Jungmin So and Nitin Vaidya University of Illinois at Urbana-Champaign

1. Multi-Channel MAC for Ad Hoc Networks: Handling Multi-Channel Hidden Terminals Using A Single Transceiver Jungmin So and Nitin Vaidya University of Illinois at Urbana-Champaign

2. 1 1 2 defer Motivation • Multiple Channels available in IEEE 802.11 • 3 channels in 802.11b • 12 channels in 802.11a • Utilizing multiple channels can improve throughput • Allow simultaneous transmissions Single channel Multiple Channels

3. 1 2 Problem Statement • Using k channels does not translate into throughput improvement by a factor of k • Nodes listening on different channels cannot talk to each other • Requires modification of coordination schemes among the nodes • Constraint: Each node has only a single transceiver • Capable of listening to one channel at a time • Goal: Design a MAC protocol that utilizes multiple channels to improve overall performance • Modify 802.11 DCF to work in multi-channel environment

4. 802.11 Distributed Coordination Function • Virtual carrier sensing • Sender sends Ready-To-Send (RTS) • Receiver sends Clear-To-Send (CTS) • RTS and CTS reserves the area around sender and receiver for the duration of dialogue • Nodes that overhear RTS and CTS defer transmissions by setting Network Allocation Vector (NAV)

5. 802.11 Distributed Coordination Function A B C D Time A B C D

6. 802.11 Distributed Coordination Function RTS A B C D Time A RTS B C D

7. NAV CTS 802.11 Distributed Coordination Function CTS A B C D Time A RTS B C SIFS D

8. NAV NAV DATA CTS 802.11 Distributed Coordination Function DATA A B C D Time A RTS B C SIFS D

9. NAV NAV ACK DATA CTS 802.11 Distributed Coordination Function ACK A B C D Time A RTS B C SIFS D

10. NAV NAV ACK CTS DATA 802.11 Distributed Coordination Function A B C D Time A RTS B C Contention Window SIFS D DIFS

11. 802.11 Power Saving Mechanism • Time is divided into beacon intervals • All nodes wake up at the beginning of a beacon interval for a fixed duration of time (ATIM window) • Exchange ATIM (Ad-hoc Traffic Indication Message) during ATIM window • Nodes that receive ATIM message stay up during for the whole beacon interval • Nodes that do not receive ATIM message may go into doze mode after ATIM window

12. 802.11 Power Saving Mechanism Beacon Time A B C ATIM Window Beacon Interval

13. Issues in Multi-Channel Environment Multi-Channel Hidden Terminal Problem

14. A C B Multi-Channel Hidden Terminals Channel 1 Channel 2 RTS A sends RTS

15. A C B Multi-Channel Hidden Terminals Channel 1 Channel 2 CTS B sends CTS C does not hear CTS because C is listening on channel 2

16. A B Multi-Channel Hidden Terminals Channel 1 Channel 2 DATA RTS C C switches to channel 1 and transmits RTS Collision occurs at B

17. Related Work Previous work on multi-channel MAC

18. Nasipuri’s Protocol • Assumes N transceivers per host • Capable of listening to all channels simultaneously • Always have information for all channels • Disadvantage: High hardware cost

19. Wu’s Protocol [Wu00ISPAN]Dynamic Channel Assignment • Assumes 2 transceivers per host • One transceiver always listens on control channel • Negotiate channels using RTS/CTS/RES • RTS/CTS/RES packets sent on control channel • Sender includes preferred channels in RTS • Receiver decides a channel and includes in CTS • Sender sends DATA on the selected data channel

20. Wu’s Protocol (cont.) • Advantage • No synchronization required • Disadvantage • Each host must have 2 transceivers • Control channel bandwidth is an issue • Too small: control channel becomes a bottleneck • Too large: waste of bandwidth • Optimal control channel bandwidth depends on traffic load, but difficult to dynamically adapt

