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Jungmin So and Nitin Vaidya Dept. of Computer Science University of Illinois Urbana_Champaign

Multi-Channel MAC for Ad Hoc Networks: Handling Multi-Channel Hidden Terminals Using A Single Transceiver. Jungmin So and Nitin Vaidya Dept. of Computer Science University of Illinois Urbana_Champaign. 1. 1. 2. defer. Motivation. Multiple channels in 802.11 standard.

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Jungmin So and Nitin Vaidya Dept. of Computer Science University of Illinois Urbana_Champaign

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  1. Multi-Channel MAC for Ad Hoc Networks: Handling Multi-Channel Hidden Terminals Using A Single Transceiver Jungmin So and Nitin Vaidya Dept. of Computer Science University of Illinois Urbana_Champaign

  2. 1 1 2 defer Motivation • Multiple channels in 802.11 standard. • 802.11 MAC is designed for one channel. • To design a new MAC to exploit multiple channels. • Many benefit, throughput gain • Assume only one transceiver. • Goal: to improve overall performance Single channel Multiple Channels

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

  4. 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

  5. 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

  6. 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

  7. 802.11 Power Saving Mechanism Beacon Time A B C ATIM Window Beacon Interval

  8. 802.11 Power Saving Mechanism Beacon Time ATIM A B C ATIM Window Beacon Interval

  9. 802.11 Power Saving Mechanism Beacon Time ATIM A B ATIM-ACK C ATIM Window Beacon Interval

  10. 802.11 Power Saving Mechanism Beacon Time ATIM DATA A B ATIM-ACK Doze Mode C ATIM Window Beacon Interval

  11. 802.11 Power Saving Mechanism Beacon Time ATIM DATA A B ATIM-ACK ACK Doze Mode C ATIM Window Beacon Interval

  12. MMAC • Idea similar to IEEE 802.11 PSM • Divide time into beacon intervals • At the beginning of each beacon interval, all nodes must listen to a predefined common channel for a fixed duration of time (ATIM window) • Nodes negotiate channels using ATIM messages • Nodes switch to selected channels after ATIM window for the rest of the beacon interval

  13. Preferred Channel List (PCL) • Each node maintains PCL • Records usage of channels inside the transmission range • High preference (HIGH) • Already selected for the current beacon interval • Medium preference (MID) • No other vicinity node has selected this channel • Low preference (LOW) • This channel has been chosen by vicinity nodes • Count number of nodes that selected this channel to break ties

  14. Channel Negotiation • In ATIM window, sender transmits ATIM to the receiver • Sender includes its PCL in the ATIM packet • 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

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

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

  17. 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

  18. 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

  19. 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

  20. Discussions • Clock Synchronization • Overhead due to message exchange • Delay • Some fundamental problems in multi-hop wireless networks remain unsolved

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