1. 1 Rate Control in Wireless Networks ECE 256

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

3. 3 Recall Role of TDMA

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

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

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

7. 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. 8 802.11b – Transmission rates Optimal rate depends on SINR: i.e., interference and current channel conditions Time 1 Mbps 2 Mbps 5.5 Mbps 11 Mbps Highest energy per bit Lowest energy per bit

9. 9 Some Basics  Friss’ Equation  Shannon’s Equation  Bit-energy-to-noise ratio C = B * log 2 (1 + SINR) E b / N 0 = SINR * (B/R) Leads to BER Varying with time and space How do we choose the rate of modulation

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

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

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

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

16. 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. 17 © 2001. Gavin Holland A Rate-Adaptive MAC Protocol for Multi-Hop Wireless Networks Gavin Holland HRL Labs Nitin Vaidya Paramvir Bahl UIUC Microsoft Research MOBICOM’01 Rome, Italy

18. 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. 19 Modulation schemes have different error characteristics Problem BER SNR (dB) 1 Mbps 8 Mbps But, SINR itself varies With Space and Time But, SINR itself varies With Space and Time

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

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

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

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

24. 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. 25  Cellular networks  Adaptation at the physical layer  Impractical for 802.11 in WLANs Adaptation  At Which Layer ? Why?

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

27. 27 Who should select the data rate? A B

28. 28 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 A B

29. 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. 30 Lucent WaveLAN “Autorate Fallback” (ARF)  Sender decreases rate after  N consecutive ACKS are lost  Sender increases rate after  Y consecutive ACKS are received or  T secs have elapsed since last attempt B A DATA 2 Mbps Effective Range 1 Mbps Effective Range

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

32. 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. 33  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 Receiver-Based Autorate (RBAR) Protocol C BA CTS (1) RTS (2) 2 Mbps 1 Mbps D DATA (1) 2 Mbps 1 Mbps

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

35. 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. 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 Frame Control Duration DA SABSSID Sequence Control Body FCS Reservation Subheader (RSH) WHY

37. 37  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) Performance Analysis BER SNR (dB) 1E-5 2 Mbps Threshold 8 Mbps Threshold

38. 38 Performance Results Single-Hop Network

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

40. 40 CBR source Packet Size = 1460 Varying Node Speed UDP Performance  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 Mean Throughput (Kbps) Mean Node Speed (m/s) RBAR ARF WHY?

41. 41 Varying Node Speed TCP Performance  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 FTP source Packet size = 1460 Mean Throughput (Kbps) Mean Node Speed (m/s) RBAR ARF

42. 42 No Mobility UDP Performance  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 Distance (m) CBR source Packet size = 1460 RBAR ARF Mean Throughput (Kbps) WHY?

43. 43 No Mobility UDP Performance  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 Distance (m) CBR source Packet size = 1460 RBAR-P Mean Throughput (Kbps) Why useful ?

44. 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. 45 Questions?

46. 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. 47 Motivation  Consider the situation below  ARF?  RBAR? A BC

48. 48 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? A BC A B C Timeshare Throughput Fairness vs Temporal Fairness

49. 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. 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

51. 51 OAR Idea  Basic Idea  If bad channel, transmit minimum number of packets  If good channel, transmit as much as possible A B CD A C Data

52. 52 Why is OAR any better ?  802.11 alternates between transmitters A and C  Why is that bad A B CD A C Data Is this diagram correct ?

53. 53 Why is OAR any better ?  Bad channel reduces SINR  increases transmit time  Fewer packets can be delivered A B CD A C Data

54. 54 OAR Protocol Steps  Transmitter estimates current channel  Can use estimation algorithms  Can use RBAR, etc.  If channel better than base rate (2 Mbps)  Transmit proportionally more packets  E.g., if channel can support 11 Mbps, transmit (11/2 ~ 5) pkts  OAR upholds temporal fairness  Each node gets same duration to transmit  Sacrifices throughput fairness  the network gains !!

55. 55 MAC Access Delay Simulation  Back to back packets in OAR decrease the average access delay  Increase variance in time to access channel Why?

