT Sébastien et al. Synchronous Ethernet and IEEE 1588 in Telecoms: Next Generation Synchronization Networks MELLOUK Abdelhamid, HOCEINI Said, TRAN Hai Anh Quality-of-Experience for Multimedia: Application to Content Delivery Network Architecture NAIT-SIDI-MOH Ahmed, BAKHOUYA Mohamed, GABER Jaafar, WACK Maxime Geopositioning and Mobility PEREZ André Voice over LTE: EPS and IMS Networks AL AGHA Khaldoun Network Coding BOUCHET Olivier Wireless Optical Communications DECREUSEFOND Laurent, MOYAL Pascal Stochastic Modeling and Analysis of Telecoms Networks DUFOUR Jean-Yves Intelligent Video Surveillance Systems EXPOSITO Ernesto Advanced Transport Protocols: Designing the Next Generation JUMIRA Oswald, ZEADALLY Sherali Energy Efficiency in Wireless Networks KRIEF Francine Green Networking PEREZ André Mobile Networks Architecture BONALD Thomas, FEUILLET Mathieu Network Performance Analysis CARBOU Romain, DIAZ Michel, EXPOSITO Ernesto, ROMAN Rodrigo Digital Home Networking CHABANNE Hervé, URIEN Pascal, SUSINI Jean-Ferdinand RFID and the Internet of Things GARDUNO David, DIAZ Michel Communicating Systems with UML 2: Modeling and Analysis of Network Protocols LAHEURTE Jean-Marc Compact Antennas for Wireless Communications and Terminals: Theory and Design RÉMY Jean-Gabriel, LETAMENDIA Charlotte Home Area Networks and IPTV PALICOT Jacques Radio Engineering: From Software Radio to Cognitive Radio PEREZ André IP, Ethernet and MPLS Networks: Resource and Fault Management TOUTAIN Laurent, MINABURO Ana Local Networks and the Internet: From Protocols to Interconnection CHAOUCHI Hakima The Internet of Things FRIKHA Mounir Ad Hoc Networks: Routing, QoS and Optimization KRIEF Francine Communicating Embedded Systems / Network Applications CHAOUCHI Hakima, MAKNAVICIUS Maryline Wireless and Mobile Network Security VIVIER Emmanuelle Radio Resources Management in WiMAX CHADUC

Jean-Marc, POGOREL Gérard The Radio Spectrum GAITI Dominique Autonomic Networks LABIOD Houda Wireless Ad Hoc and Sensor Networks LECOY Pierre Fiber-optic Communications MELLOUK Abdelhamid End-to-End Quality of Service Engineering in Next Generation Heterogeneous Networks PAGANI Pascal et al. Ultra-wideband Radio Propagation Channel BENSLIMANE Abderrahim Multimedia Multicast on the Internet PUJOLLE Guy Management, Control and Evolution of IP Networks SANCHEZ Javier, THIOUNE Mamadou VIVIER Guillaume Reconfigurable Mobile Radio Systems Wireless Networking Technology From Principles to Successful Implementation Steve Rackley Newnes Wireless Networking Technology From Principles to Successful Implementation This page intentionally left blank Wireless Networking Technology From Principles to Successful Implementation Steve Rackley AMSTERDAM BOSTON . HEIDELBERG LONDON NEW YORK OXFORD PARIS SAN DIEGO SAN FRANCISCO SINGAPORE SYDNEY TOKYO ELSEVIER Newnes is an imprint of Elsevier Newnes Newnes is an imprint of Elsevier Linacre House, Jordan Hill, Oxford OX2 8DP 30 Corporate Drive, Suite 400, Burlington MA 01803 First published 2007 Copyright © 2007, Steve Rackley. All rights reserved The right of Steve Rackley to be identified as the author of this work has been asserted in accordance with the Copyright, Designs and Patents Act 1988 No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means electronic, mechanical, photocopying, recording or otherwise without the prior written permission of the publisher Permission may be sought directly from Elsevier's Science & Technology Rights Department in Oxford, UK: phone (+44) (0) 1865 843830; fax (+44) (0) 1865 853333; email: permissions@elsevier.com. Alternatively you can submit your request online by visiting the Elsevier web site at http://elsevier.com/locate/permissions, and selecting Obtaining permission to use Elsevier material Notice No responsibility is assumed by the publisher for any injury and/or damage to perso

ns or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions or ideas contained in the material herein. Because of rapid advances in the medical sciences, in particular, independent verification of diagnoses and drug dosages should be made British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library Library of Congress Cataloguing in Publication Data A catalogue record for this book is available from the Library of Congress ISBN 13: 978-0-7506-6788-3 ISBN 10: 0-7506-6788-5 For information on all Newnes publications visit our website at www.books.elsevier.com Printed and bound in Great Britain 07 08 09 10 11 10987654321 Working together to grow libraries in developing countries www.elsevier.com www.bookaid.org www.sabre.org ELSEVIER BOOK AID International Sabre Foundation Contents Chapter 1: Introducing Wireless Networking Development of Wireless Networking The Diversity of Wireless Networking Technologies Organisation of the Book PART I: Wireless Network Architecture Introduction Chapter 2: Wireless Network Logical Architecture The OSI Network Model Network Layer Technologies Data Link Layer Technologies Physical Layer Technologies Operating System Considerations Summary Chapter 3: Wireless Network Physical Architecture Wired Network Topologies - A Refresher Wireless Network Topologies Wireless LAN Devices Wireless PAN Devices Contents Wireless MAN Devices Summary of Part I PART II: Wireless Communication Introduction Chapter 4: Radio Communication Basics The RF Spectrum Spread Spectrum Transmission Wireless Multiplexing and Multiple Access Techniques Digital Modulation Technique RF Signal Propagation and Reception Ultra Wideband Radio MIMO Radio Near Field Communications Chapter 5: Infrared Communication Basics The Ir Spectrum Infrared Propagation and Reception Summary of Part II PART III: Wireless LAN Implementation Introduction Chapter 6: Wireless LAN Standards The 802.11

WLAN Standards The 802.11 MAC Layer 802.11 PHY Layer 802.11 Enhancements Other WLAN Standards Summary Contents Chapter 7: Implementing Wireless LANs Evaluating Wireless LAN Requirements Planning and Designing the Wireless LAN Pilot Testing Installation and Configuration Operation and Support A Case Study: Voice over WLAN Chapter 8: Wireless LAN Security The Hacking Threat WLAN Security WEP - Wired Equivalent Privacy Encryption Wi-Fi Protected Access - WPA IEEE 802.11i and WPA2 WLAN Security Measures Wireless Hotspot Security VoWLAN and VoIP Security Summary Chapter 9: Wireless LAN Troubleshooting Analysing Wireless LAN Problems Troubleshooting using WLAN Analysers Bluetooth Coexistence with 802.11 WLANs Summary of Part III PART IV: Wireless PAN Implementation Introduction Chapter 10: Wireless PAN Standards Introduction Bluetooth (IEEE 802.15.1) Wireless USB Contents ZigBee (IEEE 802.15.4) Near Field Communications Summary Chapter 11: Implementing Wireless PANs Wireless PAN Technology Choices Pilot Testing Wireless PAN Security Summary of Part IV PART V: Wireless MAN Implementation Introduction Chapter 12: Wireless MAN Standards The 802.16 Wireless MAN Standards Other WMAN Standards Metropolitan Area Mesh Networks Summary Chapter 13: Implementing Wireless MANs Technical Planning Business Planning Start-up Phase Operating Phase Summary of Part V PART VI: The Future of Wireless Networking Technology Introduction Contents Chapter 14: Leading Edge Wireless Networking Technologies Wireless Mesh Network Routing Network Independent Roaming Gigabit Wireless LANs Cognitive Radio Summary of Part VI PART VII: Wireless Networking Information Resources Introduction Chapter 15: Further Sources of Information General Information Sources Wireless PAN Resources by Standard Wireless LAN Resources by Standard Wireless MAN Resources by Standard Chapter 16: Glossary Networking and Wireless Networking Acronyms Networking and Wireless Networking Glossary Subject Index This page intentionally left blank CHAPTER Introducing Wireless Netwo

rking Development of Wireless Networking Although the origins of radio frequency based wireless networking can be traced back to the University of Hawaii's ALOHANET research project in the 1970s, the key events that led to wireless networking becoming one of the fastest growing technologies of the early 21st century have been the ratification of the IEEE 802.11 standard in 1997, and the subsequent development of interoperability certification by the Wi-Fi Alliance (formerly WECA). From the 1970s through the early 1990s, the growing demand for wireless connectivity could only be met by a narrow range of expensive hardware, based on proprietary technologies, which offered no interoperability of equipment from different manufacturers, no security mechanisms and poor performance compared to the then standard 10 Mbps wired Ethernet. The 802.11 standard stands as a major milestone in the development of wireless networking, and the starting point for a strong and recognisable brand - Wi-Fi. This provides a focus for the work of equipment developers and service providers and is as much a contributor to the growth of wireless networking as the power of the underlying technologies. While the various Wi-Fi variants that have emerged from the original 802.11 standard have grabbed most of the headlines in the last decade, other wireless networking technologies have followed a similar timeline, with the first IrDA specification being published in 1994, the same year Chapter One 802.11n WUSB (Optional) WUSB (Mandatory) 802.16 (10-66 GHz) 802.16d 802.11g WiMax IrDA VFIR 802.11b WCDMA (3.5G) Bluetooth Bluetooth Bluetooth Class 1 Class 2 Class 3 IrDA SIR Zigbee Range (meters) Figure 1-1: Wireless Networking Landscape (rate vs. range) that Ericsson started research on connectivity between mobile phones and accessories that led to the adoption of Bluetooth by the IEEE 802.15.1 Working Group in 1999. During this period of rapid development, the variety of wireless networking technologies has expanded to fill the full range of require

ments for data rate (both high and low), operating range (long and short) and power consumption (low and very low), as shown in Figure 1-1. The Diversity of Wireless Networking Technologies Wireless networks now operate over four orders of magnitude in data rate (from ZigBee at 20 kbps to wireless USB at over 500 Mbps), and six orders of magnitude in range (from NFC at 5 cm to WiMAX, and also Wi-Fi, at over 50 km). To deliver this breadth of capabilities, the many companies, research institutions and individual engineers who have contributed to these developments have called into service a remarkable range of technologies; from Frequency Hopping Spread Spectrum, the inspired World War II invention of a film actress and a screen composer that is the basis of the Bluetooth radio, to Low Density Parity Check Codes, a breakthrough in high efficiency data transmission that lay gathering dust for forty years Introducing Wireless Networking after its development in 1963 and has proved to be an enabling technology in the most recent advances towards gigabit wireless networks. Technologies that started from humble origins, such as OFDM - used in the 1980s for digital broadcasting, have been stretched to new limits and combined with other concepts, SO that Ultra Wideband (UWB) radio now uses multi-band OFDM over 7 GHz of radio spectrum with a transmitted power below the FCC noise limit, while OFDM combined with Multi- Carrier Code Division Multiple Access is another gigabit wireless network enabler. Techniques to satisfy the every growing demand for higher data rates have gone beyond the relatively simple approaches of shortening the time to transmit each bit, using both the phase and amplitude of the carrier to convey data or just using more radio bandwidth, as in UWB radio, and arrived at the remarkable concept of spatial diversity - of using the same space several times over for concurrent transmissions over multiple paths - as applied in MIMO radio. This fascinating breadth and variety of technologies is the first moti

vation behind this book, which aims to give the reader an insight into these technologies of sufficient depth to gain an understanding of the fundamentals and appreciate the diversity, while avoiding getting down to the level of detail that would be required by a system developer. As well as seeking to appeal to the reader who wants to gain this technical insight, the book also aims to use this understanding of the principles of wireless networking technologies as a foundation on which, a discussion of the practical aspects of wireless network implementation can be grounded. Organisation of the Book This book is arranged in seven parts, with Parts I and II providing an introduction to wireless networking and to wireless communication that lays the foundation for the more detailed, technical and practical discussion of the local, personal and metropolitan areas scales of wireless networking in Parts III to V. Part I - Wireless Network Architecture - introduces the logical and physical architecture of wireless networks. The 7 layers of the OSI Chapter One network model provide the framework for describing the protocols and technologies that constitute the logical architecture, while wireless network topologies and hardware devices are the focus of the discussion of the physical architecture. Some of the key characteristics of wired networking technologies are also briefly described in the two chapters of Part I, in order to provide a background to the specific challenges addressed by wireless technologies. In Part II - Wireless Communication - the basics of wireless communication are described; spread spectrum, signal coding and modulation, multiplexing and media access methods and RF signal propagation including the important topic of the link budget. Several new or emerging radio communication technologies such as ultra wideband, MIMO radio and Near Field Communications are introduced. Part II closes with a similar overview of aspects of infrared communications. Part III - Wireless LAN Implementation - focuses on

what is perhaps the most important operating scale for wireless networks - the local area network. Building on the introductory description of Part I, local area wireless networking technologies are reviewed in more detail - including the full alphabet of 802.11 standards and enhancements. The practical aspects of wireless LAN implementation are then described, from the identification of user requirements through planning, pilot testing, installation, configuration and support. A chapter is devoted to the important topic of wireless LAN security, covering both the standards enhancements and practical security measures, and Part III closes with a chapter on wireless LAN troubleshooting. Part IV - Wireless PAN Implementation - takes a similar detailed look at wireless networking technologies on the personal area scale, including Bluetooth, wireless USB, ZigBee, IrDA and Near Field Communications. The practical aspects of wireless PAN implementation and security are covered in the final chapter of Part IV. Part V - Wireless MAN Implementation - looks at how the metropolitan area networking challenges of scalability, flexibility and quality of service have been addressed by wireless MAN standards, particularly WiMax. Non-IEEE MAN standards are briefly described, as well as metropolitan area mesh networks. Introducing Wireless Networking The practical aspects of wireless MAN implementation are discussed, including technical planning, business planning and issues that need to be addressed in the start-up and operating phases of a wireless MAN. Part VI - The Future of Wireless Networking Technology - looks at four emerging technologies - namely wireless mesh routing, network independent handover, gigabit wireless LANs and cognitive radio - that, taken together, look set to fulfil the promise of ubiquitous wireless accessibility and finally lay to rest the recurring technical challenges of bandwidth, media access, QoS and mobility. Finally Part VII - Wireless Networking Information Resources - provides a quick reference

guide to some of the key online information sites and resources relating to wireless networking, a comprehensive listing of acronyms and a glossary covering the key technical terms used throughout the book. This page intentionally left blank WIRELESS NETWORK ARCHITECTURE Introduction In the next two chapters, the logical and physical architecture of wireless networks will be introduced. The logical architecture is introduced in terms of the 7 layers of the OSI network model and the protocols that operate within this structure, with an emphasis on the Network and Data Link aspects that are most relevant to wireless networking - IP addressing, routing, link control and media access. Physical layer technologies are introduced, as a precursor to the more detailed descriptions later in the book, and the physical architecture of wireless networks is described, focussing on wireless network topologies and hardware devices. At each stage, some of the key characteristics of wired networking technologies are also briefly described, as a preliminary to the introduction of wireless networking technologies, in order to provide a background to the specific challenges addressed by wireless technologies, such as media access control. After this introduction, Part II will describe the basic concepts and technologies of wireless communication - both radio frequency and infrared. This page intentionally left blank CHAPTER Wireless Network Logical Architecture The logical architecture of a network refers to the structure of standards and protocols that enable connections to be established between physical devices, or nodes, and which control the routing and flow of data between these nodes. Since logical connections operate over physical links, the logical and physical architectures rely on each other, but the two also have a high degree of independence, as the physical configuration of a network can be changed without changing its logical architecture, and the same physical network can in many cases support different sets of stand

ards and protocols. The logical architecture of wireless networks will be described in this chapter with reference to the OSI model. The OSI Network Model The Open Systems Interconnect (OSI) model was developed by the International Standards Organisation (ISO) to provide a guideline for the development of standards for interconnecting computing devices. The OSI model is a framework for developing these standards rather than a standard itself - the task of networking is too complex to be handled by a single standard. The OSI model breaks down device to device connection, or more correctly application to application connection, into seven so-called "layers" of logically related tasks (see Table 2-1). An example will show Chapter Two Table 2-1: The Seven Layers of the OSI Model Layer Description Standards and Protocols 7 - Application layer Standards to define the provision HTTP, FTP, SNMP, of services to applications - such POP3, SMTP as checking resource availability, authenticating users, etc. 6 - Presentation layer Standards to control the translation of incoming and outgoing data from one presentation format to another. 5 - Session layer Standards to manage the ASAP, SMB communication between the presentation layers of the sending and receiving computers. This communication is achieved by establishing, managing and terminating "sessions". 4 - Transport layer Standards to ensure reliable TCP, UDP completion of data transfers, covering error recovery, data flow control, etc. Makes sure all data packets have arrived. 3 - Network layer Standards to define the IPv4, IPv6, ARP management of network connections - routing, relaying and terminating connections between nodes in the network. 2 - Data link layer Standards to specify the way in which devices access and share Ethernet the transmission medium (IEEE 802.3), Wi-Fi (known as Media Access Control (IEEE 802.11), or MAC) and to ensure reliability Bluetooth (802.15.1) of the physical connection (known as Logical Link Control or LLC). 1 - Physical layer Standards to

control transmission Ethernet, Wi-Fi, of the data stream over a particular Bluetooth, WiMAX medium, at the level of coding and modulation methods, voltages, signal durations and frequencies. Wireless Network Logical Architecture how these layers combine to achieve a task such as sending and receiving an e-mail between two computers on separate local area networks (LANs) that are connected via the Internet. The process starts with the sender typing a message into a PC e-mail application (Figure 2-1). When the user selects "Send", the operating system combines the message with a set of Application layer (Layer 7) instructions that will eventually be read and actioned by the corresponding operating system and application on the receiving computer. The message plus Layer 7 instructions is then passed to the part of sender's operating system that deals with Layer 6 presentation tasks. These include the translation of data between application layer formats as well as some types of security such as Secure Socket Layer (SSL) encryption. This process continues down through the successive software layers, with the message gathering additional instructions or control elements at each level. By Layer 3 - the Network layer - the message will be broken down into a sequence of data packets, each carrying a source and destination Sender writes e-mail message Recipient reads e-mail message Message is prepared and Layer 7 Message is received by the e-mail "sent" from an e-mail application Application layer application and read by the addressee Layer 6 Message is broken into presentation and Session and Presentation layer control session elements. Presentation and Presentation layer headers are successively removed. session layer control headers are Layer 5 Messages reassembled into a specific successively added format for the receiving e-mail application Session layer Message is broken into packets and Layer 4 Packet reception and sequencing controlled, transport layer control header added Transport layer data reassembled into Lay

er 5 messages. Data frame created from data packet + Layer 3 Frame headers removed, payloads network addresses + Layer 3 header Network layer reassembled into data packets Data frame encrypted, frame control Layer 2 Bit stream structured into frames, header added, network addresses decrypted, and checked for destination translated into MAC addresses Data Link layer MAC addresses Access gained to physical medium, bit Layer 1 Received signals are continuously stream coded and modulated onto PHY demodulated, decoded and bits stream layer signals and transmitted Physical layer are set to Data Link Layer Figure 2-1: The OSI Model in Practice - an E-mail Example Chapter Two IP address. At the Data Link layer the IP address is "resolved" to determine the physical address of the first device that the sending computer needs to transmit frames to the so-called MAC or media access control address. In this example, this device may be a network switch that the sending computer is connected to or the default gateway to the Internet from the sending computer's LAN. At the physical layer, also called the PHY layer, the data packets are encoded and modulated onto the carrier medium - a twisted wire pair in the case of a wired network, or electromagnetic radiation in the case of a wireless network - and transmitted to the device with the MAC address resolved at Layer 2. Transmission of the message across the Internet is achieved through a number of device-to-device hops involving the PHY and Data Link layers of each routing or relaying device in the chain. At each step, the Data Link layer of the receiving device determines the MAC address of the next immediate destination, and the PHY layer transmits the packet to the device with that MAC address. On arrival at the receiving computer, the PHY layer will demodulate and decode the voltages and frequencies detected from the transmission medium, and pass the received data stream up to the Data Link layer. Here the MAC and LLC elements, such as a message integrity check, will be extra

