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ECE 256 - Spring 2010

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. ECE 256 - Spring 2010. Acknowledgments. Slides from: Jungmin So and Nitin Vaidya

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ECE 256 - Spring 2010

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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 ECE 256 - Spring 2010

  2. Acknowledgments Slides from: Jungmin So and Nitin Vaidya http://www.crhc.uiuc.edu/wireless/groupPubs.html Modified and Presented by: Federico Gonzalez ECE 256 - Spring 2010

  3. 1 1 2 defer Motivation • ‘Exploit multiple channels to improve network throughput’ … why ? • Greater parallel communication is possible ECE 256 - Spring 2010

  4. 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 transceiver • Goal: Exploit multiple channels using a single transceiver • Requires modification of coordination schemes among the nodes ECE 256 - Spring 2010

  5. 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 - Spring 2010

  6. A B C D Time A B C D 802.11 DCF ECE 256 - Spring 2010

  7. Time A RTS B C D 802.11 DCF RTS A B C D ECE 256 - Spring 2010

  8. CTS A B C D Time A NAV RTS B CTS C SIFS D 802.11 DCF ECE 256 - Spring 2010

  9. DATA A B C D Time A NAV NAV RTS B CTS DATA C SIFS D 802.11 DCF ECE 256 - Spring 2010

  10. ACK A B C D Time A NAV NAV RTS B CTS DATA ACK C SIFS D 802.11 DCF ECE 256 - Spring 2010

  11. Beacon Time A B C ATIM Window Beacon Interval 802.11 PSM (Power Saving Mode) • Doze mode – less energy consumption but no communication • ATIM – Ad hoc Traffic Indication Message ECE 256 - Spring 2010

  12. Beacon Time ATIM A B C ATIM Window Beacon Interval 802.11 PSM (Power Saving Mode) ECE 256 - Spring 2010

  13. Beacon Time ATIM A B ATIM-ACK C ATIM Window Beacon Interval 802.11 PSM (Power Saving Mode) ECE 256 - Spring 2010

  14. Beacon Time ATIM ATIM-RES A B ATIM-ACK C ATIM Window Beacon Interval 802.11 PSM (Power Saving Mode) ECE 256 - Spring 2010

  15. Beacon Time ATIM ATIM-RES DATA A B ATIM-ACK Doze Mode C ATIM Window Beacon Interval 802.11 PSM (Power Saving Mode) ECE 256 - Spring 2010

  16. Beacon Time ATIM ATIM-RES DATA A B ATIM-ACK ACK Doze Mode C ATIM Window Beacon Interval 802.11 PSM (Power Saving Mode) ECE 256 - Spring 2010

  17. 802.11 PSM (Power Saving Mode) Summary • 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 - Spring 2010

  18. Multi-channel Hidden Terminals ECE 256 - Spring 2010

  19. 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 transceivers ECE 256 - Spring 2010

  20. 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 - Spring 2010

  21. 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 - Spring 2010

  22. 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 - Spring 2010

  23. 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 - Spring 2010

  24. Common Channel Selected Channel A Beacon B C D Time ATIM Window Beacon Interval MMAC ECE 256 - Spring 2010

  25. MMAC Common Channel Selected Channel ATIM- RES(1) ATIM A Beacon B ATIM- ACK(1) C D Time ATIM Window ECE 256 - Spring 2010

  26. 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 - Spring 2010

  27. MMAC Common Channel Selected Channel ATIM- RES(1) ATIM RTS DATA Channel 1 A Beacon Channel 1 B CTS ACK ATIM- ACK(1) ATIM- ACK(2) CTS Channel 2 ACK C Channel 2 D DATA ATIM Time ATIM- RES(2) RTS ATIM Window Beacon Interval ECE 256 - Spring 2010

  28. Experimental 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 - Spring 2010

  29. WLAN - Throughput ECE 256 - Spring 2010

  30. Multi-hop Network - Throughput ECE 256 - Spring 2010

  31. Analysis • - 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 - Spring 2010

  32. 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 • Starvation can occur with common source and multiple destinations ECE 256 - Spring 2010

  33. Questions?

  34. Backup Slides

  35. 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 - Spring 2010

  36. 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 - Spring 2010

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