Alleviating MAC Layer Self-Contention

in Ad-hoc Networks

Zhenqiang Ye, Dan Berger, Prasun Sinha† ,

Srikanth Krishnamurthy, Michalis Faloutsos, Satish K. Tripathi

Dept. of CSE, UC Riverside† Dept of CIS, Ohio State University

MotivationSelf-contention

Contention between packets of same transport connection

Inter-stream contentionContention between DATA packet

stream and ACK packet stream

Intra-stream contentionContention caused by packets of the

same stream at different nodes

source destination

destination

source

DATA stream (TCP or UDP) TCP DATA stream

ACK stream

Contention for shared mediaContention for shared media

Self-contention is best resolved at the MAC layer because…• Self-contention arises in the MAC layer• Requires no changes to widely deployed transport protocols• IEEE 802.11 is an evolving standard and is amenable to changes

prior MAC solutions: (none)prior MAC solutions: [Fu et. al., Infocom ’03]

The Main Contributions

• Propose two mechanisms:– Fast Forward alleviates intra-stream contention by

withholding transmission until previous packet has reached beyond interference range.

– Quick Exchange alleviates inter-stream contention by exchanging TCP data and TCP ACK packets in the same RTS-CTS-ACK dialogue.

• Observe significant performance improvement:– Up to 250% goodput improvement– Up to 19% backoff time reduction– 22% MAC layer overhead reduction

Fast-Forward (FF)Key Idea: don’t send next packet till previous packet is out of interference range

FrameControl

DurationDestination

AddressFCS

2 2 6 4Bytes:

Modified ACK(with RTS for next-hop)

RTS destAddress

6SourceAddress

6

Identifies the intended RTS recepient (next hop)

Needed by the next hop to respond with CTS

RTS

CTS

DATA

ACK( with implicit RTS)

Sender Receiver Next hop Receiver

CTS

ACK (with Implicit RTS)

DATA(fast forwarded packet)

ACK( with implicit RTS)

Tim

e

Lower avg back-off time per packetNo backoff precedes FFPKT tx

Fewer False Link FailuresNo explicit contention for FFPKT

Reduced control packet overheadNo RTS for FFPKT

FFPKT: Fast Forwarded Packet

Quick-Exchange (QE)Key Idea: subsume contention caused by reverse stream

FrameControl

DurationExtra

Duration()Destination

AddressFCS

FrameControl

DurationDestination

AddressHCS

Source Address

BSSIDSequence

ControlBody FCS

2 2 2 6 4

2 2 6 4 6 6 2 40 - 2308

Bytes:

Bytes:

CTS

DATA2(with ACK1)

ACK Header

MAC Header

HCS : Header Check SequenceFCS : Frame Check SequenceBSSID : Basic Service Set ID (unique network ID)

Lower avg back-off time per packetNo backoff precedes DATA2 tx

Fewer False Link FailuresNo explicit contention for DATA2

Reduced control packet overheadNo RTS/CTS for DATA2

Piggybacked ACK1

RTS

CTS

DATA1

ACK2

Sender Receiver Receiver’s NeighborSender’s Neighbor

NAV(CTS)

Tim

e

ACK1DATA2

NAV(RTS)

NAV(ACK1)

NAV(DATA1)

Performance: Goodput in String Topology

Single UDP flow in a string topology Single TCP flow in a string topology

Goodput increase up to 250% Goodput increase up to 45%

Performance: Normalized Goodput in Random Topology

TCP flows in a random topologyNormalized Goodput increases by up to 30%

• 100 nodes in 2500m 1000m• Average of 50 scenarios• 180 sec sim time per scenario

Goodput Improvement Factors(Scenario: TCP flows in random topology)

Normalized Backoff Time(backoff time per MAC packet tx)

Normalized Control Packet Overhead(#Control packets per unicast packet)

Reduction by up to 19% Reduction from 3.2 to 2.5 (approx.)

Number of Link Failures Reduction by up to 66%

Conclusions

Ongoing Work

• Quick-Exchange alleviates inter-stream self-contention• Fast-Forward alleviates intra-stream self-contention• UDP goodput improves by 250% in string topology• TCP goodput improves by 45% in string topology

• Goodput studies for scenarios with mobility• Analytical model of goodput gains for fast-forward and quick-exchange