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Using Directional Antennas for Medium Access Control in Ad Hoc Networks

This research paper explores the use of directional antennas in ad hoc networks and investigates their benefits, limitations, and potential improvements for medium access control. It compares the performance of directional antennas with omnidirectional antennas and discusses various issues and tradeoffs.

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Using Directional Antennas for Medium Access Control in Ad Hoc Networks

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  1. Using Directional Antennas for Medium Access ControlinAd Hoc Networks Romit Roy Choudhury, UIUC Xue Yang, UIUC Ram Ramanathan, BBN Nitin H. Vaidya, UIUC

  2. Ad Hoc Networks Typically assume Omnidirectional antennas A silenced node C B A D

  3. Can Directional Antennas Improve Performance? Not possible using Omni C B A D

  4. A Comparison

  5. Motivation • Are directional antennas beneficial to medium access control in ad hoc networks ? • To what extent ? • Under what conditions ?

  6. Protocol Stack Neighbor Discovery Routing Layer Transceiver Profile Hello, Data/Control Pkts DMAC / MMAC Antenna Layer

  7. Organization • 802.11 Basics • Related Work • Antenna Model • Simple Directional MAC protocol (DMAC) • Problems with DMAC – Insights • Multi-Hop MAC (MMAC) • Performance (comparison with 802.11) • Conclusion

  8. IEEE 802.11 • Sender sends Ready-to-Send (RTS) • Receiver responds with Clear-to-Send (CTS) • RTS and CTS announce the duration of the imminent dialogue • Nodes overhearing RTS/CTSdefer transmission for that duration • Network Allocation Vector (NAV) remembers duration

  9. IEEE 802.11 RTS = Request-to-Send RTS A B C D E F

  10. IEEE 802.11 RTS = Request-to-Send RTS A B C D E F NAV = 10

  11. IEEE 802.11 CTS = Clear-to-Send CTS A B C D E F

  12. IEEE 802.11 CTS = Clear-to-Send CTS A B C D E F NAV = 8

  13. IEEE 802.11 • DATA packet follows CTS. Successful data reception acknowledged using ACK. DATA A B C D E F

  14. IEEE 802.11 ACK A B C D E F

  15. IEEE 802.11 • Channel contention resolved using backoff • Nodes choose random backoff interval from [0, CW] • Count down for this interval before transmission Random backoff Data Transmit backoff Wait A Random backoff Wait backoff Data Transmit B

  16. Antenna Model 2 Operation Modes: OmniandDirectional A node may operate in any one mode at any given time

  17. Antenna Model In Omni Mode: • Nodes receive signals with Gain Go • While idle a node stays in Omni mode In Directional Mode: • Capable of beamforming in specified direction • Directional Gain Gd(Gd > Go)

  18. Directional Communication • Received Power  (Tx Gain) * (Rx Gain) • Tx Gain = Transmit gain in the direction of receiver • Rx Gain = Receive gain in the direction of the transmitter A B C Convention: A link shown by overlapping beams along the line joining the transmitter and receiver in question. Nodes C, A form a link. C, B do not.

  19. Directional Neighborhood Receive Beam Transmit Beam B A C • When C transmits directionally • Node A sufficiently close to receive in omni mode • Node C and A are Directional-Omni (DO) neighbors • Nodes C and B are not DO neighbors

  20. Directional Neighborhood Transmit Beam Receive Beam A C B • When C transmits directionally • Node B receives packets from C only in directional mode • C and B are Directional-Directional (DD) neighbors

  21. Related Work • Many proposals/analyses of directional antennas [Ko00,Ramanathan01,Nasipuri00, Balanis00, Takai02,Bandyopadhay01, Bao02, Sanchez01, Ephremides98] • MAC Proposals differ based on • How RTS/CTS transmitted (omni, directional) • Transmission range of directional antennas • Channel access schemes • Omni or directional NAVs

  22. Simple DMAC protocol Similar protocols proposed in [Takai02, Nasipuri00, Ramanathan01] • A node listens omni-directionally when idle • Sender transmits Directional-RTS (DRTS) using specified transceiver profile • RTS received in Omni mode (only DO links used) • Receiver sends Directional-CTS (DCTS) • DATA,ACK transmitted and received directionally

