1 Medium Access Control for Wireless Networks using Directional Antennas ECE 256

2 Applications Several Challenges, Protocols Internet

3 Omnidirectional Antennas

4 CTS = Clear To Send RTS = Request To Send IEEE with Omni Antenna D Y S M K RTS CTS X

5 IEEE with Omni Antenna D Y S X M K silenced Data ACK

6 IEEE with Omni Antenna D S X M K silenced Y Data ACK D silenced E A C F B G `` Interference management `` A crucial challenge for dense multihop networks

7 Managing Interference  Several approaches  Dividing network into different channels  Power control  Rate Control … Recent Approach … Exploiting antenna capabilities to improve the performance of wireless multihop networks

8 From Omni Antennas … D S X M K silenced Y D E A C F B G

9 To Beamforming Antennas D S X M K Y D E A C F B G

10 To Beamforming Antennas D S X M K Y D E A C F B G

11 Outline / Contribution  Antenna Systems  A closer look  New challenges with beamforming antennas  Design of MAC and Routing protocols  MMAC, ToneDMAC, CaDMAC  DDSR, CaRP  Cross-Layer protocols – Anycasting  Improved understanding of theoretical capacity  Experiment with prototype testbed

12 Antenna Systems  Signal Processing and Antenna Design research  Several existing antenna systems Switched Beam Antennas Steerable Antennas Reconfigurable Antennas, etc.  Many becoming commercially available For example …

13 Electronically Steerable Antenna [ATR Japan]  Higher frequency, Smaller size, Lower cost  Capable of Omnidirectional mode and Directional mode

14 Beam Steering  Steering  Mechanical steering (rotating dish antennas, cellular, etc.)  Electronic steering (wireless cards, vehicle mounted, etc.)

15 For Mesh Networks  On poletop or vehicles  Antennas bigger  No power constraint

16 Antenna Abstraction  3 Possible antenna modes  Omnidirectional mode  Single Beam mode  Multi-Beam mode  Higher Layer protocols select  Antenna Mode  Direction of Beam

17 Antenna Beam  Energy radiated toward desired direction A Pictorial Model A Main Lobe (High gain) Sidelobes (low gain)

18 Directional Communication  Directional Gain (G d ) ≥ Omni Gain (G o )  Friss’ Equation AB C DO OO DD

19 Directional Reception  Directional reception = Spatial filtering  Interference along straight line joining interferer and receiver AB C D Signal Interference No Collision at A AB C D Signal Interference Collision at A

20 Will attaching such antennas at the radio layer yield most of the benefits ? Or Is there need for higher layer protocol support ?

21 Let’s study a simple baseline MAC protocol (a directional version of ) Call this protocol DMAC and investigate its behavior through simulation

22 Let’s design a “Directional MAC”  Let’s assume 2 antenna modes  Omni (beamwidth = 360) and directional  Switching between modes require negligible latency  Several design choices appear  Mode of channel access (omni or directional)  Backing off mechanism  Mode of RTS/CTS transmissions  Mode of Data/ACK transmissions  Mode of “Idle” state  Omni or directional NAVs  Several others … [Ko00,Ramanathan01,Nasipuri00, Balanis00, Takai02,Bandyopadhay01, Bao02, Sanchez01, Ephremides98, Elbatt02, Sivakumar03, Gossain03, Tang05, Vasudevan05, Korakis03, etc.]

23 Design Choices – Carrier Sensing  How should a node carrier sense ?  Omnidirectionally or directionally ?

24 Design Choices – Carrier Sensing  How should a node carrier sense ?  Omnidirectionally or directionally ?  Omni carrier sensing inhibits spatial reuse  unsuitable ab c d ab c d Channel busy Channel idle

25 Design Choices – Backing Off  While backing off, should a node be  Omnidirectional or directional ?

26 Design Choices – Backing Off  While backing off, should a node be  Omnidirectional or directional ?  Omni back off prevents paralle communication  unsuitable ab c d ab c d Freeze Backoff Continue backoff

27 Design Choices – Virtual Carrier Sensing  Inhibit transmissions only in unsafe directions  Directional antennas extend NAV to Directional NAV ab c d DNAV inhibits transmission DNAV allows transmission

28 Design Choices – Exchanging RTS/CTS  Omnidirectional RTS/CTS  Directional RTS/CTS  Multiple directional RTS/CTS (sweep)

