1. This material is based in part upon work supported by the National Science Foundation under Grant No. 0326582. 802.11: Challenges and System Design Issues for Long-distance Networks Rabin Patra Michael Rosenblum

2. This material is based in part upon work supported by the National Science Foundation under Grant No. 0326582. Outline Why 802.11 Short Overview of 802.11 Traffic Measurements for Single Link

3. This material is based in part upon work supported by the National Science Foundation under Grant No. 0326582. Why 802.11? Commodity radio products Low cost, high volume High power Unlicensed spectrum 2.4 GHz and 5 GHz Performance 11b: upto 11 Mbps, 11a/g: upto 54 Mbps Later: Mubaraq will talk about economic analysis

4. This material is based in part upon work supported by the National Science Foundation under Grant No. 0326582. 802.11 Standard Half-duplex radios only one of: Transmit or Receive Collision avoidance Cannot listen to channel while transmit Backoff before every transmit Contention for access to broadcast channel

5. This material is based in part upon work supported by the National Science Foundation under Grant No. 0326582. 802.11 Packet Sequence Features: Stop and go MAC retransmissions for reliability Key Issues: Channel utilisation ACK timeout Collisions Packet transmission time : Tp ACK transmission time: Ta Propagation delay : Td ACK Timeout > (Tp + 2*Td + Ta)

6. This material is based in part upon work supported by the National Science Foundation under Grant No. 0326582. Throughput vs distance PRISM 2.5 Radios UDP throughput With retries enabled Results: Gradual drop as round trip time increases Sharp drop at 22 km Setup: Channel emulator RF isolated experiments

7. This material is based in part upon work supported by the National Science Foundation under Grant No. 0326582. Throughput vs distance again Atheros Radios UDP throughput With MAC retries Tweak: Use longer ACK timeouts Results: 30 % drop even before 111 km Sharp drop at 111 km 30% drop

8. This material is based in part upon work supported by the National Science Foundation under Grant No. 0326582. Bi-directional traffic vs distance Bi-directional UDP With zero MAC retries Results: Collision avoidance does not work Too far apart Upto 30% at 100km

9. This material is based in part upon work supported by the National Science Foundation under Grant No. 0326582. TCP on Lossy link 20 km link over the bay from Berkeley to SF (near Sutro Tower) 20 % loss Results: TCP : 372 kbps UDP : 4.5 mbps Frequent timeouts of 200 ms Lots of ACKs are also lost TCP timeout 200ms

10. This material is based in part upon work supported by the National Science Foundation under Grant No. 0326582. TCP on Lossy link with tweaks Change RTO to 10ms in Linux TCP Results: TCP : 1.41 Mbps

11. This material is based in part upon work supported by the National Science Foundation under Grant No. 0326582. Design Challenges Throughput: use sliding window to increase channel usage Reliablity: hardware MAC retries bulk acknowledgements coding – bursty losses/ latency type of traffic

12. This material is based in part upon work supported by the National Science Foundation under Grant No. 0326582. TCP Sequence numbers: Closer look SenderReceiver

13. This material is based in part upon work supported by the National Science Foundation under Grant No. 0326582. 802.11 Packet Sequence Features Stop and go Tight MAC ACKs MAC retries Key Issues Channel utilisation ACK timeout Collisions