  23. Directional NAV (DNAV) • Nodes overhearing RTS or CTS set up directional NAV(DNAV)for thatDirection of Arrival (DoA) B CTS D A C

  24. Directional NAV (DNAV) • Nodes overhearing RTS or CTS set up directional NAV(DNAV)for thatDirection of Arrival (DoA) B D DNAV A C

  25. Directional NAV (DNAV) • New transmission initiated only if direction of transmission does not overlap with DNAV,i.e. if (θ > 0) B D DNAV θ A C RTS

  26. DMAC Example C E B D A B and C communicate D and E cannot: D blocked with DNAV from C D and A communicate

  27. Data RTS Issues with DMAC • Two types of Hidden Terminal Problems • Due to asymmetry in gain B C A A is unaware of communication between B and C A’s RTS may interfere with C’s reception of DATA

  28. Issues with DMAC • Two types of Hidden Terminal Problems • Due to unheard RTS/CTS D B C A • Node A beamformed in direction of D • Node ADoes nothear RTS/CTS from B & C

  29. Issues with DMAC • Two types of Hidden Terminal Problems • Due to unheard RTS/CTS D B C A Node A may now interfere at node C by transmitting in C’s direction

  30. Issues with DMAC • Deafness Z RTS A B DATA RTS Y RTS X does not know node A is busy. X keeps transmitting RTSs to node A X Using 802.11 (omni antennas) X would be aware that A is busy, and defer its own transmission

  31. DMAC Tradeoffs • Disadvantages • Hidden terminals • Deafness • No DD Links • Benefits • Better Network Connectivity • Spatial Reuse

  32. Enhancing DMAC • Are improvements possible to make DMAC more effective ? • One possible improvement: Make Use of DD Links

  33. Using DD Links Exploit larger range of Directional antennas Transmit Beam Receive Beam C A A and C are DD neighbors, but cannot communicate in DMAC

  34. DO neighbors D E DD neighbors F C A B G Multi Hop RTS – Basic Idea A source-routes RTS to D through adjacent DO neighbors (i.e., A-B-C-D) When D receives RTS, it beamforms towards A, forming a DD link

  35. A transmits RTS towards D MMAC protocol D E H F C A B G

  36. MMAC protocol H updates DNAV D E DNAV H F C A B G

  37. A transmits M-RTS to DO neighbor B MMAC protocol D E H F C A B G

  38. B forwards M-RTS to C (also DO) MMAC protocol D E H F C A B G

  39. A beamforms toward D – waits for CTS MMAC protocol D E H F C A B G

  40. C forwards M-RTS to D MMAC protocol D E H F C A B G

  41. D beamforms towards A – sends CTS MMAC protocol D E H F C A B G

  42. MMAC protocol A & D communicate over DD link D E H F C A B G

  43. MMAC protocol Nodes D and G similarly communicate D E H F C A B G

  44. Performance • Simulation • Qualnet simulator 2.6.1 • Constant Bit Rate (CBR) traffic • Packet Size – 512 Bytes • 802.11 transmission range = 250meters • DD transmission range = 900m approx • Beamwidth = 60 degrees, Main-lobe Gain = 10 dBi • Side lobes ignored • Channel bandwidth 2 Mbps • Mobility - none

  45. D E F A B C Impact of Topology Aggregate throughput 802.11 – 1.19 Mbps DMAC – 2.7 Mbps Nodes arranged in linear configurations reduce spatial reuse for D-antennas Aggregate throughput 802.11 – 1.19 Mbps DMAC – 1.42 Mbps A B C

  46. Aligned Routes in Grid

  47. Unaligned Routes in Grid

  48. “Random” Topology

  49. “Random” Topology: delay

  50. MMAC - Concerns • High traffic – lower probability of RTS delivery • Multi-hop RTS may not reach DD neighbor due to • deafness or collision • We use no more than 3 DO links for each DD link • Neighbor discovery overheads may offset the advantages of MMAC

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