CMPE 255: Advanced Computer Communication LECTURE 2:

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CMPE 255: Advanced Computer Communication LECTURE 2:. Medium Access Control Protocols forAd Hoc Networks. RTS. RTS. CTS. CTS. S to R. R to S. S. S. S. RTS. CTS. time. RTS. H to R. noise is heard. FAMA: Floor Acquisition Multiple Access.
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CMPE 255: Advanced Computer CommunicationLECTURE 2: Medium Access Control Protocols forAd Hoc NetworksUCSC cmpe255RTSRTSCTSCTSS to RR to SSSSRTSCTStimeRTSH to Rnoise is heardFAMA: Floor Acquisition Multiple Access
  • Stations use carrier sensing to send any packet.
  • The CTS lasts much longer than an RTS to force the interfering sources to detect carrier (from the receiver) and back off.
  • RTS from S arrives at R with no collisions.RTS from H must start within one prop. delay from CTS from R to S.H must hear noise from CTS and backs off!UCSC cmpe255Packetready Floor Taken?yesnosend RTSdelay packettransmission k timeswait for a round-trip timeCTSback?compute randombackoff integer ksend packetnoyesBasic FAMA ProtocolNon-persistent strategy.Same basic algorithm for all CSMA/CA schemesUCSC cmpe255A packet is successful with probabilityFor we can approximate:Throughput of FAMATwo mutually exclusive events: packet is successful or a collision occurs. Therefore:The utilization period is only that portion of a packet transmission that has no overhead, that is:Notice the impact of the RTS-CTS overhead!Substituting:UCSC cmpe255Throughput of FAMA
  • FAMA (and all collision-avoidance protocols) is always below CSMA/CD.
  • UCSC cmpe255RIMA-DP timing diagramsWaiting periodXRTRDATANode X sends an RTR and after  seconds receives a DATA packet and then sends its DATADATAZWaiting periodRTRDATAXNode X sends an RTR and node Z replies with a CTS; node X sends its DATAZCTSXRTRNodes X and Z send RTRs within  seconds and therefore a collision occursZRTRchannelcollisionBACKOFFNoise detected at ZXRTRNTRBACKOFFDue to interference from node Z, node X sends an NTR to stop the handshakeZinterferenceUCSC cmpe255Throughput of RIMA-DP
  • The probability of success is the probability that an RTR is sent in the clear, because any RTR produces one or two data packets, i.e.,
  • The probability with which the polled node has data is
  • The probability with which the poled node has no data is
  • The length of an average busy period always includes an RTR, a prop delay, and the average time between the first and the last RTR of the busy period; therefore,
  • UCSC cmpe255first packet starts (A)last interfering packet starts (B)NEWNEWARTRBtimesuccessful packet:idle period:collision interval:Throughput of RIMA-DPNEWDATACTS
  • The length of the average idle period is simply 1/lambda
  • The average utilization period is
  • idleperiodUCSC cmpe255Throughput of RIMA-DP
  • The throughput is simple the length of the ave. utilization period divided by the length of the average cycle:
  • UCSC cmpe255Throughput Analysis
  • 500 Byte data packets; 1Mbps network speed; maximum distance between nodes is 1 mile; on the left a 10 node network; on the right a 50 node network
  • UCSC cmpe255Limitations of Colision Avoidance
  • Collision avoidance is meant for unicast packets.
  • A large number of network-level control and data packets are multicast and broadcast in nature.
  • UCSC cmpe255Collision-Avoidance Transmission Scheduling (CATS)
  • A contention and reservation based topology-dependent multi-channel scheduling protocol.
  • Schedules unicasting, multicasting and broadcasting traffics simultaneously.
  • Data packets are sent collision-free in the presence of hidden terminals.
  • Supports real-time applications and node mobility.
  • Provides better spatial reuse than topology-independent scheduling since frame length depends only on node degree.
  • Works well with commercial SFH radios in ISM bands.
  • UCSC cmpe255Time and Channel Organization
  • Time is slotted and slots are grouped into frames. A slot is further divided into six mini-slots.
  • Multiple channels are available: one signaling channel (SCH), one broadcast data channel (BCH) and a number of other data channels (DCHs).
  • A data link refers to a particular DCH or the BCH in a particular slot.
  • Small control packets called beacons are used to contend for and reserve data links.
  • UCSC cmpe255Identifying Reservations and Data TransmissionFrameslot 1slot 2slot 3slot LLRBUnicast Data PacketUnicastLRBLRBLRBMulticast Data PacketMulticastLRBBroadcast Data PacketBroadcastMS1MS2MS3MS4MS5MS6Signaling CHBroadcast CHReserved Data CH'sLRB: Link Reservation BeaconUCSC cmpe255Frameslot 1slot 2slot 3slot 4slot LUnsuccessfulSLSLRUBRLC/Nunicast contentionSuccessfulSLSLRUBRLCUBUCDunicast contentionUnsuccessfulSLSLRMBSMB/Nmulticast contentionSuccessful multicastSLSLRMBClearMCDcontentionUnsuccessfulSLRBBSBB/Nbroadcast contentionSuccessfulSLRBBClearBCDbroadcast contentionMS1MS2MS3MS4MS5MS6Signaling CHData CHRUB: Request Unicast Beacon, RMB: Request Multicast Beacon, RBB: Request Broadcast BeaconCUB: Concur with Unicast Breacon, SMB: Stop Multicast Beacon, SBB: Stop Broadcast BeaconUCD: UniCast Data, MCD: MultiCast Data, BCD: BroadCast Data, SL: Sender ListensRL: Receiver Listens, C/N: Clear/NoiseMaking Reservations for Data TransmissionsRLSLRLSLSLSLBroadcast CHUCSC cmpe255Frame LengthWorst-case minimum frame length L and number of DCHs C (assuming N > d2, d: the max node degree, and N: the node population in the network):
  • For broadcast: L = d2 + 1.
