Performance Evaluation of Multiple Access Protocols for Ad hoc Networks Using Directional Antenna

1. Performance Evaluation of Multiple Access Protocols for Ad hoc Networks Using Directional Antenna Tamer ElBatt, Timothy Anderson, Bo Ryu WCNC 2003, March 2003 speaker：Chen-Chi Hsieh

2. Outline • Introduction • System Model • Channel Reservation Based on Directional Antennas Information • Results and Discussion

3. Introduction • The broadcast nature of omni-directional antenna • causes multi-user interference • thus limit the spatial reuse

4. Introduction –Related Work • Three schemes • Omni-directional reservation (802.11) • Directional reservation (DRTS/DCTS) • Hybrid reservation (DMAC using DRTS/OCTS) • Decide whether to proceed with a transmission in a specific direction or not • Rely on hearing/not hearing a reservation message

5. Introduction –Related Work (cont.) • Neighbor • N(x) ：Area covered by the radiation patterns of all beams at node x • Partition N(x) into Block-Neighbor BN(x) and Unblock-Neighbor UBN(x) • BN(x) ：at least one of its beams is blocked from transmission • UBN(x) ：not hear any reservation message • Cause collision to the ongoing transmission

6. Introduction –Related Work (cont.)Omni-directional Reservation(802.11) • All neighbors become aware of the ongoing transmission N(S)=｛1,2,3,4,5,6,D｝ BN(S)=N(S) UNB(S)=Φ N(D)=｛1,4,5,6,7,8,9,S｝ BN(D)=N(D) UNB(D)=Φ • Advantage： • Minimize the possibility of collisions • Drawback： • Too conservative

7. Introduction –Related Work (cont.)Directional Reservation • D-RTS/D-CTS N(S)=｛1,2,3,4,5,6,D｝ BN(S)=｛4,5,6,D｝ UBN(S)=｛1,2,3｝ N(D)=｛1,4,5,6,7,8,9,S｝ BN(D)=｛4,5,S｝ UBN(D)=｛1,6,7,8,9｝ • Advantages： • Initiates more simultaneous • reservation attempts • Trade-off between collisions and • spatial reuse • Drawbacks： • Unaware of each other • Highly subject to collisions

8. Introduction –Related Work (cont.)Hybrid Reservation • Use different combinations of omni-directional and directional reservation messages (D-RTS/O-CTS) N(S)=｛1,2,3,4,5,6,D｝ BN(S)=｛4,5,6,D｝ UBN(S)=｛1,2,3｝ N(D)=｛1,4,5,6,7,8,9,S｝ BN(D)=｛1,4,5,6,7,8,9,S｝ UBN(D)=Φ

9. A B C D E Introduction –Related works (cont.)Hybrid Reservation • “Medium Access Control Protocols Using Directional Antennas in Ad Hoc Networks” IEEE INFOCON 2000 Exploit the spatial reuse but raise collision problem DRTS(B) OCTS(B,C) OCTS(B,C) DRTS(D) DATA OCTS(D,E) OCTS(D,E) DATA ACK ACK

10. Introduction –Related works (cont.)Hybrid Reservation (cont.) • Drawbacks • Doesn’t strike a balance • minimizing control packet collisions • May lead to further collisions

11. Introduction (cont.) • New concepts in this paper • All neighbors of the source and the destination should beaware of the intended transmission，if possible • Antenna blocking decisions should be based on the information included in the RTS/CTS packets

12. Introduction (cont.) • The contributions of this paper • Balance the trade-off • between omni-directional and directional reservations • Resolve new type of collisions • Concept • Sending reservation packets carrying “directional antennas information” to as many neighbors as possible • Knowing their locations relative to the transmitter-receiver pair

13. System Model - Antenna Model • Antenna Model • Each node • a fixed number (B) of switched beams • Each of width θ＝2π/B radians • Transmitted energy • uniformly in a beam of width θ • Nodes receive omni-directionally • Beams are non-overlapping

14. S R A A System Model –Network Assumptions (cont.) • Each node obtains neighboring information by a node-discovery scheme • Information of each node • The identities of all neighbors • The identities of neighbors that lie within the coverage of each beam • Location of a node not allowed to engage in any communications transmit on any directional beam except the one pointing towards the transmitter

15. Channel Reservation Based on Directional Antennas Information • Balance the aforementioned trade-off • Send RTS/CTS packets over all unblocked beams • Add two fields to the RTS/CTS messages • the index of the directional beam currently being used • the index of the directional beam intended to be used

16. Channel Reservation Based on Directional Antennas Information (cont.) Node 2：(1)block only the beam pointing towards node S (2)can initiate a transmission with node 10 Node 5：not engage in any communications 10 1 2 9 5 8 S D O-RTS O-CTS 11 3 6 4 7

17. RTS RTS S D A Channel Reservation Based on Directional Antennas Information (cont.) • New types of collisions • A neighbor may miss reservation messages due to lying in the coverage of a blocked beam Node A：itmay be active or inactive， May cause collision later

18. Special reservation packets A：inactive RTS S D Channel Reservation Based on Directional Antennas Information (cont.) • Solution of collision • Use auxiliary channel to transmit special reservation packets on blocked beams • Suffering collisions from active users

19. Channel Reservation Based on Directional Antennas Information (cont.) • Two solutions for solving collision problem • Use auxiliary channel(s) to transmit special reservation packets on blocked beams • Suffering collisions from active users • Send “pending” RTS/CTS packets once the blocked beam becomes unblocked • Involves synchronization complexity

20. D1 S1 S2 D2 Special RTS/CTS S1：unaware the ongoing transmission DATA RTS DATA RTS

21. S1 RTS S2 D1 Pending RTS/CTS

22. Channel Reservation Based on Directional Antennas Information (cont.) • Solution of collision • Use auxiliary channel(s) to transmit special reservation packets on blocked beams • Suffering collisions from active users • Send “pending” RTS/CTS packets once the blocked beam becomes unblocked • Involves synchronization complexity • Transmit different packets on multiple beams at the same time

23. Results and Discussion • NS-2 simulator • Under a wide variety of network loads • Schemes • omni-directional reservation • directional reservation • hybrid reservation

24. Results and Discussion –Simulation Setup • Small networks：n = 50 nodes • A rectangular area：500 meters x 500 meters • Data packet size：500 bytes • No mobility • B = 6 switched beams per node • Each of 600 width • duration：900 sec.

25. Results and Discussion –Simulation Results Average number of data packets transmitted/sec Packet arrival rate/node λ(packets/sec)

26. Results and Discussion –Simulation Results (cont.) Average number of data packets collisions/sec Packet arrival rate/node λ(packets/sec)

27. Results and Discussion –Simulation Results (cont.) MAC throughput (Kbps) Packet arrival rate/node λ(packets/sec)

28. Results and Discussion –Simulation Results (cont.) MAC throughput (Kbps) Number of beams/node (B)

29. Results and Discussion –Simulation Results (cont.) • Drawbacks • Real switched beam antennas experience some overlapping • Solutions • minimizes beam overlap • Physical layer algorithms “captures” the strongest signal from interference in the overlapped areas • Carrying out reservations in a round-robin fashion over the course of K phases，where K may take values between 2 and B

30. Conclusions • Incorporating aggressiveness in the reservation scheme • Modifying the channel occupancy criterion to depend on information carried by the RTS/CTS messages • Candidate solutions for combating new types of collisions inherent to directional antennas