mac layer design

8/8/2019 Mac Layer Design

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MAC Layer Design for Wireless

Sensor Networks

Wei Ye

USC Information Sciences Institute


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Introduction

Wireless sensor network Large number of densely distributed nodes

Battery powered

Multi-hop ad hoc wireless network Node positions and topology dynamically

change

Self-organization

Sensor-net applications Nodes cooperate for a common task

In-network data processing


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Introduction

Important attributes of MAC protocols Collision avoidance

Basic task medium access control

Energy efficiency Scalability and adaptivity

Number of nodes changes overtime

Latency

Fairness

Throughput

Bandwidth utilization


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Contention-based protocols CSMA Carrier Sense Multiple Access

Ethernet

Not enough for wireless (collision at receiver)

MACA Multiple Access w/ Collision Avoidance

RTS/CTS for hidden terminal problem

RTS/CTS/DATA

Overview of MAC protocols

A B C

Hidden terminal: A is hidden from Cs CS


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Overview of MAC Protocols

Contention-based protocols (contd.) MACAW improved over MACA

RTS/CTS/DATA/ACK

Fast error recovery at link layer

IEEE 802.11 Distributed Coordination Function

Largely based on MACAW

Protocols from voice communication area

TDMA low duty cycle, energy efficient

FDMA each channel has different frequency

CDMA frequency hopping or direct sequence


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Example: IEEE 802.11 DCF

Distributed coordinate function: ad hoc mode Virtual and physical carrier sense (CS)

Network allocation vector (NAV), duration field

Binary exponential backoff

RTS/CTS/DATA/ACK for unicast packets Broadcast packets are directly sent after CS

Fragmentation support

RTS/CTS reserve time for first (frag + ACK)

First (frag + ACK) reserve time for second

Give up tx when error happens


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Example: IEEE 802.11 DCF

Timing relationship


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Energy Efficiency in MAC Design

Energy is primary concern in sensornetworks

What causes energy waste?

Collisions Control packet overhead

Overhearing unnecessary traffic

Long idle time

bursty traffic in sensor-net apps

Idle listening consumes 50100% of thepower for receiving (Stemm97, Kasten)

Dominant in sensor nets


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Energy Efficiency in MAC Design

TDMA vs. contention-based protocols TDMA can easily avoid or reduce energy waste

from all above sources

Contention protocols needs to work hard in alldirections

TDMA has limited scalability and adaptivity

Hard to dynamically change frame size or slotassignment when new nodes join

Restrict direct communication within a cluster

Contention protocols easily accommodate nodechanges and support multi-hop communications


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TDMA Protocols

Bluetooth Clustering (piconet)

FH-CDMA between clusters

TDMA within each clusterCentralized control by cluster head

Only master-slave communication

Master polling slave and schedule tx

At most 8 active nodes can be in a cluster scalability problem


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TDMA Protocols

LEACH: Low-Energy Adaptive ClusteringHierarchy by Heinzelman, et al.

Similar to Bluetooth

CDMA between clusters

TDMA within each cluster

Static TDMA frame

Cluster head rotation

Node only talks to cluster headOnly cluster head talks to base station (long

dist.)

The same scalability problem


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TDMA Protocols

Self-Organaiztion (Contd.) Result (if a node has n neighbors)

Uses n different channels to talk to each ofthem

Schedules n time slots to send to each of themand n time slots to receive from each of them

Looks like TDMA, but actually FDMA or CDMA

Any pair of two nodes can talk at the same time

Low bandwidth utilization


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Contention-Based Protocols

PAMAS: Power Aware Multi-Access withSignalling by Singh and Raghavendra

Improve energy efficiency from MACA

Avoid overhearing by putting node into sleep

Control channel separates from data channelRTS, CTS, busy tone to avoid collision

Probe packets to find neighbors transmission time

Increased hardware complexity

Two channels need to work simultaneously, meaningtwo radio systems.


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Piconet by Bennett, Clarke, et al. Low duty-cycle operation energy efficientSleep for 30s, beacon, and listen for a while

Sending node needs to listen for receivers

beacon first, thenCarrier sense before sending data

May wait for long time before sending

Tx rate control by Woo and Culler

Carrier sense + adaptive rate control Promote fair bandwidth allocation

Helps for congestion avoidance


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Power save (PS) mode in IEEE 802.11 DCF Assumption: all nodes are synchronized and can hear each other

(single hop)

Nodes in PS mode periodically listen for beacons & ATIMs (ad hoctraffic indication messages)