21. MMAC • Assumptions • All channels have same BW and none of them are overlapping channels • Nodes have only one transceiver • Transceivers are capable of switching channels but they are half-duplex • Channel switching delay is approx 250 us, avoid per packet switching • Nodes synchronized: Begin their beacon intervals same time

22. MMAC • Steps – • - Divide time into beacon intervals • At the beginning, nodes listen to a pre-defined channel for ATIM window duration • Channel negotiation starts using ATIM messages • Nodes switch to the selected channel after the ATIM window duration

23. MMAC • Preferred Channel List (PCL) • For a node, PCL records usage of channels inside Tx range • HIGH preference – always selected • MID preference – others in the vicinity did not select the channel • LOW preference – others in the vicinity selected the channel

24. MMAC • Channel Negotiation • Sender transmits ATIM to the receiver and includes its PCL in the ATIM packet • Receiver selects a channel based on sender’s PCL and its own PCL • Receiver sends ATIM-ACK to sender including the selected channel • Sender sends ATIM-RES to notify its neighbors of the selected channel

25. Channel Negotiation Common Channel Selected Channel A Beacon B C D Time ATIM Window Beacon Interval

26. Channel Negotiation Common Channel Selected Channel ATIM- RES(1) ATIM A Beacon B ATIM- ACK(1) C D Time ATIM Window Beacon Interval

27. Channel Negotiation Common Channel Selected Channel ATIM- RES(1) ATIM A Beacon B ATIM- ACK(1) ATIM- ACK(2) C D ATIM Time ATIM- RES(2) ATIM Window Beacon Interval

28. Channel Negotiation Common Channel Selected Channel ATIM- RES(1) RTS DATA Channel 1 ATIM A Beacon Channel 1 B CTS ACK ATIM- ACK(1) ATIM- ACK(2) CTS ACK Channel 2 C Channel 2 D ATIM DATA RTS Time ATIM- RES(2) ATIM Window Beacon Interval

29. Performance Evaluation Simulation Model Simulation Results

30. Simulation Model • ns-2 simulator • Transmission rate: 2Mbps • Transmission range: 250m • Traffic type: Constant Bit Rate (CBR) • Beacon interval: 100ms • Packet size: 512 bytes • ATIM window size: 20ms • Default number of channels: 3 channels • Compared protocols • 802.11: IEEE 802.11 single channel protocol • DCA: Wu’s protocol • MMAC: Proposed protocol

31. Wireless LAN - Throughput 2500 2000 1500 1000 500 2500 2000 1500 1000 500 MMAC MMAC DCA DCA Aggregate Throughput (Kbps) 802.11 802.11 1 10 100 1000 1 10 100 1000 Packet arrival rate per flow (packets/sec) Packet arrival rate per flow (packets/sec) 30 nodes 64 nodes MMAC shows higher throughput than DCA and 802.11

32. Multi-hop Network – Throughput 2000 1500 1000 500 0 1500 1000 500 0 MMAC MMAC DCA DCA Aggregate Throughput (Kbps) 802.11 802.11 1 10 100 1000 1 10 100 1000 Packet arrival rate per flow (packets/sec) Packet arrival rate per flow (packets/sec) 3 channels 4 channels

33. Analysis • For DCA: BW of control channel significantly affects the performance and it’s difficult to adapt control channel BW • - For MMAC: • ATIM window size significantly affects performance • ATIM/ATIM-ACK/ATIM-RES exchanged once per flow per beacon interval – reduced overhead • ATIM window size can be adapted to traffic load

34. Conclusion • MMAC requires a single transceiver per host to work in multi-channel ad hoc networks • MMAC achieves throughput performance comparable to a protocol that requires multiple transceivers per host

35. Future Work • Dynamic adaptation of ATIM window size based on traffic load for MMAC • Efficient multi-hop clock synchronization • Routing protocols for multi-channel environment

36. Thank you! Sanhita Ganguly