1. ECE 256, Spring 2009 __________ Multi-Channel MAC for Ad Hoc Networks: Handling Multi-Channel Hidden Terminals Using A Single Transceiver __________________ Paper by Jungmin So & Nitin Vaidya University of Illinois at Urbana-Champaign ACM MobiHoc ‘04 Presenter: Sandip Agrawal, Duke University

2. Acknowledgments Slides courtesy: Jungmin So and Nitin Vaidya http://www.crhc.uiuc.edu/wireless/groupPubs.html

3. Topics  Introduction o Motivation o Problem Statement  Preliminaries o 802.11 DCF structure o 802.11 PSM mode  Issues in multi-channel environment  Other works in multi-channel MAC  Proposed MMAC  Simulation results  Discussions

4. Motivation  Multiple Channels available in IEEE 802.11 802.11b – 14 channels in PHY layer – 3 of them are used (1,6,11) 802.11a – 12 channels – 8 in the lower part of the band for indoor use and rest in higher for outdoor us  ‘Exploit multiple channels to improve network throughput’ … why ? Allow Simultaneous Transmissions 1 defer 1 2 Single channel Multiple Channels

5. Problem Statement  The ideal scenario – use k channels to improve throughput by a factor of k Reality is different…Nodes listening on different channels cannot talk to each other  Constraint : Single Transceiver - Can listen to only one channel at a time  Goal: Design a MAC protocol that utilizes multiple channels to improve overall performance (at least possible cost and complexity) 1 2

6. Topics  Introduction  Motivation  Problem Statement  Preliminaries  802.11 DCF structure  802.11 PSM mode  Issues in multi-channel environment  Other works in multi-channel MAC  Proposed MMAC  Simulation results  Discussions

7. 802.11 DCF (Distributed Coordinate Function)  Designed for sharing a single channel between the hosts  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)

8. 802.11 DCF ABCD A B C D Time

9. 802.11 DCF ABCD RTS A B C D Time

10. 802.11 DCF ABCD CTS A B C D RTS CTS SIFS NAV Time

11. 802.11 DCF A B C D ABCD RTS CTS DATA SIFS NAV Time DATA

12. 802.11 DCF A B C D ABCD RTS CTS DATA SIFS ACK NAV Time ACK

13. 802.11 DCF A B C D ABCD RTS CTS DATA SIFS ACK NAV Time Contention Window

14. 802.11 PSM (Power Saving Mechanism) A B C Time Beacon ATIM Window Beacon Interval  Doze mode – less energy consumption but no communication ATIM – Ad hoc Traffic Indication Message

15. 802.11 PSM (Power Saving Mechanism) A B C Time Beacon ATIM ATIM Window Beacon Interval

16. 802.11 PSM (Power Saving Mechanism) A B C Time Beacon ATIM ATIM-ACK ATIM Window Beacon Interval

17. 802.11 PSM (Power Saving Mechanism) A B C Time Beacon ATIM ATIM-ACK ATIM-RES ATIM Window Beacon Interval

18. 802.11 PSM (Power Saving Mechanism) A B C Time Beacon ATIM ATIM-ACK DATAATIM-RES Doze Mode ATIM Window Beacon Interval

19. 802.11 PSM (Power Saving Mechanism) A B C Time Beacon ATIM ATIM-ACK DATA ACK ATIM-RES Doze Mode ATIM Window Beacon Interval

20. In essence …  All nodes wake up at the beginning of a beacon interval for a fixed duration of time (ATIM window)  Exchange ATIM 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

21. Topics  Introduction  Motivation  Problem Statement  Preliminaries  802.11 DCF structure  802.11 PSM mode  Issues in multi-channel environment  Other works in multi-channel MAC  Proposed MMAC  Simulation results  Discussions

22. Multi-channel Hidden Terminals

23.  Observations 1.Nodes may listen to different channels 2.Virtual Carrier Sensing becomes difficult 3.The problem was absent for single channel  Possible approaches 1.Use multiple transcievers 2.Exploit synchronization technique available from IEEE 802.11 PSM

24. Topics  Introduction  Motivation  Problem Statement  Preliminaries  802.11 DCF structure  802.11 PSM mode  Issues in multi-channel environment  Other works in multi-channel MAC  Proposed MMAC  Simulation results  Discussions