56. 56 OAR Protocol  Rates in IEEE 802.11b: 2, 5.5, and 11 Mbps  Number of packets transmitted by OAR ~

57. 57 Simulations  Three Simulation experiments 1.Fully connected networks: all nodes in radio range of each other Number of Nodes, channel condition, mobility, node location 2.Asymmetric topology 3.Random topologies  Implemented OAR and RBAR in ns-2 with extension of Ricean fading model [Punnoose et al ‘00]

58. 58 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

59. 59 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)

60. 60 Asymmetric Topology Results  OAR maintains time shares of IEEE 802.11  Significant gain over RBAR

61. 61 OAR thoughts  OAR does not offer benefits when  OAR may not be suitable for applications like  With TCP how can OAR get affected ?

62. 62 OAR thoughts  OAR does not offer benefits when  Neighboring nodes do not experience diverse channel conditions  Coherence time is shorter than N packets  With TCP can OAR get affected ?  Back-to-back packets can increase TCP performance  However, bottleneck bandwidth can get congested quick  Also, variance of RTT can increase

63. 63 Other Ideas (Briefly)

64. 64 Exploiting Diversity in Rate Adaptation  Yet another idea exploits multiple user diversity  Among many intermediate nodes, who has best channel  Use that node as forwarding node  Forwarding node can change with time Due to channel fluctuations at different time and space WLAN Channel Conditions SNR TIME USERS AP

65. 65 The Protocol Overview SIFS DIFS GRTS ACK 0~2 CTS 1 CTS k CTS 2 … sender user 1 user 2 user k DATA 0DATA 1 DATA 2 SF MAD using Packet Concatenation (PAC) Since at least one intermediate node is likely to have good channel condition, transmitter can transmit at a high data rate or concatenate Multiple packets Choosing subset of neighbor-group is important Coherence time of channel must be greater than packet chain Group needs to really have independent channel gain Correlated channel gains can lead to performance hit.

66. 66 Summary  Rate control can be useful  When adapted to channel fluctuations (RBAR)  When opportunistically selecting transmitters (OAR)  When utilizing node diversity  Benefits maximal when  Channel conditions vary widely in time and space  Correlation in fluctuation can offset benefits  OAR and Diversity-based MAC may show negligible gain  Several more research possibilities with rate control

67. 67 Questions ?

68. 68 What lies ahead ?  Routing based on rate-control  Choosing routes that contain high-rate links  ETX metric proposed from MIT accomodates link character  Opportunistic routing from MIT again – takes neighbor diversity into account (best paper Sigcomm 2005)  Fertile area for a project …  Dual of rate-control is power control  One might be better than the other  Decision often depends on the scenario  open problem  Directional antennas for DD link for data/ack  Rate control can be introduced  Not been studied yet … many many more

69. 69 Infrequent Traffic UDP Performance  Similar results for shorter burst intervals  Similar results for TCP (see tech report) Delivery Ratio (MAC Rx/Tx) Mean Burst Length (ms) Pareto source Mean burst interval = 1 sec Packet size = 1460

70. 70 Any other tradeoff ? Can anyone think of yet another IMPORTANT tradeoff Hint: Related to the MAC Layer

71. 71 Total interference CE A D B Rate = 20 Rate = 10 More nodes free to transmit packets (A,B,E) Interference incident at receiver (D) increases More nodes free to transmit packets (A,B,E) Interference incident at receiver (D) increases

72. 72 Performance Results Multi-Hop Network

73. 73 Multi-Hop Network UDP Performance  Similar results for TCP 20 nodes CBR source Packet size = 1460 Mean Throughput (Kbps) Mean Node Speed (m/s)

74. 74 A B C D Flow 1Flow 2 Multi-Hop Network Sensitivity to RSH Loss  Aggregate throughput is unaffected by RSH loss  High loss probability results in only slight change in fairness Mean Throughput (Kbps) RSH Loss Probability Mean Throughput (Kbps) ARF RSH

75. 75 A B C D Flow 1Flow 2 Multi-Hop Network Sensitivity to RSH Loss Mean Throughput (Kbps) RSH Loss Probability Mean Throughput (Kbps) ARF RSH  Similar results  Slightly more unfairness (vs. ARF) for no loss  (overall fairness problem due to MAC backoff by node A)

76. 76 802.11b – Transmission rates  Different modulation methods for transmitting data.  Binary/Quadrature Phase Shift Keying  Quadrature Amplitude Modulation  Each packs different number of data in modulation.  The highest speed has most dense data and is most vulnerable to noise. Time 1 Mbps 2 Mbps 5.5 Mbps 11 Mbps Highest energy per bit Lowest energy per bit