I want to study this slide in 15 minutes, gimme a comprehensive description of what it consists:
Wireless D2D Networks2
Outline
• Device-to-device networks
– Principles and applications
– Main issues
– Examples3
“D2D Network” Definition
• Also called “ad hoc”, which in Latin literally
means "for this purpose only”, or machine-tomachine (M2M)
• It can be seen as a “spontaneous network,”
i.e., automatically “emerging” when nodes
gather together4
Network Scenario
• A D2D network may be:
– Pure D2D: an autonomous system of nodes
connected by wireless links forming an arbitrary
graph (stand-alone fashion), or
– Hybrid: connected to a fixed infrastructure
• Network nodes are battery-powered and free to
move5
Applications
• Personal communications
– smartphones, laptops, tablets
• Internet of Things
– sensors (also smartphones), RFIDs for smartcities
• Cooperative environments
– taxi cab network
– meeting rooms
• Emergency operations
– policing and fire fighting
• Military environments
– battlefield6
D2D Network Advantages
1. Ease and speed of deployment
2. Decreased dependence on infrastructure: content
sharing and distribution within a community, e.g
campus, concert, emergency situations -> traffic
offloading
3. Independent of network operators: safe, private, fast7
Typical Scenarios
• Distributed environment:
§ Fully symmetric environment: nodes have identical
capabilities and responsibilities
§ Asymmetric capabilities: transmission ranges may differ,
battery life at different nodes may differ, processing
capacities may differ
§ Asymmetric responsabilities: some nodes may act as
leaders of nearby nodes8
Typical Scenarios
• Traffic characteristics may differ
– bit rate
– time constraints
– reliability requirements
– unicast / broadcast / multicast / geocast / anycast9
Typical Scenarios
• Mobility patterns may differ
– people sitting at an airport lounge
– taxi cabs
– kids playing
– military squads
– work / home environments
• Mobility characteristics
– speed10
Interference
range Transmission range
An Example
Wireless node
Neighbors
Wireless linkAlso…
• Most of the wireless communication
devices are half-duplex:
– They include one transceiver only
– Hence, they can either transmit or receive (not
both at the same time)
– It might change in the future into full duplex
11Main Issues
1213
1 - Network Topology
• The network topology is the set of nodes and links
• It may change rapidly and unpredictably,
because of
• node movement
• node failure
• link failure
D
C
A
B
D
C
A
B14
Most Common Network Topologies
• Flat: All nodes are at the same hierarchical level
• Tree: Hierarchical architecture, easily extendible but not
suitable for fault-tolerant networks
• Cluster-Based:
– Nodes grouped into different sets (clusters)
– Nodes identified as cluster-heads or ordinary
• cluster-head: central controller that coordinates
communications within the cluster
– There may be several hierarchical layers15
Example: Flat
N nodes
At most Nx(N-1)/2 links16
Example: Tree
N nodes
(N-1) links17
Example: Cluster-Based18
2 - Channel Access
• Broadcast nature of the wireless medium
• Limited wireless transmission range
• Packet losses due to transmission errors
• Typically, CSMA/CA-based schemes
– Listen before transmit
– Back-off and retransmit if a collision is detected
– Problem of: Hidden and Exposed terminals22
3 - Multi-hop Communications
S=Source; D=Destination; R=Relay; N=Not participating
S1
D1
D3
D2
R
R
S2
R
S3
N
N
Each node is a router !!!23
Multi-hop Communications
• Advantages:
– Reduced transmit energy at the source
– Reduced interference, thus
– Increased network capacity since the spatial
domain can be reused for concurrent
communications
• Disadvantages:
– Increased network traffic load!
– Increased traffic delivery delay
– Increased network energy expenditure24
Multi-hop Communications
• Need for coordination among the nodes
• Easy in military environment; difficult for civil
applications (users are naturally selfish)
• Algorithm to favor cooperation (e.g., based on
monetary penalization of the user denying service
or on users’ reputation)25
Traffic Routing
A
D
E
B C26
• Mobility, battery powered devices
• Devices runtime depends on the amount of available
energy and advances in battery technology are slow
• On top of it, increasing attention to energy
consumption -> green networks
• Efficient energy management is crucial
– Energy aware link, routing, transport protocols
– Architectures, sleep modes
4 - Energy Consumption27
User Device Energy
Consumption
• Contributions:
– DSP
– Transceiver (Tx/Rx)
– Output transmit power (1mW-1W)
• Solutions
1. Power control: Reduce transmit power
2. Energy saving: Enter low-power operational
modes when idle28
Power Control
• Lower output transmit power
• Received power level is proportional
to 1/da, a ³ 2
• Reduced interference, increased
spatial reuse
• However, network topology
changes (connectivity)
A
B
C
A
C
D
B
D29
Using Max Transmission Power30
Reducing
the Transmission Power31
Energy Saving
Active
Tx/Rx
Sleep 1
RF off
Sleep 2
µproc. &
RF off
• Enter low-power state when idle
• Wake-up schedule [Singh98,Chiasserini03]:
• Synchronous
• Asynchronous
Power
consumption32
Summary
Important Issues & Requirements
• Autonomous and spontaneous nature of nodes
è Distributed algorithms
è Decisions based on local knowledge
Main issues:
1) Time-varying network topology
è Scalable mobility management techniques to face
network dynamics33
Summary
Important Issues & Requirements
2) Channel access
è Enhanced functionalities to improve link-layer
performance and QoS network support, in spite of
fluctuating link capacity
3) Multihop communication
è When is it convenient?
è How to make nodes relay traffic for others?
è What is the best route?
4) Low-power devices
è Energy saving techniques at all layers34
Examples of Technologies
• WLAN/Cellular based
• VANETs or connected cars
• WPANs
• BANs
• Sensor networks35
WLAN/Cellular based
• Several new technologies are available that allow
D2D communications
• Examples:
– AllJoyn by Qualcomm
https://allseenalliance.org/framework
– WiFi Direct (adopted by WiFi Alliance) http://www.wifi.org/discover-wi-fi/wi-fi-direct
– LTE Direct https://ltedirect.qualcomm.comVehicular Networks (Connected cars)
• Mobility: higher speed and predicatble
movements, macro and micro mobility
• Network topology: frequent network
partitions, low connectivity level, highly
dynamic topology
Wireless ad hoc networks with mobile nodes (as
MANETs) but:
• Resources: not limited by energy, memory, computation
capabilities
• Localization: equipped with satellite positioning systems such
as GPS
3840
Body Area Networks (BANs)
• Wireless communication between various
components (earphones, microphones, sensors)
attached to human body
• Possible Applications:
– transmission of body parameters (blood pressure, pulse
rate, body temperature)
– music entertainment
– help for disabled people41
Body Area Networks (BANs)
• Features
– transmission without interference
– licence free
– low transmission power
– cheap, tiny and light-weight hardware
• Data rates up to 250 Kbps
• Candidate technology: Bluetooth