1. 1 MAC Protocols for Wireless Networks: Interaction between Physical Layer and MAC Nitin H. Vaidya University of Illinois at Urbana-Champaign www.crhc.uiuc.edu/wireless © 2004 Vaidya

2. 2 Joint work with Jason Fuemmeler Xue Yang Romit RoyChoudhury Jungmin So Pradeep Kyasanur Venugopal Veeravalli Funded in part by National Science Foundation Motorola Center for Communications NSF graduate fellowship Vodafone graduate fellowship Motorola graduate fellowship

3. 3 Our Research  MAC/Routing/Transport protocols for wireless  Distributed algorithms (leader election, clock sync,...)  Misbehavior in wireless networks

4. 4 Wireless Ad Hoc Networks  Formed by wireless hosts that may be mobile  Without necessarily using infrastructure  Routes between nodes may potentially contain multiple hops

5. 5 Ad Hoc Networks E A BC D X Z Ad hoc connectivity

6. 6 Hybrid Environments  Infrastructure + Ad hoc connectivity E A BC D BS1BS2 X Z infrastructure Ad hoc connectivity

7. 7 Wireless Capacity  Wireless capacity limited  In dense environments, performance suffers  How to improve performance ?

8. 8 Improving Wireless Capacity  Exploit physical resources  Exploit diversity  Examples...

9. 9 Add Spectrum  More bandwidth  Example: Multiple channels in IEEE 802.11

10. 10 Improve Spatial Reuse Power/Rate Control ABCD A BCD

11. 11 Exploit Infrastructure  Infrastructure provides a tunnel to forward packets E A BC D BS1BS2 X Z infrastructure Ad hoc connectivity

12. 12 Exploit Antennas  Diversity antenna  Steered beam directional antenna

13. 13 Path Diversity  Multiple paths to a destination  Multiple next-hops to a destination

14. 14 Exploiting Diversity Exploiting physical layer requires suitable protocols  Routing  Medium access control (MAC) Link Network Transport Physical Layer Upper layers

15. 15 Medium Access Control (MAC)  MAC protocols coordinate wireless channel access  May be centralized or distributed  Distributed protocols suit ad hoc & hybrid networks

16. 16 MAC Protocols  Need to design MAC protocols to exploit physical layer capabilities  Proof by example …

17. 17 Outline  CSMA Protocols: Warning: Work-in-progress  Directional antennas & Multiple channels: Brief discussion, time permitting

18. 18 Carrier Sense Multiple Access (CSMA)  Listen-before-you-talk  A host may transmit only if the channel is idle

19. 19 Carrier Sense Multiple Access (CSMA) Implementation using Carrier Sense (CS) threshold  If received power < CS threshold  Channel idle  Else channel busy

20. 20 Carrier Sense Multiple Access (CSMA)  D perceives idle channel although A is transmitting A B C D distance power D’s CS Threshold

21. 21 Carrier Sense Multiple Access (CSMA)  D perceives busy channel when A transmits A B C D distance power D’s CS Threshold

22. 22 Transmission Reliability  Reliability depends on SIR = S / I (ignoring noise)  SIR requirement depends on modulation scheme (rate) A B C D distance power S I

23. 23 Warning  SIR and SINR used in this talk interchangeably  SINR = S / (I + N)S = signal I = interference N = noise  SIR = S / I (when Noise ignored)

24. 24 Packet Transmission The transmitter must choose  Transmit rate  Transmit power  CS Threshold Prior work typically focuses on choosing 1 parameter, keeping others fixed  Jointly adapting 2-of-3 potentially better strategy

25. 25 Joint Adaptation 2-of-3  Which 2-of-3 ?  Practical considerations might eventually prove one combination superior  Our on-going work  Transmit Rate & CS threshold  Transmit Power & CS threshold

26. 26 Joint Control: Transmit Power and CS Threshold

27. 27 Assumed Protocol Requirements  Distributed decision-making  Choose transmit power to meet desired SINR (despite future transmitters)  Not destroy on-going transmissions E to F A to B C to D

28. 28 Observation 1 A B distance power  Higher transmit power increases received signal power S at the intended receiver Low transmit power S

29. 29 Observation 1 A B distance power High transmit power  Higher transmit power increases received signal power S at the intended receiver S

30. 30 Observation 2 A B J distance power  For a fixed CS threshold, high transmit power  closest interferer farther  Decreases interference I Low transmit power

31. 31 Observation 2 A B J distance power  For a fixed CS threshold, high transmit power  closest interferer farther  Decreases interference I High transmit power

32. 32 Observations 1 & 2  Higher transmit power increases signal S, decreases interference I at the intended receiver  SIR = S/I improves Double dipping!

