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2. The Transport Layer 5. Multiplexing 5. Service Primitives 7.1.5.1.5.5. CCITT X.3.2.5.2. Network Layer Standards 4.2.6.2.1.2.2.1.5. Functional Units 6. Splitting and Recombining 5.1.8. The Presentation Layer 7. Segmentation 5.25 4.2. Abstract Syntax Notation One viii 66 66 69 71 72 72 65 65 67 67 67 68 69 69 69 69 70 70 70 71 72 74 74 77 77 78 79 79 80 81 82 82 83 84 84 84 87 89 89 . CCITT X.7.7. Error Checking 5.1. Transport Layer Standards 5.2.2. Classes of Protocol 5.2. TPDUs 5. SPDUs 6. Further Reading 7. Syntax 7.3. Error Reporting and Resynchronization 6. TCP 5.1.2.1.1.6.4. Summary 5. Transport Protocol 5.4.2.2. Synchronization 6.2.2. Further Reading 4.5.2. Flow Control 5.4.4. Session Layer Standards 6.1. IP 70 4.2.1.3. ISO 8473 4.1. Transport Services 5. Presentation Services 7.3. Addressing 5.2.5.4. Network Types 5.1. Session Layer Role 6.75 4.3. Tokens 6.3.2.4.1. Session Services 6. Functional Units 7.3. Activities and Dialogue Units 6.2.1. Session Protocol 6.1. Further Reading 6. The Session Layer 6.
2.1. Application Entity 8.1.2. Local Area Networks 9. Subscriber Signaling 10.1. Networks Topologies 10.3. Reliable Transfer 8. ANSI FDDI Standard 9.2.3.2.1. Association Control 8.1.4.2. Access.2.2. Basic Encoding Rules 7.1. Further Reading 9. Common Application Service Elements 8. A Simple Network 10.1. Telephone Networks 10.3. Transmission 9.3. Architecture 9. Presentation Standards 7.5.1. Token Ring Protocol 9.5. Further Reading 8.2.2.4. Topologies and Access Protocols 9.1. Interexchange Signaling Contents 89 91 93 94 94 95 95 96 97 97 97 98 98 98 100 104 108 108 109 109 110 112 113 113 114 115 116 117 118 118 119 120 121 121 122 123 125 126 127 128 ix .2.3.7.3. The Application Layer 8.1.1. Definitions in ASN.2. Application Services 8. Virtual Terminal 8. Remote Operations 8. and Management 8.2.2.2.3.3.1.1.3. IEEE 802 Standards 9.3.3. Presentation Protocol 7.1 7.1.1. Specific Application Service Elements 8.2. Other Standards 8.1.2.2.1. Logical Link Control 9. Basic Concepts 9. File Transfer.3. Token Ring 9.4.2.2. Further Reading 10.3.2. Token Bus 9.1. Signaling 10.1. Message Handling Systems 8.4. Switching Systems 10. Basic Concepts 10. Topology 9.2. CSMA/CD 9.2.
ATM Cells 12. Signaling Network Functions 10.4. V.3. PBX Networks 10. Protocol Architecture 11. Integrated Services Digital Network 11. Broadband ISDN 12.3. Further Reading 11. Cell-Based Interface 12.3.2.5.1. Corporate Networks 10. ISDN Standards 11. Intelligent Networks 10.4. Frame Relay 11. Common Channel Signaling 10.6. Operations and Maintenance Applications Part 10.2.2.2. Asynchronous Transfer Mode 12.3. Further Reading 12.2.3. Frame Relay 11.3.2. B-ISDN User-Network Interface 12.3.1. Private Telephone Networks 10. Basic Concepts 11.3.4.3. Internetworking 11. B-ISDN Services 12. Signaling Data Link 10.3. Signaling Link Control 10.5.3.1. Physical Layer 12.2.1. The Network Layer 11.120 11. The Data Link Layer 11.1. Signaling Connection Control Part 10.2. Cell Delineation x 129 131 132 132 133 134 135 136 136 136 137 138 139 140 140 141 142 144 145 146 148 151 154 155 156 157 158 159 161 161 161 163 164 165 165 167 168 168 169 170 .2.3. Broadband ISDN and ATM 12.1.2. ISDN Channels 11.1.2.1.1. Signaling System Number 7 10. The Physical Layer 11.1. User Parts 10.3.3.3.10. ISDN Services 11.2.4.2.1.3.1.1.3.4. B-ISDN Protocol Architecture 12.6. SDH-Based Interface 12.1.3.2.4.1.3. Channels and Paths 12.3.5.2.2.1. Functional Groupings and Reference Points 11.3.
ATM Adaptation Layer 12.2. Cell Loss Priority 12. HEC Generation and Verification 12. Virtual Channel Identifier 12.1.3. Virtual Path Identifier 12.4. Payload Type 12.5.5. Cell Rate Decoupling 12.5. B-ISDN Standards 12.3.2.4. Segmentation and Reassembly Sublayer 12. ATM Layer 12.4. Convergence Sublayer 12.4.6.3.4.5.7.4.12.1. Further Reading Bibliography 171 171 172 172 172 172 173 173 173 174 175 175 175 160 Contents xi .5.4. Generic Flow Control 12.4.
It also introduces the OSI reference model. Telecommunication refers to the primarily human-to-human communication facilitated by the global telephone system. Chapter 11 builds on earlier chapters by examining ISDN as the merging point of data and voice networks. Data communication refers to the communication between digital computers. and data communication is relying more than ever on telecommunication networks. Telecommunication is increasingly relying on digital computer technology. Chapter 10 provides an introduction to telecommunication. The two streams are rapidly converging. Newcomers to this field are often bewildered by the substantial wealth of information already published on the subject. facilitated by computer networks. Chapter 1 introduces computer networks and explains some of their elementary concepts. In-depth discussions of technically-involved topics are intentionally avoided in favor of more general concepts. The structure of the book is as follows. This book is aimed at this group of people. upon which later chapters are based. No previous knowledge of networks or programming is assumed. and terminology.Preface This book is concerned with post-computer communication networks and two of its important streams: data communication and telecommunication. Each of Chapters 2-8 describes one of the seven layers of the OSI model in the context of wide area data networks. Chapter 9 looks at local area networks and their applications. so as to prepare readers for more advanced discussions. x . The differences between these two streams are mainly due to historical reasons. techniques. Chapter 12 looks at the ATM technology and the potential applications that it can support. It provides a broad coverage of the key concepts.
and the OSI model. The network achieves this by providing a set of rules for communication. This chapter introduces the fundamental concepts of computer networks. The need for a protocol should be obvious: it allows different computers from different vendors and with different operating characteristics to ‘speak the same language’. After reading this chapter you should be able to:       Describe the general characteristics of a computer network. which should be observed by all participating hosts. in general.  Understand the role of the major components of a computer network. in particular.  Distinguish between different network types and understand their properties. and then describe a reference model for network protocol architectures which we will expand upon throughout the rest of this book.  Describe the role and functions of each of the OSI layers.  1 . We will also discuss the role of international standards and major standards organizations. Introduction A computer network is the infrastructure that allows two or more computers (called hosts) to communicate with each other. called protocols. We will first look at constituent network components and various network types.     Appreciate the wealth of knowledge available on communication networks.  Use sequence and state transition diagrams to interpret and describe protocols.1.      Appreciate the relevance and importance of standards.
from one point to another. Figure 1. there will be occasions when it is not necessary to distinguish between hosts and nodes. The communication lines may take many different shapes and forms. the entire network may be viewed as a black box. All communication between hosts passes through the nodes. In such cases. The network is made up of two types of components: nodes and communication lines. Network Components Figure 1. Geographic spread of nodes and hosts. Examples include: copper wire cables. radio channels. For a host to communicate with another host. A node is usually itself a computer (general or special) which runs specific network software.1 An abstract network. which in turn determine how to route messages across the network. the network is said to be a Local Area 2 . network hosts nodes Throughout the rest of this book. Each host has a unique address allocated to it by the network. optical fiber.1 shows an abstract view of a network and its hosts.1. and telephone lines. even in the same network. A host is connected to the network by a separate communication line which connects it to one of the nodes. it needs to know the latter’s address. 1. The nodes typically handle the network protocols and provide switching capabilities. When the physical distance between the hosts is within a few kilometers. Network Types Networks may be divided into different types and categories according to four different criteria: 1. In most cases. to which many other hosts are connected. From a host’s point of view.1. we will use the term station to mean either.2. more than one host may be connected to the same node.
3). as illustrated in Figure 1. or WAN type.g. airlines. LANs. but may require registration and payment of connection fees.Network (LAN). Figure 1. or the globe. the network is said to be a Metropolitan Area Network (MAN) or a Wide Area Network (WAN). 2. a university campus). hospitals.g.. on the other hand. both private and public networks may be of LAN. an office environment) or a set of closely-located buildings (e. Technically. are generally accessible to the average user.2. WANs are used to connect hosts spread across a country.2 Example of a WAN between LANs. Public networks. Most networks are for the private use of the organizations to which they belong. although public networks. LANs are typically used to connect a set of hosts within the same building (e. a message follows a specific route 3 . insurance companies. Darwin Brisbane Perth Adelaide Sydney WAN Melbourne LAN/MAN 3. and most other businesses are of this nature. and WANs usually coexist: closely-located hosts are connected by LANs which can access hosts in other remote LANs via MANs and WANs. MAN. Communication model employed by the nodes. by their size and nature. a continent. these are called private networks. MANs. Access restrictions.. Networks maintained by banks. For larger distances. Internet is the most-widely known example of a public network. tend to WANs. MANs cover distances of up to a few hundred kilometers and are used for inteconnecting hosts spread across a city. In the point-to-point model. The communication between the nodes is either based on a point-to-point model or a broadcast model (see Figure 1.
