department of information engineering university of padova, italy mathematical analysis of ieee...

Department of Information EngineeringUniversity of Padova, ITALY

Mathematical Analysis of IEEEMathematical Analysis of IEEE

802.11 Energy Efficiency.802.11 Energy Efficiency.

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Mathematical Analysis of IEEEMathematical Analysis of IEEE

802.11 Energy Efficiency802.11 Energy Efficiency

{andrea.zanella, depe}@dei.unipd.it

Andrea Zanella, Francesco De Pellegrini

WPMC 2004, 12-15 September 2004

Special Interest Group on NEtworking & Telecommunications

WPMC'04 Abano Terme, Padova (Italy) 12-15 September 2004

Motivations Wireless ad-hoc networks are becoming more and more popular

Self-organization

Mobility

Portability

IEEE 802.11 offers native support for ad-hoc networking Single cell managed by means of Distributed Coordination Function (DCF)

Terminals are battery-powered: energy consumption is a primary issue! Energy consumption in transmission and reception is of the same order of

magnitude [Feeney 01]

The carrier-sense mechanism (CSMA/CA) reduces collision probability but

draws energy [Stemm 97]

Cost of sensing is exacerbated by transmissions occurring during the backoff

Also collisions and alien traffic involve an energetic cost


Aim of the study Goal

Providing a complete statistical description of the energy spent

Characterize the impact of RTS/CTS on energy consumption

Provide a mathematical tool for the design of energy-aware

algorithms Case study

Reference scenario [Bianchi2000] Ad hoc network with n saturated IEEE 802.11 terminals

Single-hop network

• No hidden or exposed node problem

Heavy traffic conditions (saturation)

• All terminals have always a packet ready for transmission


Energy Model

Linear energetic model

Energy is drawn proportionally to the time spent in each mode [Feeney]

Each operating mode is associated to a different energetic coefficient

Transmitting ()

Receiving (R)

Sensing (S)Virtual Sensing (0)

RTSCTS

DATAACK

TRTS TSIFSTCTS

TDATA TACK

TDIFS

RTSCTS

DATAACK

TSIFS TSIFS

RTS

NAV

TNAV

A)

B)

C)

A B

C

Energy spent during SIFS periods is neglected


Detailing the Energy Consumption

B

n

jjTT EEEE

c

c

1,

Overall energy spent for successful packet delivery

Energy spent in non-colliding transmission

Energy spent in colliding transmissions

Energy spent during backoff

Number of collisions before success

Energy spent in each collision

Hypothesis are i.i.d. and

independent of ET

Probability of collision p

independent of the

system state [Bianchi01]

jTcE ,


Detailing ET & ETc,j

ET: Energy required for transmitting a packet with success

DIFSSACKRD TTT

DIFSSACKCTSRDRTS TTTTT TE

Basic Access

RTS/CTS

ETc,j: Energy spent during packet collision

Basic Access

RTS/CTS

DIFSACKSD TTT

CTSDIFSSRTS TTT jTc

E ,

TEIFS


Detailing EB

rW

jjB rE

0

EB: Energy spent in backoff

Wr: total number of tick periods spent in backoff Tick Period

time between two successive decrements (tick) in the backoff countdown process

• Idle channel: countdown 1 per time slot

• Busy channel: freeze until the channel returns idle for a DIFS, then resume countdown

j : energy spent in each tick period


Detailing j

During a tick period a node can be sensing the radio channel receiving a valid packet intended for that node discarding a valid packet for other destinations listening collided transmission on the channel

Idle Channel

Busy Channel

ccs

ssc

RTDRT

RRTTTS

EE

E

1

1

sense receive

listendiscard


Putting al pieces together...

