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ECE 256, Spring 2008

Multi-Channel MAC for Ad Hoc Networks: Handling Multi-Channel Hidden Terminals Using A Single Transceiver . Jungmin So & Nitin Vaidya University of Illinois at Urbana-Champaign (Paper presented at ACM MobiHoc ‘04) Presenter: Rahul Ghosh, ECE Dept., Duke University . ECE 256, Spring 2008.

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ECE 256, Spring 2008

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  1. Multi-Channel MAC for Ad Hoc Networks: Handling Multi-Channel Hidden Terminals Using A Single Transceiver Jungmin So & Nitin Vaidya University of Illinois at Urbana-Champaign (Paper presented at ACM MobiHoc ‘04) Presenter: Rahul Ghosh, ECE Dept., Duke University ECE 256, Spring 2008

  2. Acknowledgments Slides courtesy: Jungmin So and Nitin Vaidya http://www.crhc.uiuc.edu/wireless/groupPubs.html ECE 256 / CS 215, Spring 2008

  3. 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 ECE 256 / CS 215, Spring 2008

  4. 1 1 2 defer Motivation • ‘Exploit multiple channels to improve network throughput’ … why ? • More number of parallel communications possible • Standard supports … • 802.11b – 14 channels in PHY layer – 3 of them are used • 802.11a – 12 channels – 8 in the lower part of the spectra and rest in higher ECE 256 / CS 215, Spring 2008

  5. 1 2 Problem Statement • The ideal scenario – use k channels to improve throughput by a factor of k • Reality is different… • Nodes on listening to different channels can not talk to each other • Listen one channel at a time – constraint with single transciever • Goal: Exploit multiple channels using a single transciever • Requires modification of coordination schemes among the nodes ECE 256 / CS 215, Spring 2008

  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 ECE 256 / CS 215, Spring 2008

  7. Preliminaries • 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) ECE 256 / CS 215, Spring 2008

  8. A B C D Time A B C D 802.11 DCF ECE 256 / CS 215, Spring 2008

  9. Time A RTS B C D 802.11 DCF RTS A B C D ECE 256 / CS 215, Spring 2008

  10. CTS A B C D Time A NAV RTS B CTS C SIFS D 802.11 DCF ECE 256 / CS 215, Spring 2008

  11. DATA A B C D Time A NAV NAV RTS B CTS DATA C SIFS D 802.11 DCF ECE 256 / CS 215, Spring 2008

  12. ACK A B C D Time A NAV NAV RTS B CTS DATA ACK C SIFS D 802.11 DCF ECE 256 / CS 215, Spring 2008

  13. Beacon Time A B C ATIM Window Beacon Interval Preliminaries • 802.11 PSM (Power Saving Mode) • Doze mode – less energy consumption but no communication • ATIM – Ad hoc Traffic Indication Message ECE 256 / CS 215, Spring 2008

  14. Beacon Time ATIM A B C ATIM Window Beacon Interval Preliminaries ECE 256 / CS 215, Spring 2008

  15. Beacon Time ATIM A B ATIM-ACK C ATIM Window Beacon Interval Preliminaries ECE 256 / CS 215, Spring 2008

  16. Beacon Time ATIM ATIM-RES A B ATIM-ACK C ATIM Window Beacon Interval Preliminaries ECE 256 / CS 215, Spring 2008

  17. Beacon Time ATIM ATIM-RES DATA A B ATIM-ACK Doze Mode C ATIM Window Beacon Interval Preliminaries ECE 256 / CS 215, Spring 2008

  18. Beacon Time ATIM ATIM-RES DATA A B ATIM-ACK ACK Doze Mode C ATIM Window Beacon Interval Preliminaries ECE 256 / CS 215, Spring 2008

  19. 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 ECE 256 / CS 215, Spring 2008

  20. 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 ECE 256 / CS 215, Spring 2008

  21. Multi-channel Hidden Terminals ECE 256 / CS 215, Spring 2008

  22. Multi-channel Hidden Terminals • Observations • Nodes may listen to different channels • Virtual Carrier Sensing becomes difficult • The problem was absent for single channel • Possible approaches • Exploit synchronization technique available from IEEE 802.11 PSM • Use multiple transcievers ECE 256 / CS 215, Spring 2008

  23. 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 ECE 256 / CS 215, Spring 2008

  24. Related Works • Nasipuri et. al proposed for a scheme with 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 • Cost becomes higher ECE 256 / CS 215, Spring 2008

  25. Related Works • Wu et. al talks about a scheme with 2 transceivers per host • RTS/CTS/RES packets sent on control channel • Sender includes PCL list in RTS, receiver picks one and tells in CTS • Sender transmits RES and sends data on agreed channel • No synch is required • Per packet channel switching can be expensive • Control channel’s BW becomes an issue ECE 256 / CS 215, Spring 2008

  26. 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 ECE 256 / CS 215, Spring 2008

  27. 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 ECE 256 / CS 215, Spring 2008

  28. 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 agreed upon channel after the ATIM window duration ECE 256 / CS 215, Spring 2008

  29. 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 ECE 256 / CS 215, Spring 2008

  30. 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 ECE 256 / CS 215, Spring 2008

  31. Common Channel Selected Channel A Beacon B C D Time ATIM Window Beacon Interval MMAC ECE 256 / CS 215, Spring 2008

  32. MMAC Common Channel Selected Channel ATIM- RES(1) ATIM A Beacon B ATIM- ACK(1) C D Time ATIM Window ECE 256 / CS 215, Spring 2008

  33. MMAC 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 ECE 256 / CS 215, Spring 2008

  34. MMAC Common Channel Selected Channel ATIM- RES(1) ATIM RTS DATA Channel 1 Beacon Channel 1 CTS ACK ATIM- ACK(1) ATIM- ACK(2) CTS Channel 2 ACK Channel 2 DATA ATIM Time ATIM- RES(2) RTS ATIM Window Beacon Interval ECE 256 / CS 215, Spring 2008

  35. 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 ECE 256 / CS 215, Spring 2008

  36. 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 ECE 256 / CS 215, Spring 2008

  37. WLAN - Throughput ECE 256 / CS 215, Spring 2008

  38. Multihop Network - Throughput ECE 256 / CS 215, Spring 2008

  39. 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 ECE 256 / CS 215, Spring 2008

  40. 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 ECE 256 / CS 215, Spring 2008

  41. 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 • Beaconing mechanism may fail to synchronize in a multi-hop network – probabilistic beaconing may help • 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 ECE 256 / CS 215, Spring 2008

  42. Two 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 ECE 256 / CS 215, Spring 2008

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