A part of the message (an address) indicates for which node the message is intended. it is not necessary to reserve a path across the network for the duration of communication between A and B. Switching model employed by the nodes. as summarized by Figure 1. Because data is sent in packets.3 Communication models. The OSI Model The International Standards Organization (ISO) has developed a reference model for network design called the Open Systems Interconnection (OSI). this requires packets to carry additional addressing information. Suppose that a host A wishes to communicate with another host B. Layer Name Data Unit Main Function 4 . a dedicated communication path is allocated between A and B. However. 1. a message transmitted by any node can be received by all other nodes. nodes either employ circuit switching or packet switching.across the network in order to get from one node to another. The data is sent along the path as a continuous stream of bits. Figure 1.4.3. Each layer is characterized by a set of standard protocols which specify its behavior. and is then released. In circuit switching. all nodes share the same communication medium and. In packet switching. Different packets can be routed differently in order to spread the load between the nodes and improve performance. In the broadcast model. via a set of intermediate nodes. as a result. on the other hand. All nodes look at this address and ignore the message if it does not match their own address. Each intermediate node temporarily stores the packet and waits for the receiving node to become available to receive it. data is divided into packets (chunks of specific length and characteristics) which are sent from A to B via intermediate nodes. It proposes a seven-layer architecture for networks. Figure 1. point-to-point broadcast 4. This path is maintained for the duration of communication between A and B.4 The OSI reference model. In the point-to-point model.
and moves up the three layers of node X and down again. as illustrated in Figure 1.5 Nodes use only the lower 3 layers. Figure 1. Transmission of raw data bits over communication lines. where it moves up its seven layers. The reason for this is that the upper four layers are irrelevant to the task of communication between the nodes. Mutually-agreeable binary representation of application data (common syntax). and finally is transmitted to host B. when host A sends a message to host B. For example. This would be characterized by the presentation protocol. until it arrives at the application layer of host B. to the session layer.7 6 5 4 3 2 1 Application Presentation Session Transport Network Data Link Physical Message Message Message Message Packet Frame Bit Mutually-agreeable meaning of application data (common semantics). The nodes in a network implement only the lower three layers. we can use an imaginary line of communication between the presentation layer on host A and the same layer on host B.5. Negotiation of the establishment and termination of connections (sessions).5. 5 . from the application layer to the presentation layer. Z X Host A Application Presentation Session Transport Network Data Link Physical Node X Network Data Link Physical Node Y Network Data Link Physical Y Host B Application Presentation Session Transport Network Data Link Physical Although actual communication takes place only at the physical layer.. until it reaches the physical layer. it is often useful to think of virtual communication between corresponding layers. the message moves down the successive layers of host A. Provision of a reliable communication line to the network layer. These seven layers represent the protocol architecture for the communications component of a host. etc. Routing of data packets within the network and across multiple networks. In Figure 1. Efficient and cost-effective transportation of data across the network. Then it is transmitted to node Y where it goes through the same procedure. It is then transmitted across the communication line between host A and node X.
The additional information appears in form of a header (e. Well-defined protocols and interfaces for each of the layers make it possible for the layer to be designed and implemented in isolation from the other layers.The terms protocol and layer are often used interchangeably. each of these layers may be implemented as a set of routines which communicate with the layer above and the layer below it via parameters passed in function calls. Layer refers to a set of services and functions and their realization in hardware or software. Alternatively. Strictly speaking.. 6 . Custom chips are typically used for this purpose. each layer adds an additional piece of information to the message it is transmitting. Figure 1. The data link layer adds a header as well as a trailer to its data. which is implemented in hardware. A set of network layers is also commonly referred to as a protocol stack. each layer may be implemented as a task (in a multi-tasking environment) which communicates with other tasks by message passing.6 OSI layers as software tasks. This is harmless but not entirely accurate. Sending Host Application AH data Presentation PH AH data Session SH PH AH data Transport TH SH PH AH data Network NH TH SH PH AH data Data Link DLH NH TH SH PH AH data DLT Physical raw data bits Transport Presentation 1 data Receiving Host Application Session tasks Network Data Link Physical For house-keeping purposes. Each of the seven layers of the OSI model hides the implementation details of the lower layers from the upper layers. A layer is therefore characterized by its protocol. The same layer removes the additional piece of information on the receiving end. Figure 1. protocol refers to the rules and conventions that the functions of a layer should conform to. Except for the physical layer.6 illustrates the latter.g. 1 The Data Link layer is also often implemented in hardware for efficiency reasons. all other layers are implemented in software. TH = Transport Header). For example.
the data link layer breaks the data into data frames. voltages.g. transmits the frames sequentially over the channel. 1. and procedural nature.Each of the seven layers of the OSI model is described below in more detail.  How the circuit is terminated when no longer needed. satellites. earth stations. Physical layer standards and protocols are concerned with issues such as the following:      How a physical circuit is established between communicating devices. and the type of multiplexing technique to be used.3.  The type of modulation to be used for transmitting digital data over analog transmission lines. it appears as a logical communication channel which can send a stream of bits from one point in the network to another (but not necessarily reliably).  Whether transmission of data can take place in one or both directions over the same physical connection.            The physical layer accounts for much of the tangible components of a network. however.  Characteristics of the physical media that carry the signals (e. repeaters.. The Physical Layer The physical layer is concerned with the transmission of raw data bits over communication lines. electrical.g. To the data link layer. the most-widely respected model and has become a standard benchmark for comparing other network architectures against. Subsequent chapters examine the layers in greater depth and discuss their main protocols..  How data from a number of sources should be multiplexed before transmission and demultiplexed upon arrival.3. including cables. timing) in which data bits (binary values 0 and 1) are represented. The Data Link Layer The data link layer is concerned with the reliable transfer of data over the communication channel provided by the physical layer. The physical layer hides the above details from the higher layers. functional. It is. frequencies. and modems.  Characteristics of the connectors used for connecting the physical media. radio waves). It should be pointed out that the OSI model is not the only model in use. To do this. multiplexers. copper wire. 1. concentrators. 7 .  The physical form (e.1. Physical layer protocols and standards are of mechanical. optical fiber.2.
 Internetworking: communication between two or more networks.and checks for transmission errors by requiring the receiving end to send back acknowledgment frames. When a frame arrives corrupted or is for any reason lost in the network.  Error detection.  The interface between two hosts across the network.g. This allows the receiving end to detect where each frame begins and where it ends. including the allocation of a route and handling of congestion. In general.  Error correction. it is retransmitted.  Collection of statistical information (e.  How to delimit frames by adding special bit patterns to the beginning and end of each frame. Data link protocols are concerned with the following issues:    How to divide the data into frames.  Routing of packets across the network. This is constructed by the transmitting end based on the contents of the frame. and checked for integrity by the receiving end. Network layer protocols are concerned with the following issues:      The interface between a host and the network. number of transmitted packets) for performance measurement and accounting purposes. it appears as a reliable communication channel which can send and receive data packets as frames. Some form of error check is included in the frame header. A change in the frame bits can be detected in this way.3. which need to be detected and corrected as well. where they can be converted back into the original data.. Flow control provides a means of avoiding a slow receiver from being swamped by data from a fast transmitter. not all communication devices in a network operate at the same speed.        Flow control. The Network Layer The network layer is concerned with the routing of data across the network from one end to another.  Correct ordering of packets to reflect the original order of data.        8 . Lost acknowledgment frames may result in duplicate frames. 1.  The data link layer hides the above details from the higher layers. To the network layer. To do this. the network layer converts the data into packets and ensures that the packets are delivered to their final destination.3.
if necessary.  Multiplexing of data. The Transport Layer The aim of the transport layer is to isolate the upper three layers from the network. and demultiplexing at the other end. if necessary. To the transport layer.  Splitting of data across multiple network connections.  Type of service to be provided to the session layer (e. The address information appears as a part of the message header. This includes the setting of various communication parameters for the session (e.  Correct ordering of messages when this function is not performed by the transport layer.g.g. To the session layer.4.  9  .The network layer hides the above details from the higher layers. 1. at the node level).  Addressing of messages to their corresponding connections. Session layer protocols are concerned with the following issues:   Negotiating the establishment of a connection (a session) between user processes on communicating hosts.3.5. The Session Layer The session layer provides a structured means for data exchange between user processes on communicating hosts. to improve throughput. error-free versus error-prone connections.  Flow control between hosts. it appears as a uniform data transfer service. 1. so that any changes to the network equipment technology will be confined to the lower three layers (i.. whether messages should be delivered in the order received or not).3.. it appears as a customized data transfer service between two hosts. regardless of the location of the communicating devices and how they are connected. Transport layer protocols are concerned with the following issues:    Establishment and termination of host-to-host connections.  Efficient and cost-effective delivery of data across the network from one host to another.. and its subsequent termination. to improve use of network bandwidth.            The transport layer hides the above details from the higher layers.e. and recombining at the other end. synchronization and control). isolating the underlying network technology from it.
 Data compression to better utilize network bandwidth. 1.  The session layer hides the above details from the higher layers. and management standards for exchanging files or parts   thereof between different systems. The application layer provides standards for supporting a variety of application-independent services. it appears as a universal communication service between user processes.  Binary representation of application data. To the presentation layer.7.  Conversion between the binary representation of application data and a common format for transmission between peer applications..  Grouping of messages into a larger message. The Presentation Layer The presentation layer provides a mutually-agreeable binary representation of the application data communicated between two user processes. if necessary. 1.3.g.3. what the data means to applications. Since there are many ways of encoding application data (e. File transfer. 10 . allowing them to converse in a common syntax.e. agreement on a common representation is necessary. regardless of their system-specific idiosyncrasies. access. so that the larger message becomes available at the destination only when its constituent messages have all been delivered successfully. Presentation layer protocols are concerned with issues such as the following:      Abstract representation of application data. The Application Layer The application layer is concerned with the semantics of data.  Data encryption as a security measure. integers.6. To the application layer. text) into binary data.      The presentation layer hides the above details from the higher layers.   Recovery from interrupted transport connections.  Message handling system standards used for electronic mail.. i. Examples include:  Virtual terminal standards to allow applications to communicate with different types of terminals in a device-independent manner. if necessary. it appears as an organized communication service between user processes.
