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Effects of Multi-Rate in Ad Hoc Wireless Networks

Effects of Multi-Rate in Ad Hoc Wireless Networks. Baruch Awerbuch, David Holmer, Herbert Rubens Center for Networking and Distributed Systems Computer Science Department Johns Hopkins University Technical Report 2003. Contents. Introduction Background IEEE 802.11

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Effects of Multi-Rate in Ad Hoc Wireless Networks

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  1. Effects of Multi-Rate in Ad Hoc Wireless Networks Baruch Awerbuch, David Holmer, Herbert Rubens Center for Networking and Distributed SystemsComputer Science DepartmentJohns Hopkins University Technical Report 2003

  2. Contents • Introduction • Background • IEEE 802.11 • Receiver Based Auto Rate (RBAR) • Opportunistic Auto Rate • Performance evaluation • Conclusions

  3. Introduction • Wireless Trend • Enable devices to operate using many different transmission rates. • Multi-rate capability • Transmission can take place • At a number of rates • According to channel condition • IEEE 802.11 supports it at the physical layer • MAC mechanisms are required to exploit it • ARF (Auto Rate Fallback) • RBAR (Receiver Based Auto Rate) • OAR (Opportunistic Auto Rate)

  4. Background (1/3) • IEEE 802.11 media access • RTS/CTS mechanism • Network Allocation Vector (NAV) SIFS SIFS SIFS DIFS RTS DATA source CTS ACK destination NAV (RTS) others NAV (CTS) Channel access with backoff Delayed medium access

  5. RSH+DATA RTS CTS ACK Background (2/3) • Receiver Based Auto Rate (RBAR) • Receiver selects transmission rate • Use latest channel condition • Additional reservation sub header (RSH) source destination Choose a rate based on heuristic Analysis of RTS & SINR and determine data rate

  6. Background (3/3) SIFS SIFS SIFS DIFS DATA RTS RSH source CTS ACK destination NAV (RTS) others NAV (CTS) NAV (RSH) Delayed medium access

  7. Opportunistic Auto Rate (1/4) • Key idea • Based on RBAR • Opportunistically exploit high quality channel when they transmit multiple packets • Must limit the extent of holding the channel • Use fragmentation mechanism of IEEE802.11

  8. Opportunistic Auto Rate (2/4) • Fragmentation in IEEE 802.11 • Fragmentation fields • more-fragments, fragment number, duration time SIFS SIFS SIFS SIFS SIFS DIFS DATA/FRAG2 RTS DATA/FRAG1 source ACK1 ACK2 CTS destination NAV (RTS) NAV (DATA) others NAV (CTS) NAV (ACK) Delayed medium access

  9. Opportunistic Auto Rate (3/4) • Issues • When there are no data packets available in the interface queue • Reset the more-fragments • Reverts back to the default RBAR protocol • When channel condition significantly change during multi-packet-transmission • Continually monitor the channel quality • Use additional RSH message to notify the receiver and adapt the rate

  10. Opportunistic Auto Rate (4/4) • Example • Node 1 has a good channel (11Mbps) • Node 2 has a poor channel (5.5Mbps) 11Mbps 11Mbps 11Mbps CTS ACK0 NODE1(PKT1) ACK1 NODE1(PKT2) RTS NODE1(PKT0) OAR 11Mbps 5.5 Mbps RTS NODE1 ACK RTS CTS NODE2 ACK CTS RBAR Random backoff time

  11. Multi Rate Problem • Distance is the primary factor • Multi-rate devices must have protocols that select the appropriate rate for a given situation. High transmission rate Effective transmission range High Speed Long Range

  12. Introduction • Infrastructure based networks • Single rate nodes have the ability to select the best access point based on the received signal strength. • Multi-rate only need to add selecting the actual rate used to communicate. • physical geometry • react to the existing channel • reliably

  13. Introduction • Ad hoc multi-hop wireless networks • the routing protocol must select from the set of available links to form the path between the source and the destination • Long distance = Few hops, but low speed. • Short links = High rates, but more hops.

  14. Multi-Rate Model • Model is based on the 802.11b standard • NS2 simulations • Compare with RBAR and OAR • Lucent ORiNOCO PC Card

  15. Assumption

  16. Minimum Hop Path • Throughput Loss • The selection of minimum hop paths typically results in paths where the links operate at low rates. • Reliability Loss • Small broadcast packets to establish/maintain routing, device will have high bit error rate

  17. Throughput Phenomena • MAC • Only a single transmission can occur at a time within range of the intended receiver.

  18. Simulation • 1472 byte UDP Packets flood across a single link.

  19. Hops vs. Throughput Trade-off • Highest transmission speed more hops

  20. Even though high link rate paths must traverse more links to reach the same distance, they still provide more throughput.

  21. Temporal Fairness • One sender send at 1M and other send at 11M

  22. Conclusion • Multi Rate could enable high network throughput. • The Multi-rate protocol need to consider more phenomena in routing deccisions. • Hop path • Throughput • Quantitative • Fairness

  23. Thank You

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