25. Nasipuri’s Protocol  N transceivers per host - Capable of listening all channels simultaneously Find an idle channel and transmit – sender’s policy Channel selection should be based on channel condition on receiver side High hardware cost

26. Wu’s Protocol  2 transceivers per host One transceiver always listens on control channel Sender includes preferred channel list in RTS, receiver picks one and tells in CTS Sender sends DATA on the selected data channel  No synchronization required  Control channel’s BW becomes 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

27. Topics  Introduction  Motivation  Problem Statement  Preliminaries  802.11 DCF structure  802.11 PSM mode  Issues in multi-channel environment  Other works in multi-channel MAC  Proposed MMAC  Simulation results  Discussions

28. 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 -Multi-hop synch is achieved by other means

29. MMAC  Idea similar to IEEE 802.11 PSM - 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 agreed upon channel after the ATIM window duration

30. MMAC  Preferred Channel List (PCL) -For a node, PCL records usage of channels inside Transmission 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

31. MMAC  Channel Negotiation -In ATM window, sender transmits ATIM …. Includes its PCL -Receiver selects a channel based on sender’s PCL and its own PCL Order of preference: HIGH > MID > LOW Tie breaker: Receiver’s PCL has higher priority For “LOW” channels: channels with smaller count have higher priority -Receiver sends ATIM-ACK to sender including the selected channel -Sender sends ATIM-RES to notify its neighbors of the selected channel

32. MMAC A B C D Time ATIM Window Beacon Interval Common ChannelSelected Channel Beacon

33. MMAC A B C D ATIM ATIM- ACK(1) ATIM- RES(1) Time ATIM Window Common ChannelSelected Channel Beacon

34. MMAC A B C D ATIM ATIM- ACK(1) ATIM- RES(1) ATIM- ACK(2) ATIM ATIM- RES(2) Time ATIM Window Common ChannelSelected Channel Beacon

35. MMAC ATIM ATIM- ACK(1) ATIM- RES(1) ATIM- ACK(2) ATIM ATIM- RES(2) Time ATIM Window Beacon Interval Common ChannelSelected Channel Beacon RTS CTS RTS CTS DATA ACK DATA Channel 1 Channel 2

36. Topics  Introduction  Motivation  Problem Statement  Preliminaries  802.11 DCF structure  802.11 PSM mode  Issues in multi-channel environment  Other works in multi-channel MAC  Proposed MMAC  Simulation results  Discussions

37. Parameters 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

38. WLAN - Throughput MMAC DCA 802.11 MMAC DCA 802.11 30 nodes 64 nodes MMAC shows higher throughput than DCA and 802.11

39. Multihop Network - Throughput 3 channels4 channels MMAC DCA 802.11 DCA MMAC Packet arrival rate per flow (packets/sec) 1 10 100 1000 Packet arrival rate per flow (packets/sec) 1 10 100 1000 Aggregate Throughput (Kbps) 1500 1000 500 0 2000 1500 1000 500 0

40. Throughput of DCA and MMAC DCA MMAC 2 channels 802.11 MMAC shows higher throughput compared to DCA 6 channels 802.11 2 channels 6 channels Aggregate Throughput (Kbps) 4000 3000 2000 1000 0 4000 3000 2000 1000 0 Packet arrival rate per flow (packets/sec)

41. Analysis of Results  For DCA: -BW of control channel significantly affects the performance and it’s difficult to adapt control channel BW  For MMAC: 1.ATIM window size significantly affects performance 2.ATIM/ATIM-ACK/ATIM-RES exchanged once per flow per beacon interval – reduced overhead 3.ATIM window size can be adapted to traffic load

42. Topics  Introduction  Motivation  Problem Statement  Preliminaries  802.11 DCF structure  802.11 PSM mode  Issues in multi-channel environment  Other works in multi-channel MAC  Proposed MMAC  Simulation results  Discussions

43. Discussions  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  Instead of counting source-destination pair for calculating channel usage, counting the number of pending packets may be a better idea  Starvation can occur with common source and multiple destinations

44. Questions???  While criticizing Wu’s protocol – control channel ‘prevents the data channel from being fully utilized’ … why ?  Source and Destinations may not be in one hop distance and may not be communicated within a beacon interval

45. Thank You!