4. which are extensively used in standards and the literature. Service primitives are concerned with what interactions take place rather than how such interactions are implemented. This is issued by a service user to the service provider to request the invocation of a procedure. 1. Transaction processing standards to allow different companies with different systems to access each other’s on-line databases (e. Service primitives may be of one of the following four types:   Request Primitive. if appropriate. This section describes two popular notations.4.  Indication Primitive. Service Primitives A service primitive is an abstract representation of the interaction between a service provider and a service user.. different hardware and software architectures) without loss of meaning or usefulness. and network components. Both rely on the notion of a service primitive which is described first.  Standards for exchanging formatted documents..  Response Primitive. a set of associated parameters. in banking and airline reservation). This is issued by the service provider to a peer service user (usually in response to a request primitive) to indicate that a procedure has been requested. 1.g. followed by its 11 .       An actual service primitive consists of a command and. This is issued by a peer service user to the service provider (usually in response to an indication primitive) to indicate that the requested procedure has been invoked.1. organizations. in which data can be communicated between incompatible base systems (i.e. This is issued by the service provider to a service user to indicate that an earlier request for the invocation of a procedure has been completed. sequence diagrams and state transition diagrams .     Application layer standards have paved the way for open software systems. On-line directory standards for storing details of individuals.  Confirm Primitive. Protocol Notations OSI network protocols are specified in a variety of notations. A simple convention is used for naming primitives: a primitive name consists of the first letter of the layer to which it belongs.
According to the diagram.command name. from the service provider.4.3. Figure 1. and is labeled with the service primitive that triggers the transition. For clarity. in which case it returns to the idle state. if it issues a connection request to another station. primitive parameters are not included. Figure 1. Service primitives are represented by directed lines between the bars. The peer service user responds to the service provider which. A similar scenario applies to an incoming connection which starts with the station receiving a connection indication.8 shows an example which describes (in a simplified form) the states of a station at the network layer. Service User N-CONNECT request Service Provider Service User N-CONNECT indicate N-CONNECT confirm N-CONNECT response 1. According to the diagram. For example. it enters the attempting to connect state where it waits for a connection to be confirmed. A state transition is represented by a directed line from one state to another. assuming that a station is in the idle state. in turn. 1. or disconnected. Figure 1.2. State Transition Diagrams A state transition diagram describes the various execution states a station can assume and how service primitives cause it to transit from one state to another.4.7 A simple sequence diagram. followed by its type. a connection to a peer service user. 12 .7 shows a simplified example of requesting a connection at the network layer. Sequence Diagrams A sequence diagram defines a service protocol by specifying the permissible sequence of service primitives that may be exchanged between service users and service providers. and are labeled with a meaningful name that describes the state. Note that the NDISCONNECT primitives can be either of request or confirmation type. in which case it moves to the connected state. Service users and service providers are represented by vertical bars. States are represented by circles or boxes. a request type primitive at the network layer for initiating a connection is named ‘N-CONNECT request’. The service provider in turn issues a connection indication to the peer service user. confirms the cycle with the original service user. a service user can request.
The former specifies a service protocol from an outside observer’s point of view. including information technology. combined. ISO standards are published as ISO serial-no (e. Standards enable equipment from different vendors and with different operating characteristics to become components of the same network. ISO is active in many area of science and technology. The Consultative Committee for International Telegraph and Telephone  (CCITT) is a standards organization devoted to data and telecommunication. Standards are developed by national and international organizations established for this exact purpose. major vendors.5.. The two notations. ANSI). major vendors. Figure 1. Standards also enable different networks in different geographical locations (e.g.. with representations from governments. This is a voluntary organization with representations from national standards organizations of member countries (e. attempting to connect N-DISCONNECT N-DISCONNECT idle N-DISCONNECT connected N-CONNECT request N-CONNECT confirm N-CONNECT indication awaiting a connection N-CONNECT response 1. including the following:  The International Standards Organization (ISO) has already been mentioned.It is worth noting the complementary nature of sequence diagrams and state transition diagrams.. Standards The importance of standards in the field of communication cannot be overstressed. telecommunication  13   . provide a complete picture of how a protocol operates. From a customer’s point of view. while the latter describes the same protocol from a station’s point of view.8 A simple state transition diagram. ISO 8632). and end-users. During the course of this book we will discuss a number of important standards developed by various organizations. standards mean real cost savings: the same end-user device can be used for access to a variety of networks and services.g.g. different countries and continents) to be interconnected.
The most serious barriers a newcomer is faced with are layers of nomenclature and an inexhaustible supply of acronyms. Further Reading Communication is a vast subject area with many branches. Stamper (1991) is a well-illustrated introduction with excellent examples. These are revised and republished every four years. and Halsall (1992) are all useful readings. and because of their global market influence. De Noia (1987). IEEE 908). roughly in increasing order of detail and complexity. Unfortunately the great majority of publications in this area are not at an introductory level. and the scientific community.g. are:  Books. Hughes (1992) is an introductory text with emphasis on practice. 1. In addition to these organizations. Black (1989) and Stallings (1994) are examples of highly detailed texts. CCITT standards are very influential in the field of telecommunications and are adhered to by most vendors and carriers.g. Overall. there are four good sources of reading to look into.serial-no. I.. covering a wide range of protocols. Dickson and LLoyd (1992). large vendors occasionally succeed in establishing their products as de facto standards. which will be discussed in the next chapter. and Jain and Agrawala (1993) present balanced accounts of the OSI model.carriers..  14 . Tanenbaum (1989). Stallings (1990) serves as a good reference on communications standards. where L is a letter of the alphabet (e. IEEE standards are published as IEEE serial-no (e. One of the objectives of this book is to get the reader past these initial hurdles so that the publicly available literature becomes more accessible.6. Marshall (1990). There are numerous text and reference books available on the subject. The Electronic Industries Association (EIA) is a US trade association best known for its EIA-232 standard. Martin and Leben (1988).   The European Computer Manufacturers Association (ECMA) is a standards organization involved in the area of computer engineering and related technologies.440). CCITT standards are published as Recommendation L.  The Institute of Electrical and Electronic Engineers (IEEE) is a US standards organization with members throughout the world. These. IEEE is active in many electric and electronic-related areas. The IEEE standards for local area networks are widely adopted and will be discussed in Chapter 9. ECMA directly cooperates with ISO and CCITT. We will also look at a few standards of this nature later in the book. Brown et al (1993) provide a very useful compilation of OSI terms and acronyms.
The following are primarily US-based. MAN. It may employ a point-to-point or a broadcast communication  15  . There are many technical communications magazines and journals in circulation throughout the world. These standards are often heavily cross-referenced and intended for very technically-minded readers. The CCITT standards as well as many others are available in most university libraries. and user manuals are usually of introductory nature and easy to understand. and data books are typically at a much more technical level and therefore contain substantial detail. or WAN. but have a global readership. especially for larger products. Also worth reading are publications by vendors on their network products. Communication standards are by no means easy reading material. and are available in most university libraries:                          Bell Systems Technical Journal  Computer Communication Review  Computer Networks  Data Communications  IBM Systems Journal  IEEE Communications Magazine  IEEE Computer  IEEE Journal on selected Areas in Communication  IEEE Transactions on Communications  Journal of Telecommunication Networks  Proceedings of the IEEE  Telecommunications   Product information. Product glossies. Product handbooks. Unfortunately. such as telephone switches. and as private or public. it is not always easy to get hold of these publications. It interconnects a set of hosts which conform to the network protocols. Technical Magazines and Journals.  Published standards .  A network may be classified as a LAN. They are essential reading for those involved in the design and manufacturing of communication hardware and software. product specifications. and new ones appear every year. depending on its access restrictions.8. as they provide the necessary level of protocol specification detail required for these purposes. advertising literature. Summary   A computer network consists of nodes and communication links which implement its protocols.    1. depending on its geographic spread.
The network nodes implement only the bottom three layers.  The physical layer controls the transmission of raw data bits over communication lines. WAN. indication.model.  A sequence diagram defines a service protocol by specifying the permissible sequence of service primitives that may be exchanged between service users and service providers.1 Exercises Provide three arguments in favor of the use of a computer network in a modern organization. and may be of one of four types: request. point-to-point.2 1. and at least one argument against. or packet-switched. 1. private. public. The presentation layer provides a mutually-agreeable binary representation of application data (syntax). broadcast. and confirmation.  A state transition diagram describes the various execution states a station can assume and how service primitives cause it to transit from one state to another. The transport layer manages the efficient and cost-effective transportation of data across the network. Discuss and compare the advantages and disadvantages of circuit switching versus packet switching. MAN. response. Classify the networks operated and/or utilized by your organization as LAN.   The OSI model proposes a seven-layer architecture for networks.       Communication standards are essential in order to achieve interoperability between different equipment and networks. The session layer manages the negotiation of the establishment and termination of connections (sessions). A point-to-point model may be based on circuit switching or packet switching. while the hosts implement all the layers. Each layer is characterized by a set of protocols. circuit-switched.9. The data link layer facilitates the reliable transfer of data over communication channels. The network layer controls the end-to-end routing of data across the network.3 16 .   1. Name at least one well-known network which is based on either type of switching. The application layer provides a mutually-agreeable meaning of application data (semantics). 1.      A service primitive is an abstract representation of the interaction between a service provider and a service user.
1. Briefly describe the role of each layer and its main functions. What aspect of a protocol is better captured by either diagram? Draw a sequence diagram for the following: A service user sends a SEND request to a service provider which in turn sends a SEND indication to the peer service user. The peer service user then sends a SEND response to the service provider which in turn sends a SEND confirmation to the original service user. 1. The latter sends a DATA request to the service provider which in turn send a DATA indication to the original service user.6 1. a RESET indication will cause it to return to the notsync state.4 Explain the rationale behind the OSI seven-layer model.7 1.8 17 .5 1. While in this state. Explain how sequence and state transition diagrams can be used to specify protocols. A SYNC request or indication will cause it to enter the sync state. What is a service primitive? Describe the main four types of primitives used for defining protocols. Draw a state transition diagram for the following: A station is originally in the notsync state.
21. Distinguish between different device connection types.2. including various transmission media. After completing this chapter you should be able to:  Distinguish between different network equipment types and understand their roles.  Have a broad understanding of the different physical transmission media and their characteristics. We will first look at a categorization of networking equipment.           Have a basic knowledge of physical layer standards RS-232 and X. followed by a discussion of two important physical layer standards: RS-232 and X.  18 .  Understand how data is transmitted and the basic techniques that this process involves. The Physical Layer This chapter examines the physical layer of the OSI model in detail. and then discuss transmission-related issues.  Understand the basic multiplexing methods and their role in data transmission.21. Multiplexing methods will be described next.
