lecture 23: mobile ad-hoc networks - hajim. · pdf filelecture 6 mobile ad-hoc networks: mac...
TRANSCRIPT
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.
2
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
3
Characterization of MAC Protocols
Channel separation and accessTopologyPowerTransmission initiationTraffic load and scalabilityRange
4
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
5
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
6
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
7
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
8
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
9
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
10
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
11
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
12
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)
13
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
14
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
15
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
16
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
17
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
18
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
19
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
20
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
21
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
22
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