29 Design Choices – Exchanging RTS/CTS  Omnidirectional RTS/CTS  Limits spatial reuse – cannot always transmit omni on account of directional NAV  Limits the distance between communicable neighbors  Directional RTS/CTS  Improves reuse but requires direction of intended receiver  Suffers from a problem called “deafness” (explained later)  Multiple directional RTS/CTS (sweep) [KorakisMobihoc]  No deafness, but large overhead

30 Design Choices – Exchanging Data/ACK  Omnidirectional Data/ACK  Lower spatial reuse  Lower link quality  Lower communication range  Directional Data/ACK  The intuitive choice  higher spatial reuse, better link quality, longer range/lower power

31 Design Choices – “Idle” Mode  While “idle” a node should be  Omnidirectional or directional ?

32 Design Choices – “Idle” Mode  While “idle” a node should be  Omnidirectional or directional ?  In absence of traffic information, idle node has to be omni a c a c d Desired Signal for “a” Unwanted Interference for “a”

33 Let’s combine the design choices -- DMAC

34  Remain omni while idle  Nodes cannot predict who will trasmit to it DMAC Example D Y S X

35 RTS DMAC Example D Y S X  Assume S knows direction of D

36 RTS CTS DMAC Example D Y S X DATA/ACK X silenced … but only toward direction of D

37 Intuitively Performance benefits appear obvious

38 However … Sending Rate (Kbps) Throughput (Kbps)

39 Clearly, attaching sophisticated antenna hardware is not sufficient Simulation traces revealed various new challenges Motivates higher layer protocol design

40 Self Interference with Directional MAC New Challenges

41 Unutilized Range  Longer range causes interference downstream  Offsets benefits  Network layer needs to utilize the long range  Or, MAC protocol needs to reduce transmit power A BC Data D route

42 Enhancing MAC  MMAC  Transmit multi-hop RTS to far-away receiver  Synchronize with receiver using CTS (rendezvous)  Communicate data over long links

43 New Hidden Terminal Problems with Directional MAC New Challenges II …

44 New Hidden Terminal Problem  Due to gain asymmetry  Node A may not receive CTS from C  i.e., A might be out of DO-range from C B CA Data RTS CTS

45 New Hidden Terminal Problem  Due to gain asymmetry  Node A later intends to transmit to node B  A cannot carrier-sense B’s transmission to C B CA Data Carrier Sense RTSCTS

46 New Hidden Terminal Problem  Due to gain asymmetry  Node A may initiate RTS meant for B  A can interfere at C causing collision B CA Data RTS Collision

47 New Hidden Terminal Problems Due to missed out RTS/CTS New Challenges II …

48  While node pairs communicate  X misses D’s CTS to S  No DNAV toward D New Hidden Terminal Problem II D Y S X Data

49  While node pairs communicate  X misses D’s CTS to S  No DNAV toward D  X may later initiate RTS toward D, causing collision New Hidden Terminal Problem II D Y S X Data RTS Collision

50 Deafness with Directional MAC New Challenges III …

51 Deafness  Node N initiates communication to S  S does not respond as S is beamformed toward D  N cannot classify cause of failure  Can be collision or deafness S D N Data RTS M

52 Channel Underutilized  Collision: N must attempt less often  Deafness: N should attempt more often  Misclassification incurs penalty (similar to TCP) S D N Data RTS M Deafness not a problem with omnidirectional antennas

53 Deafness and “Deadlock”  Directional sensing and backoff...  Causes S to always stay beamformed to D  X keeps retransmitting to S without success  Similarly Z to X  a “deadlock” DATA RTS X D S Z

54 Impact on Backoff  Another possible improvement: Backoff Counter for DMAC flows Backoff Counter for ToneDMAC flows time Backoff Values

55 MAC-Layer Capture The bottleneck to spatial reuse New Challenges IV …

56  Typically, idle nodes remain in omni mode  When signal arrives, nodes get engaged in receiving the pkt  Received packet passed to MAC  If packet not meant for that node, it is dropped Capture Wastage because the receiver could accomplish useful communication instead of receiving the unproductive packet

57 Capture Example AB C D Both B and D are omni when signal arrives from A AB C D B and D beamform to receive arriving signal

58 Outline / Contribution  Antenna Systems  A closer look  New challenges with beamforming antennas  Design of Capture-aware MAC and Routing protocols  Experiment with prototype testbed

