Power-Saving Protocols for IEEE 802.11-Based Multi-Hop Ad Hoc Networks

CS710 : Special issues in Computer Architecture

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Power-Saving Protocols for IEEE 802.11-Based Multi-Hop Ad Hoc

NetworksYu-chee tseng et. al

National Chio Tung University, TaiwanINFOCOM ‘02

Presented by Joo, Jaikwan


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Contents Introduction

General power saving Power saving modes in IEEE 802.11

Three asynchronous power saving protocols for MANET Dominating-awake-interval Periodically-fully-awake-interval Quorum-based

Simulations Conclusions


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Introduction The critical issue of MANET(Mobile Ad hoc NETwork) : po

wer saving Battery technology is not likely progress as fast as computing an

d communication technologies. The category of power-saving solution

Transmission power controlTopology control

Power aware routingBase on mobile host power level

Low-power mode IEEE 802.11 has power saving mode which is a radio only needs to

be awake periodically.Bluetooth : park, hold, sniff mode.


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IntroductionMANET

Multi-hop, unpredictable mobility, no plug-in power, no clock synchronization

Two challenge of power savingClock synchronization

No central control, variable packet delay due to unpredictable mobility and radio interference.

Neighbor discoveryBecause PS host will reduce its

transmitting/receiving activityRouting problem


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IntroductionBasic idea of protocol

Enforces PS hosts send more beacon packets than the original IEEE 802.11 standard

Arrange the wake-up and sleep patterns of PS hosts such that any two hosts are guaranteed to detect each other in finite time even under PS mode


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Introduction Power saving modes in IEEE 802.11

Two power modes : active and power saving(PS) Under infrastructure(with AP)

AP monitors the mode of each mobile host. A host in PS mode only awakes up periodically to check for possible incom

ing packet from AP. A host always notifies its AP when changing mode. Periodically AP transmit beacon frames. In each beacon frame, a Traffic Indication Map(TIM) will be delivered, whic

h contains ID’s of those PS host with buffered unicast packet in the AP. A PS host, on hearing its ID, should stay awake remaining beacon interval. On DCF, awake PS host issue PS-POLL On PCF, awake PS wait for AP Poll To send Buffered broadcast packet, AP send DTIM(Delivery TIM), after that,

buffered broadcast packet will be sent.


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Introduction Under an ad hoc network

PS hosts also wake up periodicallyATIM window : short interval that PS hosts wake up.Assuming that hosts are fully connected and all synchronized. In the beginning of each ATIM window, each mobile host will con

tend to send a beacon frame.Successful beacon serve for synchronizing mobile host’s clock.This beacon also inhibits other hosts from sending their beaconTo avoid collisions among beacons, use random back-off [0-2*C

Wmin –1]


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Introduction- After the beacon, host can send a direct ATIM frame to each of its intended receivers in PS mode.

- After transmitted an ATIM frame, keep remaining awake

- On reception of the ATIM frame, reply with an ACK and remain active for the remaining period

- Data is sent based on the normal DCF access.


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IntroductionProblem statement

PS mode of 802.11 is designed for single hop(fully connected) ad hoc network.

If applied for multi-hopClock synchronization

Communication delay and mobility are all unpredictable

Neighbor discoveryA host in PS mode is reduced its chance to transmit

Network partitioning Inaccurate neighbor information may lead to long

packet delay or even network partitioning problem.


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Three asynchronous power saving protocols for MANET

Guidelines in designing protocol More beacon

To prevent the inaccurate-neighbor problemA PS host should not inhibit its beacon in ATIM window

even if it has heard other beacons. Allow multiple beacon in a ATIM window

Overlapping Awake intervalSince protocol don’t count on clock synchronizationThe wake-up pattern of two PS host must overlap with

each other. Wake-up prediction

To drive PS host’s wake-up pattern based on their time difference.


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Three power-saving protocols, each with a different wakeup pattern for PS host Beacon interval

For each PS host, it divides its time axis into a number of fixed length interval

Active window On state

Beacon window PS hosts send its beacon

MTIM window Other hosts send their MTIM frames to the PS host.

Excluding these three windows, PS host with no packet to send or receive may go to the sleep mode.


