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Multi-Channel MAC for Ad Hoc Networks: Handling Multi-Channel Hidden Terminals Using A Single Transceiver

Min Kyu HanMultimedia Communications LaboratoryHankuk University of Foreign StudiesMay 23, 2007

2007 Sensor Network

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Contents Overview Related Work Propose a protocol Issues in Multi-Channel environment Simulation Discussion

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Overview Key Point(1/2)

– Utilizes multiple channels dynamically to improve performance

– 802.11 Standard

– Issue– 802.11 MAC DCF(Distributed Coordinate Function) is designed for

sharing a single channel between hosts. – Each of current IEEE 802.11 device is equipped with one half-duplex

transceiver.– Transceiver is capable of switching channels dynamically, but it can

only transmit or listen on one channel at a time– Due to this, a new type of hidden terminal problem occurs in this multi-

channel environment multi-channel hidden terminal problem.

MACMAC

PHYPHY

IEEE 802.11 StandardIEEE 802.11 Standard

Designed only for a single channel

Allows for the use of multiple channels available

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Overview Key Point (2/2)

– Simulation precondition– network : ad-hoc non-infrastructure– there is no central authority to perform channel management

– Simulation – This paper proposed protocol enables hosts to utilize multiple

channels by switching channels dynamically, thus increasing network throughput.

– The simulation results show that proposed protocol successfully exploits multiple channels to achieve higher throughput than IEEE 802.11 & another multi-channel MAC protocol.

– Main Idea(A similar approach is used in IEEE 802.11’s PSM)– The main idea is to divide time in to fixed-time interval using

beacons,– and have a small window at the start of each interval to indicate

traffic– And negotiate channels for use during the interval.

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RELATED WORK(1) Dual Busy Tone Multiple Access(D-BTMA)

– It is divides a common channel into two sub-channels.• one data channel & one control channel• This scheme uses only one data channel and is not intended for

increasing throughput using multiple channels.

Hop Reservation Multiple Access(HRMA) & Channel-Hooping with Dual Polling– Multi-channel protocol for networks using

FHSS(Frequency Hopping Spread Spectrum)• The hosts hop from one channel to an other according to a

predefined hopping pattern(PRN Code)• They cannot be used in systems using other mechanisms such as

DSSS(Direct Sequence Spread Spectrum)

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RELATED WORK(2) Multi-channel CSMA protocol

– “soft” channel reservation• If there are N channels, the protocol assumes that each host can listen

to all N channels concurrently.– Number of listening channel : N – Preferred channel selection factor : the last successful transmission channel

» Extension : best channel based on signal power

– N’s Channel need to N’s Transceiver very expensive.

Dynamic Channel Assignment(DCA)– They maintain one dedicated channel for control messages

and other channels for data.– Each host has two transceivers, so that it can listen on

the control channel, and data packets are transmitted on the data channel.

– RTS/CTS packets are exchanged on the control channel• In RTS packet, the sender includes a list of preferred channel• On receiving the RTS, the receiver decides on a channel

– When the number of channels is small, one channel dedicated for control messages can be costly

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Propose a protocol One transceiver per host It does not require a dedicated control

channel– Provide a clock synchronization among all the hosts

At the start of each interval,

Beacon Interval

Common Channel

Data Channel N

A

BZ…

Ch. 1

Ch. 2

I’m Overhead But Don’t through

me!!!

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Preliminaries IEEE 802.11 Distributed Coordination

Function(DCF)– Random Backoff

• The counter is decrementedby on after each “time slot”

IEEE 802.11 Power Saving Mechanism(PSM)– Using ATIM– A node can save energy

by going into doze modeHey! Wake Up!!!

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Issues in Multi-Channel environment

time

A - BA - B C - DC - D

Ch .1 : Control Channel

Node c was buys receiving

on another channel

Multi-channel hidden terminal problem

– If there was only one channel that every node listens to, C would have heard the CTS and thus deferred its transmission

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Proposed MMAC(Multi-Channel MAC) Protocol Assumptions

– N channels are available for use and all channels have the same bandwidth.

– Each host is equipped with a single half-duplex transceiver.

• So a host can either transmit or listen, but cannot do both simultaneously.

– Nodes are synchronized, so that all nodes begin their beacon interval at the same time.