A DCE may be a part of a DTE or be an entirely separate device. Figure 2. 2. and convert back the incoming signal into user data. DCEs. DTEs may take many different shapes and forms.9 Network equipment types.2. Equipment This section briefly describes general networking equipment types and the types of connections that can be established between them. 2. Data Switching Equipment (DSE) refers to network equipment used to connect DCEs together. and DSEs are connected in a network.1.1. DTEs commonly reside at user sites.1. Figure 2. A DSE is commonly referred to as a switch. DSEs correspond to the nodes in a network. Data Terminal Equipment (DTE) refers to user equipment that convert outgoing user data into a transmission signal. Equipment Types Network equipment may be classified into three broad categories: 1. Data Circuit-terminating Equipment (DCE) refers to the network equipment that connect DTEs to the network communication lines. terminal adapters. electrical versus optical). and mainframes.1. DSE DCE DCE DTE DTE DCE DCE 2.9 illustrates the way DTEs.g.. In general. Digital telephone switches used in digital networks are examples. Examples include: terminals. The necessary signal conversion between these two is performed by the DCE. Modems and multiplexers are all examples of DCEs. a DTE and a network line may use different types of signals (e. 3. Connection Types Connections between devices may be classified into three categories: Chapter 2: The Physical Layer 19 . and are responsible for routing data across the network. thus providing switching capability to the network. personal computers.2.
10 Analog and digital signals. Analog signals are used in cases of equipment which date back to before the advent of digital technology. This is a unidirectional connection. Simplex connections are useful in situations where a device only receives or only sends data (e. The signal may be analog or digital.2. one representing the binary value 0 and the other representing the binary value 1. a printer).. Half-duplex.1. Existing analog telephone networks are a good example of the latter. In the analog signal example. and may travel in different media. Full-duplex. Human speech is an example: it produces a continuous variation of air pressure. Simplex.e.1. in nearly all cases.2. 2. digital signals are used. By definition. a digital signal is a restricted form of an analog signal. Figure 2. is one in which information appears as a sequence of binary values 0 and 1. i. 3. 2. In the digital signal. A full-duplex connection is equivalent to two simplex connections in opposite directions. the voltage may assume only two values: 0 Volts to represent digital value 0 and 5 Volts to represent digital value 1. data can only travel in one direction. voltage). Signal Types All signals are either analog or digital.. therefore. A digital signal. 2. 20 .. To represent these two values. A human speaker who only utters the two words zero and one is a crude example of a digital signal. signals appear as variation of some electrical property (e. with the restriction that data can travel in one direction at a time. a signal is used in which only two wave shapes are allowed. on the other hand. This is a bidirectional connection. In electrical terms. This is a bidirectional connection in which data can travel in both directions at once.g. analog 5v 5v digital 0v time 0v 1 0 1 1 time Since digital computers play a central role in data communication. Transmission Transmission is the act of transporting information from one location to another via a signal.10 illustrates.g. Figure 2. An analog signal is one in which information appears as a continuous variation of some property. the voltage freely varies between 0 and 5 Volts.
A change in the carrier signal’s phase indicates a change in the modulating digital signal’s bit value from 0 to 1 or from 1 to 0.2. In AM. A modem (modulator and demodulator) is a commonly used device which employs this technique.12 for a visual comparison): 1. represent bit values 0 and 1. two amplitude sizes (a small and a large one) may be used to. two frequency values (a low and a high one) may be used to. As illustrated in Figure 2.11 Role of modems. In PM. For example. Figure 2.11. AM’s main weakness is its susceptibility to distortion. the carrier signal’s amplitude is changed according to the modulating digital signal’s bit value. the carrier signal’s phase is changed according to the modulating digital signal’s bit value. Phase Modulation (PM). Chapter 2: The Physical Layer 21 . modem modem Three basic types of modulation are possible (see Figure 2. where the digital bit stream is modulated over an analog carrier signal. FM is more resistant to distortion than AM. In FM. 2.2.2. 3. the carrier signal’s frequency is changed according to the modulating digital signal’s bit value. respectively. Frequency Modulation (FM). Amplitude Modulation (AM). represent bit values 0 and 1. respectively. Modulation Transmission of digital data over an analog line is achieved using a technique called modulation. For example. a modem converts the outgoing digital bit stream from a device into an analog signal and converts the incoming analog signal into a digital bit stream.
14 illustrates how an analog signal is sampled. Each sample (denoted by a small black box) is a real 22 .3. Whereas in modulation a digital signal is modulated over an analog signal for transmission. Figure 2.12 Three basic modulation methods. Here the time interval for each sample is one millisecond.2. Digitization Digitization is essentially the opposite of modulation. Figure 2. digitization is only an approximate process because of sampling. For example. 1 0 1 0 Modulating digital signal (bit stream 1010) Carrier signal Amplitude Modulation (AM) Frequency Modulation (FM) Phase Modulation (PM) 2.13 Role of codecs.13 illustrates the concept.Figure 2. in digitization an analog signal is converted into digital format through a process of sampling. the analog signal resulting from human speech can be sampled and converted into digital data. codec codec It is worth noting that. and converted back to analog signal at the other end. transmitted over digital lines. Figure 2. These two functions are performed by a device called codec (coder/decoder). unlike modulation (which is an exact process since the digital signal at the source and the digital signal received at the destination are identical).
Because having a Chapter 2: The Physical Layer 23 . or be embedded in the data signal itself (see Figure 2. This limit is exercised by a popular digitization technique called Pulse Code Modulation (PCM) which uses a sampling rate twice that of the original signal frequency. a 4 kHz speech signal is sampled at a rate of 8000 samples per second. The quality of the regenerated signal can be improved by increasing the sampling rate (i.. There are two basic methods of synchronization: synchronous transmission and asynchronous transmission. the transmitter should somehow notify the receiver as to when to expect to receive data. The relatively small loss of information inherent in the process is called quantization error. The clock signal may be provided on a separate line. it is much easier to reliably transmit a digital signal over a long distance than an analog signal. This process (of representing a continuous value with a discrete value) is called quantization. due to its resistance to distortion. reducing the sampling interval). By tying the data signal to the clock signal. The main advantage of digitization is that.4. such notifications should occur frequently enough so that both devices maintain an agreement about the exact distribution of data over time. The decoding process regenerates an approximation of the original signal by fitting a smooth curve to the sampled points. Figure 2. For example. The coding process generates the sample data from the analog signal. either device can look at the clock signal to know where data bits may begin or end.e. This process is called synchronization. Synchronization When two devices are about to communicate. In synchronous transmission.14 Sampling an analog signal.15). This allows the receiver to prepare itself for receiving the data. but up to a limit dictated by the Nyquist’s theorem.2. Furthermore. a clock signal is used as a common source of reference by both the transmitter and the receiver.value which is in turn represented by an integer in the range 0-255 so that it can be represented in one byte of data. 255 127 0 5 10 15 20 miliseconds 2.
. by their physical nature.. Asynchronous In asynchronous transmission. Two types of cost are relevant: (i) the cost of installing the medium. clock 01001011 sync byte byte stop bit byte byte start bit Synchronous with separate clock signal sync byte data byte . bandwidth and data rate are usually inversely proportional to the communication distance.g.2. There is usually a need for tradeoff between cost. 3. Reliability.. and (ii) the cost of running and maintaining the medium and its equipment. for connecting personal computers). Cost. This enables the receiver to work out the byte boundaries (see Figure 2.15 Synchronous and asynchronous transmission methods. Figure 2. Also related. Low reliability translates into a higher number of errors. Transmission Media Digital data can be transmitted over many different types of media. Selecting a transmission medium is guided by comparing transmission requirements against the medium’s characteristics. For example. the terms bandwidth and data rate are sometimes used interchangeably. 24 . including the medium-specific equipment that may be needed.. This is usually expressed in kilo Hz (kHz) or mega Hz (MHz). Bandwidth.g. Four important criteria influence the choice: 1. the beginning and end of each byte of data is marked by start and stop bits. Because of its simplicity. asynchronous transmission is cheaper to implement and is therefore more widely used. bandwidth. 2. For example. and distance. it is only used for covering very short distances (e.5. which needs to be balanced against the potential cost of recovering from the errors (e. Bandwidth is the maximum frequency range that can be practically supported by a medium. more complex hardware and software). which denotes the maximum number of bits per second (bps) that can be transmitted. byte byte byte Synchronous with embedded clock signal byte byte byte . transmit data more reliably than others.15).separate clock line increases the costs. is the notion of data rate.. a data rate of 10 mbps means that 10 million bits of data can be transmitted in each second. analog transmission of human speech typically requires a bandwidth of 4 kHz. 2. Because of distortion factors. Because of their obvious relationship.. retransmission. Some media.
telephone lines) are usually organized as a much larger cable containing numerous twisted pairs. 3 Chapter 2: The Physical Layer 25 . Early installations used open wires. and this increases the costs. Coaxial cables are superior to twisted pairs both in terms of bandwidth and communication distance. Coverage. but can be used for up to a few kilometers.4. Transmission media may be classified into the following categories:  Copper Wire . which consist of a pair of insulated and twisted wires (see Figure 2. The physical characteristics of a medium dictate how long a signal can travel in it before it is distorted beyond recognition. repeaters are needed to restore the signal. which causes signal distortion. This is the oldest form of electronic transmission medium. Coaxial cables are extensively used in LANs and long distance telephone trunk lines. Twisted pairs used for long distance connections (e. They are being increasingly used by telecommunication carriers for long distance   Crosstalk is the unwanted coupling effect between two or more signal paths.16).16). it does not suffer from the various noise problems associated with electromagnetic signals. The performance of the twisted pair can be substantially improved by adding a metallic shield around the wires. which may also contain twisted pairs. multiple coaxes are usually housed within one cable..16). A coaxial cable consists of four concentric cylinders: an inner conductor. The signal is usually generated by a laser or Light Emitting Diode (LED). Twisted pairs are superior because of reduced crosstalk. Its use dates back to the development of telegraph in the 1800s and earliest telephone systems. surrounded by an insulating cylinder. Optical fibers can provide bandwidth to distance ratios in order of 100s of MHz per kilometer.g. Like other cables. Like twisted pairs. whereas the cladding (which has a different refractive index) acts as a reflector to prevent the light from escaping from the core. but these were superseded by twisted pairs. Shielded wires are much more resistant to thermal noise and crosstalk effects. This combination is called a coax (see Figure 2. The core is used for guiding a light beam. hundreds of optical fibers are usually housed within one cable.  Optical Fiber. Both the core and the cladding are made of transparent plastic or glass material (see Figure 2. An optical fiber consists of two concentric cylinders: an inner core surrounded by a cladding. and can provide bandwidth to distance ratios in order of 10s of MHz per kilometer. To cover larger areas. They are very effective for relatively short distances (a few hundred feet). surrounded by an outer conductor. surrounded by a final protective cover. Because optical fiber uses a light signal instead of electrons. A twisted pair has a bandwidth to distance ratio of about 1 MHz per kilometer. 2    Coaxial Cable.
