1 mmsn: multi-frequency media access control for wireless sensor networks gang zhou, chengdu huang,...

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MMSN: Multi-Frequency Media Access Control for Wireless Sensor Networks

Gang Zhou, Chengdu Huang, Ting Yan, Tian HeJohn. A. Stankovic, Tarek F. Abdelzaher

Department of Computer ScienceUniversity of Virginia

2University of Virginia

Outline

Motivation State of the Art Overhead Analysis Contribution – New Protocol Framework

Frequency Assignment Media Access Design

Performance Evaluation Conclusions

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Ad Hoc Wireless Sensor Networks

• Sensors• Actuators• CPUs/Memory• Radio• Minimal capacity

Self-organize

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Motivation

Limited single-channel bandwidth in WSN 19.2kbps in MICA2, 250kbps in MICAz/Telos

The bandwidth requirement is increasing Support audio/video streams (assisted living, …)

Multi-channel design needed

Hardware appearing

Multi-channel support in MICAz/Telos More frequencies available in the future

Collision-based: B-MAC Scheduling-based: TRAMA Hybrid: Z-MAC

Software still lags behind

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State of the Art: Multi-Channel MAC in MANET

① Require more powerful hardware/multiple transceivers Listen to multiple channels simultaneously

[Nasipuri 1999], [Wu 2000], [Nasipuri 2000], [Caccaco 2002]

② Frequent Use of RTS/CTS Controls For frequency negotiation Due to using 802.11

Examples: [Jain 2001], [Tzamaloukas 2001], [Fitzek 2003], [Li 2003], [Bahl 2004], [So 2004], [Adya 2004], [Raniwala 2005]

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Basic Problems for WSN

Don’t use multiple transceivers Cost Form factor

Packet Size 30 bytes versus 512 bytes (or larger) in

MANET RTS/CTS

Costly overhead

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RTS/CTS Overhead Analysis

MMAC: RTS/CTS frequency

negotiation 802.11 for data

communication

RTS/CTS are too heavyweight for WSN: Mainly due to small packet size: 30~50 bytes in WSN vs.

512+ bytes in MANET From 802.11: RTS-CTS-DATA-ACK From frequency negotiation: case study with MMAC

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Contributions

A new multi-frequency MAC, specially designed for WSN;

Single half-duplex radio transceiver; Small packets sizes;

Developed four frequency assignment schemes

Supports various tradeoffs Toggle transmission and toggle snooping

techniques for media access control; An optimal non-uniform backoff algorithm,

and a lightweight approximation;

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Frequency Assignment

F1

F2

F3

F4

F5

F6

F7

F8 Reception Frequency

Complications• Not enough frequencies• Broadcast

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Frequency Assignment

When #frequencies >= #nodes within two

hops

When #frequencies < #nodes within two

hopsExclusive Frequency

AssignmentImplicit-Consensus Even Selection Eavesdropping

Both guarantee that nodes within two hops get different frequencies

The left scheme needs smaller #frequencies

The right one has less communication overhead

Balance available frequencies within two hops

The left scheme has fewer potential conflicts

The right one has less communication overhead

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Media Access Design

F1

F2

F3

F4

F5

F6

F7

F8Issues:• Packet to Broadcast• Receive Broadcast• Send Unicast• Receive Unicast• No sending/no receiving

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Media Access Design

Different frequencies for unicast reception The same frequency for broadcast reception Time is divided into slots, each of which consists

of a broadcast contention period and a transmission period.

Tbc Ttran Tbc Ttran… ...

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Media Access Design

Case 1: When a node has no packet to transmit

Receive BC (f0)

Snoop (f0) Snoop (fself)

Snoop (f0) Snoop (fself)

Receive UNI (fself)

Signal(f0)Snoop (f0)

Signal(fself)

Tbc Ttran

(a)

(b)

(c)

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Media Access Design

Back off (f0) Receive BC (f0)

Back off (f0) Send broadcast packet (f0)

Signal(f0)

Tbc Ttran

(a)

(b)

Case 2: When a node has a broadcast packet to transmit

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Media Access Design

Receive BC (f0)

Tbc Ttran

(a) Snoop (f0) Signal(f0)

Snoop (f0) Back off (fself,fdest) Receive UNI (fself) Signal(fself)

Snoop (f0) Back off (fself,fdest) Snoop(fself) Receive UNI (fself) Signal(fdest) Signal(fself)

Snoop (f0) Back off (fself,fdest) Toggle send unicast packet(fdest)

Snoop (f0) Back off (fself,fdest) Snoop(fself)Signal(fdest)

(b)

(c)

(d)

(e)

Case 3: When a node has a unicast packet to transmit

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Toggle Snooping

During “ “, toggle snooping is usedback off (fself,fdest)

fself

fdest

TTS

fself

fdest

fself

fdest

fself

fdest

fself

fdest

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Toggle Transmission

…….

PHY Protocol Data UnitPreamble

Use fselfUse fdest

TTT

When a node has unicast packet to send Transmits a preamble

so that no node sends to me

so that no node sends to destinationdestfselff

TTS=2TTT We let

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Simulation Configuration

Components Setting

Simulator GloMoSim

Terrain (200m X 200m) Square

Node Number 289 (17x17)

Node Placement Uniform

Payload Size 32 Bytes

Application Many-to-Many/Gossip CBR Streams

Routing Layer GF

MAC Layer CSMA/MMSN

Radio Layer RADIO-ACCNOISE

Radio Bandwidth 250Kbps

Radio Range 20m~45m

Confidence Intervals The 90% confidence intervals are shown in each figure

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Performance with Different #Physical Frequencies- With Light Load

① Performance when delivery ratio > 93%② Scalable performance improvement③ Overhead observed when #frequency is small④ More scalable performance with Gossip than many-to-

many traffic

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Performance with Different #Physical Frequencies– With Higher Load

① When load is heavy, CSMA has 77% delivery ratio, while MMSN performs much better

② MMSN needs less channels to beat CSMA, when the load is heavier

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Performance with Different System Load

Observation:CSMA has a sharp decrease of packet delivery ratio, while MMSN does not.

Reason:The non-uniform backoff in time-slotted MMSN is tolerant to system load variation, while the uniform backoff in CSMA is not.

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Conclusions

First multi-frequency MAC, specially designed for WSN, where single-transceiver devices are used

Explore tradeoffs in frequency assignment Design toggle transmission and toggle snooping Theoretical analysis of an non-uniform back-off

algorithm MMSN demonstrated scalable performance in

simulation

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The End!

Thanks to anonymous reviewers for their valuable comments!

Thanks to anonymous reviewers for their valuable comments!

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Performance with Different Node Densities

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Backup Slides: Optimal Non-Uniform Backoff

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Even Selection Frequency Assignment

Beacon (multiple times) to collect nodes’ IDs within two hops

Frequency decision is made sequentially in the increasing order of nodes’ IDs

When making a decision, randomly choose one of the least chosen frequencies (once no unique ones left)

Notify neighbors of decision

NOTE: Frequency assignment happens once (or a few times)

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Back Off Period - Slotted

Backoff into a slot

Transmit at end of a slot

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Non-Uniform Backoff: Motivation & an Optimal Solution

Uniform backoff

Non-uniform backoff

Let 34 slices of length TTS;

68 nodes compete for the channel

--- a timer fires

An optimal distribution is presented in the paper Uses recursive computation Distribution depends on node density

A simple approximation is needed

TPacketTransmission

TTS …...

Backoff

Ttran

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Non-uniform Backoff: A Simple Approximation

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