Moment generating function for the energy spent by each node in the network

mcT

mcT

mcT

rcTT

xE

xEW

mE

m

rW

rEEE

Gsp H

Gsp HGsp H

Gsp HsHpsH

1

10


Case Study

Lucent WaveLAN 11 Mbps [Feeney2001] Transmitting = 1 (normalized) Receiving R = 2/3 Sensing S = 0.82R

Possible power saving policy Case 1

Energy spent during NAV phase is negligible (0=0) Case 2

Energy spent during NAV phase is not negligible (0=0.5S) Case 3

Regular sensing is performed during NAV phase (0=S)


0 10 20 30 40 50 60 70 80 90 100

Nor

mal

ized

Li

fetim

e

Number of stations

Basic Access

RTS/CTS0 = 0

0 = S

0 = 1/2S

0.2

0.4

0.6

Results: node lifetime

Normalized Lifetime Minimum theoretical

energy per pck over Average energy per pck

RTS/CTS outperforms Basic Access mode

0 =0 leads to large

gain in nodes lifetime Gain rapidly fades for

0 1/2S


0 10 20 30 40 50 60 70 80 90 100

Pay

load

[b

its]

Number of stations

Energy-based threshold

Throughput-base threshold [1]

2000

4000

6000

0 = 0 0 =1/2S 0 =S

Basic Access-RTS/CTS threshold

Energy vs Throughput perspective

With 0 1/2S payload

threshold is lower than in Throughput-base case

Threshold shows less sensitivity to the number of nodes in the network

With more than 20 nodes, the threshold remains almost const

Threshold increases as 0 gets close to S

Payload threshold after which RTS/CTS outperforms Basic Access

[1] Bianchi2000


Conclusions

Complete statistical description for energy consumption Ad-hoc network with saturated IEEE 802.11 nodes

Model allows for some interesting insights Channel sensing during backoff has a relevant energetic cost

Switching to low-power mode during NAV can potentially save

energy, but only for 0 << S

Payload length after which RTS/CTS outperforms Basic Access is

lower for Energy-base than for Throughput-base perspective

Energy-based Threshold is less sensitive to the number of nodes

in the network than Throughput-based Threshold



Mathematical Analysis of IEEE 802.11 Energy EfficiencyMathematical Analysis of IEEE 802.11 Energy Efficiency{andrea.zanella, depe}@dei.unipd.itAndrea Zanella, Francesco De Pellegrini

WPMC 2004, 12-15 September 2004

Questions?


Extra Slides…

Spare SlidesSpare Slides


Medium Access Control (MAC)

CSMA: Carrier Sensing Multiple Access (Exponential) Backoff stage

Choose a random number in the backoff window

If the channel is sensed idle, then countdown by 1 for each slot

If the channel is busy then freeze the countdown until the channel

becomes idle again for at least a DIFS

When the countdown is over transmit the packet

If no ACK is returned within a SIFS, a collision has occurred

• Double backoff window and re-enter the backoff stage

Otherwise the transmission was successfull

• Reset the backoff window and enter the backoff stage for the next packet


Collision Avoidance

Basic Access

Transmit data packet

RTS/CTS access

Try to reserve the channel before transmission

Send a very short Request To Send (RTS) packet

Receiver replies with a very short Clear To Send (CTS) packet

Stations that get RTS or CTS packets avoid transmissions in the

successive time interval (setting the NAV)


Detailing EB: backoff strategy

EB: Energy spent in backoff

Backoff strategy S(i) : backoff stage after i successive collisions

S(i) = min(i,m)

CWi: i-th backoff window

CWi=CW0 2S(i)-1

xi: i-th backoff counter

xi=random{0,1,...,CWi}

Countdown xi tick periods then retransmit the packet


Tick period (1/2)

Tick Period time between two successive decrements (tick) of the

backoff countdown process Idle channel

• countdown 1 per time slot Busy channel (valid or collided packet on the air)

• freeze until the channel returns idle for a DIFS, then resume countdown

during a tick period a node can wait (idle channel) receive valid packet intended for that node discard valid packet for other destinations listen collided transmission on the channel

Idle Channel

Busy Channel


0 20 40 60 80 100 120 140 160 180 2000

P [

E >

e]

Normalized energy: e=E / min{E}

Basic Access 0 = S

10-3

10-2

10-1

Results: complementary cdf of E Energy actually spent for a packet

transmission is many times the theoretical minimum

Jointly using RTS/CTS and smart sensing strategy drastically reduces energy costs

10-4

100

Basic Access 0 = 0

RTS/CTS 0 = S

RTS/CTS 0 = 0

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