Transcript 슬라이드 1 - University of Illinois at Urbana–Champaign
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 802.11b
– 12 channels in 802.11a
• Utilizing multiple channels can improve throughput – Allow simultaneous transmissions
1 defer
Single channel
1 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
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• 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)
A B C D
802.11 Distributed Coordination Function
A B C D Time
C D
802.11 Distributed Coordination Function
A B RTS C D Time A B RTS
802.11 Distributed Coordination Function
A B CTS C D NAV Time A B RTS CTS C SIFS D
802.11 Distributed Coordination Function
A B DATA C D NAV Time A B RTS DATA CTS C SIFS D NAV
802.11 Distributed Coordination Function
A B ACK C D NAV Time A B RTS DATA CTS ACK C SIFS D NAV
802.11 Distributed Coordination Function
A D C D A B RTS SIFS CTS B NAV C DATA NAV Time ACK DIFS
Contention Window
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
C A B
802.11 Power Saving Mechanism Beacon
Time
ATIM Window Beacon Interval
C
802.11 Power Saving Mechanism
A
Beacon ATIM
B Time
ATIM Window Beacon Interval
802.11 Power Saving Mechanism
A
Beacon ATIM
B
ATIM-ACK
C
ATIM Window Beacon Interval
Time
802.11 Power Saving Mechanism
A
Beacon ATIM ATIM-RES
B
ATIM-ACK
C
ATIM Window Beacon Interval
Time
802.11 Power Saving Mechanism
A
Beacon ATIM ATIM-RES DATA
B C
ATIM-ACK
Doze Mode ATIM Window Beacon Interval
Time
802.11 Power Saving Mechanism
A
Beacon ATIM ATIM-RES DATA
B C
ATIM-ACK ACK
Doze Mode ATIM Window Beacon Interval
Time
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 C
A sends DATA to B
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 C
A sends RTS
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 [Nasipuri99WCNC] • Extensions: channel selection based on channel condition on the receiver side [Nasipuri00VTC] • Disadvantage: High hardware cost
Wu’s Protocol [Wu00ISPAN] • 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 250us • 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
A B C D
Channel Negotiation
Common Channel Selected Channel
Beacon
Time
ATIM Window Beacon Interval
D C B A
Channel Negotiation
Selected Channel Common Channel
ATIM ATIM RES(1) Beacon ATIM ACK(1)
Time
ATIM Window Beacon Interval
D C B A
Channel Negotiation
Selected Channel Common Channel
ATIM ATIM RES(1) Beacon ATIM ACK(1) ATIM ACK(2) ATIM ATIM RES(2)
ATIM Window Beacon Interval
Time
D B C A
Channel Negotiation
Selected Channel Common Channel
ATIM ATIM RES(1) RTS DATA
Channel 1
Beacon ATIM ACK(1) ATIM ACK(2)
Channel 1
CTS ACK CTS ACK
Channel 2 Channel 2
ATIM ATIM RES(2)
ATIM Window
RTS DATA
Beacon Interval
Time
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 802.11 single channel protocol – DCA : Wu’s protocol – MMAC : Proposed protocol
Wireless LAN - Throughput 2500 2000 1500 1000 500
MMAC DCA 802.11
1 10 100 1000 Packet arrival rate per flow (packets/sec) 30 nodes 2500 2000 1500 1000 500
MMAC DCA 802.11
1 10 100 1000 Packet arrival rate per flow (packets/sec) 64 nodes MMAC shows higher throughput than DCA and 802.11
Multi-hop Network – Throughput 1500 2000
MMAC MMAC
1500 1000
DCA DCA
1000 500
802.11
0 1 10 100 1000 Packet arrival rate per flow (packets/sec) 500
802.11
0 1 10 100 1000 Packet arrival rate per flow (packets/sec) 3 channels 4 channels
Throughput of DCA and MMAC (Wireless LAN) 4000 4000
6 channels
3000 3000
6 channels
2000 2000
2 channels
1000
802.11
0 Packet arrival rate per flow (packets/sec) DCA 1000 0
2 channels 802.11
Packet arrival rate per flow (packets/sec) MMAC 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!