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

Multiple Channels available in IEEE –3 channels in b –12 channels in a Utilizing multiple channels can improve throughput –Allow simultaneous transmissions Motivation 1 defer 1 2 Single channelMultiple Channels

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 DCF to work in multi-channel environment 1 2

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)

Distributed Coordination Function A B C D ABCD Time

Distributed Coordination Function A B C D ABCD RTS Time RTS

Distributed Coordination Function A B C D ABCD RTS CTS SIFS NAV Time CTS

Distributed Coordination Function A B C D ABCD RTS CTS DATA SIFS NAV Time DATA

Distributed Coordination Function A B C D ABCD RTS CTS DATA SIFS ACK NAV Time ACK

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

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

Power Saving Mechanism A B C Time Beacon ATIM Window Beacon Interval

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

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

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

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

Related Work Previous work on multi-channel MAC

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

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

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

MMAC Assumptions -All channels have same BW and none of them are overlapping ch annels -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 switc hing -Nodes synchronized: Begin their beacon intervals same time

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

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

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

Channel Negotiation A B C D Time ATIM Window Beacon Interval Common ChannelSelected Channel Beacon

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

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

Channel Negotiation A B C D 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

Performance Evaluation Simulation Model Simulation Results

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 single channel protocol –DCA: Wu’s protocol –MMAC: Proposed protocol

Wireless LAN - Throughput 30 nodes64 nodes MMAC DCA MMAC shows higher throughput than DCA and DCA MMAC Packet arrival rate per flow (packets/sec) Aggregate Throughput (Kbps)

Multi-hop Network – Throughput 3 channels4 channels MMAC DCA DCA MMAC Packet arrival rate per flow (packets/sec) Packet arrival rate per flow (packets/sec) Aggregate Throughput (Kbps)

Analysis 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

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

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

Thank you! Sanhita Ganguly