Beacon: timing and physical layer parameters

All nodes participate in periodic beacon generation

ATIM: tell nodes in PS mode to stay awake for Rx

ATIM follows a beacon sent/received

Unicast ATIM needs acknowledgement

Broadcast ATIM wakes up all nodes no ACK


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Example of PS mode in IEEE 802.11DCF


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Energy Efficiency: Contention

Extend 802.11 PS mode for Multi-hops By Tseng, et al. at IEEE Infocom 2002 Nodes do not synchronize with each other

Designed 3 sleep patterns ensure nodes

listen intervals overlap, example:Periodically fully-awake interval: similar to S-MAC

Problem on broadcast wake up each neighbor


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S-MAC: Introduction

S-MAC by Ye, Heidemann and EstrinTradeoffs

Major components in S-MAC

Periodic listen and sleep

Collision avoidance Overhearing avoidance

Massage passing

Latency

FairnessEnergy


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Periodic Listen and Sleep

Problem: Idle listening consumessignificant energy

Solution: Periodic listen and sleep

Turn off radio when sleeping

Reduce duty cycle to ~ 10% (150mson/1.5s off)

sleeplisten listen sleep

Latency Energy


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Schedules can differ

Preferneighboring nodes have sameschedule

easy broadcast & low control overheadBorder nodes:

two schedulesbroadcast twice

Node 1

Node 2

sleeplisten listen sleep

sleeplisten listensleep

Schedule 2

Schedule 1


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Schedule Synchronization New node tries to follow an existing schedule

Remember neighbors schedules

to know when to send to them

Each node broadcasts its schedule every fewperiods of sleeping and listening

Re-sync when receiving a schedule update

Periodic neighbor discovery Keep awake in a full sync interval over long

periods


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Collision Avoidance

Problem: Multiple senders want to talkOptions: Contention vs. TDMA

Solution: Similar to IEEE 802.11 ad hoc

mode (DCF) Physical and virtual carrier sense

Randomized backoff time

RTS/CTS for hidden terminal problem RTS/CTS/DATA/ACK sequence


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Overhearing Avoidance

Problem: Receive packets destined to othersSolution: Sleep when neighbors talk

Basic idea from PAMAS (Singh, Raghavendra 1998)

But we only use in-channel signaling

Who should sleep?

All immediate neighbors of sender andreceiver

How long to sleep?

The duration field in each packet informsother nodes the sleep interval


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Message Passing

Problem: Sensor net in-network processing requires entiremessage

Solution: Dont interleave different messages

Long message is fragmented & sent in burst

RTS/CTS reserve medium for entire message

Fragment-level error recovery ACK

extend Tx time and re-transmit immediately

Other nodes sleep for whole message time

Fairness Energy

Msg-level latency


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Msg Passing vs. 802.11 fragmentation

S-MAC message passing

RTS 21 ...

...

Data 19

ACK 18CTS 20

Data 17

ACK 16

Data 1

ACK 0

RTS 3 ...

...

Data 3

ACK 2CTS 2

Data 3

ACK 2

Data 1

ACK 0

Fragmentation in IEEE 802.11 No indication of entire time other nodes keep

listening

If ACK is not received, give up Tx fairness


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Experiments: two-hop network

Topology and measured energyconsumption on source nodes

Source 1

Source 2

Sink 1

Sink 2

Each source nodesends 10 messages

Each message has400B in 10 fragments

Measure total energyover time to send allmessages

0 2 4 6 8 10

200

400

600

800

1000

1200

1400

1600

1800Average energy consumption in the source nodes

Message inter-arrival period (second)

En

ergyco

nsumption

(mJ

)

802.11-like protocol w/o sleepOverhearing avoidanceS-MAC


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Recent Progress: Adaptive Listen

Reduces latency due to periodic sleepAt end of each transmission, nodes wake

up for a short time

Next transmission can start right away

Example: data flow A BC

A sends RTS, B replies CTS, C overhears CTS C will wake up when A B is done BC can start right away

A B C


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Recent Progress: 10-hop Experiments

Compare S-MAC in 3 different modes10-hop linear network with 11 nodes

0 2 4 6 8 100

5

10

15

20

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Energy consumption on radios in the entire network

Message inter-arrival period (S)

Energyconsumption(J)

10% duty cycle without adaptive listen10% duty cycle with adaptive listenNo periodic sleep

0 2 4 6 8 100

2

4

6

8

10

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Average message latency under the lowest traffic load

Number of hops

Latency(S)

10% duty cycle without adaptive listen10% duty cycle with adaptive listenNo periodic sleep


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S-MAC Information

URL: http://www.isi.edu/scadds/Released S-MAC source code (forTinyOS 0.6.1)

Currently porting to nesC environment(TinyOS 1.0)

mac layer design

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