1. 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

2. Problem Statement Utilizing multiple channels can improve throughput
Allow simultaneous transmissions
Multiple Channels available in IEEE
3 non-overlapping channels in b
But, 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 1
defer 2
Single channel
Multiple Channels

3. Overview
Goal: Design a MAC protocol that utilizes multiple channels to improve overall performance
Modify 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
Selected Channel
Negotiate Channel A
RTS
DATA
Beacon B
CTS
ACK

4. 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 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) [Wu00 ISPAN]
RTS/CTS can’t solve Hidden Terminal Problem

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

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

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

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

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

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

11. 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)

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 A B C
Time
Beacon
ATIM
ATIM-ACK
DATA
ACK
ATIM-RES
Doze Mode
ATIM Window
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 A B C D Time Common Channel Selected Channel
Beacon B C D
Time
ATIM Window
Beacon Interval

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

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

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

19. 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 trans-mitting 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

20. 1 Sender, 2 Receivers A has packets for both B and 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 A
ATIM-ACK(1)
ATIM-ACK(2)
ATIM ATIM B C

21. Performance Evaluation
Simulation Model
Simulation Results

22. 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
Two Network Scenarios: wireless LAN, multi-hop network
Compared protocols
802.11: IEEE single channel protocol
DCA: Wu’s protocol
MMAC: Proposed protocol

23. Wireless LAN – Throughput
2500
2000
1500
1000
500
2500
2000
1500
1000
500
MMAC
MMAC
DCA
DCA
Aggregate Throughput (Kbps)
802.11
802.11
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 as network becomes saturated

24. Wireless LAN – Throughput vs. #Channels
4000
3000
2000
1000
4000
3000
2000
1000
6 channels
6 channels
2 channels
Aggregate Throughput (Kbps)
2 channels
802.11
802.11
Packet arrival rate per flow (packets/sec)
Packet arrival rate per flow (packets/sec)
DCA
MMAC
The number of channels DCA can fully utilize is limited by the capa-city of the control channel
When network load is high, the control channel could be the bottleneck

25. Multi-hop Network – Throughput
2000
1500
1000
500
1500
1000
500
MMAC
MMAC
DCA
DCA
Aggregate Throughput (Kbps)
802.11
802.11
Packet arrival rate per flow (packets/sec)
Packet arrival rate per flow (packets/sec)
3 channels
4 channels

26. 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