1. Centre for Wireless Communications Opportunistic Media Access for Multirate Ad Hoc Networks B.Sadegahi, V.Kanodia, A.Sabharwal and E.Knightly Presented by Matti Raustia

2. Contents WLAN Rate Adaptation ARF RBAR OAR Simulation Results Conclusion

3. 802.11 WLAN Specified by IEEE 802.11b (1999) –2.4 GHz DSSS –Multirate: 1, 2, 5.5, 11 Mbit/s –Proprietary modes up to 22 Mbit/s 802.11a –5 GHz OFDM –Multirate: 6, 12, 24, 36, 48, 54 Mbit/s 802.11g –2.4 GHz OFDM, 2.4GHz DSSS –Compatible with 802.11b devices –Multirate: 802.11b data rates + OFDM data rates up to 54 Mbit/s

4. Why is Rate Adaptation Needed? According to IEEE standards we can scale the data rate according to channel conditions Radio channel conditions are changing if TX, RX or something between them is moving –this causes that maximum achievable data rate in the channel is changing also –if we can use higher data rate when channel is good we can increase the throughput Higher data rates overall More traffic or users We must have some way to control the data rate adaptation

5. How to Implement It? First we have to have some knowledge about the channel so we can adapt the data rate –802.11 uses time division (TDMA/TDD) and radio channel is reciprocal –Physical layer (PHY) of destination or sender can measure the channel and inform MAC

6. Auto Rate Fallback (ARF) ARF is the first commercial implementation that exploits multi-rate capability Senders use history of previous transmission error rates to select future transmission rates –If no errors, increase the data rate –If errors, decrease the data rate Achieves performance gain over plain 802.11

7. Receiver Based Auto Rate (RBAR) Idea: Why not let the receiver decide which data rate to use? He knows the channel much better than sender because of RTS message –Well, due the CTS message the sender knows the channel also but then sender would have to signal the data rate used to the receiver Physical Layer (PHY) of the receiver analysis the channel conditions from RTS message and inserts orders of data rate to the CTS message –Other nodes hear the CTS also and can adapt their NAV accordingly Sender adapts the data rate Performance gain over IEEE 802.11 and ARF

8. 802.11 with RBAR DIFS = Distributed InterFrame Spacing SIFS = Short InterFrame Spacing RTS = Ready to Send CTS = Clear to Send ACK = Acknowledge RSH = Reservation Sub- Header NAV = Network Allocation Vector

9. Opportunistic Auto Rate Source sends Ready to Send (RTS) message at base date rate Receiver’s PHY measures channel condition from it Receiver sends Clear to Send (CTS) message to sender with orders to use specific data rate Source sends as many packets that can be sent in base rate time window (for example 11 Mbit/s / 2 Mbit/s = 5 packets) Time window is the same all the time regardless of channel conditions –Fairness

10. Fragmentation Long packets can be fragmented according 802.11 What if channel changes during transmission of long packet? –OAR receiver monitors channel conditions during transmission and if channel quality decreases, receiver can inform sender with additional RSH message to decrease data rate What if sender doesn’t have enough packets in queue? –In this case OAR sender can revert to RBAR without throughput loss

11. RBAR and OAR: A Comparison RBAR causes channel contention after each packet It seems like the Node 2 doensn’t get any channel access at all...

12. Simulation results

13. OAR and RBAR have performance nearly independent from the velocity –Slow velocity -> large channel coherence times...

14. Conclusion Nodes with good channel conditions are allowed to transmit multiple packets OAR ensures that all the nodes get equal time share –fair OAR obtains throughput gains up to 50 % if compared to RBAR OAR method is simple

15. References 1.B.Sadegahi, V.Kanodia, A.Sabharwal and E.Knightly: Opportunistic Media Access for Multirate Ad Hoc Networks. MOBICOM’02