Rate Control in Wireless Networks

1. Rate Control in Wireless Networks ECE 256

2. Recall 802.11 • RTS/CTS + Large CS Zone • Alleviates hidden terminals, but trades off spatial reuse RTS E F CTS A B C D

3. Recall Role of TDMA

4. Silenced Node No Ok Simultaneous Communication Simultaneous Communication Recall Beamforming Omni Communication Directional Communication e f f c a b c a b d d

5. Also Multi-Channel • Current networks utilize non-overlapping channels • Channels 1, 6, and 11 • Partially overlapping channels can also be used

6. Today Benefits from exploiting channel conditions • Rate adaptation • Pack more transmissions in same time

7. What is Data Rate ? Number of bits that you transmit per unit time under a fixed energy budget Too many bits/s: Each bit has little energy -> Hi BER Too few bits/s: Less BER but lower throughput

8. Highest energy per bit Lowest energy per bit 802.11b – Transmission rates Optimal rate depends on SINR: i.e., interference and current channel conditions 1 Mbps 2 Mbps 5.5 Mbps 11 Mbps Time

9. Varying with time and space How do we choose the rate of modulation Some Basics • Friss’ Equation • Shannon’s Equation • Bit-energy-to-noise ratio C = B * log2(1 + SINR) Eb / N0 = SINR * (B/R) Leads to BER

10. Static Rates SINR time # Estimate a value of SINR # Then choose a corresponding rate that would transmit packets correctly (i.e., E b / N 0 > thresh) most of the times # Failure in some cases of fading  live with it

11. Adaptive Rate-Control SINR time # Observe the current value of SINR # Believe that current value is indicator of near-future value # Choose corresponding rate of modulation # Observe next value # Control rate if channel conditions have changed

12. Is there a tradeoff ? Rate = 10 B C E A D

13. Is there a tradeoff ? Rate = 10 B C E A D Rate = 20 What about length of routes due to smaller range ?

14. Any other tradeoff ? Will carrier sense range vary with rate

15. Total interference Rate = 10 B C E A D Rate = 20 Carrier sensing estimates energy in the channel. Does not vary with transmission rate

16. Bigger Picture • Rate control has variety of implications • Any single MAC protocol solves part of the puzzle • Important to understand e2e implications • Does routing protocols get affected? • Does TCP get affected? • … • Good to make a start at the MAC layer • RBAR • OAR • Opportunistic Rate Control • …

17. A Rate-Adaptive MAC Protocol forMulti-Hop Wireless Networks Gavin Holland HRL Labs Nitin Vaidya Paramvir Bahl UIUC Microsoft Research MOBICOM’01 Rome, Italy © 2001. Gavin Holland

18. Background • Current WLAN hardware supports multiple data rates • 802.11b – 1 to 11 Mbps • 802.11a – 6 to 54 Mbps • Data rate determined by the modulation scheme

19. Problem Modulation schemes have different error characteristics 8 Mbps 1 Mbps BER But, SINR itself varies With Space and Time SNR (dB)

20. Impact Large-scale variation with distance (Path loss) 8 Mbps Path Loss Mean Throughput (Kbps) SNR (dB) 1 Mbps Distance (m) Distance (m)

21. Impact Small-scale variation with time (Fading) Rayleigh Fading SNR (dB) 2.4 GHz 2 m/s LOS Time (ms)

22. QuestionWhich modulation scheme to choose? SNR (dB) SNR (dB) 2.4 GHz 2 m/s LOS Distance (m) Time (ms)

23. Answer  Rate Adaptation • Dynamically choose the best modulation scheme for the channel conditions Desired Result Mean Throughput (Kbps) Distance (m)

24. Design Issues • How frequently must rate adaptation occur? • Signal can vary rapidly depending on: • carrier frequency • node speed • interference • etc. • For conventional hardware at pedestrian speeds, rate adaptation is feasible on a per-packet basis Coherence time of channel higher than transmission time

25. Adaptation  At Which Layer ? • Cellular networks • Adaptation at the physical layer • Impractical for 802.11 in WLANs Why?

26. RTS: 10 C 8 A B CTS: 8 D 10 Adaptation  At Which Layer ? • Cellular networks • Adaptation at the physical layer • Impractical for 802.11 in WLANs • For WLANs, rate adaptation best handled at MAC Why? RTS/CTS requires that the rate be known in advance Receiver Sender

27. A B Who should select the data rate?

28. A B Who should select the data rate? • Collision is at the receiver • Channel conditions are only known at the receiver • SS, interference, noise, BER, etc. • The receiver is best positioned to select data rate

29. Previous Work • PRNet • Periodic broadcasts of link quality tables • Pursley and Wilkins • RTS/CTS feedback for power adaptation • ACK/NACK feedback for rate adaptation • Lucent WaveLAN “Autorate Fallback” (ARF) • Uses lost ACKs as link quality indicator