cted from the data stream and executed, and the message plus instructions passed up the protocol stack. At Layer 4, a protocol such as Transport Control Protocol (TCP), will ensure that all data frames making up the message have been received and will provide error recovery if any frames have gone missing. Finally the e-mail application will receive the decoded ASCII characters that make up the original transmitted message. Standards for many layers of the OSI model have been produced by various organisations such as the Institute of Electrical and Electronics Engineers (IEEE). Each standard details the services that are provided within the relevant layer and the protocols or rules that must be followed to enable devices or other layers to call on those services. In fact, multiple standards are often developed for each layer, and they either compete until one emerges as the industry "standard" or else they peacefully coexist in niche areas. The logical architecture of a wireless network is determined principally by standards that cover the Data Link (LLC plus MAC) and PHY layers of Wireless Network Logical Architecture the OSI model. The following sections will give a preliminary introduction to these standards and protocols, while more detailed descriptions will be found in Parts III to V where Local Area (LAN), Personal Area (PAN) and Metropolitan Area (MAN) wireless networking technologies are described respectively. The next section starts this introductory sketch one layer higher - at the Network layer - not because this layer is specific to wireless networking, but because of the fundamental importance of its addressing and routing functions and of the underlying Internet Protocol (IP). Network Layer Technologies The Internet Protocol (IP) is responsible for addressing and routing each data packet within a session or connection set up under the control of transport layer protocols such as TCP or UDP (see Glossary). The heart of the Internet Protocol is the IP address, a 32-bit number that is attached to eac

h data packet and is used by routing software in the network or Internet to establish the source and destination of each packet. While IP addresses, which are defined at the Network layer, link the billions of devices connected to the Internet into a single virtual network, the actual transmission of data frames between devices relies on the MAC addresses of the network interface cards (NICs), rather than the logical IP addresses of each NIC's host device. Translation between the Layer 3 IP address and the Layer 2 MAC address is achieved using Address Resolution Protocol (ARP), which is described in the Section "Address Resolution Protocol, p. 16". IP Addressing The 32-bit IP address is usually presented in "dot decimal" format as a series of four decimal numbers between 0 and 255, for example; 200.100.50.10. This could be expanded in full binary format as 11001000.01100100.00110010.00001010 As well as identifying a computer or other networked device, the IP address also uniquely identifies the network that the device is connected to. These two parts of the IP address are known as the host ID and the network ID. The network ID is important because it allows a device Chapter Two transmitting a data packet to know what the first port of call needs to be in the route to the packet's destination. If a device determines that the network ID of the packet's destination is the same as its own network ID, then the packet does not need to be externally routed, for example through the network's gateway and out onto the Internet. The destination device is on its own network and is said to be "local" (Table 2-2). On the other hand, if the destination network ID is different from its own, the destination is a remote IP address and the packet will need to be routed onto the Internet or via some other network bridge to reach its destination. The first stage in this will be to address the packet to the network's gateway. This process uses two more 32-bit numbers, the "subnet mask" and the "default gateway". A device determines th

e network ID for a data packet destination by doing a "logical AND" operation on the packet's destination IP address and its own subnet mask. The device determines its own network ID by doing the same operation using its own IP address and subnet mask. Table 2-2: Local and Remote IP Addresses Sending Device IP Address: 200.100.50.10 11001000.01100100.00110010.00001010 Subnet Mask: 255.255.255.240 11111111.11111111.11111111.11110000 Network ID: 200.100.50.000 11001000.01100100.00110010.00000000 Local IP address IP Address: 200.100.50.14 11001000.01100100.00110010.00001110 Subnet Mask: 255.255.255.240 11111111.11111111.11111111.11110000 Network ID: 200.100.50.000 11001000.01100100.00110010.00000000 Remote IP address IP Address: 200.100.50.18 11001000.01100100.00110010.00010010 Subnet Mask: 255.255.255.240 11111111.11111111.11111111.11110000 Network ID: 200.100.50.016 11001000.01100100.00110010.00010000 Wireless Network Logical Architecture Private IP Addresses In February 1996, the Network Working Group requested industry comments on RFC 1918, which proposed three sets of so-called private IP addresses (Table 2-3) for use within networks that did not require Internet connectivity. These private addresses were intended to conserve IP address space by enabling many organisations to reuse the same sets of addresses within their private networks. In this situation it did not matter that a computer had an IP address that was not globally unique, provided that that computer did not need to communicate via the Internet. Table 2-3: Private IP Address Ranges Class Private address range start Private address range end 10.0.0.0 10.255.255.255 172.16.0.0 172.31.255.255 192.168.0.0 192.168.255.255 Subsequently, the Internet Assigned Numbers Authority (IANA) reserved addresses 169.254.0.0 to 169.254.255.255 for use in Automatic Private IP Addressing (APIPA). If a computer has its TCP/IP configured to obtain an IP address automatically from a DHCP server, but is unable to locate such a server, then the operating system will autom

atically assign a private IP address from within this range, enabling the computer to communicate within the private network. Internet Protocol Version 6 (IPv6) With 32 bits, a total of 232 or 4.29 billion IP addresses are possible - more than enough one would think for all the computers that the human population could possibly want to interconnect. However, the famous statements that the world demand for computers would not exceed five machines, probably incorrectly attributed to Tom Watson Sr., chairman of IBM in 1943, or the statement of Ken Olsen, founder of Digital Equipment Corporation (DEC), to the 1977 World Future Society convention that "there is no reason for any individual to have a computer in his home", remind us how difficult it is to predict the growth and diversity of computer applications and usage. Chapter Two The industry is now working on IP version 6, which will give 128-bit IP addresses based on the thinking that a world population of 10 billion by 2020 will eventually be served by many more than one computer each. IPv6 will give a comfortable margin for future growth, with 3.4 X 1038 possible addresses - that is, 3.4 X 1027 for each of the 10 billion population, or 6.6 X 1023 per square metre of the earth's surface. It seems doubtful that there will ever be a need for IPv7, although, to avoid the risk of joining the short list of famously mistaken predictions of trends in computer usage, it may be as well to add the caveat "on this planet". Address Resolution Protocol As noted above, each PHY layer data transmission is addressed to the (Layer 2) MAC address of the network interface card of the receiving device, rather than to its (Layer 3) IP address. In order to address a data packet, the sender first needs to find the MAC address that corresponds to the immediate destination IP address and label the data packet with this MAC address. This is done using Address Resolution Protocol (ARP). Conceptually, the sending device broadcasts a message on the network that requests the device with a c

ertain IP address to respond with its MAC address. The TCP/IP software operating in the destination device replies with the requested address and the packet can be addressed and passed on to the sender's Data Link layer. In practice, the sending device keeps a record of the MAC addresses of devices it has recently communicated with, SO it does not need to broadcast a request each time. This ARP table or "cache" is looked at first and a broadcast request is only made if the destination IP address is not in the table. In many cases, a computer will be sending the packet to its default gateway and will find the gateway's MAC address from its ARP table. Routing Routing is the mechanism that enables a data packet to find its way to a destination, whether that is a device in the next room or on the other side of the world. A router compares the destination address of each data packet it receives with a table of addresses held in memory - the router table. If it finds a Wireless Network Logical Architecture match in the table, it forwards the packet to the address associated with that table entry, which may be the address of another network or of a "next-hop" router that will pass the packet along towards its final destination. If the router can't find a match, it goes through the table again looking at just the network ID part of the address (extracted using the subnet mask as described above). If a match is found, the packet is sent to the associated address or, if not, the router looks for a default next-hop address and sends the packet there. As a final resort, if no default address is set, the router returns a "Host Unreachable" or "Network Unreachable" message to the sending IP address. When this message is received it usually means that somewhere along the line a router has failed. What happens if, or when, this elegantly simple structure breaks down? Are there packets out there hopping forever around the Internet, passing from router to router and never finding their destination? The IP header includes a control

field that prevents this from happening. The time-to-live (TTL) field is initialised by the sender to a certain value, usually 64, and reduced by one each time the packet passes through a router. When TTL get down to zero, the packet is discarded and the sender is notified using an Internet Control Message Protocol (ICMP) "time-out" message. Building Router Tables The clever part of a router's job is building its routing table. For simple networks a static table loaded from a start-up file is adequate but, more generally, Dynamic Routing enables tables to be built up by routers sending and receiving broadcast messages. These can be either ICMP Router Solicitation and Router Advertisement messages which allow neighbouring routers to ask "Who's there?" and respond "I'm here", or more useful RIP (Router Information Protocol) messages, in which a router periodically broadcasts its complete router table onto the network. Other RIP and ICMP messages allow routers to discover the shortest path to an address, to update their tables if another router spots an inefficient routing and to periodically update routes in response to network availability and traffic conditions. Chapter Two A major routing challenge occurs in mesh or mobile ad-hoc networks (MANETs), where the network topology may be continuously changing. One approach to routing in MANETs, inspired by ant behaviour, is described in the Section "Wireless Mesh Network Routing, p. 345". Network Address Translation As described in the Section "Private IP Address, p. 15", RFC 1918 defined three sets of private IP addresses for use within networks that do not require Internet connectivity. However, with the proliferation of the Internet and the growing need for computers in these previously private networks to go online, the limitation of this solution to conserving IP addresses soon became apparent. How could a computer with a private IP address ever get a response from the Internet, when its IP address would not be recognised by any router out in the Internet as a v

to private addresses as required. Needless to say, dynamic NAT is by far the most common, as it is automatic and requires no intervention or maintenance. Port Address Translation One complication arises if the private network's gateway has only a single public IP address available to assign, or if more computers in a private network try to connect than there are IP addresses available to the gateway. This will often be the case for a small organisation with a single Internet connection to an ISP. In this case, it would seem that only one computer within the private network would be able to connect to the Internet at a time. Port Address Translation (PAT) overcomes this limitation by mapping private IP addresses to different port numbers attached to the single public IP address. When a computer within the private network sends a data packet to be routed to the Internet, the gateway replaces the source address with the single public IP address together with a random port number between 1024 and 65536 (Figure 2-2). When a data packet is returned with this destination Chapter Two Table 2-5: Example of a Simple PAT Table Private IP address Public IP address:Port 192.168.0.1 129.35.78.178:2001 192.168.0.2 129.35.78.178:2002 192.168.0.3 129.35.78.178:2003 192.168.0.4 129.35.78.178:2004 address and port number, the PAT table (Table 2-5) enables the gateway to route the data packet to the originating computer in the private network. Data Link Layer Technologies The Data Link layer is divided into two sub-layers - Logical Link Control (LLC) and Media Access Control (MAC). From the Data Link layer down, data packets are addressed using MAC addresses to identify the specific physical devices that are the source and destination of packets, rather than the IP addresses, URLs or domain names used by the higher OSI layers. Logical Link Control Logical Link Control (LLC) is the upper sub-layer of the Data Link layer (Figure 2-3), and is most commonly defined by the IEEE 802.2 standard. It provides an interface that enables the Ne

twork layer to work with any type of Media Access Control layer. Logical Link Control layer (LLC) Layer 2 Data Link layer Medium Access Control layer (MAC) Layer 1 Physical layer (PHY) Physical layer OSI model layers IEEE 802 specifications Figure 2-3: OSI Layers and IEEE 802 Specifications Wireless Network Logical Architecture OSI Network layer LLC SAP Logical Link Control layer (LLC) MAC SAP Medium Access Control layer (MAC) Figure 2-4: Logical Location of LLC and MAC Service Access Points A frame produced by the LLC and passed down to the MAC layer is called an LLC Protocol Data Unit (LPDU), and the LLC layer manages the transmission of LPDUs between the Link Layer Service Access Points of the source and destination devices. A Link Layer Service Access Point (SAP) is a port or logical connection point to a Network layer protocol (Figure 2-4). In a network supporting multiple Network layer protocols, each will have specific Source SAP (SSAP) and Destination SAP (DSAP) ports. The LPDU includes the 8-bit DSAP and SSAP addresses to ensure that each LPDU is passed on receipt to the correct Network layer protocol. The LLC layer defines connectionless (Type 1) and connection oriented (Type 2) communication services and, in the latter case, the receiving LLC layer keeps track of the sequence of received LPDUs. If an LPDU is lost in transit or incorrectly received, the destination LLC requests the source to restart the transmission at the last received LPDU. The LLC passes LPDUs down to the MAC layer at a logical connection point known as the MAC Service Access Point (MAC SAP). The LPDU is then called a MAC Service Data Unit (MSDU) and becomes the data payload for the MAC layer. Media Access Control The second sub-layer of the Data Link layer controls how and when a device is allowed to access the PHY layer to transmit data, this is the Media Access Control or MAC layer. In the following sections, the addressing of data packets at the MAC level is first described. This is followed by a brief look at MAC methods Chapter

Two applied in wired networks, which provides an introduction to the more complex solutions required for media access control in wireless networks. MAC Addressing A receiving device needs to be able to identify those data packets transmitted on the network medium that are intended for it - this is achieved using MAC addresses. Every network adapter, whether it is an adapter for Ethernet, wireless or some other network technology, is assigned a unique serial number called its MAC address when it is manufactured. The Ethernet address is the most common form of MAC address and consists of six bytes, usually displayed in hexadecimal, such as 00-D0-59- FE-CD-38. The first three bytes are the manufacturer's code (00-D0-59 in this case is Intel) and the remaining three are the unique serial number of the adapter. The MAC address of a network adapter on a Windows PC can be found in Windows 95, 98 or Me by clicking Start, Run, and then typing "winipcfg", and selecting the adapter, or in Windows NT, 2000, and XP by opening a DOS Window (click Start, Programs, Accessories, Command Prompt) and typing "ipconfig/all". When an application such as a web browser sends a request for data onto the network, the Application layer request comes down to the MAC SAP as an MSDU. The MSDU is extended with a MAC header that includes the MAC address of the source device's network adapter. When the requested data is transmitted back onto the network, the original source address becomes the new destination address and the network adapter of the original requesting device will detect packets with its MAC address in the header, completing the round trip. As an example, the overall structure of the IEEE 802.11 MAC frame, or MAC Protocol Data Unit (MPDU) is shown in Figure 2-5. The elements of the MPDU are as shown in Table 2-6. Media Access Control in Wired Networks If two devices transmit at the same time on a network's shared medium, whether wired or wireless, the two signals will interfere and the result will be unusable to both devices. Acc

ess to the shared medium therefore needs to be actively managed to ensure that the available bandwidth is not wasted through repeated collisions of this type. This is the main task of the MAC layer. Wireless Network Logical Architecture Length (bytes) 0 to 2312 Frame Address Address Address Address checksum Protocol Frame Frame version sub-type Length (bits) Management, Association Request/Response Control, Data Beacon, RTS, CTS, ACK, Figure 2-5: MAC Frame Structure Carrier Sense Multiple Access/Collision Detection (CSMA/CD) The most commonly used MAC method to control device transmission, and the one specified for Ethernet based networks, is Carrier Sense Multiple Access/Collision Detection (CSMA/CD) (Figure 2-6). When a device has a data frame to transmit onto a network that uses this method, it first checks the physical medium (carrier sensing) to see if any other device is already Table 2-6: Elements of the 802.11 MPDU Frame Structure MPDU element Description Frame control A sequence of flags to indicate the protocol version (802.11 a/b/g), frame type (management, control, data), sub-frame type (e.g. probe request, authentication, association request, etc.), fragmentation, retries, encryption, etc. Duration Expected duration of this transmission. Used by waiting stations to estimate when the medium will again be idle. Address 1 to Destination and source, plus optional to and from addresses Address 4 within the distribution system. Sequence Sequence number to identify frame fragments or duplicates. The data payload passed down as the MSDU. Frame check sequence A CRC-32 checksum to enable transmission errors to be detected. Chapter Two Device A Data packet Medium busy Medium free Collision Device B attempts to send Carrier Random sensing backoff Data packet Medium busy Medium free Collision Medium busy Medium free Device C attempts to send Carrier Random Carrier sensing backoff sensing Data packet Slot time Figure 2-6: Ethernet CSMA/CD Timing transmitting. If the device senses another transmitting device it wai

ts until the transmission has finished. As soon as the carrier is free it begins to transmit data, while at the same time continuing to listen for other transmissions. If it detects another device transmitting at the same time (collision detection), it stops transmitting and sends a short jam signal to tell other devices that a collision has occurred. Each of the devices that were trying to transmit then computes a random backoff period within a range 0 to tmax, and tries to transmit again when this period has expired. The device that by chance waits the shortest time will be the next to gain access to the medium, and the other devices will sense this transmission and go back into carrier sensing mode. A very busy medium may result in a device experiencing repeated collisions. When this happens tmax is doubled for each new attempt, up to a maximum of 10 doublings, and if the transmission is unsuccessful after 16 attempts the frame is dropped and the device reports an "excessive collision error". Other Wired Network MAC Methods Another common form of media access control for wired networks, defined by the IEEE 802.5 standard, involves passing an electronic "token" between devices on the network in a pre-defined sequence. The token is similar to a baton in a relay race in that a device can only transmit when it has captured the token. Wireless Network Logical Architecture If a device does not need control of the media to transmit data it passes the token on immediately to the next device in the sequence, while if it does have data to transmit it can do SO once it receives the token. A device can only keep the token and continue to use the media for a specific period of time, after which it has to pass the token on to the next device in the sequence. Media Access Control in Wireless Networks The collision detection part of CSMA/CD is only possible if the PHY layer transceiver enables the device to listen to the medium while transmitting. This is possible on a wired network, where invalid voltages resulting from coll

isions can be detected, but is not possible for a radio transceiver since the transmitted signal would overload any attempt to receive at the same time. In wireless networks such as 802.11, where collision detection is not possible, a variant of CSMA/CD known as CSMA/CA is used, where the CA stands for Collision Avoidance. Apart from the fact that collisions are not detected by the transmitting device, CSMA/CA has some similarities with CSMA/CD. Devices sense the medium before transmitting and wait if the medium is busy. The duration field in each transmitted frame (see preceding Table 2-6) enables a waiting device to predict how long the medium will be busy. Once the medium is sensed as being idle, waiting devices compute a random time period, called the contention period, and attempt to transmit after the contention period has expired. This is a similar mechanism to the back-off in CSMA/CD, except that here it is designed to avoid collisions between stations that are waiting for the end of another station's transmitted frame rather than being a mechanism to recover after a detected collision. CSMA/CA is further described in the Section "The 802.11 MAC Layer, p. 144", where the 802.11 MAC is discussed in more detail, and variations on CSMA/CA used in other types of wireless network will be described as they are encountered. Physical Layer Technologies When the MPDU is passed down to the PHY layer, it is processed by the PHY Layer Convergence Procedure (PLCP) and receives a preamble and header, which depend on the specific type of PHY layer in use. The PLCP Chapter Two preamble contains a string of bits that enables a receiver to synchronise its demodulator to the incoming signal timing. The preamble is terminated by a specific bit sequence that identifies the start of the header, which in turn informs the receiver of the type of modulation and coding scheme to be used to decode the upcoming data unit. The assembled PLCP Protocol Data Unit (PPDU) is passed to the Physical Medium Dependent (PMD) sublayer, which tr

