Lecture 6Mobile Ad-Hoc Networks: MAC

Reading: • “MAC Protocols for Ad Hoc Wireless Networks,” in Ad Hoc Wireless

Networks: Architectures and Protocols, Chapter 6.• “A Survey, Classification and Comparative Analysis of Medium Access

Control Protocols for Ad Hoc Networks” by Raja Jurdak, Cristina VideiraLopes, and Pierre Baldi; University of California, Irvine, http://www.comsoc.org/livepubs/surveys/public/2004/jan/index.html

• E. Royer, S.-J. Lee and C. Perkins, “The Effects of MAC Protocols on Ad hoc Network Communication,” Proceedings of the IEEE Wireless Communications and Networking Conference (WCNC ’00), 2000.

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MAC ProtocolsProvide “rules” for channel accessIn MANETs, no centralized control

Nodes independently determine accessLocal nodes elected to control channel accessNodes coordinate amongst themselves locally to determine channelaccess

Goals for MAC protocols for MANETsHigh channel efficiencyLow powerScalableFairSupport for prioritizationSupport for heterogeneous nodesDistributed operationQoS supportLow control overhead

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Characterization of MAC Protocols

Channel separation and accessTopologyPowerTransmission initiationTraffic load and scalabilityRange

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Cannel Separation and AccessCommon channel vs. multiple channelsTypical use of channel

Data transmissionRTS/CTS handshakeCarrier sensingPeriodic information exchangeReservations

Can use single channel for all packetsSend some packets (e.g., overhead) on one channel and other packets (e.g., data) on other(s)Multiple channels allow more simultaneous users

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Single ChannelAll nodes share the medium for transmission of data and control messagesCollisions and contention

Handshake protocolACKsBackoff protocol

ExamplesCSMAMACA, MACAW, FAMAMACA-BI, RIMA-SP: receiver-initiated approachesMARCH: string of RTS-CTS-CTS-CTS…DPC/ALP: consecutive increase in RTS powerPS-DCC: calculate sending probability based on current network load

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Multiple ChannelsCan separate channels in time, frequency, space, etc.Typically, one channel for control, other(s) for dataExamples

BTMA, DBTMA: separate busy-tone channelPAMAS: RTS/CTS sent on control channelDCAPC: one control channel, multiple data channelsGRID-B: channel borrowing from neighboring cells

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Multiple Channels (cont.)TDMA-based separation

Time segmented into frames, slotsNodes maintain synchronizationBest with real-time, periodic dataExamples

FPRP, CATA, SRMA/PA: each slot has reservation and information subslotsMarkowski: traffic classes, window-splitting contention resolutionADAPT: nodes “own” slots but others may useD-PRMA: continuous reservations for voice

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Multiple Channels (cont.)FDMA-based separation

Allows multiple nodes to transmit simultaneouslyExamples

MCSMA: CSMA on each channel

CDMA-based separationSimultaneous transmissions via code separationExamples

MC-MAC: one common control signal code, N data codesIEEE 802.11: DSSS or FHSS channel separationRICH-DP: reserve hops in frequency hopping scheme, RTR scheme

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Multiple Channels (cont.)SDMA-based separation

Directional antennas to transmit in particular directionExamples

Lal: poll direction with RTR, directional RTS and CTS returnedMMAC: directional carrier sensing, directional RTS

Hybrid schemesCombine channel separation methodsExamples

PRMA: TDMA and FDMAJin, Bluetooth: CDMA/TDMA

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TopologyAd hoc network features

MobilityHeterogeneous node capabilities

Types of topologiesCentralized

Base station used for network control and managementNot useful for MANETs

Flat: single and multi-hopCompletely distributed approach

ClusteredLocal cluster head elected and used for network control

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Flat TopologiesNodes make independent decisions to access the channel

Local coordination via handshaking, carrier sensingSingle-hop: concerned only with immediate neighbors

Scalability issuesCSMA, MACA, FAMA, MACA-BI, RIMA-SP, 802.11, etc.

