2. August 8, 2015 Computer Networks COE 549 Directional Antennas for Ad- hoc Networks Tarek Sheltami KFUPM CCSE COE http://faculty.kfupm.edu.sa/coe/tarek/coe549.htm

3. 2 Outline  Introduction  IEEE 802.11 (CSMA/CA) overview  Motivations  Problem statement  Beamforming: Definition, types and advantages.  Basic DMAC  Challenges in Ad-hoc Networks using directional antennas.  Multi-Hop MAC (MMAC)  Beamforming with Power Control  Performance Evaluation

4. Ad Hoc Networks 8/8/20153 A silenced node A B C D Typically assume Omnidirectional antennas

5. Can Directional Antennas Improve Performance? A B C D Not possible using Omni 8/8/20154

6. A Comparison IssuesOmniDirectional Spatial ReuseLow High Connectivity LowHigh Interference OmniDirectional Cost & Complexity LowHigh 8/8/20155

7. Motivation Are directional antennas beneficial to medium access control in ad hoc networks ? –To what extent ? –Under what conditions ? 8/8/20156

8. 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/CTS defer transmission for that duration –Network Allocation Vector (NAV) remembers duration IEEE 802.11 8/8/20157

9. CFABED RTS RTS = Request-to-Send IEEE 802.11 8/8/20158

10. CFABED RTS RTS = Request-to-Send IEEE 802.11 NAV = 10 8/8/20159

11. CFABED CTS CTS = Clear-to-Send IEEE 802.11 8/8/201510

12. CFABED CTS CTS = Clear-to-Send IEEE 802.11 NAV = 8 8/8/201511

13. CFABED DATA DATA packet follows CTS. Successful data reception acknowledged using ACK. IEEE 802.11 8/8/201512

14. CFABED ACK IEEE 802.11 8/8/201513

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 Random backoff Wait backoff Data Transmit Wait A B 8/8/201514

16. Antenna Model 2 Operation Modes: Omni and Directional A node may operate in any one mode at any given time 8/8/201515

17. Antenna Model In Omni Mode: Let us assume that nodes receive signals with Gain G o In Directional Mode: Directional Gain G d (G d > G o ) 8/8/201516

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. Nodes C, A form a link. C, B do not. 8/8/201517

19. B Directional Neighborhood A 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 C Transmit Beam Receive Beam 8/8/201518

20. Directional Neighborhood A B C When C transmits directionally Node B receives packets from C only in directional mode C and B are Directional-Directional (DD) neighbors Transmit Beam Receive Beam 8/8/201519

21. A technique in which the antenna pattern is switched (or steered) to a desired direction. Two types: switched & steered beam. 8/8/201520 Antenna Beamforming - Steered beam: can direct the beam to the desired direction. (cost more but better performance) - Switched beam: can select one from a set of predefined beams/antennas S D S D

22. 21 1. Longer range Why? higher antenna gain in the desired direction Benefits: better connectivity and lower end-to-end delay 2. Higher spatial reuse Why? Reduced interference (narrower beamwidth) Benefits: increased capacity and throughput 8/8/2015 Antenna Beamforming

23. 22 Identify the challenges encountered in MAC when beamforming antennas are used in Ad hoc networks and find the possible solutions of those problems in the literature. Research Problem 8/8/2015

24. 23 The two most impacted networking mechanisms as a result of using beamforming antennas are 1. Neighbor discovery identifies the one-hop neighbors 2. MAC provides distributed access to the channel Challenges in Ad-hoc Networks 8/8/2015

25. 24 DMAC is MAC with directional (beamforming) Antennas. Omni Two Operation Modes: Omni and Directional A node may operate in any mode at any given time DMAC 8/8/2015

26. Basic DMAC Assumption: Location of neighbors is known. Sender transmits Directional-RTS (DRTS) A node listens omni-directionally when idle, –RTS received in Omni mode. Receiver sends Directional-CTS (DCTS) DATA, ACK transmitted and received directionally. Operation is the same as 802.11 but with directional antennas and, and with the use of DNAV (directional NAV)!! 8/8/201525

27. 26 Basic DMAC Why DNAV (directional Network allocation Vector)? Asnwer: to combat directional exposed terminal problem.  increased spatial reuse and throughput A C B E D 8/8/2015

28. 27 Neighbor discovery New notions of neighbors: B A OO Nodes A and B are OO neighbors. OO Nodes C and A are not OO DO Nodes C and B are not DO C DO but DO neighbors. DD but DD neighbors. Transmit antenna Receive antenna OOOmni DODir.Omni DDDir 8/8/2015