ansmits the PPDU over the physical medium, whether that is twisted-pair, fibre-optic cable, infra-red or radio. PHY layer technologies determine the maximum data rate that a network can achieve, since this layer defines the way the data stream is coded onto the physical transmission medium. However, the MAC and PLCP headers, preambles and error checks, together with the idle periods associated with collision avoidance or backoff, mean that the PMD layer actually transmits many more bits than are passed down to the MAC SAP by the Data Link layer. The next sections look at some of the PHY layer technologies applied in wired networks and briefly introduces the key features of wireless PHY technologies. Physical Layer Technologies - Wired Networks Most networks that use wireless technology will also have some associated wired networking elements, perhaps an Ethernet link to a wireless access point, a device-to-device Fire Wire or USB connection, or an ISDN based Internet connection. Some of the most common wired PHY layer technologies are described in this section, as a precursor to the more detailed discussion of local, personal and metropolitan area wireless network PHY layer technologies in Parts III to V. Ethernet (IEEE 802.3) The first of these, Ethernet, is a Data Link layer LAN technology first developed by Xerox and defined by the IEEE 802.3 standard. Ethernet uses Carrier Sense Multiple Access with Collision Detection (CSMA/CD), described above, as the media access control method. Ethernet variants are known as "A" Base-"B" networks, where "A" stands for the speed in Mbps and "B" identifies the type of physical medium Wireless Network Logical Architecture used. 10 Base-T is the standard Ethernet, running at 10 Mbps and using an unshielded twisted-pair copper wire (UTP), with a maximum distance of 500 metres between a device and the nearest hub or repeater. The constant demand for increasing network speed has meant that faster varieties of Ethernet have been progressively developed. 100 Base-T, or Fast Ethern

et operates at 100 Mbps and is compatible with 10 Base-T standard Ethernet as it uses the same twisted-pair cabling and CSMA/CD method. The trade-off is with distance between repeaters, a maximum of 205 metres being achievable for 100 Base-T. Fast Ethernet can also use other types of wiring - 100 Base-TX, which is a higher-grade twisted-pair, or 100 Base-FX, which is a two strand fibre-optic cable. Faster speeds to 1 Gbps or 10 Gbps are also available. The PMD sub-layer is specified separately from the Ethernet standard, and for UTP cabling this is based on the Twisted Pair-Physical Medium Dependent (TP-PMD) specification developed by the ANSI X3T9.5 committee. The same frame formats and CSMA/CD technology are used in 100 Base-T as in standard 10 Base-T Ethernet, and the 10-fold increase in speed is achieved by increasing the clock speed from 10 MHz to 125 MHz, and reducing the interval between transmitted frames, known as the Inter-Packet Gap (IPG), from 9.6 us to 0.96 us. A 125 MHz clock speed is required to deliver a 100 Mbps effective data rate because of the 4B/5B encoding described below. Input bit stream 4B/5B encoding 4-bit 5-bit 4-bit 5-bit nibble symbol nibble symbol 11110 10010 01001 10011 Feedback Shift Register 10100 10110 10101 10111 01010 11010 01011 11011 01110 11100 MLT-3 coding Output 01111 11101 Figure 2-7: 100 Base-T Ethernet Data Encoding Scheme Chapter Two To overcome the inherent low-pass nature of the UTP physical medium, and to ensure that the level of RF emissions above 30 MHz comply with FCC regulations, the 100 Base-T data encoding scheme was designed to bring the peak power in the transmitted data signal down to 31.25 MHz (close to the FCC limit) and to reduce the power in high frequency harmonics at 62.5 MHz, 125 MHz and above. 4B/5B encoding is the first step in the encoding scheme (Figure 2-7). Each 4-bit nibble of input data has a 5th bit added to ensure there are sufficient transitions in the transmitted bit stream to allow the receiver to synchronise for reliable decoding. In th

e second step an 11-bit Feedback Shift Register (FSR) produces a repeating pseudo-random bit pattern which is XOR'd with the 4B/5B output data stream. The effect of this pseudo-randomisation is to minimise high frequency harmonics in the final transmitted data signal. The same pseudo-random bit stream is used to recover the input data in a second XOR operation at the receiver. The final step uses an encoding method called Multi-Level Transition 3 (MLT-3) to shape the transmitted waveform in such a way that the centre frequency of the signal is reduced from 125 MHz to 31.25 MHz. MLT-3 is based on the repeating pattern 1, 0, -1, 0. As shown in Figure 2-8, an input 1-bit causes the output to transition to the next bit in the pattern while an input 0-bit causes no transition, i.e. the output level remaining unchanged. Compared to the Manchester Phase Encoding (MPE) scheme used in 10 Base-T Ethernet, the cycle length of the output signal is reduced by a factor of 4, giving a signal peak at 31.25 MHz instead of 125 MHz. On the physical UTP medium, the 1, 0 and -1 signal levels are represented by line voltages of +0.85, 0.0 and -0.85 Volts. 1111111111111111 Input bit stream MPE coded bit stream MLT-3 coded bit stream Figure 2-8: Ethernet MPE and Fast Ethernet MLT-3 Encoding Wireless Network Logical Architecture ISDN, which stands for Integrated Services Digital Network, allows voice and data to be transmitted simultaneously over a single pair of telephone wires. Early analogue phone networks were inefficient and error prone as a long distance data communication medium and, since the 1960s, have gradually been replaced by packet-based digital switching systems. The International Telephone and Telegraph Consultative Committee (CCITT), the predecessor of the International Telecommunications Union (ITU), issued initial guidelines for implementing ISDN in 1984, in CCITT Recommendation I.120. However, industry-wide efforts to establish a specific implementation for ISDN only started in the early 1990s when US industry members

agreed to create the National ISDN 1 standard (NI-1). This standard, later superseded by National ISDN 2 (NI-2), ensured the interoperability of end user and exchange equipment. Two basic types of ISDN service are defined - Basic Rate Interface (BRI) and Primary Rate Interface (PRI). ISDN carries voice and user data streams on "bearer" (B) channels, typically occupying a bandwidth of 64 kbps, and control data streams on "demand" (D) channels, with a 16 kbps or 64 kbps bandwidth depending on the service type. BRI provides two 64 kbps B channels, which can be used to make two simultaneous voice or data connections or can be combined into one 128 kbps connection. While the B channels carry voice and user data transmission, the D channel is used to carry Data Link and Network layer control information. The higher capacity PRI service provides 23 B channels plus one 64 kbps D channel in the US and Japan, or 30 B channels plus one D channel in Europe. As for BRI, the B channels can be combined to give data bandwidths of 1472 kbps (US) or 1920 kbps (Europe). As noted above, telephone wires are not ideal as a digital communication medium. The ISDN PHY layer limits the effect of line attenuation, near- end and far-end crosstalk and noise by using Pulse Amplitude Modulation (PAM) technology (see the Section "Pulse Modulation Methods, p. 104") to achieve a high data rate at a reduced transmission rate on the line. This is achieved by converting multiple (often two or four) binary bits into a single multilevel transmitted symbol. In the US, the 2B1Q method Chapter Two Input bit stream Input "dibit" stream +2.5 V +0.83 V -0.83 V 2B1Q line voltage -2.5 V Figure 2-9: 2B1Q Line Code Using in ISDN is used, which converts two binary bits (2B) into a single output symbol, known as a "quat" (1Q), which can have one of four values (Figure 2-9 and Table 2-7). This effectively halves the transmission rate on the line, SO that a 64 kbps data rate can be transmitted at a symbol rate of 32 ksps, achieving higher data rates within the lim

ited bandwidth of the telephone system. As well as defining a specific PHY layer, ISDN also specifies Data Link and Network layer operation. LAP-D (Link Access Protocol D-channel) is a Data Link protocol, defined in ITU-T Q.920/921, that ensures error free transmission on the PHY layer. Two Network layer protocols are defined in ITU-T Q.930 and ITU-T Q.931 to establish, maintain and terminate user-to-user, circuit-switched, and packet-switched network connections. FireWire FireWire, also known as IEEE 1394 or i.Link, was developed by Apple Computer Inc. in the mid-1990s as a local area networking technology. At that time it provided a 100 Mbps data rate, well above the Universal Serial Bus (USB) speed of 12 Mbps, and it was soon taken up Table 2-7: 2B1Q Line Code Used in ISDN Input "DIBIT" Output "QUAT" Line voltage +.833 -.833 Wireless Network Logical Architecture Computer A Digital video camera Firewire Bridge Digital video recorder Firewire splitter Printer 1 Firewire repeater Printer 2 Computer B Figure 2-10: FireWire Network Topology: Daisy-chain and Tree Structures by a number of companies for applications such as connecting storage and optical drives. Fire Wire is now supported by many electronics and computer companies, often under the IEEE 1394 banner, because of its ability to reliably and inexpensively transmit digital video data at high speeds, over single cable lengths of up to 4.5 metres. The standard data rate is 400 Mbps, although a faster version is also available delivering 800 Mbps and with plans for 3.2 Gbps. Range can be extended up to 72 metres using signal repeaters in a 16-link daisy chain, and Fire Wire to fibre transceivers are also available that replace the copper cable by optical fibre and can extend range to 40 km. A generic Fire Wire topology is shown in Figure 2-10. The FireWire standard defines a serial input/output port and bus, a 4 or 6 wire dual-shielded copper cable that can carry both data and power, and the related Data Link, Network and Transport layer protocols. Fire Wire

is based on the Control and Status Register Management (CSR) architecture, which means that all interconnected devices appear as a single memory of up to 256 Terabytes (256 X 1012 bytes). Each transmitted packet of data contains three elements: a 10-bit bus ID that is used to determine which FireWire bus the data packet originated from, a 6-bit ID that identifies which device or node on that bus sent the data packet, and a 48-bit offset that is used to address registers and memory in a node. While primarily used for inter-device communication, The Internet Society has combined IP with the FireWire standard to produce a standard Chapter Two called IP over IEEE 1394, or IP 1394. This makes it possible for networking services such as FTP, HTTP and TCP/IP to run on the high speed FireWire PHY layer as an alternative to Ethernet. An important feature of Fire Wire is that the connections are "hot-swappable", which means that a new device can be connected, or an existing device disconnected, while the connection is live. Devices are automatically assigned node IDs, and these IDs can change as the network topology changes. Combining the node ID variability of FireWire with the IP requirement for stable IP addresses of connected devices, presents one of the interesting problems in enabling IP connections over FireWire. This is solved using a special Address Resolution Protocol (ARP) called 1394 ARP. In order to uniquely identify a device in the network, 1394 ARP uses the 64-bit Extended Unique Identifier (EUI-64), a unique 64-bit number that is assigned to every FireWire device on manufacture. This is an extended version of the MAC address, described in the Section "Media Access Control, p. 21" that is used to address devices other than network interfaces. A 48-bit MAC address can be converted into a 64-bit EUI-64 by prefixing the two hexadecimal octets "FF-FF". Universal Serial Bus The Universal Serial Bus (USB) was introduced in the mid-1990s to provide a hot-swappable "plug-and-play" interface that would replace differ

ent types of peripheral interfaces (parallel ports, serial ports, PS/2, MIDI, etc.) for devices such as joysticks, scanners, keyboards and printers. The maximum bandwidth of USB 1.0 was 12 Mbps, but this has since increased to a FireWire matching 480 Mbps with USB 2.0. USB uses a host-centric architecture, with a host controller dealing with the identification and configuration of devices connected either directly to the host or to intermediate hubs (Figure 2-11). The USB specification supports both isochronous and asynchronous transfer types over the same connection. Isochronous transfers require guaranteed bandwidth and low latency for applications such as telephony and media streaming, while asynchronous transfers are delay-tolerant and are able to wait for available bandwidth. USB control protocols are designed specifically to Wireless Network Logical Architecture Device Device Device Device Device Device Root Hub Tier 1 Device Device Tier 2 Device Device Tier 3 Device Device Device Figure 2-11: USB Network Topology: Daisy-chain and Tree Structures give a low protocol overhead, resulting in highly effective utilisation of the available bandwidth. This available bandwidth is shared among all connected devices and is allocated using "pipes", with each pipe representing a connection between the host and a single device. The bandwidth for a pipe is allocated when the pipe is established, and a wide range of different device bit rates and device types can be supported concurrently. For example, digital telephony devices can be concurrently accommodated ranging from 1 "bearer" plus 1 "demand" channel (64 kbps - see ISDN above) up to T1 capacity (1.544 Mbps). USB employs NRZI (Non Return to Zero Inverted) as a data encoding scheme. In NRZI encoding, a 1-bit is represented by no change in output voltage level and a 0-bit is represented by a change in voltage level (Figure 2-12). A string of 0-bits therefore causes the NRZI output to toggle between states on each bit cycle, while a string of 1-bits causes a period wit

h no transitions in the output. Chapter Two Data bits Non Return to zero (NRZ) Non Return to zero inverted (NRZI) period Figure 2-12: USB NRZI Data Encoding Scheme NRZI has the advantage of a somewhat improved noise immunity compared with the straight encoding of the input data stream as output voltages. Physical Layer Technologies - Wireless Networks The PHY layer technologies that provide the Layer 1 foundation for wireless networks will be described further in Parts III, IV and V, where LAN, PAN and MAN technologies and their implementations will be covered in detail. Each wireless PHY technology, from Bluetooth to ZigBee, will be described in terms of a number of key aspects, as summarised in Table 2-8. The range and significance of the issues vary depending on the type of technology (Ir, RF, Near-field) and its application (PAN, LAN or WAN). Operating System Considerations In order to support networking, an operating system needs as a minimum to implement networking protocols, such as TCP/IP, and the device drivers required for network hardware. Early PC operating systems, including Windows versions prior to Windows 95, were not designed to support networking. However, with the rise of the Internet and other networking technologies, virtually every operating system today qualifies as a network operating system (NOS). Individual network operating systems offer additional networking features such as firewalls, simplified set-up and diagnostic tools, remote access, Wireless Network Logical Architecture Table 2-8: Aspects of PHY and Data Link Layer Wireless Technologies Technology aspect Issues and considerations Spectrum What part of the electromagnetic spectrum is used, what is the overall bandwidth available, how is this segmented into channels? What mechanisms are available to control utilised bandwidth to ensure coexistence with other users of the same spectrum? Propagation What power levels are permitted by regulatory authorities in the spectrum in question? What mechanisms are available to control the tra

nsmitted power or propagation pattern to minimise co-channel interference for other users, maximise effective range or utilise spatial diversity to increase throughput? Modulation How is encoded data carried on the physical medium, for example by modulating one or more carriers in phase and/or amplitude, or by modulating pulses in amplitude and/or position? Data encoding How are the raw bits of a data frame coded into symbols for transmission? What functions do these coding mechanisms serve, for example increasing robustness to noise or increasing the efficient use of available bandwidth? Media access How is access to the transmission medium controlled to ensure that the bandwidth available for data transmission is maximised and that contention between users is efficiently resolved? What mechanisms are available to differentiate media access for users with differing service requirements? and inter-connection with networks running other operating systems, as well as support for network administration tasks such as enforcing common settings for groups of users. The choice of a network operating system will not be covered in detail here, but should be based on a similar process to that described in Parts III and IV for selecting WLAN and WPAN technologies. Start by determining networking service requirements such as security, file sharing, printing and messaging. The two main network operating systems are the Microsoft Windows and Novell NetWare suites of products. A key differentiator between these two products may be a requirement for interoperability support in networks that include other operating systems such as UNIX or Linux. NetWare is often the preferred NOS in mixed Chapter Two operating-system networks, while simplicity of installation and administration makes Windows the preferred product suite in small networks where technical support may be limited. Summary The OSI network model provides the conceptual framework to describe the logical operation of all types of networks, from a wireless PAN link between

a mobile phone and headset to the global operation of the Internet. The key features that distinguish different networking technologies, particularly wired and wireless, are defined at the Data Link (LLC and MAC) and physical (PHY) layers. These features will be covered in detail in Parts III to V which, above all, reveal the fascinating variety of different techniques that have been harnessed to bring wireless networks to life. CHAPTER Wireless Network Physical Architecture Wired Network Topologies - A Refresher The topology of a wired network refers to the physical configuration of links between networked devices or nodes, where each node may be a computer, an end-user device such as a printer or scanner, or some other piece of network hardware such as a hub, switch or router. The building block from which different topologies are constructed is the simple point-to-point wired link between two nodes, shown in Figure 3-1. Repeating this element results in the two simplest topologies for wired networks - bus and ring. For the ring topology, there are two possible variants depending on whether the inter-node links are simplex (one-way) or duplex (two-way). In the simplex case, each inter-node link has a transmitter at one end and a receiver at the other, and messages circulate in one direction around the ring, while in the duplex case each link has both transmitter and receiver (a so-called transceiver) at each end, and messages can circulate in either direction. Bus and ring topologies are susceptible to single-point failures, where a single broken link can isolate sections of a bus network or halt all traffic in the case of a ring. The step that opens up new possibilities is the introduction of specialised network hardware nodes designed to control the flow of data between Chapter Three Figure 3-1: Point-to-point, Bus and Ring Topologies other networked devices. The simplest of these is the passive hub, which is the central connection point for LAN cabling in star and tree topologies, as shown in Figure 3-2. An

active hub, also known as a repeater, is a variety of passive hub that also amplifies the data signal to improve signal strength over long network connections. For some PAN technologies, such as USB, star and tree topologies can be built without the need for specialised hardware, Figure 3-2: Star and Tree Topologies Wireless Network Physical Architecture Passive hub First PC Scanner Second PC Printer Figure 3-3: A Passive Hub in a Physical Star Network because of the daisy-chaining capability of individual devices (see Figure 2-11). An active or passive hub in a star topology LAN transmits every received data packet to every connected device. Each device checks every packet and decodes those identified by the device's MAC address. The disadvantage of this arrangement is that the bandwidth of the network is shared among all devices, as shown in Figure 3-3. For example, if two PCs are connected through a 10 Mbps passive hub, each will have on average 5 Mbps of bandwidth available to it. If the first PC is transmitting data, the hub relays the data packets on to all other devices in the network. Any other device on the network will have to wait its turn to transmit data. A switching hub (or simply a switch) overcomes this bandwidth sharing limitation by only transmitting a data packet to the device to which it is addressed. Compared to a non-switching hub, this requires increased memory and processing capability, but results in a significant improvement in network capacity. The first PC (Figure 3-4) is transmitting data stream A to the printer and the switch directs these data packets only to the addressed device. At the same time, the scanner is sending data stream B to the second PC. Chapter Three Switching hub First PC Scanner Second PC Printer Figure 3-4: Switching Hub in Physical Star Network. The switch is able to process both data stream concurrently, SO that the full network bandwidth is available to every device. Wireless Network Topologies Point to Point Connections The simple point to point connection sh

own in Figure 3-1 is probably more common in wireless than in wired networks, since it can be found in a wide variety of different wireless situations, such as: peer-to-peer or ad-hoc Wi-Fi connections wireless MAN back-haul provision LAN wireless bridging Bluetooth Star Topologies in Wireless Networks In wireless networks the node at the centre of a star topology (Figure 3-5), whether it is a WiMAX base station, Wi-Fi access point, Bluetooth Master device or a ZigBee PAN coordinator, plays a similar role to the hub in a wired network. As described in Parts III to V, the different wireless Wireless Network Physical Architecture Figure 3-5: Star Topologies in Wireless Networks networking technologies require and enable a wide range of different functions to be performed by these central control nodes. The fundamentally different nature of the wireless medium means that the distinction between switching and non-switching hubs is generally not relevant for control nodes in wireless networks, since there is no direct wireless equivalent of a separate wire to each device. The wireless LAN switch or controller (Figure 3-6), described in the Section "Wireless LAN Switches or Controllers, p. 48", is a wired network device that switches data to the access point that is serving the addressed destination station of each packet. The exception to this general rule arises when base stations or access point devices are able to spatially separate individual stations or groups of stations using sector or array antennas. Figure 3-7 shows a wireless MAN example, with a switch serving four base station transmitters each using a 90° sector antenna. With this configuration, the overall wireless MAN throughput is multiplied by the number of transmitters, similar to the case of the wired switching hub shown in Figure 3-4. Access point switch Figure 3-6: A Tree Topology Using a Wireless Access Point Switch Sector antennas Switching hub Figure 3-7: Switched Star Wireless MAN Topology Wireless Network Physical Architecture In the wireless

