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 Modified and Presented by Yong Yang University of Illinois at Urbana-Champaign

Problem Statement • Utilizing multiple channels can improve throughput – Allow simultaneous transmissions 1 1 defer Single channel 2 Multiple Channels • Multiple Channels available in IEEE 802. 11 – 3 non-overlapping channels in 802. 11 b • But, 802. 11 MAC protocol is only for a single channel – Doesn’t work well due to Multi-channel Hidden Terminal Problem • Naïve method: a node has a transceiver for each channel

Overview • Goal: Design a MAC protocol that utilizes multiple channels to improve overall performance – Modify 802. 11 DCF to work in multi-channel environment • Constraint: Each node has only a single transceiver – Capable of listening to one channel at a time • MMAC (Multi-channel MAC) – Divide time into fixed-time interval using beacons – Have a small window at the start of each interval – Senders and receivers negotiate channels for this interval Common Channel A B Negotiate Channel Selected Channel RTS DATA Beacon CTS ACK

Multi-Channel Hidden Terminals • Consider the following naïve protocol – – – Each node has one transceiver One channel is dedicated for exchanging control msg Reserve channel as in IEEE 802. 11 DCF Sender indicates preferred channels in RTS Receiver selects a channel and includes it in CTS Sender and Receiver switch to the selected channel • This protocol is similar to DCA (Dynamic Channel assignment) [Wu 00 ISPAN] • RTS/CTS can’t solve Hidden Terminal Problem

Multi-Channel Hidden Terminals Time B A C D RTS(2, 3) Channel 1 Channel 2 Channel 3

Multi-Channel Hidden Terminals Time B A C D RTS(2, 3) CTS(2) Channel 1 Channel 2 Channel 3

Multi-Channel Hidden Terminals Time B A C D RTS(2, 3) CTS(2) DATA Channel 1 Channel 2 Channel 3

Multi-Channel Hidden Terminals Time B A D C RTS(2, 3) CTS(2) DATA RTS(2, 3) Channel 1 Channel 2 Channel 3

Multi-Channel Hidden Terminals Time B A D C RTS(2, 3) CTS(2) DATA RTS(2, 3) CTS(2) Channel 1 Channel 2 Channel 3

Multi-Channel Hidden Terminals Time B A D C RTS(2, 3) CTS(2) DATA Collision Channel 1 Channel 2 Channel 3

Proposed Protocol (MMAC) • Assumptions – Each node is equipped with a single transceiver – The transceiver is capable of switching channels – Multi-hop synchronization is achieved by other means • Out-of-band solutions (e. g. GPS) • In-band solutions (e. g. beaconing)

MMAC • Idea similar to IEEE 802. 11 PSM A B C Beacon ATIM-RES ATIM-ACK DATA Time ACK Doze Mode ATIM Window Beacon Interval – 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

Some facts of MMAC • Outside the ATIM window, the default channel is also used for data transmission • To avoid ATIM collision, from the start of the ATIM window, each node waits for a random backoff interval before transmitting an ATIM packet • Two closed transmissions may choose the same channel – RTS/CTS are still used • Nodes refrain from transmitting a packet if the time left in the current beacon interval is not long enough

1 Sender, 2 Receivers • A has packets for both B and C A ATIM-ACK(1) ATIM-ACK(2) ATIM B ATIM C • C must wait until the next beacon interval • But packets for C may block the queue • Solutions: – Multiple queues – Randomly choose which one to send packets to first – Send ATIM to the destination of the first packets in the queue

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 • Two Network Scenarios: wireless LAN, multi-hop network • 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 as network becomes saturated

Aggregate Throughput (Kbps) Wireless LAN – Throughput vs. #Channels 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 • The number of channels DCA can fully utilize is limited by the capacity of the control channel – When network load is high, the control channel could be the bottleneck

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

Conclusion • 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 – Requirement for synchronization