Telecommunication carriers and TV stations are the primary users of microwave transmission. Another increasingly-popular form of radio is cellular radio.digital trunk lines. These operate in the VHF band and subdivide their coverage area into conceptual cells. They are particularly attractive for long distance communication over difficult terrain or across the oceans. satellites are capable of supporting an enormous number and variety of channels. where the cost of installing cables can be too prohibitive. Because of their high bandwidths.  Infra-red. and retransmitted to other earth stations by the satellite. ranging from hundreds of Hz to hundreds of giga Hz (GHz).  26 . The signal is generated and received using optical transceivers. represents a major investment and typically has a limited lifetime (at most a few decades).  Radio. The signals transmitted by earth stations are received. which is essentially a microwave system plus a large repeater in the sky (see Figure 2. because there is no cabling involved and the necessary equipment is relatively cheap. providing data rates in order of 100s of mbps. A huge range of transmission bandwidths are therefore possible. applications are limited because of distance limitations (of about one kilometer). Current trends promise that they will replace twisted pair residential loops in the near future. the bandwidth is subdivided into channels of 10s of MHz each. However. Radio signals have been used for a long time to transmit analog information. its communication is handed over from one station to another. Data rates similar to those of twisted pairs are easily possible.16). The satellite itself. where each cell represents a limited area which is served by a low-power transmitter and receiver station. It operates in the GHz range with data rates in order of 100s of mbps per channel. One recent use of infra-red has been for interfacing hand-held and portable computing devices to LANs (see Figure 2. telephone. however. As the mobile user moves from one cell area to another. Like other microwave systems. which is currently being used by carriers for providing mobile telephone networks. A minimum radio system consists of a transmitter and a receiver. Infra-red systems represent a cheap alternative to most other methods. It may operate at a variety of frequency bands. amplified. Microwave is by far the most widely used form of radio transmission.16). including TV. and data. Infra-red signals are suitable for transmission over relatively short distances (the signal is easily reflected by hard objects). An important form of microwave system is a satellite system.
Chapter 2: The Physical Layer 27 .3.Figure 2.16 Transmission media.17 Relative comparison of transmission media. Figure 2. Twisted Pair Satellite Repeater Coax Cover Inner Conductor Earth Station Outer Conductor Insulation LAN Optical Fiber Infra-red Earth Station Core Cladding Hand-held Computer Figure 2. There are three basic multiplexing methods. It is important to note that the figures provided are approximate and continually improve as the technology moves forward.17 compares the characteristics of these media using the criteria mentioned earlier. Medium Copper Cable Coaxial Cable Optical Fiber Radio Infra-red Bandwidth 1 MHz 10s of MHz 100s of MHz 100s of MHz 1 MHz Data Rates 1-10 mbps 10-100 mbps 100s of mbps 100s of mbps 1-10 mbps Cost Medium/km High/km High/km Very High Low Reliability Low-Medium Medium-High Very High Very High Low-Medium Coverage Kilometers 10s of Kilometers 10s of Kilometers 1000s of Kilometers Kilometer 2. The objective of multiplexing should be obvious: to reduce costs by better utilizing the capacity of a line. these are separately described below. Multiplexing Multiplexing is a technique which makes it possible to cram a number of logical channels (each capable of supporting an independent connection) into the same physical channel or line.
Radio and TV broadcasting represent the oldest examples of FDM. a line that has a bandwidth of 30 kHz can be divided into 3 times 10 kHz channels. the frequency bandwidth of the line is divided into a number of partitions.1. SDM has the unique advantage of not requiring any multiplexing equipment.18 Space division multiplexing. each of which is used as a separate logical channel. Figure 2. A cable that has. For example. 50 twisted pairs inside it can support 50 channels.2. cable enclosure 2. each of which consists of 8 kHz of bandwidth for data and two gaps of 1 kHz on either side.19).18).3. It is usually combined with other multiplexing techniques to better utilize the individual physical channels. for example.3.19 Frequency division multiplexing. 8kHz 30kHz 8kHz MUX 30kHz 20kHz 10kHz 8kHz time MUX time 28 . Figure 2. Space Division Multiplexing (SDM) SDM is the simplest (and crudest) form of multiplexing. To avoid neighboring channels from interfering with one another. It involves grouping many separate wires into a common cable enclosure.2. Frequency Division Multiplexing (FDM) In FDM. FDM requires special multiplexing/demultiplexing hardware (MUX) at either end of the line (see Figure 2. the extreme ends of the channel frequencies are left unused to provide a gap. There is therefore a one-to-one correspondence between physical and logical channels (see Figure 2.
each logical channel is allocated a time slot to transmit over a shared physical channel. a predetermined bandwidth is reserved for each of the logical channels. each logical channel may be given a 5 millisecond time slot to transmit.3. the sum of which for all the logical channels equates the bandwidth of the line. For example. Concentration In multiplexing.4. Since. where each channel is allocated a time slot only when it has data to transmit. some means of initial synchronization is also needed. TDM requires special multiplexing/demultiplexing hardware (MUX) at either end of the line (see Figure 2. and can work everything else out from this reference point.20). a 9600 bps line could be used to serve 10 times 2400 bps channels. assuming that no more than 4 channels are used at any one time. This is easily achieved by having each time slot to contain two fields: the address of the channel to which it belongs. the receiving end needs to know which time slot belongs to the first channel when the connection is established. Consequently. Because the channels are spread across time. then some cost savings could be achieved by using a lower capacity line. if the bandwidth of each of the channels could be dynamically adjusted according to its traffic. Time Division Multiplexing (TDM) In TDM.3.21). none of the logical channels is fully utilized at all times by the equipment attached to them. during which time it will have the entire bandwidth of the line to itself. For example. Like FDM. In practice. Basically. Chapter 2: The Physical Layer 29 .2. time slot 101 011 001 101 001 MUX MUX 011 2. in this case time slots do not occur in a predetermined order.3. Figure 2. This is made possible by a variation of TDM called concentration. and the channel data (see Figure 2.20 Time division multiplexing. some means of indicating to which channel a time slot belongs is needed.
4. many of which are inter-related.22). and can support simplex.28 defines the electrical characteristics of RS-232.28 standards and ISO’s 2110 standard. EIA-232-D.Figure 2. A large number of the existing standards deal with transmission over telephone lines. is based on CCITT’s V. ISO 2110 defines the mechanical appearance of the RS-232 connectors (see Figure 2. 2.23.1. CCITT. V. It allows for connection distances of up to 20 meters and data rates of up to 20 kbps. 001 10101 001 11111 010 01110 001 11011 Channel data Channel address Concentration is a popular method for connecting a set of character-based terminals to a central computer. The connector provides 25 pins for connecting the circuits derived from the V. as summarized in Figure 2.21. 2. conveying of control signals. The circuits are used for data transfer.24 standard. 30 . RS-232 RS-232 has dominated the computer industry as the most-widely used standard for physically connecting devices.21 Time slots in concentration. It originated in the late 1950s.24 and V.4. half-duplex. and has been revised a number of times over the years. and -5 to -15 Volts to represent binary value 1. It is an analog standard. The latest revision. Physical Layer Standards The most commonly-used physical layer standards are those published by ISO. IEEE. Below we will look at two very popular standards for connecting DTEs and DCEs: the analog standard RS-232 and the digital standard X. and full-duplex connections in synchronous as well as asynchronous mode. and EIA. defining the physical layer interface between a DTE and a DCE.28 uses 5 to 15 Volts to represent binary value 0. The CCITT V series of standards fall into this category and are by far the most-widely adopted. Line capacity requirements are greatly reduced due to the fact that terminals tend to be idle for most of their operating period. V. and conveying of clocking signals for synchronization.
Figure 2.22 RS-232 connector (based on ISO 2110). Pin Circuit 1 AA 2 BA 3 BB 4 CA 5 CB 6 CC 7 AB 8 CF 9 -10 -11 -12 SCF 13 SCB 14 SBA 15 DB 16 SBB 17 D 18 LL 19 SCA 20 CD 21 RL/CG 22 CE 23 CH 23 CI 24 DA 25 TM Direction -DTE to DCE DCE to DTE DTE to DCE DCE to DTE DCE to DTE -DCE to DTE ---DCE to DTE DCE to DTE DTE to DCE DCE to DTE DCE to DTE DCE to DTE -DTE to DCE DTE to DCE DCE to DTE DTE to DCE DTE to DCE DCE to DTE DTE to DCE -Description Protective ground shield / common return Transmitted Data Received Data Request to Send Clear to Send DCE Ready Signal Ground Carrier Detect Reserved for testing Reserved for testing Unassigned Secondary Carrier Detect Secondary Clear to Send Secondary Transmission Data Transmitter Signal Element Timing (transmitter clock) Secondary Received Data Receiver Signal Element Timing (receiver clock) Local Loopback Secondary Request to Send Data Terminal Ready Signal Quality Detector / Remote Loopback Ring Indicator Data Signal Rate Selector Data Signal Rate Selector Transmitter Signal Element Timing (transmitter clock) Test Mode In most applications.Figure 2.23 V. It illustrates how a PC may be connected to a modem.24 serves as an example.24 circuits. Chapter 2: The Physical Layer 31 . 1 14 2 15 3 16 4 17 5 18 6 19 7 20 8 9 21 22 10 23 11 12 24 13 25 Figure 2. only a few of the circuits specified by RS-232 are actually used.
