2007

Sensor Network

Multi-Channel MAC for Ad Hoc Networks:

Handling Multi-Channel Hidden Terminals Using A Single Transceiver

Min Kyu Han Multimedia Communications Laboratory Hankuk University of Foreign Studies May 23, 2007

     

Contents

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

Overview



Key Point(1/2)

– – Utilizes multiple channels dynamically to improve performance 802.11 Standard MAC PHY Designed only for a single channel Allows for the use of multiple channels available – IEEE 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.

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.

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)

 

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 channel.

two transceivers

, so that it can listen on the control channel, and data packets are transmitted on the data 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

 

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, I’m Overhead But Don’t through me!!!

Beacon Interval Common Channel B … Z Ch. 2 Ch. 1 Data Channel N A

Preliminaries

 IEEE 802.11 Distributed Coordination Function(DCF) – Random Backoff • The counter is decremented by on after each “time slot”  IEEE 802.11 Power Saving Mechanism(PSM) – – Using ATIM A node can save energy by going into doze mode Hey! Wake Up!!!

Issues in Multi-Channel environment

 Multi-channel hidden terminal problem A - B C - D Ch .1 : Control Channel time

Node c was buys receiving on another channel

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

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

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

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) Update channel state Vicinity of S S D Vicinity of D Check Channel & Last decision & Confirm Select one channel Based on the Sender’s PCL & own PCL Ready to Receive

New type of packet

wait

Rules for Selecting the Channel(1/2)

 I’m free 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

Best channel : the least scheduled traffic .

based on information included in the sender’s PCL and its own PCL.

Example • Case 1. Selected HIGH state channel HIGH MID LOW

Ch. 1 Ch. 2 Ch. 3 …

Counter : 2 Counter : 3 • Case 2. Selected LOW state channel LOW LOW LOW Counter : 1

Ch. 1 Ch. 2 Ch. 3 …

Rules for Selecting the Channel(2/2)

 Compare to Src.PCL vs Dest.PCL

S D Multiple channels in this state ???

Multiple channels in this state ???

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

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

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

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

Simulation(WLAN : Single hop – 2/2)

Simulation(WLAN : multi hop – 1/3)

 Keypoint Parameter – – – Control channel saturation(DSA) • Packet size Number of channel ATIM Window(MMAC)

Simulation(WLAN : multi hop – 2/3)

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.

Simulation(MMAC – 1/2)

 Extended ATIM Window – ATIM Window is affects the throughput of MMAC Protocol.

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)

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 Data send Common Channel Common Channel The node might miss the ATIM packets sent by other nodes.

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

Discussion(2/2)

 Case 3. 1(Source):N(Destination) Problem – – Head of line blocking problem Starvation problem