1. Medium Access Control Protocols Using Directional Antennas in Ad Hoc Networks CIS 888 Prof. Anish Arora The Ohio State University

2. A paper by Young-Bae Ko, Vinaychandra Shankarkumar and Nitin.H.Vaidya Department of Computer Science Texas A&M university http://www.cs.tamu.edu/faculty/vaidya/papers/mobile-computing/infocom00.ps

3. Background and Motivation Wireless mobile networks Omnidirectional antennas Wastage of network capacity Directional antennas Improve network capacity Improve routing performance Physical size limitations

4. Related work: Packet Radio Networks Slotted ALOHA using RTDMA SDMA for performance improvement Mobile Broadband Systems Dynamic slot assignment protocol RTDMA – Random Time Division Multiple Access SDMA – Space Division Multiple Access (simultaneous multiple receptions)

5. Network Model: Shared wireless channel Multiple Directional antennas Interference assumption Hidden terminal problem Fixed transmission range Unidirectional transmission

6. IEEE 802.11 MAC Protocol: RTS/CTS mechanism Sender Broadcasts RTS packet Intended Receiver replies with CTS packet Sender transmits data packet Receiver sends an ACK RTS – Request To Send CTS – Clear To Send

7. RTS/CTS Mechanism in 802.11 ABCDE RTS CTS DATA ACK

8. RTS/CTS (Contd…) RTS and CTS contain proposed duration of data transmission All in-range nodes MUST wait for this duration before transmitting Adv – Elimination of Hidden terminals Disadv – Wastage of network capacity (D cannot send anything to E)

9. D-MAC Schemes (Overview) Similar to IEEE 802.11 – only on a per antenna basis. 802.11 if node N is aware of an on-going transmission, N cannot send or receive itself D-MAC if antenna A at node N is aware, N cannot send or receive using antenna T.

10. Contd… Even if one antenna is blocked, the node may transmit using unblocked antennas. Leads to performance enhancement Can be used omnidirectionally as well

11. Scheme 1 – Using DRTS packets Directional RTS packets Omnidirectional CTS packets DRTS – sender’s location OCTS – sender’s and receiver’s locations Data packet and ACK sent using Directional antennas

12. Scheme 1 (contd…) Other nodes CAN transmit AB  C x D  E A  BC  D x E A  BC  D x E All DRTS may not get an OCTS reply ( D & E in the above scenarios cannot send OCTS if anyone sends them a DRTS because one of their antennas is blocked) Control packets may collide AB- -> C  DE

13. Scheme 2 – Using DRTS/ORTS Send either DRTS or ORTS based on rule “if all D-antennas are unblocked, send ORTS but if any D-antenna is blocked, send DRTS” A  B  C  DE C sends ORTS but B can only send DRTS Reduces, not eliminates control packet collisions

14. Performance evaluation Modified ns-2 simulator to include D- antennas (90°) and location information Simulation model 5 x 5 mesh – 200m apart 250m range for each node 2Mbps wireless link bandwidth

15. 5 4 10 1 3 2 9 8 7 15 14 2520 13 12 11 19 18 16 1722 23 24 6 21 Network Topology

16. Case 1: ConnectionsIEEE 802.11Scheme 1 No.1(6-11)1130.42771.27 No.2(16-21)214.571040.21 Total Throughput 1344.991811.48 Use of DRTS allows simultaneous transmissions hence throughput for Scheme 1 is better than 802.11 Fairness is also much better in Scheme 1

17. Case 2: ConnectionsIEEE 802.11Scheme 1 No.3(6-1)653.641250.14 No.4(11-16)634.581251.64 Total Throughput 1288.222501.79 Directions of data transfers differ, hence fewer collisions Best case scenario for D-antennas

18. Case 3: ConnectionsIEEE 802.11Scheme 1Scheme 2 No.1(1-21)179.66207.41210.20 No.2(1-5)179.46209.53216.53 Total Throughput 359.12416.94426.73 Scheme 2 performs better than Scheme 1 because probability of control packet collisions decreases.

19. Case 4: ConnectionsIEEE 802.11Scheme 1Scheme 2 No.7(1-21)157.50146.73165.89 No.8(2-22)89.9085.3181.30 No.9(3-23)22.0091.39105.03 No.10(4-24)89.2982.3082.83 No.11(5-25)157.94153.30163.37 Total Throughput 516.63559.03598.42 Border connections have much higher throughput Percentage performance enhancement of DMAC not so high because of increase in number of connections Reasonable fairness (esp. to conn. 9) in DMAC schemes

20. Case 5: ConnectionsIEEE 802.11Scheme 1Scheme 2 No.12(1-21)76.38112.5787.00 No.13(2-22)23.9340.2625.27 No.14(3-23)7.0836.0323.66 No.15(4-24)36.9132.8037.50 No.16(5-25)128.7598.10120.23 No.17(1-5)74.67117.0885.96 No.18(6-10)21.6042.1728.98 No.19(11-15)6.8040.4626.73 No.20(16-20)36.4836.8735.76 No.21(21-25)125.36101.27122.11 Total Throughput 537.96657.61593.20

21. Case 5 (contd…) DMAC 1 outperforms DMAC 2 The use of ORTS in scheme 2 reduces the possibility of simultaneous transmissions by neighbouring nodes Trade-off between probability of collisions and loss of simultaneous transmissions

22. Case 5(contd…) Scheme 1 Scheme 2 E C A F D B E C A F D B DRTS ORTS DRTS

23. Optimization: Using DWTS DWTS – Directional Wait-To-Send Aim: to avoid DRTS retransmissions ABCDE DRTS (B) OCTS (B,C) DATA ACK DRTS (E)

24. DWTS (contd…) DWTS – short control packet, includes time-to-wait before RTS retransmission ABCDE DRTS (B) OCTS (B,C) DATA ACK DRTS (E) DWTS (D)

25. Conflict-Free ACK 802.11 – Immediate ACK for reliability Minimal ACK collisions due to reserved transmission range D-MAC – No guarantee on ACK collisions Possible solutions – Use separate channels for DATA/ACK & RTS/CTS Use RTS/CTS for ACK packets

26. Location Information DRTS – accurate node locations required Hard to achieve in mobile nodes Solution: If location unknown - send ORTS (no loss of correctness) If location known – send DRTS

27. (Contd…) Stale location data: Include location information in RTS/CTS Set a threshold for DRTS transmissions If no response, switch to ORTS

28. Conclusions: Current MAC protocols – wasted bandwidth D-MAC – utilizes directional transmissions Scheme 1 – DRTS/OCTS Scheme 2 – DRTS, ORTS / OCTS Optimization using DWTS D-MAC outperforms 802.11 by allowing simultaneous transmissions