29. 28 Neighbor discovery How to know the direction of the intended node? –CTS, DATA, ACK are much easier than RTS –Two possible ways: From the AOA (Angle_of_Arrival ) of RTS and CTS. Or from self location information included in RTS and CTS. –Directing the beam towards the destination for DRTS is challenging. Possible solutions: Most MAC proposal assumes that this information is available by routing protocol. Each node know its location (by GPS or any location estimation method). By AoA cashing of overheard packets (ex. Takai et al.[2]) Circular DRTS ORTS. 8/8/2015

30. 29 DMAC by Takai et al. [2] Goals: send RTS directionally without location knowledge. Employs DNAV –It is set according to AoA of the RTS/CTS dialog Employs AoA cashing –The direction of neighbors is cashed based on the estimation of AoA of the overheard packets. RTS is send directionally if the direction of the intended destination is available in the cash RTS is sent omnidirectionally if the direction of the destination is not available in the AoA cash or CTS is not received after directional RTS transmission. 3 to 4 times improvement in throughput compared to 802.11 8/8/2015 Neighbor discovery

31. 30 Extended transmission range –Beamforming enables longer range –Advantages: reduced # of hops, e2e delays and better connectivity (sparse networks) –Most of MAC proposals are not able to achieve the maximum possible range OO, OD link only, –For Maximum range: DD link –MMAC by Choudhury et al. [3] 8/8/2015 Neighbor discovery

32. 31 MMAC by Choudhury et al. [3] - Knowledge of neighbors location is assumed - Goal: improve system performance (e2e delay and throughput) by extending the range of transmission (DD link). -Similar to basic DMAC + DD link -DD link can be established by multi-hop RTS (MHRTS) D B A C E DO Link DD Link MHR TS DATA MHR TS DRTSDCTS 8/8/2015 Neighbor discovery

33. Multi Hop RTS – Basic Idea A B C DE F G DO neighbors DD neighbors 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 8/8/201532

34. MMAC protocol B C DE F G H A transmits RTS towards D A 8/8/201533

35. MMAC protocol B C DE F G H DNAV A H updates DNAV 8/8/201534

36. MMAC protocol B C DE F G H A transmits M-RTS to DO neighbor B A 8/8/201535

37. MMAC protocol C DE F G H A B forwards M-RTS to C (also DO) B 8/8/201536

38. MMAC protocol B C DE F G H A beamforms toward D – waits for CTS A 8/8/201537

39. MMAC protocol B DE F G H A C forwards M-RTS to D C 8/8/201538

40. MMAC protocol B E F G H C A D beamforms towards A – sends CTS D 8/8/201539

41. MMAC protocol B E F G H C D A A & D communicate over DD link 8/8/201540

42. MMAC protocol B E F H C A Nodes D and G similarly communicate G D 8/8/201541

43. 42 Problems in DMAC There are two main problems associated with DMAC: 1. New Hidden Terminals 2. Deafness 8/8/2015

44. 43 Case 1. E is out of RTS/CTS range of A/C communication A C E A E D The node is hidden to the ongoing communication of other node when it didn’t hear the RTS/CTS transmission while it can interfere Case 2. Loss in channel state D C Collision The antenna of E is directed twards D RTS/CTS of A/C CANNOT be heard by E Problems in DMAC 1. New Hidden Terminals 8/8/2015

45. 44 A node A is deaf with respect to nodes X, Z, if it cannot receive from nodes X, Z due to beam direction while it can receive if it was in omni mode. Effects: –Waste the capacity and energy (due unproductive control packets). –Introduce unfairness (increased backoff interval). RTS AB X Z DATA X and Z do not know node A is busy. They keep transmitting RTSs to node A Problems in DMAC 2. Deafness 8/8/2015

46. 45 Hidden terminals and deafness are the two critical problems in DMAC. Possible Solution: –Send RTS and/or CTS omnidirectionally while DATA/ACK are sent directionally. Example: DMAC by Ko et al. [5] Problems in DMAC 8/8/2015

47. 46 - Knowledge of neighbors location is assumed - Multiple directional antennas for each nodes (switched beam) - Goal: increase spatial reuse while reducing control packet collisions. - DATA/ACK is directional - CTS is omnidirectional = OCTS - Two schemes for RTS: -Scheme 1 : DRTS (Directional RTS) only -Scheme 2 : ORTS/DRTS A B S D X S can send to D but not to X Both schemes send DRTS D S Scheme 2 sends RTS in all directions (ORTS) if no antenna is blocked A B Problems in DMAC DMAC by Ko et al. [5] 8/8/2015