30. DATA 2 Mbps A B 2 Mbps Effective Range 1 Mbps Effective Range Lucent WaveLAN “Autorate Fallback” (ARF) • Sender decreases rate after • N consecutive ACKS are lost • Sender increases rate after • Y consecutive ACKS are receivedor • T secs have elapsed since last attempt

31. Performance of ARF • Slow to adapt to channel conditions • Choice of N, Y,T may not be best for all situations SNR (dB) Time (s) Dropped Packets Rate (Mbps) Time (s) Failed to Increase Rate After Fade Attempted to Increase Rate During Fade

32. RBAR Approach • Move the rate adaptation mechanism to the receiver • Better channel quality information = better rate selection • Utilize the RTS/CTS exchange to: • Provide the receiver with a signal to sample (RTS) • Carry feedback (data rate) to the sender (CTS)

33. 1 Mbps C 2 Mbps RTS (2) 1 Mbps CTS (1) A B DATA (1) 1 Mbps D 2 Mbps Receiver-Based Autorate (RBAR) Protocol • RTS carries sender’s estimate of best rate • CTS carries receiver’s selection of the best rate • Nodes that hear RTS/CTS calculate reservation • If rates differ, special subheader in DATA packet updates nodes that overheard RTS

34. Performance of RBAR SNR (dB) Time (s) Rate (Mbps) Time (s) RBAR Rate (Mbps) Time (s) ARF

35. Question to the class • There are two types of fading • Short term fading • Long term fading • Under which fading is RBAR better than ARF ? • Under which fading is RBAR comparable to ARF ? • Think of some case when RBAR may be worse than ARF

36. Implementation into 802.11 • Encode data rate and packet length in duration field of frames • Rate can be changed by receiver • Length can be used to select rate • Reservations are calculated using encoded rate and length • New DATA frame type with Reservation Subheader (RSH) • Reservation fields protected by additional frame check sequence • RSH is sent at same rate as RTS/CTS • New frame is only needed when receiver suggests rate change Sequence Control Frame Control Duration DA FCS SA BSSID FCS Body Reservation Subheader (RSH) WHY

37. Performance Analysis • Ns-2 with mobile ad hoc networking extensions • Rayleigh fading • Scenarios: single-hop, multi-hop • Protocols: RBAR and ARF • RBAR • Channel quality prediction: • SNR sample of RTS • Rate selection: • Threshold-based • Sender estimated rate: • Static (1 Mbps) BER 1E-5 SNR (dB) 8 Mbps Threshold 2 Mbps Threshold

38. Performance ResultsSingle-Hop Network

39. Single-Hop Scenario Mean Throughput (Kbps) Distance (m) A B

40. Varying Node SpeedUDP Performance RBAR • RBAR performs best • Declining improvement with increase in speed • Adaptation schemes over fixed • RBAR over ARF • Some higher fixed rates perform worse than lower fixed rates ARF Mean Throughput (Kbps) CBR source Packet Size = 1460 Mean Node Speed (m/s) WHY?

41. Varying Node SpeedTCP Performance RBAR • RBAR again performs best • Overall lower throughput and sharper decline than with UDP • Caused by TCP’s sensitivity to packet loss • More higher fixed rates perform worse than lower fixed rates ARF Mean Throughput (Kbps) FTP source Packet size = 1460 Mean Node Speed (m/s)

42. No MobilityUDP Performance CBR source Packet size = 1460 • RSH overhead seen at high data rates • Can be reduced using some initial rate estimation algorithm • Limitations of simple threshold-based rate selection seen • Generally, still better than ARF RBAR ARF Mean Throughput (Kbps) Mean Throughput (Kbps) Distance (m) Distance (m) WHY?

43. No MobilityUDP Performance CBR source Packet size = 1460 RBAR-P • RBAR-P – RBAR using a simple initial rate estimation algorithm • Previous rate used as estimated rate in RTS • Better high-rate performance • Other initial rate estimation and rate selection algorithms are a topic of future work Mean Throughput (Kbps) Distance (m) Why useful ?

44. RBAR Summary • Modulation schemes have different error characteristics • Significant performance improvement may be achieved by MAC-level adaptive modulation • Receiver-based schemes may perform best • Proposed Receiver-Based Auto-Rate (RBAR) protocol • Implementation into 802.11 • Future work • RBAR without use of RTS/CTS • RBAR based on the size of packets • Routing protocols for networks with variable rate links

45. Questions?

46. OAR: An Opportunistic Auto-Rate Media Access Protocol for Ad Hoc Networks B. Sadeghi, V. Kanodia, A. Sabharwal, E. Knightly Rice University Slides adapted from Shawn Smith

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

48. 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

49. 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

50. Opportunistic Auto-Rate (OAR) • In multihop networks, there is intrinsic diversity • Exploiting this diversity can offer benefits • Transmit more when channel quality great • Else, free the channel quickly • RBAR does not exploit this diversity • It optimizes per-link throughput