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Medium access control in Wireless Ad-hoc Networks

Presented byJin Xu and Lan Nguyen

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Outline

Classifications of MAC protocols

Five Phase Reservation Protocol (FPRP)

Distributed Wireless Ordering Protocol (DWOP)

Conclusions

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Classifications of MAC protocols

Contention-basedA node contends with its neighbors to access the channelNo QoS guaranteesMACAW, FAMA, BTMA

Contention-based with reservationReserve bandwidth a priori Can provide QoS support to real time trafficD-PRMA, CATA, FPRP

Contention-based with schedulingFocus on transmission scheduling of the nodesFair and no starvationDPS, DWOP, DLPS

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Five-Phase Reservation Protocol (FPRP)

Contention-based with reservationSingle channel time division multiple accessFully distributed (synchronized)Slot reservation using a 5 phase processParallel Localized processScalable (insensitive to network size)

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FPRP: the model

Nodes keep perfect timingA link between 2 nodes is noiseless, symmetricThe network topology not change when FPRP is performedWhen multiple packets arrive at a node, they are destroyedA node can tell whether 0, 1, or multiple packets are transmitted when in receiving modeEvery node has a unique ID

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FPRP: overview

Time is divided into frames

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FPRP: 5 phase

Reservation request (RR)Collision report (CR)Reservation confirmation (RC)Reservation acknowledgement (RA)Packing and elimination (P/E)

A node keeps global timing, and knows when a 5-phase cycle starts.

A node can transmit or receive, but cannot do both at the same time.

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Phase 1: Reservation request

A node which wants to make a reservation sends a Reservation Request packet (RR) with probability pOther nodes listen

Requesting Node

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Phase 2: Collision report

If a node receives multiple RR’s in phase 1, it transmits a Collision Report packet (CR)o.w. silentRequesting node (RN) transmission node (TN)

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Phase 3: Reservation confirmation

TN sends Reservation Confirmation packet (RC)Every node (1 hop away) which receives RC know the slot has been reserved cease contention, receiving

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Phase 4: Reservation acknowledgement

A node ack a RC just received by sending a Reservation Ack packet (RA)

Inform nodes 2 hops awayNot transmitting

Prevent isolated node from transmittingResolve isolated deadlock

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Phase 5: packing / elimination

Every node 2 hops away from TN sends a Packing packet (PP)

A node receiving PP learns there is a TN 3 hops awayAdjust its contention prob.to reuse time slot efficiently

TN sends an Elimination packet (EP) with a probability of 0.5

Attempt to resolve a non-isolated deadlock

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An example

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How to calculate the contention probability

nc : # of nodes that contend within 2 hopsnb : # of nodes within 2 hops that have to contend in next slot due to a nearby success (cannot contend in current slot)

R1: a portion of 1 hop neighbors from success cease to contend in the current slotR2: 2 hop neighborsR3: 3 hop neighbors

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How to calculate the contention probability

At the beginning of a reservation slot, a node resets its nc and nb:

nc = nb nb = 0

After every reservation cycle, on hearing an:Idle: nc = nc – 1Collision: nc = nc + 1 / (e - 2)Success:

0 hop: done;1 hop: nc = nc – 1 nb = nb + nc R1 nc =nc(1 – R1)2 hops: nc = nc – 1 nb = nb + nc R2 nc =nc(1 – R2)3 hops: nb = nb + nc R3 nc =nc(1 – R3)

P = 1/nc16

Simulation results

The # of nodes is 100, 200, 300 and 400 from bottom to top

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Effects of nodal mobility

The observation time t is 0, 0.5, 1, 2, 4, 8, 16, 32, 64, 128 seconds from bottom to top

BM RCSt = 018

Protocol considerations and applications

Time synchronization issue:GPS can provide accurate global time

Some applications:TDMA schedule produced can be used to transmit user generated packetsMake reservations for network control trafficCan provide good service for multimedia (voice) traffic

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Conclusions for FPRP

Fully distributed, requiring no a prior knowledge about the networkGenerate transmission schedules with low amount of overheadNot affected much by the network size and nodal mobilitySuitable for use in large, mobile ad hoc network

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Problems of IEEE 802.11Unfair channel access

Due to random access nature of wireless 802.11

Hard to support QoS (priority routing/schedulinge.g. diverges significantly from FIFO order

Even worse in more complex topologies, e.g.Asymmetric informationPerceived collision

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Asymmetric informationSymmetric information sharing

All nodes are within radio range of each other, hence can hear all the RTS and CTSAll nodes have the same probability of accessing the channel

Asymmetric information sharing (opposite)All nodes are not within radio range of each otherNodes have unequal channel access probabilities

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Example Node 1 does not know about flow B, so it has to send RTS randomly

Node 2 may not be able to send CTS due to either:

Deferral for flow B transmissionDidn’t receive RTS due to collision with flow B

Node 3 knows about flow A via node 2, hence it knows the “right time” to send RTS

Flow A: 5%Flow B: 95%

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A possible solutionMACAW

Node 2 sends Request-for-RTS (RRTS) to node 1Upon receiving RRTS, node 1 sends RTS immediately

Works only if node 2 has received RTSNot included in IEEE 802.11

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Perceived collisionIn the previous example, flow B has info about flow A (not the vice versa) -> flow B gets more channel access

But this is not always the case

As knowing more information about other flows makes a flow defer access to more flows

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Perceived collision (cont.)