Multi-hop: some notion of nodes outside local neighborhood

Can aid in scalability and power efficiencyMost use multiple channelsPAMAS, DCA-PC, DCP/ALPMARCH: directly uses notion of multi-hop path

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Clustered TopologiesElect local cluster head to perform control/management of network resourcesReduces burden on nodes, increases burden on cluster head

Good for heterogeneous networksClustering protocols differ in

Election of cluster headCluster maintenanceChannel access

ExamplesVBA: elect CH based on lowest IP addressWCA: elect CH based on weighting of distance to nbrs, battery power, mobility and connectivity; allows roaming between clustersJin, GPC: elect CH based on battery powerBluetooth: elect CH (Master) as node that initiated cluster (piconet)

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Power ConsumptionRadio operates in 3 modes: transmit, receive, standbyRelative powers

PTX > PRX >> PSB for long-range communicationPTX ~ PRX > PSB for short-range, low power transceivers

Different MAC protocols will be “low-power” depending on model of transceiver power dissipationTime delay and power dissipation switching between states

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Reducing Energy ConsumptionReduce transmit power

Use “just enough” to reach intended destinationExamples

GPC, DCAPC, DCA-PC, DPC/ALPPlace nodes in standby mode as much as possible

Nodes do not need to be on when not receiving dataRequires nodes to know when they must listen to the channel and when they can “sleep”MAC protocols cannot use “promiscuous” mode to listen to other conversationsNode must know when other nodes have data to tx to itExamples

PAMAS, Bluetooth, HIPERLAN

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Reducing Energy Consumption (cont.)

Tradeoff energy consumption and delay in receiving a messageApproaches

Directory approach: BS broadcasts directory of packets waiting in its queue

Node receives directory and knows when to wake up and listen for data

Grouped-TDMA approach: nodes grouped and each group wakes up at given slot to determine if data needs to be receivedPseudo-random approach: nodes have unique pseudo-random sleep/wake cycles known to BS

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Reducing Energy Consumption (cont.)

Collisions should be minimizedRetransmissions expend energyIntroduce delaysReduce number of ACKs requiredUse contention for reservations and contention-free for data transmission

Allocate contiguous slots for transmission/receptionAvoids power/time in switching from Tx to Rx

Have node buffer packets and transmit all packets at once

Allows node to remain asleep for long timeTrade-off in delay to receive packets and buffer size

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Reducing Energy Consumption (cont.)

Make protocol decisions based on battery levelChoose cluster head to have plenty of energyGive nodes with low energy priority in contentionExamples

WCA, DPC/ALP, Jin GPC

Reduce control overheadNeed control to avoid collisions, but reduce as much as possibleExamples

MARCH

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Reducing Energy Consumption (cont.)

Centralized scheduling is most energy-efficientEnergy advantages depend on relative power in the transmit and receive modeAdapt protocol to traffic and network for most energy efficient approach

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Transmission InitiationSender-initiated

Most protocols follow this approachSender attempts to access channel when it has data

Receiver-initiatedReceiver attempts to clear channel for transmissionsSend request-to-transmit (RTR) to all neighbors or specific nodePolling for dataOnly efficient if large amount of traffic on network

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Traffic Load and ScalabilityHighly loaded networks

Receiver-initiated approachesAdjust sending probability based on network loadChannel borrowing for non-uniform loadTDMA approaches for periodic sources

Dense networksTransmission power controlDirectional antennas

Voice and real-time trafficPrioritiesReservations

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Interaction Between MAC and Routing

Why should the MAC protocol affect the routing protocol? What affects would you expect the MAC protocol to have on the routingprotocol and vice versa?Royer et al. study

Routing protocolsWRP: triggered + periodic routing updatesFSR: non-uniform updates with more accurate information for closer destinationsAODV: reactive protocol, uses Hello messages

MAC protocolsCSMA: non-persistentMACA: RTS/CTS with no CSFAMA: RTS/CTS with CSIEEE 802.11: CSMA/CA with RTS/CTS/ACK

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Simulation ResultsPackets delivered

AODV only protocol to vary depending on MAC– why should the MAC affect this protocol more?With 802.11, AODV performs best– why might this be the case?

Control overheadWRP: control traffic increases as mobility increases– why?FSR: control traffic relatively constantAODV: overhead varies with MAC and mobility– why?

Normalized routing loadWRP consistently highFSR and AODV: similar performance, varies based on MAC

Overall, AODV more dependent on MAC than on-demand protocols