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Access procedure Back-off delay

[0 ~ 2*CWmin –1 slot]

Notation used in this paper BI : length of a beacon interval AW : length of an active window BW : length of a beacon window MW : length of an MTIM window


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Dominating-awake-interval PS host stay awake sufficiently long so as to ensure

that neighboring host can know each other. Dominating awake property

AW >= BI/2 + BWThis guarantees any PS host’s beacon window to

overlap with any neighboring PS host’s active window. In every two beacon interval, PS host can receive all its

neighbor’s beacon short response timesuitable for highly mobile

The sequence of beacon intervals are alternatively labeled as odd and even interval


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Periodically–fully-awake-intervalTwo types of beacon interval

Low power intervals AW is reduced to the minimum PS host send out its beacon to inform others its existence AW = BW + MW, in the rest of the time , the host can go to the sleep mode.

Fully awake intervals AW is extended to the maximum Arrives periodically every T intervals AW = BI, rest of the time must remain awake PS hosts discover who are in its neighborhood. By collecting other host’s beacons, hosts predict when its neighboring host will

wake up. a lot of power, so they only appear periodically and are interleaved by low

power intervals.Response time to get aware of a newly appearing host

T beacon intervalSuitable for slowly mobile environments


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BI

Quorum-based PS host only needs to send beacon O(1/n) of the all

beacon intervals. Design PS host’s wakeup pattern so as to guarantee a PS

host’s beacons can always be heard by other’s active windows.

Quorum intervalBeacon + MTIM, AW = BI

Non quorum intervalsStart with an MTIM window, after that, host may go to sleep

mode, AW=MW As long as n=>4, this amount of awaking time is less

than 50% Suitable for expensive transmission cost


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Communication protocols for power-saving hosts

Since the PS host is not always active, the sending host has to predict when the PS host will wake up.

Beacon packet has to carry the clock value of the sending host so as for other hosts to calculate their time differences.

S predict the receiving side’s MTIM window, S contends to MTIM packets to notify the receiver, after which the buffered data packet can be send. Unicast

During the receiver’s MTIM window, sender contends to send its MTIM packet to the receiver.

Receivers will reply an ACK after SIFS, stay awake in the remaining of the beacon interval.

After the MTIM window, sender will contend to send the buffered packet to the receiver based on the DCF procedure.


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BroadcastTo reduce the number of transmissions, divide these

asynchronous neighbors into group and notify them separately in multiple runs.

When S intends broadcast a packet, it first check the arrival time of the MTIM windows of all neighbors.

S picks the host, whose first MTIM window arrives earliest, S picks those MTIM window have overlapping with Y’s first MTIM.

After the these notification, S repeats the same process.A neighbor, on receiving a MTIM carrying a broadcast

indication, should remain awake until broadcast received or time value expires.

Broadcast packet should send based on DCF procedure.


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Simulation experiments Environment

Implemented by C Transmission radius : 250m Rate : 2Mbps Traffic load : Poisson distribution 5~30pkts/sec “On-off model” to simulate mobility(in every 5 sec)

On probability : uniform distribution 50%~100% Beacon interval : 100ms~500ms Simulation time : 100sec Assume all hosts are in the PS mode Metric

Power consumption The average power consumption per mobile host thru one simulation run

Power efficiency average power consumption for each successful packet transmission

Neighbor discovery time Average time to discover a newly approaching neighbor


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Impact of beacon interval length

[Traffic load=10pkts/sec, “ON” probability=80%] [For unicast Traffic load=10pkts/sec, “ON” probability=80%]

[For broadcast Traffic load=10pkts/sec, “ON” probability=80%]

P(4) is good for both


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Impact of mobility

For unicast(traffic load=10pkts/sec, Beacon interval=300ms) In case of broadcast

Broadcast packet is counted as successful as long as some neighbors are thereto receive the packet.


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Impact of traffic load

For unicast(“on”=80%, Beacon interval=300ms) For unicast(“on”=80%, Beacon interval=300ms)

Higher traffic load incurs higher power consumptionsince hosts have less chance to sleep.

Higher load makes transmitting a packetless costly because multiple packets may betransmitted in one beacon interval.


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Conclusions Three power saving protocol based on IEEE802.11, m

ulti-hop, asynchronous MANETs. Dominating awake interval

Most power consumption, the lowest neighbor discovery time. Quorum based

The most power saving, the longest neighbor discovery time. Periodically-fully-awake interval

Balance both power consumption and neighbor discovery time.

Power-Saving Protocols for IEEE 802.11-Based Multi-Hop Ad Hoc Networks

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