OAR: An Opportunistic Auto-Rate Media Access Protocol for Ad Hoc Networks

1. OAR: An Opportunistic Auto-Rate Media Access Protocol for Ad Hoc Networks B. Sadeghi, V. Kanodia, A. Sabharwal, E. Knightly Presented by Shawn Smith

2. Highest energy per bit Lowest energy per bit 802.11b – Transmission rates • Different modulation methods for transmitting data. • Binary/Quadrature Phase Shift Keying • Quadrature Amplitude Modulation • Each packs different quantities of data into the modulation. • The highest speed has most dense data and is most vulnerable to noise. 1 Mbps 2 Mbps 5.5 Mbps 11 Mbps Time

3. Transmission Throughput • Why would a node ever want to slow down? • Longer transmission distance • More robust modulation • Moving node rapidly changes channel conditions • Must adapt to channel conditions based on SNR Image courtesy of G. Holland

4. Background IEEE 802.11 multi-rate • Support of higher transmission rates in better channel conditions • Auto Rate Fallback(ARF) • Use history of previous transmissions to adaptively select future rates • Error free transmissions indicates high channel quality • Lucent ARF implemention reduces rate after 2 lost ACKs, then attempts to speed up after a time interval • Receiver Based Auto Rate(RBAR) • Use RTS/CTS to communicate a transmission rate based on channel quality. Receiver determines rate.

5. B C A Motivation • Consider the situation below • ARF? • RBAR?

6. Timeshare A C B B C A Motivation • What if A and B are both at 56Mbps, and C is often at 2Mbps? • Slowest node gets the most absolute time on channel? Throughput Fairness vs Temporal Fairness

7. Opportunistic Scheduling Goal • Exploit short-time-scale channel quality variations to increase throughput. Issue • Maintaining temporal fairness (time share) of each node. Challenge • Channel info available only upon transmission

8. Coherence Interval OAR Transmission Coherence Interval • The time duration over which a channel is statistically likely to remain stable. • This interval ranges from (122ms) - (5ms) based on node motion at speeds of (1 m/s) - (20 m/s). • OAR was designed such that transmissions do not exceed the coherence interval “most” of the time.

9. Opportunistic Auto Rate (OAR) • Poor connections transmit one data packet per RTS/CTS connection. • Good connections, hence faster rate, transmit multiple data packets. • But maintain temporal fairness between good & bad connections by balancing the time using channel, not the number of packets. • i.e. (1 packet@2Mbps ~= 5 fast packets@11Mbps) • OAR: Higher overall throughput, while maintaining temporal fairness properties of single rate IEEE 802.11

10. OAR Protocol • Rates in IEEE 802.11b: 2, 5.5, and 11 Mbps • Number of packets transmitted by OAR ~

11. ACK DATA CTS RTS destination source OAR Protocol (RBAR Based) Review: Receiver Based AutoRate (RBAR) [Bahl’01] • Receiver controls the sender’s transmission rate • Control messages sent at Base Rate

12. ACK ACK ACK DATA DATA DATA CTS RTS destination source OAR Protocol (Multi-packet) OAR - Opportunistic Auto Rate • Once access granted, it is possible to send multiple packets if the channel is good

13. RBAR R D1 R D2 R D3 Transmitter C A C A C A Receiver OAR Observation II The total time in contention by OAR is approximately equal to total time spent in contention by single-rate IEEE802.11 for an experiment spanning T seconds Observation I Time spent in contention per packet by RBAR is exactly equal to the average time per packet spent in contention for single-rate IEEE802.11 R D1 D2 D3 Transmitter C A A A Receiver Performance Comparison IEEE 802.11 R D1 Transmitter C A Receiver

14. MAC Access Delay Simulation • Back to back packets in OAR decrease the average access delay • Increase variance in time to access channel • Figure • On the left is 2Mbps • On the right is 5.5 Mbps

15. Simulations • Three Simulation experiments • Fully connected networks: all nodes in radio range of each other • Number of Nodes, channel condition, mobility, node location • Asymmetric topology • Random topologies • Implemented OAR and RBAR in ns-2 with extension of Ricean fading model [Punnoose et al ‘00]

16. #1 Fully Connected Setup • Every node can communicate with everyone • Each node’s traffic is at a constant rate and continuously backlogged • Channel quality is varied dynamically

17. #1 Fully Connected Throughput Results • OAR has 42% to 56% gain over RBAR • Increase in gain as number of flows increases • Note that both RBAR and OAR are significantly better than standard 802.11 (230% and 398% respectively) • Variation in line of sight (K), mobility, and location distribution throughput all showed improvements with OAR.

18. #2 Asymmetric TopologySetup Low speed (L) High Speed (H) B A • Asymmetric topology simulated above in 4 different combinations of channel conditions • A and B are simulated at slow (2Mbps) and fast (11Mbps) • Each combination of slow/fast i.e. LL, HL, LH, HH compared between A & B concurrently communicating • Sender of Flow B hears A and knows when to contend for channel, but sender in A has to discover a time slot

19. #2 Asymmetric Topology Results • OAR maintains time shares of IEEE 802.11 • Significant gain over RBAR

20. #3 Random TopologiesSetup • A pair are moved across a communication range • Nodes are uniformly distributed over area similar to test setup #1

21. #3 Random TopologiesResults • Gains are similar as before despite changes • Throughput is 40-50% improved as compared to RBAR despite motion of a node pair.

22. Integration with IEEE 802.11 • Options to hold the channel and send multiple packets • Fragmentation* • A mechanism in IEEE 802.11 to send multiple frames • Each frame/ACK acts as virtual RTS/CTS • Use of more-fragment-flag in Data packets • Contention window set to zero • Packet bursting (802.11e) • Transmit as many frames as you like up to threshold *Method used in study

23. Discussion Issues • Not enough packets to fill a slot • If running at “Good” 11Mbps with 5 packets allowed, but only have 2 packets to send. Then other nodes NAV tables are wrong (silent for 5 instead of 2). • Authors Fix: “More Fragments” indicator in the data packet. Upon hearing, nodes revert to RBAR. • Problem: Hidden terminals would still have incorrect NAV tables, and would remain silent longer than needed. (Unless the data ACK has a “More Fragments ACK.”)

24. Discussion Issues • Channel condition changes during multi-packet transmission. • Channel gets worse • Later packets get corrupted • Channel gets better • Wasted channel capacity waiting for packets to finish • Authors propose adding RSH messages to notify receiver of these updates and adapt the rate. • The RSH is in the header of the data packet, and would allow changing speed mid transmission.

25. Discussion Issues • Ad Hoc Networks considerations • Needed more variety in the network topology. Fully connected isn’t very interesting in Ad Hoc Networks • Data traffic patterns. I.e. short bursts of traffic vs continuous traffic. • No power considerations studied or mentioned

26. Discussion Issues • Increase variance in time to access channel • Real-time traffic (like voice) is impacted. Sometimes there would be more delay before you hear “something.” • Short term fairness gets worse! • Trade throughput for a higher worst case time to access channel