59 Beamforming for transmission and reception only is not sufficient Antenna control necessary during idle state also Impact of Capture AB C D AB C D

60  Capture-Aware MAC (CaDMAC)  D monitors all incident traffic  Identifies unproductive traffic  Beams that receive only unproductive packets are turned off  However, turning beams off can prevent useful communication in future MAC Layer Solution AB C D Idle Beam

61  CaDMAC turns off beams periodically  Time divided into cycles  Each cycle consists of 1.Monitoring window + 2. Filtering window 12 CaDMAC Time Cycles 1122 cycle time All beams remain ON, monitors unproductive beams Node turns OFF unproductive beams while it is idle. Can avoid capture

62 CaDMAC Communication  Transmission / Reception uses only necessary single beam  When node becomes idle, it switches back to appropriate beam pattern  Depending upon current time window AB C D AB C D

63  During Monitoring window, idle nodes are omni Spatial Reuse in CaDMAC AB C E F D

64  At the end of Monitoring window CaDMAC identifies unproductive links Spatial Reuse in CaDMAC AB C E F D

65  During Filtering window  use spatial filtering Spatial Reuse in CaDMAC AB C E F D Parallel Communications CaDMAC : 3 DMAC & others : ≤ 2 Omni : 1

66 Network Transport Capacity  Transport capacity defined as: bit-meters per second (like man-miles per day for airline companies)  Capacity analysis

67 Directional Capacity  Existing results show  Capacity improvement lower bounded by  Results do not consider side lobes of radiation patterns  Consider main lobe and side lobe gains (g m and g s )  Capacity upper bounded by  i.e., improvement of CaDMAC still below achievable capacity

68  CaDMAC cannot eliminate capture completely  Happens because CaDMAC cannot choose routes  Avoiding capture-prone links  A routing problem Discussion Y X A B

69 Routing using Beamforming Antennas Incorporating capture-awareness

70 Motivating Capture-Aware Routing Y X A B Y X A B D S Z D Z S  Find a route from S to D, given A  B exists  Options are SXYD, SXZG Capture No Capture

71  Source routing protocol (like DSR)  Intermediate node X updates route cost from S - X  Destination chooses route with least cost (U route )  Routing protocol shown to be loop-free Protocol Design X D S C1C1 C2C2 C3C3 C5C5 U SX U SD = U SX + C 2 + C 5 + P D + 1

72  U route = Weighted Combination of 1. Capture cost (K) 2. Participation cost (P) 3. Hop count (H)  Weights chosen based on sensitivity analysis Unified Routing Metric

73 CaRP Vs DSR

74 CaRP Vs DSR

75 CaRP Vs DSR

76 CaRP Vs DSR

77 CaRP Vs DSR

78 CaRP Vs DSR

79 CaRP Vs DSR

80 CaRP Vs DSR

81 CaRP Vs DSR DSRCaRP CaRP prefers a traffic-free direction “Squeezes in” more traffic in given area

82 Performance of CaDMAC CaDMAC DMAC CBR Traffic (Mbps) Aggregate Throughput (Mbps) CMAC

83 Throughput with CaRP CaRP + CaDMAC DSR + CaDMAC DSR Aggregate Throughput (Mbps) Topology Number Random Topologies

84 Conclusion / Criticism  State of the art used omnidirectional antennas  Antenna community advancements was critical  However, smart antennas cannot be used  Unless, protocols become antenna-aware  MMAC paper identifies several awareness issues  Hidden terminal, deafness, capture, interference, etc.  Proposes one solution  Many missing pieces - neighbor discovery, multipath, mobility  Capture  Intelligence even during idle state  Solution assumes stable traffic, high resolution antennas …

85 Questions

86 Announcements  Example reviews  Posted on course websites  Students not signed up for ppt  One marathon class with all presentations  I will stay through, you are welcome to attend  Project Groups  Please start thinking about it  Feb 21 is deadline for ing project topic + rough plan  If you don’t have partners  Please stay back after class next Tuesday

87 BackUp Slides

88 Testbed Prototype  Network of 6 laptops using ESPAR antennas  ESPAR attached to external antenna port  Beams controlled from higher layer via USB  Validated basic operations and tradeoffs  Neighbor discovery Observed multipath 60 degrees beamwidth useful  Basic link state routing Improves route stability Higher throughput, less delay