48. 47 Performance Offers about 50% better throughput compared to IEEE 802.11, depends on Topology Scheme 1 vs. Scheme 2: –Scheme 2 tries to reduce collision of control packets at the source while scheme 1 tries maximize spatial reuse in the vicinity of the source. –No significant performance difference Problems in DMAC DMAC by Ko et al. (Cont.) 8/8/2015

49. 48 Problems with DMAC Possible Solution to unfairness caused by Deafness: ToneDMAC by Choudury et al. [6] Goal: to reduce the effect of unfairness caused by Deafness by identify Deafness from congestion RTS/CTS/DATA/ACK are sent directionally After RTS/CTS/DATA/ACK exchange, A and B send their tones omnidirectinally. neighboring nodes that overhear the tones will know that node A or B was engaged in communication. Throughput is 2 times better than DMAC. –Fairness is improved. C will know that B was deaf. It will reset the backoff window to the minimum value. A_TONE AB C DATA B_TONE RTS A_TONE 8/8/2015

50. DMAC Tradeoffs Benefits –Better Network Connectivity –Spatial Reuse Disadvantages –Hidden terminals –Deafness –No DD Links 8/8/201549

51. Impact of Beamforming on Ad-hoc Networking: MAC, Neighbor discovery, Route discovery Our Goal is to study the impact of Antenna beamforming on MAC. Examples: (Assume CSMA/CA ) Without beamforming With beamforming A B CDA B CD Exposed terminal problem No problem A B A B E No problem Deafness Problem C D E C D 8/8/201550

52. 51 Beamforming with power control Power control by it self can achieve higher performance –Reduce interference –Lower energy consumption Power control + beamforming can substantially improve the performance No power control or beamforming Area = A r/2 r Power control only Area = A/4 r/2 Beamforming only Area = A/6 Power control or beamforming Area = A/144 !!! A rough comparison of relative interference reduction, assuming 10 degrees directional beamwidth, and r 4 propagation. [1] 8/8/2015

53. 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 –Channel bandwidth 2 Mbps –Mobility - none 8/8/201552

54. MMAC Hop Count Max MMAC hop count = 3 –Too many DO hops increases probability of failure of RTS delivery –Too many DO hops typically not necessary to establish DD link A B C DE F G DO neighbors DD neighbors 8/8/201553

55. MMAC - Concerns Neighbor discovery overheads may offset the advantages of MMAC High traffic – lower probability of RTS delivery Multi-hop RTS may not reach DD neighbor due to deafness or collision No more than 3 DO links is used for each DD link 8/8/201554

56. Aligned Routes in Grid 8/8/201555

57. Unaligned Routes in Grid 8/8/201556

58. “Random” Topology 8/8/201557

59. “Random” Topology: delay 8/8/201558

60. Nodes moving out of beam coverage in order of packet-transmission-time –Low probability Antenna handoff required –MAC layer can cache active antenna beam –On disconnection, scan over adjacent beams –Cache updates possible using promiscuous mode –Evaluated in [RoyChoudhury02_TechReport] Mobility 8/8/201559

61. Broadcast Several definitions of “broadcast” –Broadcast region may be a sector, multiple sectors –Omni broadcast may be performed through sweeping antenna over all directions [RoyChoudhury02_TechReport] A Broadcast Region 8/8/201560

62. 61 References 1.Basagni, M. Conti, S. Giordano, I. Stojmenovic, eds, Mobile Ad Hoc Networking, IEEE Press/Wiley, August 2004. 2.M. Takai, et al., “Directional virtual carrier sensing for directional antennas in mobile ad hoc networks”, ACM MobiHoc 2002, pp 39-46, June 2002 3.R.R. Choudhury, X. Yang, N.H. Vaidya, and R. Ramanathan, “Using directional antennas for medium access control in ad hoc networks”, MOBICOM 2002, pp 59-70, September 2002 4.N.S. Fahmy, T.D. Todd and V. Kezys, “Ad hoc networks with smart antennas using IEEE 802.11-based protocols”, IEEE ICC 2002, pp 3144-3148, May 2002 5.Y-B Ko, V. Shankarkumar and N.H. Vaidya, “Medium access control protocols using directional antennas in ad hoc networks”, IEEE INFOCOM 2000, pp 13-21 6.Choudhury, R.R.and Vaidya, N.H., “Deafness: a MAC problem in ad hoc networks when using directional antennas” ICNP 2004, Proceedings of the 12th IEEE International Conference on Network Protocols, pp:283 - 292, 2004