LAN case, a similar spatial separation can be achieved using a new class of device called an access point array, described below in the Section "Wireless LAN Arrays, p. 52", which combines a wireless LAN controller with an array of sector antennas to multiply network capacity. The general technique of multiplying network throughput by addressing separate spatial zones or propagation paths is known as space division multiplexing (Section "Space Division Multiple Access, p. 94"), and finds its most remarkable application in MIMO radio, described in the Section "MIMO Radio, p. 124". Mesh Networks Mesh networks, also known as mobile ad hoc networks (MANETs), are local or metropolitan area networks in which nodes are mobile and communicate directly with adjacent nodes without the need for central controlling devices. The topology of a mesh, shown generically in Figure 3-8, can be constantly changing, as nodes enter and leave the network, and data packets are forwarded from node-to-node towards their destination in a process called hopping. The data routing function is distributed throughout the entire mesh rather than being under the control of one or more dedicated devices. This is similar to the way that data travels around the Internet, with a packet hopping from one device to another until it reaches its destination, although in mesh networks, the routing capabilities are included in every node rather than just in dedicated routers. This dynamic routing capability requires each device to communicate its routing information to every device it connects with, and to update this as nodes move within, join and leave the mesh. This distributed control and continuous reconfiguration allows for rapid re-routing around overloaded, unreliable or broken paths, allowing mesh networks to be self-healing and very reliable, provided that the density of nodes is sufficiently high to allow alternative paths. A key challenge in the design of the routing protocol is to achieve this continuous re-configuration capability with a manag

eable overhead in terms of data bandwidth taken up by routing information messages. One approach to this problem, the biologically inspired AntHocNet, is described in the Section "Wireless Mesh Network Routing, p. 345". Chapter Three Internet Point of Presence Access points routers devices Wired connection Peer to peer connection Network connection Device to network connection Figure 3-8: Mesh Network Topology The multiplicity of paths in a mesh network has a similar impact on total network throughput as the multiple paths shown in Figures 3-4 and 3-7 for the case of wired network switches and sectorised wireless networks. Mesh network capacity will grow as the number of nodes, and therefore the number of usable alternative paths, grow, SO that capacity can be increased simply by adding more nodes to the mesh. As well as the problem of efficiently gathering and updating routing information, mesh networks face several additional technical challenges such as: Wireless link reliability - a packet error rate that may be tolerable over a single hop in an hub and spokes configuration will quickly compound over multiple hops, limiting the size to which a mesh can grow and remain effective. Seamless roaming - seamless connection and reconnection of moving nodes has not been a requirement in most wireless network standards, although 802.11 Task Groups TGr and TGs are addressing this. Security - how to authenticate users in a network with no stable infrastructure? From a practical standpoint, the self-configuring, self-optimising and self-healing characteristics of mesh networks eliminate many of the Wireless Network Physical Architecture management and maintenance tasks associated with large-scale wireless network deployments. ZigBee (Section "ZigBee (IEEE 802.15.4), p. 273") is one standard which explicitly supports mesh networks and the IEEE 802.11 Task Group TGs is in the process of developing a standard which addresses WLAN mesh networks. Two industry bodies have already been established to promote 802.11s mesh propos

als, the Wi-Mesh Alliance and SEEMesh (Simple, Efficient and Extensible Mesh). Wireless LAN Devices Wireless Network Interface Cards The wireless network interface card (NIC) turns a device such as a PDA, laptop or desktop computer into a wireless station and enables the device to communicate with other stations in a peer-to-peer network or with an access point. Wireless NICs are available in a variety of form factors (Figure 3-9), including PC (Type II PCMCIA) and PCI cards, as well as external USB devices and USB dongles, or compact flash for PDAs. Most wireless NICs Figure 3-9: A Variety of Wireless NIC Forms (courtesy of Belkin Corporation, D-Link (Europe) Ltd. and Linksys (a division of Cisco Systems Inc.)) Chapter Three have integrated antennas, but a few manufacturers provide NICs with an external antenna connection or detachable integrated antenna which can be useful to attach a high-gain antenna when operating close to the limit of wireless range. There are few features to distinguish one wireless NIC from another. Maximum transmitter power is limited by local regulatory requirements and, for standards based equipment, certification by the relevant body (such as Wi-Fi certification for 802.11) will ensure interoperability of equipment from different manufacturers. The exception will be proprietary extensions or equipment released prior to standard ratification, such as "pre-n" hardware announced by some manufacturers in advance of 802.11n ratification. High-end mobile products, particularly laptop computers, are increasingly being shipped with integrated wireless NICs, and with Intel's Centrino® technology the wireless LAN interface became part of the core chipset family. Access Points The access point (AP) is the central device in a wireless local area network (WLAN) that provides the hub for wireless communication with the other stations in the network. The access point is usually connected to a wired network and provides a bridge between wired and wireless devices. The first generation of access point

s, now termed "fat" access points, began to appear after the ratification of the IEEE 802.11b standard in 1999, and provided a full range of processing and control functions within each unit, including: security features, such as authentication and encryption support access control based on lists or filters SNMP configuration capabilities Transmit power level setting, RF channel selection, security encryption and other configurable parameters required user configuration of the access point, typically using a web-based interface. As well as providing this basic functionality, access points designed for home or small office wireless networking typically include a number of additional networking features, as shown in Table 3-1. Wireless Network Physical Architecture Table 3-1: Optional Access Point Functionality Feature Description Internet gateway Supporting a range of functions such as: routing, Network Address Translation, DHCP server providing dynamic IP addresses to client stations, and Virtual Private Network (VPN) passthrough. Switching hub Several wired Ethernet ports may be included that provide switching hub capabilities for a number of Ethernet devices. Wireless bridge or repeater Access point that can function as a relay station, to extend the operating range of another access point, or as a point-to-point wireless bridge between two networks. Network storage server Internal hard drives or ports to connect external storage, providing centralised file storage and back-up for wireless stations. Figure 3-10 illustrates a range of access point types, including weatherproofed equipment for outdoor coverage. In contrast to the first generation "fat" access point described above, slimmed-down "thin" access points are also available that limit access point capabilities to the essential RF communication functions and rely on the centralisation of control functions in a wireless LAN switch. Daink Figure 3-10: First Generation Wireless Access Points (courtesy of Belkin Corporation, D-Link (Europe) Ltd. and Linksys

(a division of Cisco Systems Inc.)) Chapter Three Wireless LAN Switches or Controllers In a large wireless network, typically in a corporate environment with tens and perhaps hundreds of access points, the need to individually configure access points can make WLAN management a complicated task. Wireless LAN switches simplify the deployment and management of large-scale WLANs. A wireless LAN switch (also known as a wireless LAN controller or access router), is a networking infrastructure device designed to handle a variety of functions on behalf of a number of dependent, or "thin", access points (Figure 3-11). As shown in Table 3-2, this offers several advantages for large-scale WLAN implementations, particularly those supporting voice services. The driver behind the development of the wireless switch is to enable the task of network configuration and management, which becomes increasingly complex and time consuming as wireless networks grow. A wireless switch provides centralised control of configuration, security, performance monitoring and troubleshooting, which is essential in an enterprise scale wireless LAN. Taking security as an example, with WEP, WPA, and 802.11i, all potentially in use at the same time in a large WLAN deployment, if security Table 3-2: "Thin" Access Point Advantages Advantage Description Lower cost A "thin" access point is optimised to cost effectively implement wireless communication functions only, reducing initial hardware cost as well as future maintenance and upgrade costs. Simplified access point Access point configuration, including security functions, management is centralised in order to simplify the network management task. Improved roaming Roaming handoffs are much faster than with conventional performance access points, which improves the performance of voice services. Simplified network The centralised command and control capability makes it upgrades easier to upgrade the network in response to evolving WLAN standards, since upgrades only have to be applied at the switch leve

l, and not to individual access points. Wireless Network Physical Architecture configuration has to be managed for individual access points, the routine management of encryption keys and periodic upgrade of security standards for each installed access point quickly becomes unmanageable. With a centralised security architecture provided by a wireless switch, these management tasks only need to be completed once. WLAN switches also provide a range of additional features, not found in first generation access points, as described in Table 3-3. Access point switch Access points Figure 3-11: Wireless LAN Topology Using a Wireless Switch Lightweight Access Point Protocol Centralising command and control into a wireless LAN switch device introduces the need for a communication protocol between the switch and its dependent access points, and the need for interoperability requires that this protocol is based on an industry standard. The Lightweight Access Point Protocol (LWAPP) standardises communications between switches or other hub devices and access points, and was initially developed by the Internet Engineering Task Force (IETF). Chapter Three Table 3-3: Wireless LAN Switch Features Feature Description Layout planning Automated site survey tools that allow import of building blueprints and construction specifications and determine optimal access point locations. RF management Analysis of management frames received from all access points enables RF signal related problems to be diagnosed and automatically corrected, by adjusting transmit power level or channel setting of one or more access points. Automatic configuration Wireless switches can provide automatic configuration by determining the best RF channel and transmit power settings for individual access points. Load balancing Maximising network capacity by automatic load balancing of users across multiple access points. Policy-based access Access policies can be based on access point control groupings and client lists that specify which access points or groups spec

To use centralised network computing power to execute the bridging, forwarding, authentication, encryption and policy enforcement functions for a wireless LAN. To provide a generic encapsulation and transport mechanism for transporting frames between hub devices and access points, which will enable multi-vendor interoperability and ensure that LWAPP can be applied to other access protocols in the future. Wireless Network Physical Architecture Table 3-4: LWAPP Functions LWAPP function Description Access point device An access point sends a Discovery Request frame discovery and and any receiving access router responds with a information exchange Discovery Reply frame. The access point selects a responding access router and associates by exchanging Join Request and Join Reply frames. Access point After association, the access router will provision the certification, access point, providing a Service Set Identifier (SSID), configuration, security parameters, operating channel and data rates provisioning and to be advertised. The access router can also configure software control MAC operating parameters (e.g. number of transmission attempts for a frame), transmit power, DSSS or OFDM parameters and antenna configuration in the access point. After provisioning and configuration, the access point is enabled for operation. Data and management LWAPP encapsulates data and management frames frame encapsulation, for transport between the access point and access fragmentation and router. Fragmentation of frames and re-assembly of formatting fragment will be handled if the encapsulated data or management frames exceed the Maximum Transmission Unit (MTU) supported between the access point and access router. Communication LWAPP enables the access router to request statistical control and reports from its access points, including data about the management between communication between the access point and its access points and associated devices (e.g. retry counts, RTS/ACK associated devices failure counts). The main communication

and control functions that are achieved using LWAPP are summarised in Table 3-4. Although the initial draft specification for LWAPP expired in March 2004, a new IETF working group called Control and Provisioning of Wireless Access Points (CAPWAP) was formed, with most working group members continuing to recommend LWAPP over alternatives such as Secure Light Access Point Protocol (SLAPP), Wireless LAN Control Protocol (WICOP) and CAPWAP Tunnelling Protocol (CTP). It seems likely that LWAPP will be the basis of an eventual CAPWAP protocol. Chapter Three Wireless LAN Arrays The so-called "3rd generation" architecture for WLAN deployment uses a device called an access point array, which is the LAN equivalent of the sectorised WMAN base station illustrated in Figure 3-7. A single access point array incorporates a wireless LAN controller together with 4, 8 or 16 access points, which may combine both 802.11a and 802.11b/g radio interfaces. A typical example uses 4 access points for 802.11a/g coverage, employing 180° sector antennas offset by 90°, and 12 access points for 802.11a covering, with 60° sector antennas offset by 30°, as illustrated in Figure 3-12. This type of device, with 16 access points operating 802.11a and g networks at an individual headline data rate of 54 Mbps, offers a total wireless LAN capacity of 864 Mbps. The increased gain of the sector antennas also means that the operating range of an access point array can be double or more the range of a single access point with an omnidirectional antenna. For high capacity coverage over a larger operating area, multiple access point arrays, controlled by a second tier of WLAN controllers would Combined beam pattern for 12 sector antennas, each 60° beamwidth, with 30° offset between adjacent antennas Combined beam pattern for 4 sector antennas, each 180° beam width, 90° offset between adjacent antennas Figure 3-12: Antenna Configuration in a 16-sector Access Point Array Wireless Network Physical Architecture Access point array coverage area; x 802.11g + 4 X

802. 11a channels Access point switch Access point arrays Client stations Figure 3-13: WLAN Tree Topology Employing Access Point Arrays create a tree topology, as shown in Figure 3-13, with multi giga-bit total WLAN capacity. Miscellaneous Wireless LAN Hardware Wireless Network Bridging Wireless bridge components that provide point-to-point WLAN or WMAN links are available from a number of manufacturers, packaged in weather proof enclosures for outdoor use (Figure 3-14). The D-Link DWL 1800 is one example which bundles a 16 dBi flat panel antenna with a 2.4 GHz radio providing a transmit power of 24 dBm (under FCC) or 14 dBm (under ETSI regulations), to deliver a range of 25 km under FCC or 10 km under ETSI. Many simple wireless LAN access points also support network bridging, or can be upgraded with a firmware upgrade to provide this capability. Configuring these devices simply involves entering the MAC address of the other endpoint into each station's access control list, SO that each station only decodes packets transmitted by the other endpoint of the bridge. Wireless Printer Servers A wireless printer server allows a printer to be flexibly shared among a group of users in the home or office without the need for the printer to be hosted by one computer or to be connected to a wired network. Chapter Three Dolitick Figure 3-14: Outdoor Wireless Bridges (courtesy of D-Link (Europe) Ltd. and Linksys (a division of Cisco Systems Inc.)) Typically, as well as wired Ethernet and wireless LAN interfaces, this device may include one or more different types of printer connections, such as USB or parallel printer ports, as well as multiple ports to enable multiple printers to be connected - such as a high-speed black and white laser and a separate colour printer. Figure 3-15: Wireless Printer Servers (courtesy of Belkin Corporation, D-Link (Europe) Ltd. and Linksys (a division of Cisco Systems Inc.)) Wireless Network Physical Architecture A printer server for home or small office wireless networking may also be bundled

with a 4-port switch to enable other wired network devices to share the printer and use the wireless station as a bridge to other devices on the wireless network. Figure 3-15 shows a range of wireless printer servers. Wireless LAN Antennas Traditional Fixed Gain Antennas Antennas for 802.11b and 11g networks, operating in the 2.4 GHz ISM band, are available to achieve a variety of coverage patterns. The key features that dictate the choice of antenna for a particular application are gain, measured in dBi (see the Section "Antenna Gain, p. 107") and angular beamwidth, measured in degrees. The most common WLAN antenna, standard in all NICs and in most access points, is the omnidirectional antenna, which has a gain in the range from 0 to 7 dBi and a beamwidth, perpendicular to the antenna axis, of a full 360°. A range of WLAN antennas is shown in Figure 3-16 and typical parameters are summarised in Table 3-5. For sector antennas with a given horizontal beamwidth, the trade-off for higher gain is a narrower vertical beamwidth, which will result in a smaller coverage area at a given distance and will require more precise alignment. A further important feature of an antenna is its polarisation, which refers to the orientation of the electric field in the electromagnetic wave emitted Outdoor omnidirectional Indoor directional panel Indoor omnidirectional High gain Yagi Figure 3-16: Wireless LAN Antenna Types (courtesy of D-Link (Europe) Ltd.) Chapter Three Table 3-5: Typical Wireless LAN Antenna Parameters for 2.4 GHz Operation Antenna type Sub-type Beamwidth (Degrees) Gain (dBi) Omnidirectional Patch / Panel 15-75 Sector 10-17 Directional 10-30 Parabolic reflector 14-30 by the antenna. Most common antennas, including all those listed in the table above, produce linearly polarised waves, with the electrical field oriented either vertically or horizontally - hence vertical or horizontal polarisation. WLAN antennas that produce circular polarisation are also available (helical antennas) but are less common. It is importan

t that the polarisations of transmitting and receiving antennas are matched, since a vertical polarised receiving antenna will be unable to receive a signal transmitted by a horizontally polarised transmitting antenna, and vice versa. It is equally important for antennas to be correctly mounted, as rotating an antenna 90° about the direction of propagation will change its polarisation by the same angle (e.g. from horizontal to vertical). Although WLAN operation in the 5 GHz band has developed more recently than in the 2.4 GHz band, a similar selection of antennas is available for the higher-frequency band. A variety of dual band omnidirectional and patch antennas are also available to operate in both WLAN bands. Smart Antennas The data throughput of a wireless network that uses a traditional antenna of the type described above is limited because only one network node at a time can use the medium to transmit a data packet. (Other so-called Wireless Network Physical Architecture Transmitter switches to beam 6 -9 dB to give maximum gain in the direction of the target station Target station direction Figure 3-17: Beam Pattern of a Six Element Switched Beam Array multiple access techniques will be described in the Section "Wireless Multiplexing and Multiple Access Techniques, p. 87"). Smart antennas aim to overcome this limitation by allowing multiple nodes to transmit simultaneously, significantly increasing network throughput. There are two varieties of smart antenna - switched beam and adaptive array. A switched beam antenna consists of an array of antenna elements each having a predefined beam pattern with a narrow main lobe and small side- lobes (Figure 3-17). Switching between beams allows one array element to be selected that provides the best gain in the direction of a target node, or the lowest gain towards an interfering source. The simplest form of switched beam antenna is the pair of diversity receiver antennas often implemented in wireless LAN access points to reduce multipath effects in indoor environmen

ts. The receiver senses which of the two antennas is able to provide the highest signal strength and switches to that antenna. Adaptive beams or beam-forming antennas consist of two or more antenna elements in an array and a so-called beam-forming algorithm, which assigns a specific gain and phase shift to the signal sent to or received from each antenna element. The result is an adjustable radiation Chapter Three Constructive interference in the desired direction Phase shift between two antenna signals Figure 3-18: Phase Shift Between Two Antennas Resulting in a Directed Beam pattern that can be used to steer the main lobe of the beam in the direction of the desired maximum gain (Figure 3-18). As well as focussing its beam pattern towards a particular node, the adaptive beam antenna can also place a "null" or zero gain point in the direction of a source of interference. Because the gain and phase shift applied to individual array elements is under real-time software control (Figure 3-19), the antenna can dynamically adjust its beam pattern to compensate for multipath and other sources of interference and noise. Summing Receiver amplifier Antenna array Amplitude and Adaptive elements phase control control algorithm Figure 3-19: Adaptive Beam Antenna Wireless Network Physical Architecture Like adaptive beam arrays, Multi-input Multi-output or MIMO radio, described in the Section "MIMO Radio, p. 124", also uses multiple antennas to increase network capacity. The key difference between these two techniques is that MIMO radio exploits multi-path propagation between a single transmitter and receiver, while adaptive beam arrays use multiple antennas to focus a single spatial channel. Some other differences are described in Table 3-6. Table 3-6: Adaptive Beam Arrays and MIMO Radio Compared Adaptive beam array MIMO radio Objective Focus the propagation pattern along Exploit multi-path propagation a single desired spatial direction, to increase data capacity by to allow multiple access, reduce multiplexing data streams ov