89 Neighbor Discovery  Non LOS and multipath important factors  However, wide beamwidth (60 degrees)  reasonable envelope Anechoic ChamberOffice Corridor

90 Route Reliability  Routes discovered using sweeping – DO links  Data Communication using DD links  Improved SINR improves robustness against fading

91 Summary  Future = Dense wireless networks  Better interference management necessary  Typical approach = Omni antennas  Inefficient energy management  PHY layer research needs be exploited

92 Impact of Hidden Terminals, Deafness, Capture, unutilized range …

93 Conclusion  Directional antennas intuitively beneficial  However, closer examination shows several tradeoffs  We designed a simple DMAC protocol  Considered several design choices, including carrier-sensing, backoff, RTS/CTS mode, idle mode, etc.  Observed several problems with DMAC  Such problems do not appear with omnidirectional antennas  Glanced at some approaches to optimize DMAC  Optimizations offer encouraging benefits in performance  But several problems still remain to be resolved …

94 Many open problems … good project topics  Neighbor discovery with directional antennas Especially under mobile scenarios  Directional antennas and routing Vectorial, Zig-Zag routing  Choose routes using direction info.  Beamwidth & power control Control network topology based on user need  Capacity of directional communication How much theoretical improvements possible over omni ?  TDMA based protocols Probably worth considering with directional antennas … and many more

95 Thoughts !!  Directional/MIMO antennas heavily considered for next generation networks  s (for mesh) advocating such technologies  Lot of research papers in the near past  many open problems  However, can mobility be supported ??  Antennas impact higher layers  Impact on performance of omni routing protocol studied  Routing protocols designed for directional antennas  Is cross layer necessary ?  Testbed prototypes being built  Both for adhoc and mesh networks

96  Previous protocols assumed omnidirectional antennas  Omni antennas radiate energy in unwanted directions  Wasteful / unnecessary  Recent advances in signal processing and antenna design principles  Interfere only toward desired direction  Feasible at smaller size and lower cost  We ask …  Can we utilize directional antennas in multihop networking  What are the benefits ? What protocols should be designed ? Why adopt new antenna technology ?

97 But first, … Some basic antenna concepts

98 Shifting from WLAN to Multihop

99 Intuitive Benefit (1) – Spatial Reuse Omni Communication Directional Communication ab c d Silenced Node ab c d e f Simultaneous Communication No Simultaneous Communication Ok f

100 Intuitive Benefit (2): Range Extension Omni Communication Directional Communication ab c d ab c d Link Between Nodes b and d No Link Between Nodes b and d Ok Many more benefits...

101 No Free Lunch  Several issues arise with directional beams  Determining direction to transmit  New hidden terminal problems  Broadcast with directional beams  Deafness  Capture  And many more … Insufficient to only add directional antennas, without appropriate support from protocols

102  Is carrier sensing necessary at all ?  Transmission from M to N does not interfere B  If B transmitting data to C, then collision likely anyway  Directional carrier sensing seems unnecessary  In reality, node B has sidelobes  Carrier sensing necessary for situation when M close to B Why Carrier Sense ? B C M Data Carrier Sense N

103 Combining Design Choices: DMAC  Idle nodes remain omni  When packet arrives from network layer  Consult directional NAV  Carrier sense directionally toward receiver  Wait for backoff in directional mode  Transmit directional RTS  RTS received omnidirectionally  Receiver determines Direction of Arrival (DoA) of RTS  Following CTS, Data, ACK exchange directional  Switch back to omnidirectional mode

104 Routing with Higher Range  Directional routes offer  Better connectivity, fewer-hop routes  However, broadcast difficult  Sweeping necessary to emulate broadcast  Evaluate tradeoffs  Designed directional DSR

105 Today’s Discussions  Introducing the role of antennas  Recall lecture 2  Motivate need for antenna technology  Basic directional antenna concepts  Applying directional antennas to networking  Design choices and tradeoffs  Designing the first simple protocol  DMAC  Several problems / challenges with DMAC  Investigate optimizations  MMAC, CaMAC  What lies ahead ?  Research issues in MAC, routing, and higher layers …

106  Sum capture costs of all beams on the route  Capture cost of a Beam j = how much unproductive traffic incident on Beam j  Route’s hop count  Cost of participation  How many intermediate nodes participate in cross traffic Measuring Route Cost X D S