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Alleviating MAC Layer Self-Contention in Ad-hoc Networks

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. Motivation. Self-contention

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Alleviating MAC Layer Self-Contention in Ad-hoc Networks

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  1. 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

  2. Motivation Self-contention Contention between packets of same transport connection Intra-stream contention Contention caused by packets of the same stream at different nodes Inter-stream contention Contention between DATA packet stream and ACK packet stream DATA stream (TCP or UDP) TCP DATA stream destination source source destination ACK stream Contention for shared media Contention for shared media prior MAC solutions: [Fu et. al., Infocom ’03] prior MAC solutions: (none) • 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

  3. 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

  4. Fast-Forward (FF)Key Idea: don’t send next packet till previous packet is out of interference range Receiver Sender Next hop Receiver RTS CTS Lower avg back-off time per packet No backoff precedes FFPKT tx Fewer False Link Failures No explicit contention for FFPKT Reduced control packet overhead No RTS for FFPKT DATA Time ACK (with Implicit RTS) ACK( with implicit RTS) CTS DATA (fast forwarded packet) FFPKT: Fast Forwarded Packet ACK( with implicit RTS) 2 6 4 6 6 2 Bytes: Modified ACK (with RTS for next-hop) Frame Control Duration Destination Address FCS RTS dest Address Source Address Needed by the next hop to respond with CTS Identifies the intended RTS recepient (next hop)

  5. Quick-Exchange (QE)Key Idea: subsume contention caused by reverse stream Receiver Sender’s Neighbor Sender Receiver’s Neighbor RTS Lower avg back-off time per packet No backoff precedes DATA2 tx Fewer False Link Failures No explicit contention for DATA2 Reduced control packet overhead No RTS/CTS for DATA2 Piggybacked ACK1 NAV (RTS) CTS Time DATA1 NAV (CTS) ACK1 DATA2 NAV (DATA1) NAV (ACK1)  ACK2 2 2 6 4 2 Bytes: HCS : Header Check Sequence FCS : Frame Check Sequence BSSID : Basic Service Set ID (unique network ID) Frame Control Duration Extra Duration() Destination Address FCS CTS 2 6 4 6 2 6 2 0 - 2308 4 Bytes: DATA2 (with ACK1) Frame Control Duration Destination Address HCS Source Address BSSID Sequence Control Body FCS ACK Header MAC Header

  6. 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%

  7. Performance: Normalized Goodput in Random Topology • 100 nodes in 2500m  1000m • Average of 50 scenarios • 180 sec sim time per scenario TCP flows in a random topology Normalized Goodput increases by up to 30%

  8. 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%

  9. Conclusions • 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 Ongoing Work • Goodput studies for scenarios with mobility • Analytical model of goodput gains for • fast-forward and quick-exchange

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