33. 33 Towards a Protocol  Node A transmits a packet to B  Interference sources: 1.Transmitters already active before A begins talking 2.Transmitters that transmit after A begins talking AB C D distance y x z E F

34. 34 Interference from “Already Active” Sources  E is transmitting before A starts transmitting  Such interference by design less than CS threshold AB C D E F

35. 35 Interference from Future Sources  C begins transmitting after A starts its transmission  C must have sensed idle channel despite A’s transmission AB C D E F

36. 36 Transmit Power & CS Threshold Selection [Fuemmeler04]  How to ensure that a transmission will be completed reliably ?  Analysis suggest that transmit power and CS threshold should be adapted together

37. 37 Notation  g(x) = gain at distance x from the transmitter Received power = transmit power * gain Distance x Gain g(x)

38. 38 Notation  P t = transmit power  P cs = Carrier-sense threshold   SINR threshold for reliability   = noise  k = estimated number of interferers

39. 39 Power Control & CS Threshold Two Constraints  = design constant

40. 40 Sensitivity Constraint  Loud transmitters should be sensitive  Whisperers may be insensitive  Limits interference to ongoing transmissions

41. 41 Interference Constraint  CS threshold = interference margin per interferer  Initial interference < CS Threshold  it can be modeled as an equivalent future interferer (alternatively it can be added to η)

42. 42 Interference Constraint  Chosen transmit power also serves as an indication of tolerable interference to the corresponding transmission  Chosen transmit power serves dual roles of limiting interference to others, and limiting interference to self

43. 43 Performance Evaluation Static Protocol

44. 44 Static Protocol  Pick a fixed value of k  Solve the two constraints to obtain transmit power and CS threshold

45. 45 Ring Topology  N nodes spaced equally around a ring  Each node transmits to its immediate counterclockwise neighbor  Symmetric topology

46. 46 k = number of interfererers Aggregate throughput (kbps) N = 128 N = 8 Ring Topology (Static Protocol: k fixed) Single flow throughput = 767 kbps SINR > 10 dB

47. 47 Random Topology x = Tx o = Rx

48. 48 Random Topology (Fixed Receive Power) CS Threshold (Watts) Aggregate throughput (kbps) x = Topology 1 o = Topology 2

49. 49 Random Topology (Fixed Transmit Power) CS Threshold (Watts) Aggregate throughput (kbps) x = Topology 1 o = Topology 2

50. 50 Static Protocol (fixed k) Aggregate throughput (kbps) x = Topology 1 o = Topology 2 k

51. 51 Failure Rate (Static Protocol: fixed k) Packet failure rate x = Topology 1 o = Topology 2 k

52. 52 Performance Evaluation Dynamic Protocol

53. 53 Dynamic Protocol  Estimate k = number of interferers  Solve the two constraints to determine transmit power and CS threshold

54. 54 Parameter k  k : the tunable knob  Co-location approximation introduces error: Error handled by increasing dynamic range of k  Intuitively, k is a measure of total interference as a multiple of that from a “worst-case interferer”

55. 55 Estimation of Optimal k Preliminary protocol : Details omitted here Obtain an initial estimate on “reasonable” non-zero k starting at k = 0:  Increase k on packet failure  Decrease k on success Gradient-descent search:  Motivated by the throughput and failure rate versus k curves

56. 56 Throughput Comparison (kbps) Random Topologies ProtocolTopology 1Topology 2 Fixed receive power30713710 Fixed transmit power 36273944 Static k47805218 Dynamic k49555365

57. 57 Dynamic Protocol  Allows each node to use different k, transmit power and CS threshold  Can improve performance compared to globally constant parameters

58. 58 Ongoing Work  Current dynamic protocol slow to adapt, may not work well in presence of rapid mobility  Small scale effects (fading) not accounted for  May potentially be accounted for by adding interference margin  Need to co-exist with other dynamic mechanisms such as backoff in 802.11  Fairness issues

59. 59 Related Work  Power-Controlled Multiple Access (PCMA): Monks et al.  Receiver-based mechanism using busy-tone (BT)  Data power & BT power chosen independently  We use data for both purpose: only 1 power to choose  Zhu et al. (Intel): Globally constant CS threshold  Muqattash et al.: Need separate control channel, or a priori coordination  Congnitive radios: Sahai et al. (Allerton’04)

60. 60 Summary  Joint adaptation of CS threshold & transmit power beneficial  Need further work to make protocol more robust

61. 61 Impact of Protocols Overheads on CS Threshold Skip?

62. 62 Goal  To identify how protocol overheads affect optimal carrier-sense threshold  Assume fixed transmit power

63. 63 Observation A B power  Higher CS Threshold brings closest interferer closer  Higher interferences at B  Lower SIR  Lower transmit rate IJ CS threshold

64. 64 Small Carrier Sense Threshold (Sensitive Nodes)  Sensitive transmitters must be far away  With fixed transmit power, interference reduces  Transmit rate can be increased

65. 65 Small Carrier Sense Threshold (Sensitive Nodes) A B X Y P Q  Smaller CS threshold  Fewer simultaneous transmissions, but at higher rate

66. 66 Large Carrier Sense Threshold (Insensitive Nodes) A B  Higher CS threshold  More simultaneous transmissions, but at lower rate M N R S

67. 67 Optimal Carrier Sense Threshold  CS Threshold = 0  Only one transmission at a time  CS Threshold = ∞  All hosts transmit together Optimal somewhere in between Protocol overhead affect the optimal CS threshold …