Figure 2. X.27 illustrates how a DTE and a DCE may be connected using X. Other similar standards have been devised to overcome these limitations.24 Typical half-duplex connection using RS-232. V.21.10 uses 4 to 6 Volts to represent binary value 0. as summarized in Figure 2. and it has a maximum bandwidth of 20 kbps. PC Ring Indicator 22 Data Terminal Ready 20 Carrier Detect 8 Signal Ground 7 DCE Ready 6 Clear to Send 5 Request to Send 4 Received Data 3 Transmitted Data 2 RS-232 Modem 22 Ring Indicator 20 Data Terminal Ready 8 Carrier Detect 7 Signal Ground 6 DCE Ready 5 Clear to Send 4 Request to Send 3 Received Data 2 Transmitted Data RS-232 has two important limitations which reduce its usefulness: it is not suitable for distances of more than about 50 meters. Unlike RS-232. 32 .Figure 2. Figure 2.25).24 standard. It can be used for connections of up to 1 km in length and data rates of up to 10 mbps (for distances less than 10 m).21 are defined by V. CCITT X.2.26 or V11/X. For example. It allows for connection distances of up to 1 km.27. RS-449 and EIA-530 can both support data rates of up to 2 mbps over longer distances.26. The connector provides 15 pins for connecting the circuits derived from the X. and -4 to -6 Volts to represent binary value 1.21 connector (based on ISO 4903). the same transmit and receive circuits (T and R) are used for the exchange of control as well as data signals.10/X.4.21 X.21 is a widely-accepted standard for interfacing a DTE to a DCE of a digital network. 1 9 2 10 3 11 4 12 5 13 6 14 7 8 15 The electrical characteristics of X.21 uses a connector based on the ISO 4903 standard (see Figure 2.25 X. 2.
Blahut (1990). modulation. DCE (connect DTE to network).27 Typical full-duplex connection using X. error-handling. DTE X.5. Stone (1982) describes detailed examples of (mainly RS-232) physical layer interfaces for microcomputers. McClimans (1992) describes different transmission media and their properties. and standards. coding.21 standard with similarities to RS-232: it uses the V. synchronization. and DSE (perform switching between DCEs). 2. transmission. Summary   Network equipment are classified into DTE (user equipment). Further Reading Black (1988).  A connection may be of type simplex.21 bis is a variation of the X.24 circuits and is usually used with the 25-pin connector of ISO 2110.24 circuits. 2. Circuit G Ga Gb T R C I S B F X Direction -DTE to DCE DCE to DTE DTE to DCE DCE to DTE DTE to DCE DCE to DTE DCE to DTE DCE to DTE DCE to DTE DTE to DCE Description Protective ground shield / Common Return DTE Common Return DCE Common Return Transmit Receive Control Indication Signal Element Timing Byte Timing Frame Start Identification DTE Signal Element Timing Figure 2.21.26 X.   Chapter 2: The Physical Layer 33 . half-duplex.21 Ground DTE Common Return Transmit Receive Control Indication Signal Element Timing Byte Timing DCE X. or full-duplex.6.Figure 2. and Gitlin et al (1992) provide detailed descriptions of physical layer topics such as interfaces. Bic et al (1991).
Name the devices that perform these functions. PCM is a popular digitization method for voice signals.11 2.  Concentration is a variation of TDM where time slots are allocated on demand. and name an example of each device type. illustrate the difference between synchronous and asynchronous transmission. Using a sample bit stream and a diagram. coaxial cable.12 34 . Modulation method are classified into AM.     RS-232 is a popular analog standard for the physical interface between a DTE and a DCE.21 is a popular digital standard for the physical interface between a DTE and a DCE.  Multiplexing methods are divided into SDM (multiple wires in a common enclosure). 2.  Popular transmission media include: copper wire . and infra-red. optical fiber. 2. Provide an example of either signal type.          Transmission methods are classified into synchronous (clock-based) and asynchronous . DCEs. A signal may be analog (continuous variation of some property) or digital (sequence of binary values 0 and 1). FM. Describe the differences between modulation and digitization. FDM (subdivision of the frequency bandwidth into logical channels).  X. radio. and TDM (allocation of time slots to each logical channel).   2. These two functions are performed by a modem.9 Exercises Describe the role and functions of DTEs. and PM. DSEs. Converting an analog signal into digital is called digitization and is performed by a codec. State the differences between an analog signal and a digital signal.10 2. Digital data is transmitted over analog lines using modulation and converted back to digital format using demodulation.8.
16 Chapter 2: The Physical Layer 35 .2.15 2.13 Consider the problem of providing a 2 mbps physical connection between two LAN sites which are 10 kms apart and are located in the same city.21 standards. Provide an application example of either standard. 2. What is the purpose of multiplexing? Compare the strengths and weaknesses of FDM and TDM.14 2. Discuss the merits of using different types of transmission media for this purpose. Describe how a 100 MHz line with a data rate of 200 mbps can be divided into 20 channels using FDM and TDM. Describe the differences between RS-232 and X.
we will discuss two popular data link standards: BSC and HDLC.  Describe the BSC character-oriented data link protocol.  Explain how the CRC error checking method works and how a CRC code is calculated. Finally.  Describe the HDLC bit-oriented protocol.  Understand the sliding window protocol and explain how it can be used for flow control. Have a general understanding of the various data link protocol functions. and subsets. The Data Link Layer This chapter looks at the data link layer of the OSI model.3. different frame types. and then describe the constituent functions of link protocols.           36 . These functions are embodied by a general technique called the sliding window protocol. The data link layer transforms the logical communication channel provided by the physical layer into a reliable channel by splitting the data into frames which are subjected to error control and flow control procedures. and flow control. including its modes. frame format. including its block format and functions. error checking. We will first look at various link protocol types. such as acknowledgment of frames. After completing this chapter you should be able to:  Distinguish between different data link protocol types and know the characteristics of each type. which will be described next.
These two are distinct and essential in their own right: physical layer synchronization ensures that the transmitter and the receiver share a common clock signal so that bit boundaries can be detected. Figure 3. ASCII or EBSDIC) and are therefore characterset dependent. Checksum 01111110 The delimiting bit pattern used is 01111110 and is called a flag. no specific character set is assumed. and two delimiting bit patterns at either end of the frame. To avoid this bit pattern occurring in user data. These are described below.3. Data Contains the actual user data and is a bit sequence of arbitrary length.1 Link Protocol Types Data link protocols are divided into two basic categories: synchronous and asynchronous. data link layer synchronization ensures that user data is not confused with control data. End flag: marks the end of the frame. A checksum field for error detection. Figure 3. error checksum.g. address data. The unit of transmission is a frame. 3.28 illustrates the frame structure for HDLC protocols (discussed later in this chapter).1.. control data. Synchronous protocols may be character-oriented or bit-oriented. etc. frame sequence. user data consists of a sequence of characters and is delimited by two unique control characters (SYN and EOT). In a character-oriented protocol. In a bit-oriented protocol. Address of the host for which the frame is intended.28 HDLC frame structure. the transmitter inserts a 0 bit after every five Chapter 3: The Data Link Layer 37 . It is important not to confuse synchronization at the data link layer with synchronization at the physical layer. Field 01111110 Address Control Description Start flag: marks the beginning of the frame. The biggest disadvantage of character-oriented protocols is that they are based on specific character sets (e. Record frame type. Synchronous Protocols Synchronous protocols operate by delimiting user data with unique bit patterns which the receiver uses to detect where the user data begins and where it ends. flow control. which consists of user data (an arbitrary bit sequence).1.
a set of terminals are connected to a host via a shared communication line. the master ‘polls’ each slave in turn by sending a message to it and asking if it has any data to send. Link Protocol Functions This section describes various functions performed by link layer protocols. the station starts transmitting. To receive data from the slaves.consecutive 1 bits it finds. These two bits indicate to the receiver where the character starts and where it ends. The collided stations then wait for some random period of time before attempting to retransmit. The slave responds by either sending data or by sending a rejection message to indicate it has nothing to send. 3. Communication between the master and its slaves is governed by a general technique called polling. polling works as follows. The master can send data in form of a message to any of the slaves. 38 . where all stations share a common communication channel. a station monitors the channel for transmission by other stations and awaits its becoming free. The receiver extracts the user data by removing the start and stop bits. Bit-oriented protocols are by comparison more recent than other protocols and have dominated the market. 3.1.2. 3.2. The majority of network link protocols fall into this category. Peer-to-Peer Protocols In peer-to-peer protocols. Collisions (when two stations attempt to transmit simultaneously) are resolved by requiring a transmitting station to monitor the channel at the same time and stop the transmission if it detects another station also transmitting. The message contains an address which uniquely identifies the slave for which it is intended. which removes every 0 bit that occurs after every five consecutive 1 bits. Polling is typically used in a multidrop line configuration where. Asynchronous Protocols Asynchronous protocols are character-oriented and operate by having the transmitter surround each character with a start and a stop bit. for example. Master-Slave Protocols Master-slave protocols are used in situations where a primary station (such as a host) controls one or more secondary stations (such as peripheral devices).3. its effect is canceled by the receiver. To transmit. The carrier-sense protocol serves as an example.4. Once the channel is free.1. 3. In its simplest form. This is called bit stuffing. To acknowledge their importance. all stations have similar status and communicate in a democratic fashion. most of this chapter is devoted to the description of this class of protocols.1.