er interference or increase range. several spatial channels. Antenna 2 or more antenna elements at Typically 2 X 2 or 4 X 4 configuration Tx and/or Rx. Tx and Rx (Tx X Rx). Tx and Rx are configurations are independent. linked by the digital signal processing algorithm. Spatial Single spatial channel focussed Multiple spatial channels, diversity between transmitter and receiver. exploiting multi-path propagation. Single bit stream encoded to all Data stream multiplexed over multiplexing transmit antennas. spatial channels. Signal Simple phase and gain modification Complex processing algorithm processing for each antenna. to decode signals over multiple spatial channels. Application 3rd generation WLAN access PHY layer for the 802.11n example points - Section "Wireless LAN standard - Section "MIMO Arrays, p. 52". and data rates to 600 Mbps (802.11n), p. 165". Also under development is a new type of switched beam wireless LAN antenna called a plasma antenna, which uses a solid-state plasma (an ionised region in a silicon layer) as a reflector to focus and direct the emitted RF beam. A plasma antenna will be able to switch a medium gain (10-15 dBi) beam with approximately 10° beamwidth to 1 of 36 directions within a full 360° coverage, with a switching time that is less than the gap between transmitted frames. Chapter Three Wireless PAN Devices Wireless PAN Hardware Devices Bluetooth Devices In Part IV the wide range of PAN technologies will be described, from Bluetooth, which is most commonly identified with personal area networking, to ZigBee, an emerging technology primarily aimed at networking home and industrial control devices. In fact with high powered (Class 1) Bluetooth radios, which can equal the range of Wi-Fi devices, the boundary between personal and local area networking is blurred and, within the limitations of achievable data rates, most of the WLAN devices described in the Section "Wireless LAN Devices p. 45" could equally well be built using Bluetooth technology. The most common types of Bluetooth d

evices and some of their key features are summarised in Table 3-7 and shown in Figure 3-20. Access point and Printer adapter print server Dial-up modem adapters Figure 3-20: A Variety of Bluetooth Devices (courtesy of Belkin Corporation, D-Link (Europe) Ltd., Linksys (a division of Cisco Systems Inc.) and Zoom Technologies, Inc.) As developing wireless PAN technologies such as wireless USB mature, a comparable range of devices will be developed to support these networks. Novel capabilities inherent in these new technologies will also result in new device types offering new services, an example being the Wireless Network Physical Architecture Table 3-7: Bluetooth Devices and Features Bluetooth device Key features Mobile phone Interface with a Bluetooth hands-free headset. Connect to PDA or PC to transfer or back-up files. Exchange contact details (business cards), calendar entries, photos, etc. with other Bluetooth devices. Connect to PC to transfer or back-up files. Connect to the Internet via a Bluetooth access point. Exchange contact details (business cards), calendar entries, photos, etc. with other Bluetooth devices. Headset or headphones Hands-free mobile telephony. Audio streaming from PC, TV, MP3 player or hi-fi system. Audio transceiver Audio streaming from a PC or hi-fi system to Bluetooth headphones. Access point Extend a LAN to include Bluetooth enabled devices. Internet connectivity for Bluetooth devices. Bluetooth adapters Bluetooth enable a range of devices, such as laptops or PDAs. As for WLAN NICS, they are available in a range of form factors, with USB dongles being the most popular. Serial adapter for plug-and-play connectivity to any serial RS-232 device. Print adapter Print files or photos from Bluetooth enabled device. PC input devices Wireless connectivity to a PC mouse or keyboard. GPS receiver Provide satellite navigation capabilities to Bluetooth-enabled devices loaded with required navigation software. Dial-up modem Provide wireless connectivity from a PC to a dial-up modem. capability o

f the multi-band OFDM radio to spatially locate a wireless USB station, offering the potential for devices that rely on location based services. ZigBee Devices ZigBee is an emerging low data rate, very low power, wireless networking technology, described in the Section "ZigBee (IEEE 802.15.4), p. 273", that will initially focus on home automation but is likely to find a wide Chapter Three range of applications, including a low cost replacement for Bluetooth in applications that do not require higher data rates. The key features of a range of currently available and expected ZigBee devices is summarised in Table 3-8, and some of these devices are shown in Figure 3-21. Wireless PAN antennas In practice, since PAN operating range is generally under ten metres, Bluetooth and other PAN devices will typically use simple integrated omnidirectional antennas. However, for PANs such as Bluetooth that share the 2.4 GHz ISM radio band with 802.11b/g WLANs, the wide range of external WLAN antennas described above are also available to enable PAN devices to be operated with extended range. Table 3-8: ZigBee Devices and Features ZigBee device Key features PC input devices Wireless connectivity to a PC mouse or keyboard. Automation devices Wireless control devices for home and industrial automation functions such as heating, lighting and security. Wireless remote control Replacing Ir remote for TV etc., and eliminating the line-of-sight and alignment restriction. Sensor modem Provides a wireless networking interface for a number of existing current loop sensors for home or industrial automation. Ethernet gateway A ZigBee network coordinator that enables command of ZigBee end devices or routers from an Ethernet network. Wireless MAN Devices While wireless LANs and PANs present a wide diversity of topologies and device types, to date wireless MAN devices have serviced only fixed point-to-point and point to multi-point topologies, requiring in essence only two device types, the base station and the client station. However, followin

g the ratification of the 802.16e standard (also designated 802.16-2005), broadband Internet access will soon be widely Wireless Network Physical Architecture Network Sensor Ethernet coordinator Modem gateway Figure 3-21: A Variety of ZigBee Devices (courtesy of Cirronet Inc.) available to mobile devices and a range of new mobile wireless MAN devices is emerging, driving the convergence of mobile phones and PDAs. Fixed Wireless MAN Devices Wireless networking devices for fixed wireless MAN applications, essentially for last mile broadband Internet access, fall into two categories - station equipment and customer premises equipment (CPE). Some examples of base station equipment to support wireless MANs of differing scales are shown in Figure 3-22. The macro scale base station shown can potentially support thousands of subscribers in dense Figure 3-22: Micro and Macro WMAN Base Station Equipment (courtesy of Aperto Networks Inc.) Chapter Three Table 3-9: Wireless MAN Devices and Features WMAN device Typical features Basic self installed Basic WMAN connectivity to a customer PC or indoor CPE network. Multiple diversity or adaptive array antennas to improve non line-of-sight reception. Outdoor CPE External antenna and radio. Provides higher antenna gain and longer range. Base station equipment Modular and scalable construction. Macro and micro configurations for dense metropolitan or sparse rural installations. Flexible RF channel usage, from one channel over multiple antenna sectors to multiple channels per antenna sector. Integrated network MAN interface with network gateway functions (Routing, gateway NAT and firewall capabilities). Optionally with integrated wireless LAN access point. Integrated CPU to support additional WISP services such as VoIP telephony. metropolitan area deployments, while the micro scale equipment is designed to support lower user numbers in sparse rural areas. Some of the types and key features of base station and CPE hardware are summarised in Table 3-9. Figure 3-23 shows a variety of dif

ferent wireless MAN CPE equipment. Fixed Wireless MAN Antennas Antennas for fixed wireless MAN applications similarly divide into base station and CPE. The general types of antennas summarised earlier in Table 3-5 for LANs are equally applicable to MAN installations, with appropriate housings for outdoor service and mountings designed for wind and ice loading. The factors that determine the choice of antenna are summarised in Table 3-10. Depending on its elevation relative to the target area, a base station may comprise two sets of sector antennas as illustrated in Figure 3-24. A set of intermediate gain antennas, with higher vertical beamwidth, provide coverage over short-to-medium distances, with a second set of high-gain, low vertical beamwidth antennas providing coverage at longer range. aperto Figure 3-23: Fixed Wireless MAN CPE Equipment (courtesy of Aperto Networks Inc.) Table 3-10: - Factors Determining Wireless MAN Antenna Choice Location Antenna type Application Omnidirectional Low gain requirement - for subscribers located close to the base station. Patch Intermediate gain - mid range equipment should be applicable to most subscribers. Directional (Yagi High gain, high cost equipment to or parabolic maximise data rate at the edge of the reflector) operating area. Base station Sector, intermediate Wide area coverage close to the base station. Wider vertical beamwidth required to provide coverage close to the base station. Sector, high gain Wide area coverage at a distance from the base station. High gain adds range with narrower vertical beamwidth. Directional High gain antenna for point-to-point applications, such as backhaul, bridging between base stations, etc. Chapter Three High-gain, low beamwidth for long range coverage Medium-gain, high beamwidth for short range coverage Figure 3-24: WMAN Base Station Sector Antenna Configuration Mobile Wireless MAN Devices The first implementation of mobile wireless MAN services and devices, delivering broadband Internet access to the user on the move, has been

in the South Korean market, driven by the rapid development of the WiBro standard (a sub-set of 802.16e, described in the Section "TTA WiBro, 320"). Commercial uptake has also been speeded by the use of licensed spectrum, and the granting in 2005 of operating licences to three telecom companies. The form factor for devices to deliver mobile Internet services reflects the need to combine telephony and PDA capabilities - a larger screen and QWERTY input to overcome the limitations experienced with WAP phones. Figure 3-25 shows two early WiBro phones developed by Samsung. Summary of Part I Chapters 2 and 3 have introduced the basic logical and physical architecture of wireless networks, the software and hardware elements that are the building blocks for the construction and operation of all wireless networks. Although all networks require protocols and standards operating at all layers in the OSI model in order to operate, it is primarily the protocols and mechanisms of the Data Link and PHY layers which distinguish wired from wireless networking because of their specific design to Wireless Network Physical Architecture Figure 3-25: Wireless MAN Enabled Phones (courtesy of Samsung Electronics) address the challenges of data transmission across the wireless medium. These protocols and mechanisms are explored in greater detail in Parts III to V, where the main wireless networking standards, such as 802.11 (WLAN), 802.15 (WPAN) and 802.16 (WMAN) are discussed. Chapter 3 has also provided an overview of the different types of devices available for wireless networking at these three operating scales - personal, local and metropolitan. Device convergence is a common feature at the personal-local and local-metropolitan interfaces, and devices are increasingly appearing with multi-mode radios, enabling a single device to participate in a variety of different wireless networks. This page intentionally left blank WIRELESS COMMUNICATION Introduction The OSI network model illustrates how data and protocol messages from the appl

ication level cascade down through the logical layers and result in a series of data frames to be transmitted across the physical network medium. In a wireless network that physical layer is provided by radio frequency (RF) or infrared (Ir) communications and in Part II the basics of these methods of wireless communication will be covered. Starting with the RF spectrum, the regulation of spectrum use is briefly described and spread spectrum techniques are then introduced. This is a key technology that enables high data link reliability by making RF communications less susceptible to interference. Multiple access methods that enable many users to simultaneously use the same communication channel are then discussed. Signal coding and modulation is the step that encodes the data stream onto the RF carrier or pulse train, and a range of coding and modulation techniques applied in wireless networking will be covered, from the simplest to some of the most complex. The various elements that impact on RF signal propagation will be described, enabling a calculation of the link budget - the balance of power available to overcome system and propagation losses to bring the transmitted signal to the receiver at a sufficient power level for reliable, low error rate reception. The link budget calculation is an essential part of the toolkit of the wireless network designer in defining the basic power Part Two requirements for a given network installation. This will usually be supplemented by practical techniques such as site surveys, which will be covered in Parts III and V. Ultra wideband (UWB) radio has emerged as a key technology for short range wireless network applications, and some of the varieties of UWB radio that are applied in wireless USB, wireless Fire Wire and ZigBee will be described. Chapter 5 gives a similar overview of aspects of infrared communications, as used in IrDA connections, covering the infrared spectrum, Ir propagation and reception, and Ir link calculation. CHAPTER Radio Communication Basics The RF Sp

ectrum The radio frequency, or RF, communication at the heart of most wireless networking operates on the same basic principles as everyday radio and TV signals. The RF section of the electromagnetic spectrum lies between the frequencies of 9 kHz and 300 GHz (Table 4-1), and different bands in the spectrum are used to deliver different services. Recalling that the wavelength and frequency of electromagnetic radiation are related via the speed of light, SO that wavelength (A) = speed of light (c) / frequency (f), or wavelength in metres = 300 / frequency in MHz. Table 4-1: Subdivision of the Radio Frequency Spectrum Transmission type Frequency Wavelength Very low frequency (VLF) 9-30 kHz 33-10 km Low frequency (LF) 30-300 kHz 10-1 km Medium frequency (MF) 300-3000 kHz 1000-100 m High frequency (HF) 3-30 MHz 100-10 m Very high frequency (VHF) 30-300 MHz 10-1 m Ultra high frequency (UHF) 300-3000 MHz 1000-100 mm Super high frequency (SHF) 3-30 GHz 100-10 mm Extremely high frequency (EHF) 30-300 GHz 10-1 mm Chapter Four Beyond the extremely high frequency (EHF) limit of the RF spectrum lies the infrared region, with wavelengths in the tens of micrometre range and frequencies in the region of 30 THz (30,000 GHz). Virtually every Hz of the RF spectrum is allocated for one use or another (Figure 4-1), ranging from radio astronomy to forestry conservation, and some RF bands have been designated for unlicensed transmissions. The RF bands which are used for most wireless networking are the unlicensed ISM or Instrument, Scientific and Medical bands, of which the three most important lie at 915 MHz (868 MHz in Europe), 2.4 GHz and Space Space Earth Fixed Mobile Research Operation Exploration Space Research Fixed Mobile Amateur Broadcasting Satellite Mobile Radio Location Amateur Fixed Mobile Radiodetermination Satellite Mobile Satellite Broadcasting Fixed Satellite Earth Radio Space Astronomy Research Exploration Satellite Aeronautical Meteorological Radionavigation Radio Location 2.9GHz Figure 4-1: FCC Spectrum Allocation A

round the 2.4GHz ISM Band Radio Communication Basics Table 4-2: Radio Frequency Bands in Use for Wireless Networking RF band Wireless networking specification 915/868 MHz ISM ZigBee 2.4 GHz ISM IEEE 802.11b, g, Bluetooth, ZigBee 5.8 GHz IEEE 802.11a 5.8 GHz (Table 4-2). As well as these narrow band applications, new networking standards such as ZigBee (Section "ZigBee (IEEE 802.15.4), p. 273)" will make use of the FCC spectrum allocation for ultra wideband radio (UWB - see Section "Ultra Wideband Radio p. 119") that permits very low power transmission across a broad spectrum from 3.1 to 10.6 GHz. Radio Frequency Spectrum Regulation The use of the radio frequency spectrum, in terms of the frequency bands that can be used for different licensed and unlicensed services, and the allowable transmission power levels for different signal formats, are controlled by regulatory authorities in individual countries or regions (Table 4-3). Although there is an increasing trend towards harmonisation of spectrum regulation across countries and regions, driven by the International Telecommunications Union's World Radio Communication Conference, there are significant differences in spectrum allocation and other conditions such as allowable transmitter power levels which have an impact on wireless networking hardware design and interoperability. As an example, in the 5.8 GHz ISM band used for IEEE 802.11a networks, the FCC in the USA allows a maximum transmitted power of Table 4-3: Radio Frequency Spectrum Regulatory Bodies Country/Region Regulatory body Federal Communications Commission Canada Industry Canada Europe European Telecommunications Standards Institute Japan Association of Radio Industries and Businesses Chapter Four Table 4-4: 2.4 GHz ISM Band Regulatory Differences by Region Regulator 2.4 GHz ISM specifications FCC (USA) 1 W maximum transmitted power 2.402-2.472 GHz, 11 X 22 MHz channels ETSI (Europe) 100 mW maximum EIRP 2.402-2.483 GHz, 13 X 22 MHz channels ARIB (Japan) 100 mW maximum EIRP 2.402-2.497 GHz, 14 X 22 M

Hz channels 1 W, while in Europe the ETSI permits a maximum EIRP (equivalent isotropic radiated power) of just 100 mW EIRP or 10 mW/MHz of bandwidth, with variations in other countries. Table 4.4 shows a range of other regulatory differences that apply to the 2.4 GHz ISM band used for IEEE 802.b/g networks. The pace of regulatory change also differs from region to region. For example, the FCC developed regulations governing ultra wideband radio in 2002, while in Europe ETSI Task Group 31 was still working on similar regulations in 2006. Although the regulatory bodies impose conditions on the unlicensed use of parts of the RF spectrum, unlike their role in the licensed spectrum, these bodies take no responsibility for or interest in any interference between services that might result from that unlicensed use. In licensed parts of the RF spectrum, the FCC and similar bodies have a role to play in resolving interference problems, but this is not the case in unlicensed bands. Unlicensed means in effect that the band is free for all, and it is up to users to resolve any interference problems. This situation leads some observers to predict that the 2.4 GHz ISM band will eventually become an unusable junk band, overcrowded with cordless phone, Bluetooth, 802.11 and a cacophony of other transmissions. This impending "tragedy of the commons" may be prevented by the development of spectrum agile radios, described in Chapter 14. Further information on current spectrum regulation and future developments, including the further development of regulations on ultra Radio Communication Basics Table 4-5: Web Sites of Spectrum Regulators Regulator Country/Region www.fcc.gov Industry Canada Canada www.ic.gc.ca Europe www.etsi.org Japan www.arib.or.jp/english wideband radio outside the USA, can be found from the regulators web sites at the URLs shown in Table 4-5. Radio Transmission as a Network Medium Compared to traditional twisted-pair cabling, using RF transmission as a physical network medium poses a number of challenges, as out

lined in Table 4-6. Security has been a significant concern since RF transmissions are far more open to interception than those confined to a cable. Security issues will be covered in detail in Chapters 8 and 11. Data link reliability, bit transmission errors resulting from interference and other signal propagation problems, are probably the second most significant challenge in wireless networks, and one technology that resulted in a quantum leap in addressing this problem (spread spectrum transmission) is the subject of the next section. Controlling access to the data transmission medium by multiple client devices or stations is also a different type of challenge for a wireless medium, where, unlike a wired network, it is not possible to both transmit and receive at the same time. Two key situations that have the potential to Table 4-6: The Radio Frequency Networking Challenge Challenges Considerations and solutions Link reliability Signal propagation, interference, equipment siting, link budget. Media access Sensing other users (hidden station and exposed station problems), Quality of service requirements. Security Wired equivalent privacy (WEP), Wi-Fi Protected Access (WPA), 802.11i, directional antennas. Chapter Four Hidden Station: C is hidden from A A will not sense C's transmission and A's transmission to B will fail due to interference from C. Range of station C's transmitter Station A Station B Station C Exposed Station: B is exposed to C B will be prevented from transmitting to A while station C is transmitting, although A would be able to receive successfully. Figure 4-2: Hidden and Exposed Station Challenges for Wireless Media Access Control degrade network performance are the so-called hidden station and exposed station problems. The hidden station problem occurs when two stations A and C are both trying to transmit to an intermediate station B, where A and C are out of range and therefore one cannot sense that the other is also transmitting (see Figure 4-2). The exposed station problem occurs when a

transmitting station C, prevents a nearby station B from transmitting although B's intended receiving station A is out of range of station C's transmission. The later sections of this chapter look at digital modulation techniques and the factors affecting RF propagation and reception, as well as the practical implications of these factors in actual wireless network installations. Spread Spectrum Transmission Spread spectrum is a radio frequency transmission technique initially proposed for military applications in World War II with the intention of making wireless transmissions safe from interception and jamming. These techniques started to move into the commercial arena in the early Radio Communication Basics Baseband signal Spread Spreading function signal Frequency Frequency De-spread Narrowband Baseband interference signal Spread signal De-spreading function Interference Frequency Frequency Figure 4-3: A Simple Explanation of Spread Spectrum 1980s. Compared to the more familiar amplitude or frequency modulated radio transmissions, spread spectrum has the major advantage of reducing or eliminating interference with narrowband transmissions in the same frequency band, thereby significantly improving the reliability of RF data links. Unlike simple amplitude or frequency modulated radio, a spread spectrum signal is transmitted using a much greater bandwidth than the simple bandwidth of the information being transmitted. Narrow band interference (the signal I in Figure 4-3) is rejected when the received signal is "de-spread". The transmitted signal also has noise-like properties and this characteristic makes the signal harder to leavesdrop on. Types of Spread Spectrum Transmission The key to spread spectrum techniques is some function, independent of the data being transmitted, that is used to spread the information signal over a wide transmitted bandwidth. This process results in a transmitted signal bandwidth which is typically 20 to several 100 times the information bandwidth in commercial applications, or 100