Flows A and C can access the channel simultaneouslyFlow B contains information about both flows A & C, i.e. it has to wait for both flows A & C

Flow A: 36%Flow B: 28%Flow C: 36%

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DWOP protocolGoalsFIFO-like behavior Piggy-backing arrival timesModifying IEEE 802.11Receiver participationStale entry detection

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Goals of DWOPProviding fair channel access

Scheduling packets in the order that approximates a reference scheduler (e.g. FIFO)

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FIFO-like behavior Wireless contention for the channel access requires a scheduler for all the contending nodes

For FIFO, packet priority is packet’s arrival time

Contending nodes share the arrival times

A scheduling table is needed

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Piggy-backing the arrival timesA sender (receiver) piggy-backs the arrival time of its current (highest priority) packet into RTS (CTS)

A sender (receiver) also piggy-backs the arrival time of its next highest priority packet into the DATA (ACK) packet’s transmission

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Piggy-backing (cont.)Nodes overhearing RTS, CTS, DATA, ACK packets add the attached arrival time into their sorted scheduling tables along with their own packets’ arrival times

Also, nodes remove the arrival time entry when overhearing the completion of DATA and ACK

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Modifying IEEE 802.11 If the channel is busy, behave like IEEE 802.11

If the channel is idle, check the scheduling table

If its packet has the highest priority, send RTSElse defer as in IEEE 802.11

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Does it work all the time ?

Only if all nodes are within radio range of each otherIf not -> asymmetric information

Node 1 is not aware of flow B -> behaves like IEEE 802.11 -> try to contend the access continuously and randomlyFlow B knows the arrival time of flow A -> defers if it has lower priority -> less aggressive than flow A -> Flow B gets less bandwidth

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Receiver participationReceiver of A (node 2)

know the arrival time of flow B“warn” node 1 include an out-of-order notification in CTS/ACK

Upon receiving the out-of-order notification

Node 1 finishes it current transmissionNode 1 goes into a backoff

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Receiver participation (cont.) Node 1 is allowed to complete its current out-of-order transmission

Only approximate FIFO is achieved

DWOP can be modified to achieve perfect FIFO

But tradeoff between perfect FIFO scheduling and network utilization

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Stale entry elimination

Stale arrival time entry occurs in a node if:Does not receive ACK (due to collision)

Stale entry detectionIf there is a deletion below its packet’s position in the scheduling table

ReactionRemove the first entry in the scheduling table

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Simulation experimentsExperiment setup

ns-2 simulator with cmu-wireless extensionData packet size: 1000 bytesChannel bandwidth: 2MbpsCBR flows

Three topologies:Asymmetric information topologyPerceived collision topologyMore complex topology

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Simulation experiments (cont.)Performance metrics

Fair channel accessFIFO-like behavior

Ideal FIFOSwitch to another flow after one packet transmission

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Asymmetric information

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Perceived collision topology

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More complex topology

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Summary of resultsDWOP has less deviation from FIFO

DWOP: deviation bounded up to 4 packetsIEEE 802.11: unbounded deviation

DWOP achieves better fairness of accessing the channel than IEEE 802.11

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Limitations of DWOPFlows of one single hopFixed packet sizeFIFO schedulerTotal number of received packets is about 1/3 less than IEEE 802.11Does not consider

MobilityChannel errors (ECF [3])Variable packet sizeOther reference schedulers

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DWOP conclusionsIEEE 802.11 does not take care of fair channel access wellIEEE 802.11 diverges significantly from FIFO DWOP provides fairness of channel accessDWOP approximates the FIFO order betterDWOP can be applied to other schedulersQoS can be integrated in DWOP

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Reference [1]. A Five-Phase Reservation Protocol (FPRP) for Mobile Ad Hoc Networks, C Zhu, M S Corson, Proceedings of IEEE INFOCOM, 1998.

[2]. Ordered Packet Scheduling in Wireless Ad Hoc Networks: Mechanisms and Performance Analysis, V. Kanodia, C. Li, A.Sabharwal, B. Sadeghi, Proceedings of ACM MOBIHOC, 2002.

[3]. Fair scheduling in wireless ad-hoc networks of location dependent channel errors, Chen J., Somani A. K., Proceedings of the 2003 IEEE International, 2003.

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Question ?