1. However. this is a responsibility of higher layers. This is facilitated by the use of acknowledgments. The control field of a frame can contain two sequence numbers: a send sequence number and a receive sequence number. of which there are two types:   An acknowledgment (ACK) is sent by a receiver to a transmitter to indicate that a frame has been received seemingly intact. they are implemented as acknowledgment frames. the exact number and function of which is protocol-dependent.2.  A negative acknowledgment (NAK) is sent by a receiver to a transmitter to indicate that a corrupt frame has been received. acknowledgments are implemented as control characters. The receiver compares the send sequence number of a frame against the send sequence numbers of earlier frames to check for lost frames. the transmitter resets T1 and retransmits the frame.2.   In character-based protocols. Link protocols make use of a number of different timers. T1 is set by a transmitter upon transmitting a frame. In piggybacking.3. Instead. the corresponding process is notified to take appropriate action. instead of sending a separate acknowledgment frame. This is intended to avoid the transmitter having Chapter 3: The Data Link Layer 39 .2. Acknowledgments Data link layer does not provide end-to-end accountability. T2 is set by a receiver to ensure that an acknowledgment frame is sent to the transmitter before its T1 timer expires. the receiver waits until it has data to send to the receiver and embeds the acknowledgment in that frame. or that a frame is missing. 3. A timer is set by a process and has a predefined expiry period. The transmitter expects to receive an acknowledgment frame before T1 expires. In bit-oriented protocols. A NAK frame uses the receive sequence number to identify a corrupted frame. while an ACK frame uses the receive sequence number to identify the next frame it expects to receive. Otherwise. Timers Timers provide a mechanism for placing a time limit on certain operations to be completed. The frames dispatched by a transmitter are sequentially numbered using the send sequence number. To improve the use of network bandwidth. implying that it has correctly received all frames with sequence numbers less than that number. an acknowledgment method known as piggybacking is often used. each station is responsible for ensuring that the data received by it is correctly passed to the next station. two important timers (called T1 and T2) are used by most protocols. When the timer expires. The protocol allows a certain number of retries before the problem is propagated to a higher layer.
to retransmit frames because the receiver has been slow in acknowledging their receipt. When piggybacking is used, the receiver will send a separate acknowledgment frame if T2 expires. 3.2.3. Error Checking No physical medium can guarantee error-free transmission of data. Errors occur because of interference from the environment surrounding the transmission medium or because of artifacts introduced by the equipment or the medium itself. In case of copper wires, for example, thermal noise, electrical noise, and impulse noise are some of the most common causes. The most important role of the data link layer is to provide error-free transmission. To do this, it uses an error checking scheme to detect errors, and overcomes them by retransmitting corrupted frames. Many different error detection methods have been devised. We will discuss two popular methods in this section: parity checking and cyclic redundancy code. Parity checking is a simple error detection method used with characteroriented protocols. One bit in every character bit sequence is reserved for parity. This bit is set by the transmitter to 0 or 1 so that the total number of 1 bits in the character is always even (in case of even parity checking) or always odd (in case of odd parity checking). The receiver checks the parity bit of each character to see if it is as expected. Figure 3.29 illustrates the method for an even parity checking scheme.
Figure 3.29 Even parity error checking.
An octet consists of eight consecutive bits. In most systems this is the same as a byte, but there are also systems, where a byte has seven, or nine, or some other number of bits.
A window denotes a continuous subrange within the permitted range of values for the sequence numbers.3.g. The transmitter must have enough buffer space to store the maximum possible number of unacknowledged frames. The receiver must have enough buffer space to store the maximum possible number of frames that can be received out of order.30). When the receiver senses that it can no longer accept incoming data.. When necessary. 3. S. The protocol described in the next section uses exactly such a strategy.device to another so that the receiver has enough time to consume the data in its receive buffer. R. A receiver window size of 1 means that frames must be received in transmission order. which denotes the sequence number of the next frame it expects to receive. the receiver maintains a variable. before it overflows. The receiver window denotes frames that are expected to be received.  Figure 3. Once the receiver has consumed enough of the data in its receive buffer so that it can receive more. it sends an XON character to the transmitter.g. the transmitter maintains a variable. For example. Larger window sizes allow the receiver to receive as many frames out of order. the receiver can use this fact to slow down the transmitter by withholding ACK frames. modulo 8). the ranges 0-3 and 6-1 both represent windows of size 3 (see Figure 3. Similarly. causing it to resume transmission.. which causes the latter to stop transmitting. Sliding Window Protocol Sliding window is a general protocol used with bit-oriented protocols.30 Two windows of size 3. Since the transmitter needs to keep a copy of its transmitted but yet unacknowledged frames in a buffer (in case they are corrupted and need to be retransmitted). In bit-oriented protocols. The receiver window size is fixed. which denotes the sequence number of the next frame to be transmitted. the size of the buffer imposes an upper limit on the number of such frames. from empty to the entire range. This window can vary in size. 0 through 7) by using modulo arithmetic (e. In this protocol. Chapter 3: The Data Link Layer 43 . flow control is handled through the use of ACK frames. it sends an XOFF character to the transmitter. Both variables are restricted to a limited range of values (e. Both the transmitter and the receiver have their own window:  The transmitter window denotes the frames that have been transmitted but remain unacknowledged. flow control is usually based on two control characters: XON and XOFF. In character-oriented protocols.
It should be now clear that flow control is straightforward in the sliding window protocol. If the frame’s sequence number matches the lower bound of the window. it increments S (and the upper bound of its window). If the receiver withholds ACK frames. the window is rotated clockwise by one position (or more positions if succeeding frames within the window have already been received). it increments R and sends an ACK to the transmitter. with the sequence number range 0-7. When the transmitter sends a frame. If the frame’s sequence number matches any position other than the lower bound of the window. 44 . it increments the lower bound of its window. the transmitter soon reaches its maximum window size and has to stop transmitting.31 illustrates the protocol for a sliding window of size 3. it can reduce its window size and transmit more frames. Once it receives further ACK frames. Figure 3.7 6 5 0 1 2 Range 0-3 6 5 7 0 1 2 Range 6-1 4 3 4 3 The sliding window protocol operates as follows. When the transmitter receives an ACK for a transmitted frame. it notes the fact that the corresponding frame has now been received. When the receiver receives a frame whose sequence number falls within its window.
Data Link Layer Standards The most commonly-used data link layer standards are a number of de facto standards established by major computer manufacturers (e.Figure 3. character-oriented protocol devised by IBM in the 1960s for halfduplex communication. and IEEE. Chapter 3: The Data Link Layer 45 . CCITT. one character-oriented and one bit-oriented.1.31 Sliding window of size 3.. BSC Binary Synchronous Control (BSC. BSC by IBM and DDCMP by DEC) and others published by ISO. Transmitter 7 6 5 4 3 0 1 2 Receiver Sl = 0 Sh = 0 Transmitter is idle 6 5 4 3 7 0 1 2 Rl = 0 Rh = 3 Receiver is idle 7 6 5 4 0 1 2 3 Sl = 0 Sh = 1 Transmitter sends frame 1 6 5 7 0 1 2 Rl = 0 Rh = 3 Receiver is idle 4 3 7 6 5 4 0 1 2 3 Sl = 0 Sh = 2 Transmitter sends frame 2 6 5 7 0 1 2 Rl = 1 Rh = 4 Receiver receives frame 1 and returns ACK 4 3 7 6 5 4 0 1 2 3 Sl = 1 Sh = 2 Transmitter receives ACK for frame 1 6 5 7 0 1 2 Rl = 2 Rh = 5 Receiver receives frame 2 and returns ACK 4 3 7 6 5 4 0 1 2 3 Sl = 2 Sh = 2 Transmitter receives ACK for frame 2 6 5 7 0 1 2 Rl = 2 Rh = 5 Receiver is idle 4 3 3. with sequence numbers 0-7.4. Below we will look at two popular standards.g.4. also known as BISYNC) is a widely-used synchronous. 3.
another control character.32 Sample BSC block. If a control character occurs in user data. If the receiver receives a corrupted block. Figure 3. the transmitter repeatedly sends a SYN character which the receiver looks for. Information is exchanged in character blocks. Once the receiver has detected the SYN character. and ISO 7809 standards. 3. which is an arbitrary sequence of characters. it is simply preceded by a DLE character. Since control characters may also occur in user data. It is specified by ISO 3309. ISO 4335. Start-of-Header character. is used to avoid their being interpreted as control codes. the address of the receiver. the address of the transmitter. Start-of-Text character.as well as full-duplex 46 . Field SYN SOH Header STX Description Synchronization character. etc. A redundancy check concludes the block. Parity checking is also used on individual characters. the two stations handshake to confirm that they are synchronized and can start exchanging data.To synchronize. The receiver treats a DLE as meaning ‘accept the next character as literal’. Error checking char(s).4. HDLC The High-level Data Link Control (HDLC) is a more recent bit-oriented protocol which enjoys widespread acceptance throughout the world. such as the block sequence number. Data Actual user data (of variable length and arbitrary content). A block starts with a SYN character. DLE (Data Link Escape). STX and ETX mark the beginning and end of user data.2. Error handling in BSC is fairly simple. ETX Check End-of-Text character. SOH marks the beginning of a header which contains additional control information. The transmitter then retransmits that block. it returns a NAK block which contains the sequence number of the offending block. Used for control purposes.32 shows a sample block. A literal DLE itself appears as two consecutive DLEs. Figure 3. and supports half.
   Asynchronous Response Mode (ARM).communication. where a central station (e. This mode is best-suited to point-to-point configurations. printers. and may be one of the three shown in Figure 3. The Data field is of arbitrary length. PCs. Each station may transmit on its own accord.g. a slave station is unable to initiate a transmission. all stations are of the hybrid form with equal status. The Address field and the Control field are one or two octets each.28. This mode is now largely obsolete. Unnumbered frames are used for link control functions such as link connection and disconnection. it marks the last transmitted frame in response to a poll. terminals. The structure of the control field depends on the type of frame.). HDLC offers three modes of operation:   Normal Response Mode (NRM). In this mode. The Supervisory code consists of two bits and determines the type of supervisory commands (maximum of four commands or four responses). and may be zero for some messages. It is also possible for a station to assume a hybrid identity (master and slave) so that it can both issue commands and send responses. In this mode. it acts as a polling request to the slave station.g..33 Frame types and associated control field structures. The Unnumbered code consists of five bits and determines the type of unnumbered commands (maximum of 32 commands or 32 responses). In this mode. They may also be used for acknowledging receipt of data in a piggyback fashion. a host computer) polls a set of other stations (e. When set by the slave station. Figure 3. a slave station can initiate a transmission on its own accord. Supervisory frames are used for control purposes such as ACK and NAK frames and flow control. However. Most vendors now tend to support this protocol (or one of its derivatives) in their networking products. Information frames are used for exchange of user data between two stations. The Checksum field is two octets and uses the CRC method. This mode is typically used for multipoint lines. the master station is still responsible for managing the transmission. When set by the master station. The role of Send and Receive sequence numbers were discussed earlier in the chapter.33. The Poll/Final (P/F) bit is used in a HDLC master-slave arrangement. Information Frame Supervisory Frame Unnumbered Frame Chapter 3: The Data Link Layer 47 . HDLC offers a master-slave arrangement in which the master station (in charge of the link) issues command frames and the slave stations reply with response frames. The master station is responsible for managing the transmission.. etc.  The HDLC frame structure is shown in Figure 3. it can only transmit in response to a command from the master station.   Asynchronous Balanced Mode (ABM).