0 to 1 million times in military systems. Several different methods of spread spectrum transmission have been developed, which differ in the way the spreading function is applied to the information signal. Two methods, direct sequence spread spectrum and frequency hopping spread spectrum, are most widely applied in wireless networking. Chapter Four In Direct Sequence Spread Spectrum (DSSS) (Figure 4-4), the spreading function is a code word, called a chipping code, that is XOR'd with the input bit stream to generate a higher rate "chip stream" that is then used to modulate the RF carrier. Baseband Spread De-spread Baseband signal Spreading function signal Correlation function signal Frequency Frequency Frequency Figure 4-4: A Simple Explanation of DSSS In Frequency Hopping Spread Spectrum (FHSS) (Figure 4-5), the input data stream is used directly to modulate the RF carrier while the spreading function controls the specific frequency slot of the carrier within a range of available slots spread across the width of the transmission band. Time Hopping Spread Spectrum (THSS) (Figure 4-6), is a third technique in which the input data stream is used directly to modulate the RF carrier which is transmitted in pulses with the spreading function controlling the Frequency Frequency Baseband Frequency signal Baseband signal Transmitter frequency Receiver frequency hopping pattern; hopping pattern; 12 > 16 > 5 > 10 > 12 > 16 > 5 > 10 > Figure 4-5: A Simple Explanation of FHSS Radio Communication Basics Data burst transmitted at a position within the time slot determined by the spreading function Time slot; Ts Figure 4-6: A Simple Explanation of THSS timing of each data pulse. For example, impulse radio uses pulses that are SO short, typically in the region of 1 nanosecond (nS), that the spectrum of the signal is very wide and meets the definition of an ultra wideband (UWB) system. The spectrum is effectively spread as a result of the narrowness of transmitted pulses, but time-hopping, with each user or node being assigned a

unique hopping pattern, is a simple technique for impulse radio to allow multiple user access (see Section "Time Hopping PPM UWB (Impulse Radio), p. 121"). Two other less common techniques are Pulsed FM systems and Hybrid systems (Figure 4-7). In Pulsed FM systems, the input data stream is used directly to modulate the RF carrier, which is transmitted in frequency Linear chirp - frequency sweeping down Linear chirp - frequency sweeping up WWWWW Warble - frequency sweeping up then down WWW.VVA Warble - frequency sweeping down then up Data pulse with FM pattern determined by the spreading function Time slot; Ts Figure 4-7: A Simple Explanation of Pulsed FM Systems Chapter Four modulated pulses. The spreading function controls the pattern of frequency modulation, which could for example be a linear "chirp" with frequency sweeping up or down. Hybrid systems also use combinations of spread spectrum techniques and are designed to take advantage of specific characteristics of the individual systems. For example, FHSS and THSS methods are combined to give the hybrid frequency division - time division multiple access (FDMA/TDMA) technique (see the Section "Wireless Multiplexing and Multiple Access Techniques, p. 87"). Of these alternative spread spectrum techniques, DSSS and FHSS are specified in the IEEE 802.11 wireless LAN standards, although DSSS is most commonly used in commercial 802.11 equipment. FHSS is used by Bluetooth, and FHSS and chirp spread spectrum are optional techniques for the IEEE 802.15.4a (ZigBee) specification. Chipping, Spreading and Correlating The spreading function used in DSSS is a digital code, known as a chipping code or pseudo-noise (PN) code, which is chosen to have specific mathematical properties. One such property is that, to a casual listener on the broadcast band, the signal is similar to random noise, hence the "pseudo-noise" label. Under the IEEE 802.11b standard, the specified PN code for 1 Mbps and 2 Mbps data rates is the 11-bit Barker code. Barker codes are binary sequences that h

Chipping Codes One of the desirable mathematical properties of PN codes is that it enables the receiver's PN code generator to very rapidly synchronise with the PN code in the received signal. This synchronisation is the first step in the de-spreading process. Fast synchronisation requires that the position of the code word can be quickly identified in a received signal, and this is achieved as a result of the low auto-correlation property of the Barker codes. Another benefit of low auto-correlation is that the receiver will reject signals that are delayed by more than one chip period. This helps to make the data link robust against multipath interference, which will be discussed in the Section "RF Signal Propagation and Losses, p. 112". A second key property of chipping codes that is important in applications where interference between multiple transmitters must be avoided, for example in mobile telephony, is low cross-correlation. This property reduces the chance that a correlator using one PN code will experience interference from a signal using a different code (i.e. that it will incorrectly decode a noisy signal that was encoded using a different chipping code). Ideally codes in use in this type of multiple access application should have zero cross-correlation, a property of the orthogonal codes used in CDMA (Section "Code Division Multiple Access, p. 94"). Code orthogonality for multiple access control is not required for wireless networking applications, such as IEEE 802.11 networks, as these standards use alternative methods to avoid conflict between overlapping transmitted signals from multiple users, which are described in the Section "Wireless Multiplexing and Multiple Access Techniques, p. 87". Complementary Code Keying An alternative to using a single chipping code to spread every bit in the input data stream is to use a set of spreading codes and to select one code from the set depending on the values of a group of input data bits. This scheme is known as complementary code keying (CCK). CCK was pro

posed to the IEEE by Lucent Technologies and Harris Semiconductor (now part of Intersil Corp.) in 1998, as a means to raise the IEEE 802.11b data rate to 11 Mbps. Instead of using the Barker code, they proposed to use a set of codes called Complementary Sequences, Radio Communication Basics based on the Walsh/Hadamard transforms (see the Section "Code Division Multiple Access, p. 94"). Using CCK, a chipping code word is chosen from a set of 64 unique codes depending on the value of each 6-bit segment of the input data stream. The encoded data sequence comprises a series of code words, and this chip sequence is modulated onto the RF carrier using one of a variety of modulation techniques that will be described in the Section "Digital Modulation Techniques, p. 93". The main advantage of CCK modulation is spectral efficiency, since each transmitted code word represents 6 input data bits instead of the single bit represented by the Barker code. CCK can achieve 11 Mbps using the same 22 MHz bandwidth used to transmit 1 Mbps with the Barker code. However, the price of this high data rate is complexity. A receiver using the Barker code requires just one correlator to pick out the chipping code, while a CCK system needs 64 correlators, one on the lookout for each of the complementary codes. Direct Sequence Spread Spectrum in the 2.4 GHz ISM Band As noted above, in DSSS the data signal is combined with a code word, the chipping code, and the combined signal is used to modulate the RF carrier, resulting in a transmitted signal spread over a wide bandwidth. For example, in the 2.4 GHz ISM band, a spread bandwidth of 22 MHz is specified for IEEE 802.11 networks, as shown in Figure 4-9. The 2.4 GHz ISM band has a total allowed width of 83.5 MHz and is divided into a number of channels (11 in the USA, 13 in Europe, 14 in Japan), with 5 MHz steps between channels. To fit 11 or more 22 MHz 22 MHz channel bandwidth 25 MHz channel separation 2.400 2.410 2.420 2.430 2.440 2.450 2.460 2.470 GHz 11 channels in 5 MHz steps Figure 4-9:

802.11 DSSS Channels Chapter Four Channel 5 Channel 10 2.432 GHz 2.457 GHz Channel 4 Channel 9 2.427 GHz 2.452 GHz Channel 3 Channel 8 2.422 GHz 2.447 GHz Channel 2 Channel 7 2.417 GHz 2.442 GHz Channel 1 Channel 6 Channel 11 2.412 GHz 2.437 GHz 2.462 GHz 2.400 2.410 2.420 2.430 2.440 2.450 2.460 2.470 GHz 11 channels in 5 MHz steps Figure 4-10: DSSS Channels in the 2.4 GHz ISM Band (US) wide channels into an 83 MHz wide band results in considerable overlap between the channels (as shown in Figure 4-10), resulting in the potential for interference between signals in adjacent channels. The 3 non-overlapping channels allow 3 DSSS networks to operate in the same physical area without interference. The use of DHSS in 802.11 wireless networks is described further in Chapter 6. Frequency Hopping Spread Spectrum in the 2.4 GHz ISM Band In frequency hopping spread spectrum transmission (FHSS) the data is modulated directly onto a single carrier frequency, but that carrier frequency hops across a number of channels within the RF band using a pseudo-random hopping pattern. In the 2.4 GHz ISM band for example, a maximum channel width of 1 MHz is specified for FHSS systems, and 79 such channels are available. A transmitter switches between these channels many times a second, moving on to the next channel in its sequence after a predetermined time, known as the "dwell time". The IEEE 802.11b standard specifies that the hop must be to a new channel a minimum of 6 MHz from the previous channel, and that hops must occur at least 2.5 time per second (Figure 4-11). The spectrum regulators specify the allowable limits for transmission parameters such as Radio Communication Basics Over 2.5 hops per second 6 MHz minimum hop 1 MHz channel width 78 hopping patterns 2.400 2.410 2.420 2.430 2.440 2.450 2.460 2.470 79 X 1 MHz Bluetooth channels Figure 4-11: FHSS Channels Within the 2.4Ghz ISM Band the maximum dwell time and individual standards, like IEEE 802.11, have to work within these boundary conditions. In the receiver, a PN genera

tor recreates the same hopping pattern. This allows the receiver to make the same channel-to-channel hops as the transmitter, SO that the data signal can be decoded. Because the probability of two networks selecting the same channel at the same time is very low, many more FHSS networks can overlap physically without interference than is the case for DSSS networks. Frequency hopping spread spectrum in the 2.4 GHz ISM band is specified alongside DSSS as an option in the IEEE 802.11b standard and is also used in Bluetooth networks. All 79 available channels are normally used in Bluetooth, although an alternative hopping sequence that uses 23 channels (2.454 to 2.476 GHz), is available for use in France where special regulatory conditions apply. Frequency hops occur after each data packet, which will be a multiple of 1-, 3- or 5-times the time slot duration of 625 microseconds (320 to 1600 hops/second). The frequency hopping pattern is determined by the unique 48-bit device ID of the master device in each Bluetooth piconet, and synchronisation to the hopping pattern is part of the process of device discovery when a new device joins the piconet. The use of FHSS in Bluetooth is described further in Chapter 10. Time Hopping Spread Spectrum In a time hopping spread spectrum system, time is divided into frames, with each frame divided into a number of transmission slots. Within each Chapter Four User 1 bit stream = 1101... User 2 bit stream = 0011 Bit time Tb User 1 pulses TH code = 2767 User 2 pulses TH code = 4483 Frame time Tf Transmission slot number Figure 4-12: Time Hopping Spread Spectrum frame, data is transmitted only during one time slot and the specific time slot used during each frame is determined using a PN code. Figure 4-12 shows a THSS system with two clients using different hopping codes. Impulse radio is an ultra-wide band transmission technique that is a candidate for the IEEE 802.15.4a (ZigBee) physical layer specification. This is a time hopping spread spectrum technique where a very short pulse is tr

ansmitted in each transmission time slot. Information is encoded via pulse position or pulse amplitude modulation (PPM, PAM). The spreading effect of time hopping, together with the short pulse duration, results in a transmitted signal spread across an ultra-wide bandwidth. Table 4-8: Benefits of Common Spread Spectrum Techniques Frequency Hopping Direct sequence Simple to design and manufacture Higher data speeds Cheaper to implement Increased range Higher density of overlapping Throughput is interference-tolerant up to networks a threshold level Gradual degradation of throughput with interference Radio Communication Basics Spread Spectrum in Wireless Networks - Pros and Cons The advantages of spread spectrum techniques, such as resistance to interference and eavesdropping and the ability to accommodate multiple users in the same frequency band, make this an ideal technology for wireless network applications (Table 4-8). Although the good interference performance is achieved at the cost of relatively inefficient bandwidth usage, the available radio spectrum, such as the 2.4 GHz ISM band, still permits data rates of up to 11 Mbps using these techniques. Since speed and range are important factors in wireless networking applications, DSSS is the more widely used of the two techniques although, because of its simpler and cheaper implementation, FHSS is used for lower rate, shorter-range systems like Bluetooth and the now largely defunct HomeRF. Wireless Multiplexing and Multiple Access Techniques Introduction Multiplexing techniques aim to increase transmission efficiency by transmitting multiple signals or data streams on a single medium. The resulting increased capacity can be used either to deliver a higher data rate to a single user, or to allow multiple users to access the medium simultaneously without interference. User access to the bandwidth can be separated by a numbers of means: in time (TDMA), in frequency (FDMA or OFDMA), in space (SDMA) or by assigning users unique codes (CDMA). These methods will be d

escribed in turn in the following sections. Time Division Multiple Access Time division multiple access (TDMA) allows multiple users to access a single channel without interference by allocating specific time slots to each user. As shown in Figure 4-13, the time axis is divided into time slots that are assigned to users according to a slot allocation algorithm. A simple form of TDMA is time division duplex (TDD), where alternate transmit periods are used for uplink and downlink in a duplex communication system. TDD is used in cordless phone systems to accommodate two-way communication in a single frequency band. Chapter Four TDMA time slot allocation Station 1 Station 2 Station 3 Station 4 Time slot; Ts TDD time slot allocation Uplink Downlink Figure 4-13: Time Division Multiple Access (TDMA) and Duplexing (TDD) TDMA is used in Bluetooth piconets (see Chapter 10). The master device provides the system clock that determines the timing of slots and, within each time slot, the master first polls slave devices to see which devices need to transmit and then allocates transmission time slots to devices that are ready to transmit. Frequency Division Multiple Access In contrast to TDMA, frequency division multiple access (FDMA) provides each user with a continuous channel that is restricted to a fraction of the total available bandwidth. This is done by dividing the available bandwidth into a number of channels that are then allocated to individual users as shown in Figure 4-14. Frequency division duplex (FDD) is simple form of FDMA in which the available bandwidth is divided into two channels to provide continuous duplex communication. Cellular phone systems such as GSM (2G) and UMTS (3G) use FDD to provide separate uplink and downlink channels, while 1G cellular phone systems used FDMA to allocate bandwidth to multiple callers. Radio Communication Basics Channel allocation Channel allocation Frequency User A User B Downlink User C User D Uplink Figure 4-14: Frequency Division Multiple Access (FDMA) and Duplexing (FDD)

In practice, FDMA is often used in combination with TDMA or CDMA to increase capacity on a single channel in an FDMA system. As shown in Figure 4-15, FDMA/TDMA divides the available bandwidth into channels and then divides each channel into time slots that are allocated to individual users. FDMA/TDMA is used by GSM cellular phones, with eight time slots available in each 200 kHz radio channel. Orthogonal Frequency Division Multiplexing Orthogonal frequency division multiplexing (OFDM) is a variant of frequency division multiplexing (FDM), in which a number of discrete subcarrier frequencies are transmitted within a band with frequencies chosen to ensure minimum interference between adjacent subcarriers. This is achieved by controlling the spectral width of the individual subcarriers (also called tones) SO that the frequencies of subcarriers coincide with minima in the spectra of adjacent subcarriers, as shown in Figure 4-16. Chapter Four Frequency User 1 User 2 User 3 User 4 TDMA time slot allocation within each FDMA channel User in User n+ 1 User n+2 User n+3 Time slot; Ts Figure 4-15: FDMA/TDMA Multiple Access System as Used in GSM Cellular Phones Frequency Figure 4-16: Orthogonality of OFDM Subcarriers in the Frequency Domain In the time domain, the orthogonality of OFDM tones means that the number of subcarrier cycles within the symbol transmission period is an integer, as illustrated in Figure 4-17. This condition can be expressed as: Ts = Ni / V or Vi=ni/Ts where Ts is the symbol transmission period and Vi is the frequency of the ith subcarrier. The subcarriers are therefore evenly spaced in frequency, with separation equal to the reciprocal of the symbol period. Radio Communication Basics Figure 4-17: Orthogonality of OFDM Subcarriers in the Time Domain There are a number of ways in which the multiple subcarriers of OFDM can be used: OFDM can be used as a multiple access technique (OFDMA), by assigning single subcarriers or groups of subcarriers to individual users according to their bandwidth needs. A ser

ial bit stream can be turned into a number of parallel bit streams each one of which is encoded onto a separate subcarrier. All available subcarriers are used by a single user to achieve a high data throughput. A bit stream can be spread using a chipping code and then each chip can be transmitted in parallel on a separate subcarrier. Since the codes can allow multiple user access, this system is known as Multi-Carrier CDMA (MC-CDMA). MC-CMDA is under consideration by the WIGWAM project as one of the building blocks of the 1 Gbps wireless LAN (see the Section "Gigabit Wireless LANs, p. 350"). A significant advantage of OFDM is that, since the symbol rate is much lower when spread across multiple carriers than it would be if the same total symbol rate were transmitted on a single carrier, the wireless link much less susceptible to inter-symbol interference (ISI). ISI occurs when, as a result of multi-path propagation, two symbols transmitted at different times arrive together at the receiving antenna after traversing different propagation paths (Figure 4-18). Although OFDM is inherently less Chapter Four Inter-symbol interference Delayed signal Symbol n Symbol n+1 Symbol n+2 Direct signal Symbol n Symbol n+1 Symbol n+2 Figure 4-18: Inter Symbol Interference (ISI) susceptible to ISI, most OFDM systems also introduce a guard interval between each symbol to further reduce ISI. OFDM radios also use a number of subcarriers, called pilot tones, to gather information on channel quality to aid demodulation decisions. These subcarriers are modulated with known training data at the start of each transmitted data packet. Decoding this known data enables the receiver to determine and adaptively correct for the frequency offset and phase noise between the reference oscillators in the transmitter and the receiver and for fading during propagation. Figure 4-19 shows a schematic block diagram of a simple OFDM transmitter and receiver. From the left, the input bit stream at a rate of R bps passes through a series to parallel conver

ter and is split into N bit stream of rate R/N bps. Each of these bit streams drives one modulator, which maps each bit or symbol onto a point in the modulation constellation being used (Section "Digital Modulation Technique, p. 95"). The N resulting amplitude and phase points drive the inputs of an Inverse Fast Fourier Transform (IFTT), the output of which is the sum of the subcarriers, each modulated according to the individual input bit streams. At the receiver, after removing any guard interval, a Fast Fourier Transform (FFT) determines the amplitude and phase of each subcarrier in the received signal. The amplitude and phase are adjusted using information gathered from the pilot tones. A demodulation decision is made by mapping this amplitude and phase onto the modulation constellation and the corresponding input bit or bits are generated. The resulting N parallel R/N bps bit streams are then combined in a parallel to series converter to give the original R bps bit stream. The IEEE 802.11a/g standards uses OFDM in the unlicensed 2.4 and 5 GHz ISM bands respectively to provide data rates up to 54 Mbps. modulation Serial bit or Inverse symbol stream Serial to Guard Digital to analogue parallel Data coding Fourier interval conversion and RF up conversion (QAM) conversion Transform insertion filtering (IFFT) Sub-carrier phase Noise, multi path and Transmission and amplitude other interference channel Serial bit or symbol stream Channel Parallel Guard Fourier Analogue to digital to serial equalization RF down interval conversion and and data Transform conversion conversion removal filtering decoding (FFT) demodulation Figure 4-19: Schematic Block Diagram of an OFDM Transmitter and Receiver Chapter Four The system uses 52 subcarriers of which 48 are used to carry data and are modulated using binary or quadrature phase shift keying (BPSK/QPSK), 16-quadrature amplitude modulation (QAM) or 64-QAM. The remaining four subcarriers are used as pilot tones. Space Division Multiple Access Space division multiple access (SD

