MultiChannel MAC for Ad Hoc Networks Handling MultiChannel

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

Introduction Motivation Problem Statement

Motivation • Multiple Channels available in IEEE 802. 11 – 3 channels in 802. 11 b – 12 channels in 802. 11 a • Utilizing multiple channels can improve throughput – Allow simultaneous transmissions 1 1 defer Single channel 2 Multiple 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 1 2 • 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 802. 11 DCF to work in multi-channel environment

Preliminaries 802. 11 Distributed Coordination Function (DCF) 802. 11 Power Saving Mechanism (PSM)

802. 11 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)

802. 11 Distributed Coordination Function A A B C D Time

802. 11 Distributed Coordination Function RTS A C D Time A B B RTS

802. 11 Distributed Coordination Function CTS A RTS C D C NAV A B B SIFS D Time

802. 11 Distributed Coordination Function A RTS CTS SIFS D Time DATA C D C NAV A B B DATA NAV

802. 11 Distributed Coordination Function ACK A RTS D Time DATA CTS C D C NAV A B B SIFS ACK NAV

802. 11 Distributed Coordination Function A RTS D Time DATA CTS C D C NAV A B B SIFS ACK Contention Window NAV DIFS

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

802. 11 Power Saving Mechanism Beacon Time A B C ATIM Window Beacon Interval

802. 11 Power Saving Mechanism Beacon A Time ATIM B C ATIM Window Beacon Interval

802. 11 Power Saving Mechanism Beacon A B Time ATIM-ACK C ATIM Window Beacon Interval

802. 11 Power Saving Mechanism Beacon A B ATIM-RES ATIM-ACK C ATIM Window Beacon Interval Time

802. 11 Power Saving Mechanism Beacon A B ATIM-RES Time DATA ATIM-ACK Doze Mode C ATIM Window Beacon Interval

802. 11 Power Saving Mechanism Beacon A B ATIM-RES ATIM-ACK Time DATA ACK Doze Mode C ATIM Window Beacon Interval

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

Hidden Terminal Problem A DATA B C C does not hear A’s transmission

Hidden Terminal Problem A DATA B C C starts transmitting – collides at B

Solution: Virtual Carrier Sensing D A RTS B C A sends RTS D overhears RTS and defers transmission

Solution: Virtual Carrier Sensing D A CTS B C B sends CTS C overhears CTS and defers transmission

Solution: Virtual Carrier Sensing D A DATA B A sends DATA to B C

Solution: Virtual Carrier Sensing D A RTS B C D overhears RTS and defers transmission

Multi-Channel Hidden Terminals • Consider the following naïve protocol – Static channel assignment (based on node ID) – Communication takes place on receiver’s channel • Sender switches its channel to receiver’s channel before transmitting

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

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

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

Related Work Previous work on multi-channel MAC

Nasipuri’s Protocol • Assumes N transceivers per host – Capable of listening to all channels simultaneously • Sender searches for an idle channel and transmits on the channel [Nasipuri 99 WCNC] • Extensions: channel selection based on channel condition on the receiver side [Nasipuri 00 VTC] • Disadvantage: High hardware cost

Wu’s Protocol [Wu 00 ISPAN] • 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 transmits RES (Reservation) – Sender sends DATA on the selected data channel

Wu’s Protocol (cont. ) • Advantage – No synchronization required • Disadvantage – Each host must have 2 transceivers – Per-packet channel switching can be expensive – 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

Protocol Description Multi-Channel MAC (MMAC) Protocol

Proposed Protocol (MMAC) • Assumptions – Each node is equipped with a single transceiver – The transceiver is capable of switching channels – Channel switching delay is approximately 250 us • Per-packet switching not recommended • Occasional channel switching not to expensive – Multi-hop synchronization is achieved by other means

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

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

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

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

Channel Negotiation Common Channel A B Selected Channel ATIM RES(1) Beacon ATIMACK(1) C D Time ATIM Window Beacon Interval

Channel Negotiation Common Channel A B C D Selected Channel ATIM RES(1) Beacon ATIMACK(1) ATIMACK(2) ATIMRES(2) Time ATIM Window Beacon Interval

Channel Negotiation Common Channel A B C D ATIM RES(1) Selected Channel RTS DATA Channel 1 Beacon Channel 1 ATIMACK(1) ATIMACK(2) CTS ACK Channel 2 ATIMRES(2) RTS DATA ATIM Window Beacon Interval Time

Performance Evaluation Simulation Model Simulation Results

Simulation Model • • • ns-2 simulator Transmission rate: 2 Mbps Transmission range: 250 m Traffic type: Constant Bit Rate (CBR) Beacon interval: 100 ms • Packet size: 512 bytes • ATIM window size: 20 ms • Default number of channels: 3 channels • Compared protocols – 802. 11: IEEE 802. 11 single channel protocol – DCA: Wu’s protocol – MMAC: Proposed protocol

Aggregate Throughput (Kbps) Wireless LAN - Throughput 2500 MMAC 2000 DCA 1500 1000 500 2000 MMAC 1500 DCA 1000 802. 11 1 10 1000 Packet arrival rate per flow (packets/sec) 30 nodes 500 1 802. 11 10 1000 Packet arrival rate per flow (packets/sec) 64 nodes MMAC shows higher throughput than DCA and 802. 11

Aggregate Throughput (Kbps) Multi-hop Network – Throughput 1500 MMAC DCA 1000 2000 MMAC 1500 DCA 1000 500 802. 11 0 1 10 1000 Packet arrival rate per flow (packets/sec) 3 channels 500 802. 11 0 1 10 1000 Packet arrival rate per flow (packets/sec) 4 channels

Aggregate Throughput (Kbps) Throughput of DCA and MMAC (Wireless LAN) 4000 6 channels 3000 6 channels 2000 2 channels 1000 802. 11 0 0 Packet arrival rate per flow (packets/sec) DCA Packet arrival rate per flow (packets/sec) MMAC shows higher throughput compared to DCA

Analysis of Results • DCA – Bandwidth of control channel significantly affects performance – Narrow control channel: High collision and congestion of control packets – Wide control channel: Waste of bandwidth – It is difficult to adapt control channel bandwidth dynamically • MMAC – ATIM window size significantly affects performance – ATIM/ATIM-ACK/ATIM-RES exchanged once per flow per beacon interval – reduced overhead • Compared to packet-by-packet control packet exchange in DCA – ATIM window size can be adapted to traffic load

Conclusion & Future Work

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! jso 1@uiuc. edu