Medium Access Control Protocols Using Directional Antennas in Ad Hoc Networks

1. Medium Access Control Protocols Using Directional Antennas in Ad Hoc Networks Young- Bae Ko Vinaychandra Shankarkumar Nitin Vaidya Computer Science Texas A&M University

2. Ad Hoc Networks • Network formed by wireless nodes, all or some of which may act as routers • Nodes may or may not be mobile

3. Wireless Medium • Shared broadcast medium • Half duplex mode: Collision avoidance, instead of collision detection • Need suitable medium access control (MAC) protocol • IEEE802.11 standard

4. Traditional MAC Protocols • Omnidirectional antenna is typically assumed. • packet transmission intended for a single receiver may cause interference at all neighbors. • IEEE 802.11 designed for omnidirectional antennas

5. Directional Antennas • Using directional antennas can be beneficial • interference can be reduced, and the probability of simultaneous transmissions may be increased • Need new MACprotocols to best utilize directional antennas

6. This Paper Directional MAC Quantitatively Improving IEEE 802.11 Qualitatively Location-aided routing (Mobicom’98 Best Student Paper award) Location-based multicasting Optimizing MAC using location information

7. A B C D A B C Carrier Sense Mechanisms • Hidden terminal problem • Exposed terminal problem Nodes A and C are hidden from each other Nodes A and B are exposed to each other

8. IEEE802.11Medium Access Control • Sender sends Request-to-Send (RTS) • Receiver replies Clear-to-Send (CTS) if not “busy” • Sender sends data on receiving CTS • Receiver sends ack on receiving data

9. IEEE 802.11 • Collisions due to hidden terminals prevented • Conflict-free ACK • Capacity wasted - any other node that overhears RTS/CTS may not communicate

10. IEEE 802.11 F A B C D E RTS RTS CTS CTS DATA DATA ACK ACK Reserved area

11. Directional MAC (D-MAC) • Directional antenna can limit transmission to a smaller region (e.g., 90 degrees). • Basic philosophy: MAC protocol similar to IEEE 802.11, but on a per-antenna basis

12. D-MAC • IEEE802.11: Node X is blocked if node X has received an RTS or CTS for on-going transfer between two other nodes • D-MAC: Antenna T at node X is blocked if antenna T received an RTS or CTS for an on-going transmission • Transfer allowed using unblocked antennas

13. D-MAC Protocols • Based on location information of the receiver, sender selects an appropriate directional antenna • Several variations are possible

14. D-MAC Scheme 1 • Uses directional antenna for sending RTS, DATA and ACK in a particular direction, whereas CTS sent ominidirectionally • Directional RTS (DRTS) and Omnidirectional CTS (OCTS)

15. D-MAC Scheme 1: DRTS/OCTS A B C D E DRTS(B) DRTS(B) - Directional RTS including OCTS(B,C) OCTS(B,C) location information of node B DRTS(D) OCTS(B,C) - Omnidirectional CTS OCTS(D,E) OCTS(D,E)) including location information DATA of nodes B and C DATA ACK ACK

16. Drawback of Scheme 1 • Collision-free ACK transmission not guaranteed ? A B C D DRTS(B) OCTS(B,C) OCTS(B,C) DRTS(A) DATA DRTS(A) ACK

17. D-MAC Scheme 2 • Scheme 2 is similar to Scheme 1, except for using two types of RTS • Directional RTS (DRTS) / Omnidirectional RTS (ORTS) both used • If none of the sender’s directional antennas are blocked, send ORTS • Otherwise, send DRTS when the desired antenna is not blocked

18. Trade-off D-MAC Scheme 2 • Probability of ACK collision lower than scheme 1 • Possibilities for simultaneous transmission by neighboring nodes reduced compared to scheme 1

19. Variations • Paper discusses further variations on the theme • Reducing ACK collisions • Reducing wasteful transmission of RTS to busy nodes

20. Performance Comparison • Which scheme will perform better depends on • location of various hosts • traffic patterns • This paper presents preliminary evaluation using a simplified model

21. 5 10 15 20 25 4 9 14 19 24 3 8 13 18 23 2 7 12 17 22 1 6 11 16 21 Performance Evaluation • Mesh topology • No mobility • Bulk TCP traffic • 2 Mbps channel

22. Performance Measurement • Reference throughput of single TCP connection using IEEE 802.11 • 1 hop (1383 Kbps) • 2 hops (687 Kbps) • 3 hops (412 Kbps) • 4 hops (274 Kbps)

23. 1 6 11 16 21 No.1 No.2 Connections IEEE802.11 Scheme1 Scheme2 No.1 1130.42 771.27 51.03 No.2 214.57 1040.21 1303.64 Total Throughput1344.991811.48 1354.67 Performance Measurement • Scenario 1

24. 1 6 11 16 21 No.3 No.4 Performance Measurement • Scenario 2: Best case for scheme 1 Connections IEEE802.11 Scheme1 Scheme2 No.3 653.64 1250.14 884.82 No.4 634.58 1251.64 867.69 Total Throughput1288.22 2501.78 1752.51

25. 5 4 3 2 1 6 11 16 21 Performance Measurement • Scenario 3 No.6 Connections IEEE802.11 Scheme1 Scheme2 No.5 179.66 207.41 210.20 No.6 179.46 209.53 216.53 Total Throughput359.12416.94 426.73 No.5

26. No.11 No.10 No.9 No.8 2 7 12 17 22 No.7 Connections IEEE802.11 Scheme1 Scheme2 No.7 157.50 146.73 165.89 No.8 89.90 85.31 81.30 No.9 22.00 91.39 105.03 No.10 89.29 82.30 82.83 No.11 157.94 153.30 163.37 Total516.63559.03 598.42 1 3 5 4 10 8 9 6 11 14 15 13 20 19 18 16 21 25 23 24 Performance Measurement • Scenario 4

27. Limitations of D-MAC • Physical size of directional antennas • Unless using higher frequency bands • No guarantee of collision-free ACK • Some improvements suggested in paper • Inaccurate/outdated location information can degrade performance

28. Limitations of the Evaluation • Propagation model • Model for the behavior of directional antennas • Network topology / mobility and traffic patterns

29. Conclusions • Pro: Can allow more simultaneous transmissions • Con: Can increase Ack collisions • Need additional evaluation of all MAC alternatives • propagation / mobility / traffic models • Alternatives for determining location information should be considered

30. Thank You !www.cs.tamu.edu/faculty/vaidya Announcement • MobiHoc - Workshop on Mobile Ad Hoc Networks • To be held on August 11 in conjunction with ACM MobiCom, Boston