MA) is a technique which aims to multiply the data throughput of a wireless network by using spatial position as an additional parameter to control user access to the transmission medium. As a simple example, if a base station is equipped with sector antennas with a 30° horizontal beamwidth, it can separate users into twelve spatial segments or channels depending on their location around the base station. (Figure 3-17 shows the beam pattern for a base station with 6 such elements.) This arrangement would enable the network to achieve a potential twelve-fold increase in data capacity compared with a base station using a single isotropic antenna. As well as simple sector antennas, smart antenna systems are being developed which combine an array of antennas with digital signal processing capabilities in order to achieve spatial control of transmission and reception. Smart antenna systems can adapt their directional characteristics in response to the signal environment and system demands, and can provide the basis for SDMA. Generally a second multiple access technique, such as TDMA or CDMA, is also used in combination with SDMA in order to allow multiple user access within a single spatial segment. Space division multiplexing (SDM), as opposed to SDMA, is based on the use of multiple propagation paths to simultaneously transmit multiple data channels using the same RF spectrum. This is the basis of MIMO radio (see the Section "MIMO Radio, p. 124") which is specified in the IEEE 802.11n standard. Code Division Multiple Access CDMA is closely related to DSSS, where a pseudo-noise code is used to spread a data signal over a wide bandwidth in order to increase its immunity to interference. As noted above, if two or more transmitters use different, orthogonal pseudo-noise (PN) codes in DS spread spectrum, Radio Communication Basics Figure 4-20: Construction of the Walsh Codes they can operate on the same frequency band and in the same physical area without interfering. This is because a correlator using one PN code will n

ot detect a signal encoded using another orthogonal code, since orthogonal codes by definition do not correlate with each other. Examples of an orthogonal code set are the Walsh codes, which can be easily generated from the procedure called the Hadamard transform. With each step to the right in Figure 4-20, the three light matrices are the same as the full matrix to the left, while the darker shaded matrix is the inverse of the one to the left. The Walsh codes can be read off as the lines in each matrix, SO the Walsh codes of length 4 are; 0000, 0101, 0011 and 0110. The property of orthogonality is the basis of CDMA and is used in 3G mobile telephony to ensure that many users, each assigned a unique orthogonal access code, can transmit and receive without interference within a single network cell. Digital Modulation Technique Introduction Modulation is the step in the digital signal processing sequence that transforms and encodes the data stream onto the transmitted RF or infrared signal. The spectrum spreading and multiple access techniques will result in a bit-stream transformed into a chip-stream which must now be modulated onto either a single or multiple carrier frequencies, or used to modulate the position or shape of a transmitted RF or Ir pulse. Chapter Four A wide variety of modulation techniques are used in wireless networking. These range from the simple return to zero inverted (RZI) used in IrDA at low data rates, through a variety of phase shift and code keying methods of increasing complexity, such as BPSK and CCK, used for example in IEEE 802.11b at intermediate data rates, to more complex methods, such as the HHH (1,13) code used in IrDA at high data rates. The selection of the best digital modulation technique for a specific application is driven by a number of criteria, the most important being: Spectral efficiency - achieving the desired data rate within the available spectral bandwidth (see Table 4.9). Bit error rate (BER) performance - achieving the required error rate given the specific fact

ors causing performance degradation in the particular application (interference, multipath fading, etc.). Power efficiency - particularly important in mobile applications where battery life is an important user acceptance factor. Modulation schemes with higher spectral efficiency (in terms of data bits per Hz of bandwidth) require higher signal strength for error-free detection. Implementation complexity - which translates directly into the cost of hardware to apply a particular technique. Some aspects of modulation complexity can be implemented in software, which has less impact on end-user costs. Table 4-9: Spectral Efficiency of Typical Modulation Techniques Modulation technique Spectral efficiency (Bits/Hz) 16-QAM 128-QAM 256-QAM Radio Communication Basics Simple Modulation Techniques On/Off keying (OOK) is perhaps the simplest modulation technique, where the carrier is turned off during a 0-bit and turned on during a 1-bit. OOK is a special case of amplitude shift keying (ASK) in which two amplitude levels represent 0- and 1-bits. The magnitude of the amplitude shift between these two levels is called the modulation index. Return to zero inverted (RZI) is the modulation technique used in IrDA for data rates up to 1.152 Mbps. It is a derivation of the non-return to zero (NRZ) modulation used in UART data transmission (Figure 4-21), in which a 1-bit is represented by a high state, a 0-bit by a low state, and the transition from high state to low state only occurs when a 1-bit is followed by a 0-bit. In contrast, a return to zero (RZ) transmission has a low-high-low pulse during the bit time for each 1-bit, while the RZI scheme inverts this to give a pulse for each zero bit or symbol. When the RZI modulated signal is received, the bit stream is recovered by triggering a high to low transition for each received pulse, as shown in Figure 4-22. The low state of the decoded signal returns to a high state and a high state remains high at the end of each bit period, unless another pulse is received. Data bits Non Ret

urn to zero (NRZ) Return to zero (RZ) Return to zero inverted (RZI) period Figure 4-21: NRZ, RZ and RZI Modulation Techniques Chapter Four Start RZI modulated signal Decoded signal Data bits Figure 4-22: RZI Bit Stream Decoding The advantage of the RZI scheme for IrDA is that it allows the transmitting LED to be off for most of the bit time, in order to conserve battery power. Phase Shift Keying Phase shift keying is a modulation technique in which the phase of the carrier is determined by the input bit or chip stream. There are several types of phase shift key (PSK) modulation including binary (BPSK) and quadrature (QPSK). Binary Phase Shift Keying BPSK is the simplest technique in this class, with the carrier phase taking one of two states, as shown in Table 4-10. A 0 input symbol (whether it is a bit or a chip) corresponds to a zero phase carrier while a 1-symbol corresponds to a 180° phase shifted carrier, resulting in the output waveform shown in Figure 4-23. Table 4-10: Binary Phase Shift Keying Symbol Carrier phase 0 degrees 180 degrees Radio Communication Basics Bit stream Zero phase carrier modulated carrier Figure 4-23: Binary Phase Shift Keying Modulation (BPSK) BPSK modulation is used by IEEE 802.11b at a data rate of 1 Mbps, and by IEEE 802.11a, in combination with OFDM, to achieve data rates of 6 and 9 Mbps. Quadrature Phase Shift Keying Instead of the two phase states used in BPSK, QPSK uses four distinct carrier phases, each of which is used to encode a symbol comprised of two input bits or chips. These four carrier phases are illustrated in Figure 4-24, which represents the phase of the carrier signal in the IQ plane (I = In phase, Q = Quadrature or 90 degrees out of phase). The angle of a given point amplitude circle QPSK constellation II/4-QPSK constellation Figure 4-24: QPSK Phase Constellation Chapter Four Table 4-11: Quadrature Phase Shift Keying Symbol Carrier phase 0 degrees 90 degrees 180 degrees 270 degrees on the plane from the I axis represents the phase angle and the distance of a poi

nt from the origin represents the signal amplitude. The four points 00, 01, 11, and 10 shown in Table 4-11, are known as the modulation constellation, and represent the four carrier phases each with unit amplitude. QPSK modulation is used by IEEE 802.111 at a data rate of 2 Mbps, and by IEEE 802.11a, in combination with OFDM, to achieve data rates of 12 and 18 Mbps. 4-QPSK is a variant of QPSK, as shown in Figure 4-24, which uses carrier phases offset by 45 degrees (i.e. 45, 135, 225 and 315 degrees). Offset QPSK (O-QPSK) is another variation on QPSK in which transmission of the quadrature phase is delayed by half a symbol period. The consequence is that, unlike QPSK, carrier phase transitions can never be more than 90 degrees and, as a result, the carrier phase and amplitude never passes through zero. The advantage is a narrower spectral width, which is important in applications where interference between adjacent channels must be avoided. O-QPSK is part of the IEEE 802.15.4 radio specification used by ZigBee, where 16 channels, each 5 MHz wide, are used in the 2.4 GHz ISM band to enable 16 co-located networks. Use of O-QPSK helps to reduce interference between these closely spaced channels. Differential Phase Shift Keying Differential phase shift keying is a variation on BPSK and QPSK in which the input symbol results in a differential change of phase instead of defining the absolute phase of the carrier. With BPSK, a 0-symbol corresponds to a period of zero phase carrier, while in DBPSK, a 0-symbol corresponds to no change in carrier phase from the previous bit-period. Radio Communication Basics Table 4-12: Differential Quadrature Phase Shift Keying Symbol Phase change 0 degrees 90 degrees 180 degrees 270 degrees Similarly in DQPSK, each symbol translates to a change of phase rather than an absolute carrier phase, as shown in Table 4-12. Although BPSK or QPSK are conceptually simpler, differential phase shift keying, whether DBPSK or DQPSK, has the practical advantage that the receiver only needs to detect rel

ative changes in carrier phase. The BPSK or QPSK receiver always needs to know the absolute phase reference of the carrier and this reference can be difficult to maintain, for example, if the phase of the received signal is varying due to multipath interference. Other variants on PSK and DPSK include 8-DPSK, which extends the DQPSK keying table to encode 8 data symbols using phase changes separated by 45 degrees rather than by 90 degrees, and t/4-DQPSK which, by analogy with 4-QPSK, uses carrier phase changes similar to Table 4-12 but offset by 45 degrees (i.e. 45, 135, 225 and 315 degrees). n/4-DQPSK and 8-DPSK are used in the enhanced data rate (EDR) Bluetooth 2.0 radio for 2 Mbps and 3 Mbps data rates respectively. Frequency Shift Keying Frequency shift keying (FSK) is a simple frequency modulation method in which data symbols correspond to different carrier frequencies, as shown in Table 4-13 for BFSK. Table 4-13: Binary Frequency Shift Keying Symbol Carrier frequency fo - f1 fo + f1 Chapter Four The sudden carrier waveform changes in simple FSK generate significant out-of-band frequencies and, as a result, FSK is inefficient in terms of spectrum usage. This situation can be improved by passing the input bit stream through a filter to make the frequency transitions more gradual. A Gaussian filter is one type of filter with a specific mathematical form, and use of this as a pre-modulation filter results in Gaussian frequency shift keying (GFSK). GFSK is used in the Bluetooth radio for standard data rate transmission, with a carrier frequency fo of 2.40 to 2.48 GHz and frequency deviation f1 of between 145 and 175 kHz. Spectral efficiency is particularly important as the FHSS frequency hopping channels are only separated by 1 MHz. Quadrature Amplitude Modulation Quadrature amplitude modulation (QAM) is a composite modulation technique that combines both phase modulation and amplitude modulation. In BPSK or QPSK, a constant carrier amplitude with 2 or 4 different phases is used to represent the input data symbol

s, as described above. Instead of using 2 or 4 points, QAM defines a constellation of 16, 64 or more points, each with a particular phase and amplitude, and each representing a 4- or 6-bit (or chip) data symbol. 16-QAM and 64-QAM modulation techniques are used in the IEEE 802.11a and g specification, together with OFDM, to achieve data rates of 24 to 54 Mbps. Figure 4-25 shows the 16-QAM constellation - the 16 points on the IQ plane - used to achieve data rates of 24 Mbps and 36 Mbps. The points in the 16-QAM constellation can be alternatively numbered according to a Gray coding in which adjacent points differ only in the switching of one bit, as shown in Figure 4-26. Using this numbering reduces the chance of two-bit errors in the receiver - if a point is erroneously detected as a neighbouring point only one bit will be incorrect. This makes it easier to recover the bit error using error correction techniques. The next step, a 256-QAM modulation scheme, would further improve achievable data rate with no increase in the occupied bandwidth, but Radio Communication Basics Figure 4-25: 16-QAM Constellation Figure 4-26: Gray Coded 16-QAM Constellation Chapter Four generating and processing 256-QAM modulated signals is currently a significant challenge for hardware performance and cost. Dual Carrier Modulation Dual carrier modulation is a technique applied in multi-carrier systems such as OFDM to combat the loss of data due to the destructive interference, or fading, of individual carrier signals in a multi-path environment (see the Section "Multipath Fading, p. 114"). By modulating data onto two carrier frequencies rather than one, the transmission can be made more robust, although at the cost of using additional bandwidth. In multi-band OFDM (see the Section "Multiband UWB, p. 122") a 4-bit symbol is mapped onto two different 16-QAM constellations and the resulting symbols are transmitted on two OFDM carrier tones separated by at least 200 MHz. If reception of one of the tones is affected by fading, the data can be

recovered from the other tone, the wide separation assuring that the probability of both tones being affected is very small. Pulse Modulation Methods Several wireless network standards specify the use of pulsed rather than continuous transmission of a carrier wave, and a number of specific modulation techniques are used in these systems. Pulse Position Modulation In pulse position modulation (PPM), each pulse is transmitted within a reference time frame, and the information carried by the pulse is determined by the specific transmission time of the pulse within its frame. For example, a 4-PPM system will define four possible positions for a pulse within the reference frame, with each possible position coding one of four input data symbols (Table 4-14). More generally, an m-PPM system will have m possible pulse transmission slots within a frame. An 8-PPM modulation system is shown in Figure 4-27. PPM is specified in the IrDA standard at a 4 Mbps data rate, and is also used in Impulse Radio (IR as opposed to Ir!). Radio Communication Basics Table 4-14: Data Symbols for 4-PPM Modulation Input data symbol 4-PPM data symbol Pulse position Code Word Pulse position Code Word (000) (100) (001) (101) (010) (110) (011) (111) Pulse sequence Code sequence (000) (011) (101) (100) Figure 4-27: 8-PPM Modulation Pulse Shape Modulation Pulse shape modulation (PSM) encodes the input data stream in the shape of the transmitted pulse. The simplest form of PSM is pulse amplitude modulation (PAM) in which, typically, two or four distinct pulse amplitudes are used to encode data symbols, as shown in Table 4-15. Similarly, pulse width modulation (PWM) uses the width of transmitted pulses and more generally, PSM may use some other pulse shape characteristics - such as the derivative of the pulse waveform - to encode data onto the pulse train. Pulse amplitude and pulse shape modulation are candidates for use in the ultra wideband radio physical layer of the ZigBee specification. Chapter Four Table 4-15: PAM Encoding Table Input data symbo

l Pulse amplitude RF Signal Propagation and Reception The first part of this chapter described the various techniques that are used to encode and modulate an input data stream onto a radio frequency carrier. The following four sections discuss the various factors that impact on the transmission, propagation and reception of radio waves, which will enable an estimate to be made of the power requirements for a given wireless networking application. The key factors are transmitter power, antenna gain at the transmitter, propagation or link losses, antenna gain once again at the receiving station, and finally receiver sensitivity. Taken together these factors make up the link budget - the balance of power plus gain required to compensate for losses in the link SO that sufficient signal strength is available at the receiver to allow data decoding at an acceptable error rate. Transmitter Power Every RF transmitter generates a certain amount of power (PTX), which is the first major factor in determining the range of a radiated signal. Transmitter power is measured in one of two ways, either in the familiar unit of Watts (or milliwatts) or alternatively using a relative unit called "dBm". The power in dBm is calculated as dBm = 10 X log 10 (Power in milliwatts), SO a transmitter of 100 mW (0.1 Watts) is equivalent to 20.0 dBm (Table 4-16). The dB (or dBm) unit is useful for two reasons. First, when considering the various factors affecting signal strength, these effects can be easily combined when using dB units by simply adding the relevant dB numbers together. Second, it is easy to translate dB into relative power by remembering that +3 dB represents a doubling of power and -3 dB similarly a halving of power. The additive rule applies here too, SO -6 dB is 1/4 the power, -9 dB is 1/8 and SO on. Radio Communication Basics Table 4-16: Power in mW and dBm Power (m W) Power (dBm) Transmitter power levels for typical wireless networking products are in the region of 100 milliwatts to 1 Watt (20 to 30 dBm). For example, in t

he US the FCC specifies a maximum transmitter power of 1 Watt for FHSS and DSSS transmitters in the 2.4 and 5.8 GHz ISM bands. In the UK the Radio Communications Agency (RA) specifies that these devices must have a maximum effective isotropic radiated power (EIRP) in the 2.4 GHz band of 100 mW or 20 dBm. As described below, EIRP is a combination of the transmitter's power and the antenna gain. Antenna Gain An antenna converts the power from the transmitter into electro-magnetic waves that are radiated to the receiver, and the type of antenna affects the pattern and power density of this radiation, and therefore the strength of signal seen by a receiver. For example, a simple dipole antenna emits radiation relatively evenly in all directions apart from along its axis, while a directional antenna emits radio waves in a narrow beam. Typical radiation patterns of a dipole and a directional antenna are shown in Figure 4-28. The ratio of the maximum power density at the centre of the radiation pattern of any antenna to the power density of the radiation from a reference isotropic antenna is known as the antenna gain (GTX or GRX), and is measured in dBi units. The effective isotropic radiated power, or EIRP, of a radiating antenna is then, the sum of the dBm power arriving at the antenna from the transmitter plus the dBi antenna gain. The types of antenna that can be used in wireless Chapter Four -3 dB -3 dB -6 dB -6 dB -9 dB Figure 4-28: Radiation Pattern from Dipole and Yagi Antennas networks were described in the Section "Wireless LAN Antennas, p. 55" which illustrated antennas with gain ranging from 0 dBi for an omnidirectional dipole antenna to +20 dBi or more for a narrow beam directional antenna. The cables and connectors that link the transmitter or receiver to the antenna also introduce a loss into the system that can range from a few dB to tens of dB depending primarily on the length and quality of cabling. An equally important aspect of transmitter to antenna or receiver to antenna connections, is the matchin

g of impedance between these components. For example, maximum power will only be transmitted if the impedances of the transmitter, connecting cable and antenna are equal, otherwise power will be lost as a result of reflections at the connections between components. In the case of equipment with integrated antennas, this will be part of the design dealt with by the equipment manufacturer. However, this aspect will have to be considered when attaching an external antenna to a wireless network adapter or access point. Receiver Sensitivity As the strength of the signal reaching the receiver input drops, the decoding of data will be increasingly affected by noise and, as a result, will become increasingly error prone. The sensitivity limit of the receiver is determined by the allowable bit error rate and the receiver noise floor. Radio Communication Basics As these factors are discussed in the following sections, an example will be worked through, based on the following parameters: 802.11b DSSS system (2.4 GHz, 22 MHz spread bandwidth) DQPSK modulation 2 Mbps data rate or 2 MHz de-spread bandwidth required bit error rate of 1 in 105 receiver noise figure of 6 dB 20°C ambient temperature. Bit Error Rate The rate at which decoding errors occur is measured by the bit error rate (BER), with a BER of 1 in 105 being typical at the receiver sensitivity limit. Since data is transmitted in packets containing several hundreds or thousands of bits of data, even a 1 in 105 chance of an error in decoding any single bit will multiply up to a significant probability of an error in a large data packet, and the resulting packet error rate (PER) can be in the range of several percent. For example, with a BER of 1 in 105 the PER for a 100 bit data packet will be: -PER) = (1 - 10-5) 000 or PER = 0.1%, rising to 1% for a 1 kb data packet. The BER is a function of the signal-to-noise ratio in the receiver, and also depends on the specific type of modulation method being used. The signal-to-noise ratio of a communication channel is given by