  • For unicast:
  • L = 2d, C = d, if each node unicasts once in each frame; Or
  • L = 2(2d -1), C = 2d -1, if each node unicasts to each neighbor once in each frame.
  • UCSC cmpe255Approximate Unicast Throughput Analysis ResultsBAMA: d=10, L=20 slots, C=10 DCH's, AFL in slots10.90.80.70.6Throughput per Node S0.50.40.30.2AFL=100AFL=10AFL=20.1AFL=1000.20.40.60.811.2Offered Load per Node Gd: node degreeL: frame lengthAFL: average flow (message) lengthUCSC cmpe255BAMA: N=16 nodes, L=16 slots, AFL in slots10.90.80.70.6Throughput per Node S0.50.40.30.2AFL=100AFL=10AFL=20.1AFL=1000.10.20.30.40.50.60.70.80.9Offered Load per Node GApproximate Broadcast Throughput Analysis ResultsN: number of nodesL: frame lengthAFL: average flow (message) lengthUCSC cmpe255Approximate Performance Analysis
  • Throughput is analyzed for two cases: unicast traffic over a hyper-cube topology and broadcast traffic over a fully-connected topology.
  • Each node can reserve at most one slot for transmission in each frame with the worst-case minimum frame length and number of data channels.
  • We consider Poison sources and geometrically distributed variable-length flows (messages).
  • Throughput is defined as the probability that a node has a reserved link for transmission in a frame.
  • UCSC cmpe255Likmitations in CATS
  • Collision avoidance dialogue is needed!
  • How can we eliminate the CA in CATS?
  • Goal is to have a topology-dependent transmission schedule!
  • Protocol needs to implement a distributed election of schedules and such schedules must be transmitted persistently without eating too much bandwidth!
  • UCSC cmpe255Collision Resolution and Backoff Strategies
  • Used to stabilize the system by preventing traffic loads that exceed its capacity.
  • Collision resolution: Let packet that collide resolve when each is transmitted and block new traffic from entering the system.
  • Backoff strategies: Increase the time between retransmissions when traffic load (that creates collisions) increases.
  • UCSC cmpe255Nodes 1 to 49 can try again; node 5 succeeds! (must be only node in range)Nodes 5, 50, 70, 80 collideNodes 76 to 100 must wait;Node 80 waitsNodes 50 to 100 must wait for all collisions from 1 to 49 to be resolvedNodes 50 to 75 can try;50 and 70 collideNodes 50 to 62 can try;50 succeeds(must be only in range)Node 70 waitsNodes 63 to 70 can try;70 succeedsNodes 76 to 100 can try;80 succeeds!Collision Resolution Algorithm
  • Assume a fully-connected network.
  • Each node maintains a stack, a HighID, a LowID and knows the maximum ID in the system
  • UCSC cmpe255Average Delay of MAC Protocols
  • We want to measure or compute the average time from the instant the first bit of a packet is first transmitted to the moment the last bit is received correctly at the destination.
  • Assume that arrivals (of new and retransmitted data or control packets) to the channel are Poisson.
  • Assume fully-connected networks.
  • UCSC cmpe255The average number of transmissions needed for a packet to be received correctly isTherefore, the number of retransmissions is Average Delay in ALOHAAssumptions:A satellite channel with propagation delay NxP, where P is the packet length and NxP >> PA retransmission is sent after an average backoff time of BxP seconds.Direct method:A packet is transmitted (G/S-1) times in error (due to collisions) and each such transmission wastes P+NxP +BxP seconds.The last transmission is successful and must take P+NxP seconds.Therefore, the average delay incurred is: UCSC cmpe255START ENDBACKOFFAverage Delay in ALOHAIndirect Method:Based on the fact that the success of a transmission is independent of others, and knowing how many times we have retransmitted does not change the likelihood of success in the next transmission!We use a diagram showing possible states, probabilities of transition, and delay incurred in that transition. From the diagram. we obtain a number of simultaneous equations that we solve to obtain delay from START to END.UCSC cmpe255From the diagram we have:Substituting we obtain the same result.Average Delay in ALOHASolving these two equations:The same method can be applied on the other MAC protocols!UCSC cmpe255Average Delay of ALOHA
  • The delay increases exponentially with heavy load, which is not acceptable for real-time applications.
  • UCSC cmpe255UCSC cmpe255UCSC cmpe255UCSC cmpe255UCSC cmpe255UCSC cmpe255
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