68. 68 Impact of Protocol Overheads Two components  Rate-independent overhead: Propagation delay, backoff slot in 802.11  Rate-dependent overhead: Collision cost

69. 69 Impact of Protocol Overhead Rate-independent Rate-dependent Single rate R transmission at a time Two rate R/2 transmissions at a time

70. 70 Impact of Protocol Overhead  Can be beneficial to have more simultaneous transmission at smaller transmit rate  One approach: Divide bandwidth into multiple channels  Previously investigated by others  Our approach: Increase “density” of transmitters

71. 71 Impact of Protocol Overheads [Yang05]  Increase density of transmitters, while decreasing transmission rate  Reduces rate-independent overhead  Conclusion: Optimal CS threshold larger when rate-independent overhead is considered

72. 72 Outline  CSMA Protocols  Directional Antennas & Multiple Channels

73. 73 Antenna Capabilities  Fixed beam antennas  Omnidirectional antennas, directional antennas with a fixed beam pattern  Movable beam antennas  Switched, steered, reconfigurable, adaptive, smart …

74. 74 Antenna Capabilities  Protocols designed for fixed beam antennas inadequate with movable beam antennas  Movable directional beams introduce additional benefits and challenges  Deafness  Neighbor discovery  Utilizing long links

75. 75 Deafness [RoyChoudhury04]  S transmitting to D  A cannot carrier-sense S  A sends packet to S, but gets no response D A S

76. 76 Deafness Two difficult-to-distinguish possibilities  Collision at S  S is looking in a different direction D A S

77. 77 Deafness Two difficult-to-distinguish possibilities  Collision at S  S is looking in a different direction Need different responses to the two events Similarities to the problem of distinguishing between packet losses due to congestion and corruption in context of TCP-over-wireless.

78. 78 Deafness  Deafness arises due to directionality & the ability to change beam directions  Need to adapt MAC protocol to handle deafness

79. 79 Multi-Channel Environments Skip

80. 80 Multi-Channel Environments  Multiple Channels available in IEEE 802.11  3 channels in 802.11b  12 channels in 802.11a  Utilizing multiple channels may improve throughput 1 defer 1 2 Single channelMultiple Channels

81. 81 Issues  Using k channels does not necessarily translate into proportional throughput improvement  Hosts limited by number of transceivers  Nodes on different channels cannot talk to each other 1 2

82. 82 Multi-Channel Hidden Terminals Consider the following naïve protocol  Static channel assignment  Communication takes place on receiver’s channel  Sender switches to receiver’s channel to transmit

83. 83 Multi-Channel Hidden Terminals A B C Channel 1 Channel 2 A transmit on channel 1 C is on channel 2

84. 84 Multi-Channel Hidden Terminals A B C C switches to channel 1 and transmits Channel 1 Channel 2 Collision occurs at B

85. 85 Multi-Channel Environments [So04,So03,Kyasanur04] Protocols need to be designed to maximize channel utilization despite  Transceiver limitations  Multi-channel hidden terminals  Switching delays

86. 86 Conclusion

87. 87 Conclusion  Newer chipsets & radios are allowing more flexibility  Timescale over which parameters can be changed shrinking Ideal scenario: Per-packet switching  Makes it possible to exert more control on the physical layer

88. 88 Conclusion  Necessary to exploit physical layer capabilities to maximize performance  Need suitable MAC and Routing protocols  Many physical layer characteristics interesting:  Transmit power, transmit rate, carrier-sense threshold  Antenna  Channel diversity  Switching delays  Multi-user diversity

89. 89 Conclusion  State-of-the-Art: Protocol mechanisms designed for particular physical layer capability  Holy grail: Adaptive protocols that learn physical layer characteristics and adapt to them

90. 90 Thanks! nhv@uiuc.edu www.crhc.uiuc.edu/wireless

91. 91 Thanks! nhv@uiuc.edu www.crhc.uiuc.edu/wireless

92. 92 Additional Slides

93. 93 Physical Layer Parameters Affecting MAC  Transmit rate  Transmit power  Antenna  Single or multi-channel  Single or multi-band  Switching delays  …

94. 94 Analysis (details omitted) [Fuemmeler04] Approximations to facilitate analysis:  Noise known  Sender & receiver effectively co-located  Sender senses same power level as the receiver  Approximation error handled by adaptation mechanism in the protocol

95. 95 Interference  From already on-going transmissions  From future interferers  Interference from on-going transmissions  can be modeled explicitly  or as an equivalent future interferer We choose the latter option, which fortuitously works out (but former can be used as well)

96. 96 Illinois Wireless Wind Tunnel  New project : To develop an environment for repeatable experiments on wireless networks  To be built in an anechoic chamber  “Scaling” of wireless network and environment to account for differences in mobility and obstacle size in the real network and experimental network

97. 97 Other Research  Energy efficient protocols for wireless networks  TCP over wireless networks  Handling misbehavior in wireless networks

98. 98 Ring Topology (Transmit Power fixed) CS Threshold (Watts) Aggregate throughput (kbps) N = 128 N = 8