: SNR=(Eg/N)*(f/ W) (4.1) where Eb is the energy required per bit of information (Joules), fb is the bit rate (Hz), No is the noise power density (Watts/Hz) and W is the bandwidth of the modulated carrier signal (Hz). Note that for our example, considering a DSSS system, it is the de-spread bandwidth that is considered in Eq. 4.1. Using a spread spectrum rather than a narrow band transmission results in an additional gain known as the processing gain. Processing Gain = 10 logio(C) dB (4.2) Chapter Four where C is the code length in chips (11 for the Barker code discussed above). This processing gain is effectively included in the calculation of channel SNR by using the de-spread bandwidth in Eq. 4.1. The bit rate per Hz of bandwidth, fb/, is a function of the modulation method employed, as discussed in the Section "Introduction, p. 95". BER is then given by: BER = 1/2 erfc (SNR) 1/2 (4.3) where erfc is the SO called complementary error function which can be looked up in mathematical tables. Figure 4-29 shows BER as a function of SNR for some of the common modulation methods. 16-PSK 8-PSK DQPSK DBPSK Eb/No (dB) Figure 4-29: Bit Error Rate (BER) for Some Common Modulation Methods The figure shows that for the example DQPSK modulated signal, with 1 bit per Hz of bandwidth, a signal-to-noise ratio of 10.4 dB is required to achieve a bit error rate of 1 in 105. Receiver Noise Floor The receiver noise floor has two components, the theoretical thermal noise floor (N) for an ideal receiver, and the receiver noise figure (NF) Radio Communication Basics which is a measure of the additional noise and losses in a particular receiver. The thermal noise is given as: N = kTW (4.4) where k is the Boltzmann constant (1.38 X 10-23 Joules/°K), T is the ambient temperature in °K and W is the bandwidth of the transmission (Hz)¹. Receivers for wireless networking will typically have noise figures in the range from 6 to 15 dB. The receiver noise floor (RNF) is then the sum of these two terms: RNF = kTW + NF (4.5) For the example 802.11

b receiver with a 2 MHz de-spread bandwidth, operating at 20°C (290°K) and with a noise figure of 10 dB: N = 1.38 X 10-23 J/K X 290°K X 2 106 Hz = 8.8 X 10-12 mW = - -110.6 dBm RNF = -110.6 dBm + 10 dB =-100.6dBm Receiver Sensitivity The receiver sensitivity, PRX, is the sum of the receiver noise floor (RNF) and the signal-to-noise ratio (SNR) required to achieve the desired bit error rate: PRx=RNF + SNR (4.6) For the example; PRX = 100.6 + 10.4 dBm = -90.2 dBm From this discussion it can be seen that as the data rate in the example increases from 2 Mbps towards the 802.11b maximum of 11 Mbps, different modulation methods will be needed to achieve the higher bandwidth efficiency (more bits per Hz of bandwidth, or fb/W in Eq. 4.1). 1 Again, the de-spread bandwidth is used here. Chapter Four Table 4-17: PRX Versus Data Rate for a Typically 802.11b Receiver Data Rate (Mbps) Modulation technique PRX (dBm) 256 CCK + DQPSK 16 CCK + DQPSK Barker + DQBSK Barker + DBPSK This will result in a higher signal-to-noise ratio requirements for the same bit error rate, SO that the receiver sensitivity will decrease at higher data rates. This is shown in Table 4-17 for a typical 802.11b receiver. This dependence of PRX on data rate underlies the gradual deterioration in wireless network throughput as signal strength decreases. There is no abrupt cut-off in performance, but rather a gradual reduction in throughput as the transmitter and receiver switch to a lower data rate at which a low BER can be maintained. RF Signal Propagation and Losses Between the transmitting and receiving antennas, the RF signal is subject to a number of factors that affect signal strength. These are considered in the following sections. Free Space Loss Once the signal is radiating outwards from the antenna, the signal power falls off with distance due to the spreading out of the radio waves. This is known as free space loss, and overall is the most significant factor affecting received signal strength. Free space loss is measured in dB, and depends on the

Figure 4-30: Free Space Loss at 2.4 GHz and 5.8 GHz Chapter Four a straight line between the two antennas. The volume of space around this straight line affects signal propagation as well, and any obstructions that come close to the direct line-of-sight will also cause signal loss. Fresnel Zone Theory The theory used to calculate the effect of obstructions is called Fresnel zone theory. The Fresnel zone is a region between the two antennas with an oval shape similar to a rugby ball. There are actually a series of such regions, called the 1st, 2nd, 3rd, etc. Fresnel zones (Figure 4-31), and at the mid point between transmitter and receiver, the radius of the nth Fresnel zone in metres is calculated from the formula: R = 0.5 (nxAxD) (4.10) where the wavelength and the transmitter to receiver distance are also in metres. For a 2.4 GHz signal, with a wavelength of 12.5 cm (0.125 m), the first Fresnel zone has a mid-point radius of 1.8 m for a 100 m range, or 5.6 m for a 1 km range. Any obstructions within the first Fresnel zone will cause signal loss through reflection, refraction or diffraction. Multipath Fading Multipath fading occurs when reflected, refracted or diffracted signals travel to the receiver along different paths, resulting in a range of different arrival times known as the multipath delay spread. Signals arriving along different paths will be phase-shifted with respect to the direct path signal, First Fresnel Zone Second Fresnel Zone Figure 4-31: The Fresnel Zones Around a Propagation Path Radio Communication Basics Other paths omitted for simplicity Out-of-phase arrival of two signals Figure 4-32: Multi-path Fading in an Indoor Environment as shown in Figure 4-32, and will therefore cause some degree of destructive interference at the receiving antenna. This is familiar in UHF TV reception as a ghost image caused by an interfering signal reflected from a nearby building or other large object. Interference between these multiple delayed signals can substantially reduce the signal strength at the recei

ving antenna, introducing a loss that can be as much as 20 to 30 dB. It is possible to compute multipath losses using complex ray tracing or other algorithms, but this is rarely done in practice. Signal Attenuation Indoors For a typical wireless network in a home or small office, multiple obstructions such as walls, floors, furniture and other objects will obstruct the propagation path from transmitter to receiver, and signal reception will tend to be very variable. Depending on its construction, transmission through a wall can introduce a loss of 3 to 6 dB or more, as shown in Table 4-18, and an additional allowance will be required in the link budget to account for this loss. In a multi-storey building, losses between floors will also depend on the building materials used, and will be very high in buildings Chapter Four Table 4-18: Typical Attenuation for Building Materials at 2.4 GHz Attenuation range Materials Loss (dB) Non tinted glass, wooden door, cinder block wall, plaster. Medium Brick wall, marble, wire mesh or metal tinted glass. Concrete wall, paper, ceramic bullet- 10-15 proof glass. Very high Metal, silvering (mirrors). with sheet steel construction. More typically, a loss of approximately 6 dB is seen between adjacent floors, rising to around 10 dB per additional floor for separations of two to three floors. The typical losses shown in Table 4-18 are highly dependent on the specific materials used and methods of construction. For example, even a stud wall can introduce a significant loss if it contains a fire retarding foil membrane. In common with multi-path fading, complex algorithms are required to calculate the various types of losses indoors, and it is therefore convenient to combine these loss components to give a single additional term in the link budget. This term, the fade margin (LFM), will generally be estimated by a rule-of-thumb, or determined by an on-site survey. Link Budget The factors considered above, transmitter power (PTX), antenna gain at the transmitter (GTX) and receiver (GRX

), receiver sensitivity (PRX), free-space loss (LFS) and other losses combined in the fade margin (LFM), together Table 4-19: Balancing factors in the link budget Reducing required PTX Increasing required PTX Lower receiver sensitivity (bigger negative) PRX Higher free space loss LFS Higher transmitter antenna gain GTX Higher fade margin LFM Higher transmitter antenna gain GRX Radio Communication Basics define the link budget that is available to bring the data signal successfully from transmission to detection (Table 4-19). It is convenient to express the link budget in terms of the transmitter power (PTX) required to deliver a signal to the receiver at its sensitivity limit (PRX). Expressed in dBm, this is: PTX = - - dBm (4.11) For example, a system comprising a directional transmitting antenna with a gain of 14 dBi, a patch receiving antenna (6 dBi), and a receiver with a sensitivity of -90 dBm, operating over 100 metres at 2.4 GHz (LFS = 80 dB) with a 36 dB fade margin (LFM) results in a required transmitter power of: PTX = -90 dBm - 14 dBi - -6 dBi + 80 dB + 36 dBm = +6 dBm To ensure that the signal at the receiving antenna is above the receiver sensitivity, the required transmitter power is therefore +6 dBm (4 mW), as shown graphically in Figure 4-33. This configuration would be comfortably achieved with a 100 mW (20 dBm) transmitter, with an extra 14 dB link margin for unaccounted losses or noise. Required PTx Free space Fade margin Antenna Receiver gain Rx sensitivity Antenna gain Tx Figure 4-33: Link Budget Expressed as Required Transmitter Power Chapter Four Ambient Noise Environment As well as the receiver noise floor, which defines the limit of receiver sensitivity, other sources of external RF noise will also have an impact on the reliability of RF signal detection and data decoding. The total RF noise entering a radio antenna at any particular location is termed the ambient noise environment and is made up of two components: Ambient noise floor - the aggregate background noise from distant sources s

uch as car ignition, power distribution and transmission systems, industrial equipment, consumer products, distant electrical storms and cosmic sources. Incidental noise - the aggregate background noise from localised man-made sources. The ambient noise floor is generally "white noise", with constant power per unit bandwidth, while incidental noise may be either broadband or narrow band. In implementing local or metropolitan area wireless networks, the ambient noise floor will be established during an RF site survey, and should be explicitly included in the link budget if it is above the receiver sensitivity of planned equipment. For example, in the link budget calculation above, if an ambient noise floor of -85 dBm was measured, using this in place of the receiver sensitivity of -90 dBm would result in a required transmitter power of 11 dBm. This type of environmental noise will limit the range that can be achieved for a given equipment configuration (transmitter power, antenna gains, etc.), but will not degrade the performance of wireless networks when operating within that limit. Narrow band incidental noise, from nearby man-made sources such as a microwave oven or a narrow band transmitter, is more likely to result in unpredictable and unreliable network performance. Interference Mitigation Techniques Wireless networking specifications are increasingly including a range of measures to mitigate the effect of interference on network performance. Wireless USB, covered in Chapter 10, is a good example of the approach which starts with establishing information about the quality of the RF link and then provides measures to control various link characteristics. Radio Communication Basics Table 4-20: Wireless USB Interference Mitigation Controls Control Description Transmit power (TPC) Host can control its own transmit power level as well as querying and controlling transmit power of devices in the cluster. Transmitted bit rate Host can adjust the transmitted bit rate for both outward (host to device) and inward (dev

ice to host) transfers. Data payload size When interference causes PER to rise, reducing packet size can improve throughput by reducing uncorrectable errors. RF channel selection Wireless USB's MB-OFDM radio provides multiple alternative channels which can be used by a host if supported by all devices in the cluster. Host schedule control Allowing isochronous data transfers to temporarily use channel time allocated for asynchronous transfers, in order to retransmit failed isochronous data packets. Dynamic bandwidth Host control of the spectral shaping capabilities of control the MB-OFDM UWB radio, described in the following section. In wireless USB, the host and other devices can maintain statistical information on packet error rate and on link indicators such as received signal strength (RSSI) and link quality (LQI). The latter indicator measures the error in the received modulation of successfully decoded symbols. The main RF link controls available in wireless USB are described in Table 4-20. Transmit power control and RF channel selection are also included in the network optimisation measures introduced in the 802.11k extension to the Wi-Fi networking standard, covered in Chapter 6. Ultra Wideband Radio Introduction Ultra Wideband wireless communication systems are based on impulse radar technology that was developed for military applications by the Chapter Four USA and USSR in the 1960s. Impulse radar or radio transmits extremely short electromagnetic pulses, typically less than 1 ns (nanosecond) in length, with no underlying carrier signal. Such short pulses result in an effective bandwidth of the transmission that may be from 500 MHz up to several GHz. In 2002 the FCC in the USA opened 7.5 GHz of radio spectrum for UWB applications, from 3.1 to 10.6 GHz, and adopted a definition of UWB as any intentional transmission in which the bandwidth to -3 dB points was at least 20% of the mean frequency of the transmission, with a minimum bandwidth of 500 MHz. Since UWB transmissions cover a wide swath of the radio

spectrum, an important requirement is that they do not result in harmful interference with other RF transmitted services, whether current or planned. To ensure this coexistence, the FCC has defined strict EIRP limits on UWB transmission, as shown in Figure 4-34. The maximum permitted power density (EIRP) of -41.3 dBm/MHz is below the FCC Part 15 noise power limit for unintentional emitters such as computers and other electronic devices. As a result of this very low EIRP specification, UWB wireless is suited for applications where very long battery life is required. FCC Part 15 Limit UWB passband 3.1 to 10.6 GHz Indoors Outdoors Frequency (GHz) Figure 4-34: FCC UWB Passband Specification Radio Communication Basics A second characteristic of some UWB implementations is spectral shaping - the capability to control the radiated power spectrum in order to avoid transmission at particular narrow band frequencies. UWB comes in three varieties for data communication applications: time-hopping pulse position modulation (or Impulse radio) direct sequence spread spectrum-UWB (DS-UWB) multiband-UWB (such as multiband OFDM). Of these, MB-OFDM offers the greatest flexibility in spectral shaping, with a wide range of course and fine control options easily implemented in software. Time Hopping PPM UWB (Impulse Radio) Impulse radio (IR) is the name given to UWB radio based on time- hopping, pulse position modulation. Data is transmitted as a discontinuous series of very short pulses, with one pulse per user in each time hopping frame of length Tf. The nominal transmission time of a pulse in a given frame is determined by a pseudo-noise (PN) code that is specific for each user of the communication channel. Finally, whether a pulse represents a 1-bit or a 0-bit depends on the actual transmission time relative to the nominal transmission time (the pulse position modulation). For example, in an early/late PPM system, if the pulse is transmitted a time offset S ahead of the nominal time it represents a 1-bit, or if an offset 8 after t

he nominal time then it represents a 0-bit. In the example shown in Figure 4-35, a TH code of length 4 is used, SO that four pulses are transmitted for each bit, each pulse in one of the eight code slots (Tc) in each of four successive frames. Pulse amplitude modulation (PAM) or pulse shape modulation (PSM) can be used as alternatives to PPM, with 1-bit and 0-bit then being determined by the amplitude or shape of each individual pulse. Impulse radio is one of two optional physical layer specifications selected in March 2005 by the IEEE 802.15 Task Group 4a as part of the enhancement of the 802.15.4 standard. (The other optional PHY is a chirp spread spectrum operating in the 2.4 GHz ISM band.) Chapter Four User n bit stream = 1101. User n TH code = 2767 Four pulses transmitted per bit Nominal pulse transmission time Early pulse (t = T2/2 - 8) signifies 1-bit Late pulse (t = Tc/2 + 8) signifies 0-bit Figure 4-35: Pulse Train in a TH-PPM Impulse Radio Transmission Direct sequence UWB (DS-UWB) Direct sequence, discussed in the Section "Direct Sequence Spread Spectrum in the 2.4 GHz ISM Band, p. 83" as the spread spectrum technique underlying the IEEE 802.11b and 802.11g physical layer, can also be applied in UWB radios. Instead of the chipping code being used to spread the carrier spectrum by increasing the symbol transmission rate, the spectrum is spread to UWB proportions as a result of the very narrow pulse that is used to transmit each symbol. The chipping code then plays a multiple access role (CDMA) with individual user codes determining the exact times at which individual users of the channel will transmit or receive a pulse. A variety of modulation methods (PAM, PSM) can be used to code the data stream onto the pulse stream. So far, DS-UWB has not been identified as a target technology for any wireless networking applications. Multiband UWB In multiband UWB, ultra-wide bandwidth is achieved by dividing the frequency band of interest into multiple overlapping or adjacent bands, Radio Communication Basics Chan

nel 1 Channel 2 Channel 3 Channel 4 Channel 5 10296 528 MHz wide bands, 128 OFDM tones, 100 data, 12 pilots, 10 guards, 6 nulls Figure 4-36: MB-OFDM Frequency Bands and Channels and operating simultaneously on all available bands. Currently, the most important example of this technique is multiband (MB) OFDM, which is being promoted by the MB-OFDM Alliance (MBOA) and has been adopted as the basis for Wireless USB and Wireless FireWire (Wireless 1394). MB-OFDM, as proposed by the MBOA, uses a bandwidth from 3.168 GHz to 10.560 GHz, which is divided into 14 bands of 528 MHz full width - thus meeting the FCC's 500 MHz minimum bandwidth specification. The 14 bands are grouped into 5 band groups or channels, as shown in Figure 4-36. Frequency hopping between bands within a band group can be used to enable overlapping piconets to be formed but unlike Bluetooth, which makes 1600 hops per second across 79 frequencies, the MBOA radio as specified for wireless USB makes 3 million hops per second, one hop after every transmitted symbol, across just 3 frequencies. MBOA specifies two types of time-frequency codes (TFC) as shown in Table 4-21. Time-frequency interleaving (TFI) codes define frequency hopping patterns, while fixed frequency interleaving (FFI) codes define continuous transmission on a single OFDM band. The FFI option can be used to improve the performance of two or more simultaneously operating piconets, by assigning a single OFDM band to each piconet. Within each 528 MHz band, 128 OFDM subcarriers are transmitted, with data modulated onto 100 of these and the remainder used as pilot, guard and null tones. For data rates of up to 200 Mbps, MBOA specifies data modulation using QPSK, while rates of 320 to 480 Mbps use dual carrier modulation (DCM). Chapter Four Table 4-21: MBOA Time-frequency Codes Code number Code type Band number (Band group 1) Spectral shaping is used to avoid interference with other RF services and can be changed under software control to respond to specific local regulations or time-varying co

nditions. Coarse control can be achieved by dropping whole bands (or in extreme circumstances whole band groups), but extremely precise shaping is also possible by "nulling out" a certain number of tones within a single band. MIMO Radio As described in the Section "Multipath Fading, p. 114", the multiple paths that a radio signal takes between transmitter and receiver often lead to a degradation of signal strength through multi-path fading. Multi-input multi-output (MIMO) radio takes advantage of this characteristic of RF propagation by sending multiple data streams across multiple transmitters to receiver paths in order to achieve a higher data capacity (Figure 4-37). Mathematical modelling of the propagation paths, using a channel calibration period during each transmitted data packet, allows the different signal paths and data streams to be identified and correctly recombined in the receiver. This technique, space division multiplexing (SDM), is analogous to FDM in the frequency domain but instead of different frequencies carrying data in parallel, here different spatial paths carry data in parallel. Effectively the same bandwidth is being used simultaneously to create multiple communication paths. If these paths are equally strong and can Radio Communication Basics Single input, Single input, single output multiple output Multiple input, Multiple input, single output multiple output Figure 4-37: MIMO Radio Definition be perfectly separated, the overall capacity of the communication channel increases linearly with the number of independent paths used. In a system with M transmitters and N receivers, the number of independent paths is the minimum of M and N. In practice, all paths will not be equally strong or perfectly separated, and performance will be determined by coefficients, known as singular values, which characterise each path between a transmitting and receiving antenna. These singular values are determined by including a short "training period" in the preamble of each transmitted data packet, during

which known and different signals are transmitted from each antenna. These signals provide information about the transmission channel (so-called Channel State Information or CSI), and with this information the receiver can compute the singular values that are used to decode the remainder of the data packet. The increased capacity of MIMO radio can be used to achieve a higher data rate or to increase link robustness or range for a given data rate. The IEEE 802.11n specification (described in the Section "MIMO and Data Chapter Four Rates to 600 Mbps (802.11n), p. 165") will use MIMO to increase the PHY layer data capacity of the 802.11a/g radio from 54 Mbps to in excess of 200 Mbps. Space time block coding (STBC) is a related technique which combines space and time diversity to increase the robustness or range of an RF link. STBC breaks the transmitted data into blocks and transmits multiple time- shifted copies of each block of data from each transmitting antenna to the receiving antenna. STBC is thus a Multi-Input Single-Output (MISO) technique (see Figure 4-37), although multiple receiving antennas can further improve performance. Near Field Communications Introduction Near field communications (NFC) is a very short range radio frequency communications technology that has been extensively developed for use in RF identification (RFID) tags and other smart labelling applications. These applications have typically employed a RF carrier frequency of 13.56 MHz, which is internationally allocated as an unlicensed ISM band. NFC is distinct from so-called far field RF communication used in personal area and longer range wireless networks, since it relies on direct magnetic field coupling between transmitting and receiving devices. There are two types of NFC devices, active and passive, which operate quite differently. Passive devices do not have an internal power source, but derive their power from an active initiating device by inductive coupling. A passive device also does not transmit data by generating a magnetic fi