the transmitting station sends a SARM to the receiving station. often referred to as HDLC subsets. Black (1989). in which case the roles are reversed. The Logical Link Control (LLC) is an HDLC subset designed as a part of the IEEE 802 series of standards for use with LANs. as discussed earlier.5. It is used for establishing a link between a DCE and a DTE. The supervisory frames use the receive sequence number for acknowledgment and rejection.    3. The latter responds with a UA. There are a number of HDLC-related protocols. LAN data link layers are discussed in Chapter 9. 48 . It will be described in Chapter 11. and only supports a limited number of the unnumbered commands. The Link Access Protocol. LAP-B does not support the SREJ command. Under LAP.25 (see Chapter 4). Further Reading Martin and Leben (1988). and Stallings (1994) provide comprehensive descriptions of the data link layer protocols and standards. At its discretion.34 summarizes some of the supervisory and unnumbered commands and responses. The Link Access Protocol Balanced (LAP-B) is an ABM subset of HDLC designed for use with X. Gitlin et al (1992). Chapter 11 describes the ISDN data link layer.0 1 2 3 4 5 6 7 0 Send Sequence Number P/F Receive Sequence Number 1 0 Supervisory Code P/F Receive Sequence Number 1 1 Unnumbered Code P/F Unnumbered Code Figure 3. It will be described in Chapter 10. D channel (LAP-D) is an HDLC subset designed for use with ISDN. These include:  The Link Access Procedure (LAP) is based on the SARM command of HDLC. Halsall (1992). the receiving station may interpret the receiving of a SARM command as a request for transmission in the opposite direction.
Set Initialization Mode. Unnumbered Poll. Used for exchange of data in unsequenced frames. and flow control (to manage the differences in device speeds). DM Disconnect Mode. UA Unnumbered Acknowledgment. Station is ready to receive frames and acknowledged receipt of earlier frames. CRC is a more sophisticated error checking method used with bit-oriented protocols. SREJ Selective Reject.   Link layer protocols comprise a number of functions: acknowledgment (to ensure that data is correctly passed on by stations). Reset. RR Receive Ready. Forces a slave station into disconnect mode. Summary   Data link protocols are divided into synchronous and asynchronous . retransmission (to overcome errors). UI Unnumbered Information.Figure 3. Enables stations to reset their send/receive sequence numbers.  Chapter 3: The Data Link Layer 49  . but acknowledged receipt of earlier frames. timers (to impose time limits on operations).6. Unsequenced poll frame.  Parity checking is a simple error checking method used with characteroriented protocols. Mode Information Supervisory Command I RR RNR Response Description I Used for exchange of user data. Set Normal Response Mode. Set Asynchronous Response Mode. Station is unable to receive frames.  A data link protocol may use the master-slave model (based on the polling   technique) or the peer-to-peer model (all stations have the same status). REJ SREJ SNRM SARM SABM SIM UI UP Unnumbered DISC RSET 3. Set Asynchronous Balanced Mode. Asynchronous protocols are all character-oriented. Unsequenced acknowledgment frame. RNR Receive Not Ready.34 Sample HDLC commands and responses. Disconnect. error checking (to facilitate error-free transmission). Rejects a frame according to the Go-BackN scheme. A synchronous protocol is either character-oriented (user data is a sequence of delimited characters) or bit-oriented (user data is encapsulated by frames). Used by a slave to indicate it is in disconnect mode. Selectively rejects a frame. REJ Reject.
Explain how the polling technique works.21 5 3 0 Assuming the generator polynomial x + x + x .22 3.17 Explain the difference between synchronization at the physical layer and synchronization at the data link layer. Transmitter sends frames 1 and 2 3.19 3. 3. supervisory frames. illustrate how the sliding window protocol (with a maximum transmitter window size of 5 and receiver window size of 2) operates for the following scenario: 1.  The HDLC is a widely-used bit-oriented protocol with a number of variations: LAP.8.18 Describe what is meant by bit stuffing.23 3. and ABM). LAP-D. HDLC offers three modes of operation (NRM.   3.20 3. What problems are likely to occur in absence of a flow control scheme? Using a diagram. LAP-B. and LLC. of which there are three types: information frames. The sliding window protocol is a widely-used flow control protocol which relies on the transmitter keeping track of transmitted but unacknowledged frames. ARM. 3. and unnumbered frames. what is the CRC code for the following bit sequence: 0110111001111011110111111010111? Describe the role of T1 and T2 timers and how they can be used to handle a lost frame situation using the Selective Reject and G0-Back-N methods. Exercises 3.  The BSC (or BISYNC) protocol is a widely-used synchronous. Describe an application for which polling may be appropriate. It transmits information in frames. Describe the role of frame sequence numbers and how they are used for acknowledgment purposes. and the receiver keeping track of frames that are expected to be received. Rewrite the following bit sequence after bit stuffing it: 0111110011101111111111010111.24 50 . character- oriented protocol devised by IBM.
Transmitter receives ACK for frame 1 3. Chapter 3: The Data Link Layer 51 .25 Describe the role of information. supervisory. Use a sequence diagram to illustrate the use of SREJ command in a data transfer situation. and unnumbered frames in HDLC. Transmitter receives ACK for frames 2 and 3 3. Transmitter sends frame 3 4.2.
and ISO 8473.  Understand the three basic routing algorithms (flooding. we will look at four widelyaccepted standards for networking as well as internetworking.4.75.  Appreciate the importance of congestion control. and discuss the protocol sublayering provided for this purpose. We will then turn our attention to the problem of interconnecting two networks. static routing. Describe how packet switching works and distinguish between the virtual circuit and datagram methods and their packet formats. After completing this chapter you should be able to:  Understand the nature of network services and use network primitives to describe network service scenarios. X. This is followed by a description of data packets and their handling by the network layer.  Have a basic knowledge of network layer standards X.25.            52 . and dynamic routing) and their characteristics. IP. Then we will look at two switching methods and their use for routing. The network layer handles the routing of data packets across the network.  Appreciate the need for internetworking and the sublayers provided to support it. We will first discuss the use of network primitives for defining network services. The Network Layer This chapter describes the network layer of the OSI model. Finally. and defines the interface between a host and a network node. Describe how circuit switching works and appreciate its strengths and weaknesses.
Figure 4.1. these denote entities at the network layer that act as the interface to service users. User data refers to actual user data provided by service users for transfer by the service provider. Quality Of Service (QOS) denotes a set of parameters (such as error rate. Addresses refer to Network Service Access Points (NSAP). Network services are defined in terms of network service primitives. Network service content is therefore of considerable importance.35 summarizes the primitives together with their possible types and parameters.. Options denote a variety of options that may be activated with some commands (e. These services characterize the interface between the two layers and isolate the network details (lower three layers) from the network users (upper four layers). Please note that these primitives only serve as a modeling tool and do not imply anything about how network services are implemented. delays.g. failure likelihood.4. whether data transfers can be confirmed). Chapter 4: The Network Layer 53 . the network layer provides a set of services to the transport layer. throughput) which collectively describe the quality of the network service. cost. Network Services At the outset.
Used for normal data transfer (with or without confirmation). reason. Then A expedites some data which is indicated to B by the network. user data) (user data. (addresses.36 illustrates the use of the network services in a sample scenario.35 Network service primitives. Used by the service user or provider for resetting the network service. Used for high priority data transfer. Network service user A first requests a connection. B responds to the request and the network confirms with A. 54 . quality of service. Used for confirmation of data transfer. Network service user (A) N-CONNECT request Network service provider Network service user (B) N-CONNECT indication N-CONNECT response N-CONNECT confirm N-EXPEDITED-DATA request N-DATA request N-ACK-DATA indication N-RESET indication N-RESET response N-EXPEDITED-DATA indication N-DATA indication N-ACK-DATA request N-RESET indication N-RESET response N-DISCONNECT request N-DISCONNECT indication Figure 4. options. Primitive N-CONNECT Types request indicate response confirm N-DATA request indicate N-EXPEDITED-DATA request indicate N-ACK-DATA request indicate N-RESET request indicate response confirm N-DISCONNECT request indicate Parameters (addresses.36 Sample scenario of network services. user data) Used by the service user or provider for disconnecting a connection. which is indicated to network service user B by the service provider.Figure 4. Figure 4. A normal data transfer from A to B follows which includes confirmation. confirm option) (user data) () () Purpose Used for initiating a connection (connections are always initiated by service users).
(Although we have used an Chapter 4: The Network Layer 55 . 4. (However. the two crossing wires are not connected). These are separately discussed below.38). b..38 shows a simple circuit switch which consists of a 3×3 matrix.1. which both respond to. One of the most important functions of the network layer is to employ the switching capability of the nodes in order to route messages across the network.. Finally. two communicating stations are connected by a dedicated communication path which consists of intermediate nodes in the network and the links that connect these nodes. Each node has the capability to ‘switch’ to a neighboring node (i. and terminates the connection. What is significant about circuit switching is that the communication path remains intact for the duration of the connection. Circuit Switching In circuit switching. and f). Figure 4. engaging the nodes and the links involved in the path for that period.e.. which is indicated to A. Switching Methods Switching is the generic method for establishing a path for point-to-point communication in a network. 4. There are two basic methods of switching: circuit switching and packet switching. B requests a disconnection. A solid circles means that the crosspoint is on (i.2.37 A ‘switched’ path. It involves the nodes in the network utilizing their direct communication lines to other nodes so that a path is established in a piecewise fashion (see Figure 4. these nodes and links are typically capable of supporting many channels. The switch can support up to three simultaneous but independent connections.) Figure 4. capable of connecting any of its inlets (a.2. a node to which it is directly connected) to further stretch the path until it is completed. Each crosspoint appears as a circle.Then the network encounters an error and simultaneously sends reset indications to A and B.e. the crossing wires are connected). so only a portion of their capacity is taken away by the circuit. e.e. A hollow circle means that the crosspoint is off (i. and c) to any of its outlets (d.

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