Proposed scheme in detail– Preferable Channel List(PCL)– Channel Negotiation during ATIM Window– Rules for Selecting the Channel

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Preferable Channel List(PCL) PCL records the usage of channels inside the

transmission range of the node. Based on this information, the

channels are categorized into three states.– High preference(HIGH)– Medium preference(MID)– Low Preference(LOW)

• There is a counter for each channel in the PCL to record how many source-destination pairs plan to use the channel for the current interval

The channel states are changed in the following way– Reset to MID state : Power up, start of each beacon interval– HIGH state : S-D nodes agree upon a channel– LOW state : if a node overhears an ATIM-ACK or ATIM-RES

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Channel Negotiation during ATIM Window In MMAC, periodically transmitted beacons divide

time into beacon intervals. A small window called the ATIM window is placed

at the start of each beacon interval– Similar to IEEE 802.11 PSM(different purpose in proposed

protocol)

S DATIM(include PCL & NAV)

ATIM-ACK(NAV)

ATIM-RES

Select one channel Based on the

Sender’s PCL & own PCL

Check Channel& Last decision

& Confirm

Ready to Receive

New type of packet

wait

ATIM-RES

Vicinity of S Vicinity of D

Update channel

state

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Rules for Selecting the Channel(1/2) When a node receives an ATIM packet,

– It selects a channel and notifies the sender by including the channel information in the ATIM-ACK packet.

– The receiver tries to select the “best” channel based on information included in the sender’s PCL and its own PCL.

• Best channel : the least scheduled traffic.

– Example• Case 1. Selected HIGH state channel

• Case 2. Selected LOW state channel

…Ch. 1 Ch. 2 Ch. 3

HIGH MID LOWI’m free

…Ch. 1 Ch. 2 Ch. 3

LOW LOW LOWCounter : 2

Counter : 3

Counter : 1

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Rules for Selecting the Channel(2/2) Compare to Src.PCL vs Dest.PCL

S DATIM(include PCL & NAV)

ATIM-ACK(Ch.1 , NAV)

ATIM-RES

S.PCL(1:MID) & D.PCL(1:MID)Selected Channel No. 1 ???

S.PCL(1:MID) & D.PCL(1:MID)Selected Channel No. 1 ???

If D.PCL(1:HIGH) Selected Channel No. 1If D.PCL(1:HIGH) Selected Channel No. 1

S.PCL(1:MID) |D.PCL(1:LOW)S.PCL(1:LOW) |D.PCL(1:MID)Selected Channel No. 1 ???

S.PCL(1:MID) |D.PCL(1:LOW)S.PCL(1:LOW) |D.PCL(1:MID)Selected Channel No. 1 ???

A.PCL(1:HIGH)Selected Channel No. 1A.PCL(1:HIGH)Selected Channel No. 1

Multiple channels

in this state ???

Multiple channels

in this state ???

S.PCL(*:LOW) & D.PCL(*:LOW)The channel with the least count is selected.

S.PCL(*:LOW) & D.PCL(*:LOW)The channel with the least count is selected.

OK : ATIM-RES Send ( Channel Selected Confirm)Otherwise : Does not send an ATIM-RES packet Retry Negotiation

OK : ATIM-RES Send ( Channel Selected Confirm)Otherwise : Does not send an ATIM-RES packet Retry Negotiation

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Simulation Model For simulations,

– Used ns-2– Two network scenario : wireless LAN, multi-hop networks– Bit rate for each channel is 2Mbps(Channel Num : 3)– Transmission range of each node : 250m– Beacon interval : 100ms– Each simulation was performed for a duration of 40sec– Packet size : 512 bytes

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Simulation(WLAN : Single hop – 1/2) In the simulated wireless LAN, all nodes are within

each other’s transmission range.– First, we examine the throughput and packet delay

varying the network load– MMAC uses all 3 channels for data exchange. – Single-hop network

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Simulation(WLAN : Single hop – 2/2)

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Simulation(WLAN : multi hop – 1/3) Keypoint Parameter

– Control channel saturation(DSA)• Packet size

– Number of channel– ATIM Window(MMAC)

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Simulation(WLAN : multi hop – 2/3)

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Simulation(WLAN : multi hop – 3/3) Keypoint

– ATIM window overhead in MMAC does not increase with the number of channels

– DCA does not benefit from having one more channel because of control channel saturation.

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Simulation(MMAC – 1/2) Extended ATIM Window

– ATIM Window is affects the throughput of MMAC Protocol.

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Simulation(MMAC – 2/2) For simulations,

– WLAN Node # : 30– Packet Size : 512Bytes– Number of Channel : 3

Keypoint– Hidden Terminal Problem– DCA(Control Channel) vs MMAC(ATIM Window)

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Discussion(1/2) Case 1. Clock Synchronization

– Out-of-band solution(GPS)– In-band solution ATM window overhead

• Beaconing mechanism problem• Clocks of (A,B) and (C,D) may drift away, because they never

exchange beacons

Case 2.

Beacon Interval

Common Channel

Data Channel N

RTS

CTS

Data send

Common

Channel

The node might miss the ATIM packets

sent by other nodes.

Common

Channel

AIT

M

If (Current Beacon Interval < transmission time of the packet) then nodes refrain from Transmitting packet

If (Current Beacon Interval < transmission time of the packet) then nodes refrain from Transmitting packet

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Discussion(2/2) Case 3. 1(Source):N(Destination) Problem

– Head of line blocking problem– Starvation problem