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 Inter-stream contention Contention between DATA packet stream and ACK packet stream Intra-stream contention Contention caused by packets of the same stream at different nodes sourcedestination source DATA stream (TCP or UDP)TCP DATA stream ACK stream Contention 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]

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 Frame Control Duration Destination Address FCS 2 264 Bytes: Modified ACK (with RTS for next-hop) RTS dest Address 6 Source Address 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) Time 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 FFPKT: Fast Forwarded Packet

5. Quick-Exchange (QE) Key Idea: subsume contention caused by reverse stream Frame Control Duration Extra Duration(  ) Destination Address FCS Frame Control Duration Destination Address HCS Source Address BSSID Sequence Control BodyFCS 2 2264 2 2646 6240 - 2308 Bytes: CTS DATA2 (with ACK1) ACK Header MAC Header HCS : Header Check Sequence FCS : Frame Check Sequence BSSID : Basic Service Set ID (unique network ID) 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 RTS CTS DATA1 ACK2 Sender Receiver Receiver’s NeighborSender’s Neighbor NAV (CTS) Time ACK1 DATA2 NAV (RTS) NAV (ACK1) NAV (DATA1) 

6. Performance: Goodput in String Topology Single UDP flow in a string topologySingle TCP flow in a string topology Goodput increase up to 250%Goodput increase up to 45%

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

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 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