On Optimizing the Backoff Interval On Optimizing the Backoff Interval for Random Access Schemefor Random Access Scheme

Zygmunt J. Hass and Jing DengZygmunt J. Hass and Jing Deng

IEEE Transactions on Communications,IEEE Transactions on Communications, Dec 2003 Dec 2003

OutlineOutline

IntroductionIntroductionRelated WorkRelated WorkSBA (Sensing Backoff Algorithm)SBA (Sensing Backoff Algorithm)Performance EvaluationPerformance EvaluationConclusionsConclusions

IntroductionIntroduction

Two fundamental characteristics of a good Two fundamental characteristics of a good backoff algorithmbackoff algorithmHigh channel throughputHigh channel throughputLow delayLow delay

FairnessFairness among competing nodes should among competing nodes should also be consideredalso be considered

Related Work(1)Related Work(1)

Binary exponential backoff (BEB)Binary exponential backoff (BEB)

collision upon ),,2min(ion transmisssuccessful upon ,

max

min{ Bxx

Bx

Related Work(2)Related Work(2)

A B

CW=Bmin=7 CW=Bmin=7

A B

CW=Bmin=7 CW=Bmin=15

A B

CW=Bmin=7 CW=Bmin=255

……

…

Related Work(3)Related Work(3)

Multiplicative increase linear decrease (MILD), Multiplicative increase linear decrease (MILD), introduced in MACAWintroduced in MACAW

Related Work(4)Related Work(4)

MILDMILDMaintains a backoff interval for each stream inMaintains a backoff interval for each stream in

stead of each nodestead of each node Improves the fairness performanceImproves the fairness performance

Related Work(5)Related Work(5)

MILD still has 2 disadvantagesMILD still has 2 disadvantagesBackoff interval stored into the transmitted paBackoff interval stored into the transmitted pa

ckets increases the overhead and the probabilckets increases the overhead and the probability of packet collisionity of packet collision

Backoff intervals migrate to the whole network,Backoff intervals migrate to the whole network, the channel throughput in these areas will be the channel throughput in these areas will be degradeddegraded

SBA(1)SBA(1)

α( > 1)θ( < 1)β( steps)γ( transmission time of a packet

SBA(2)SBA(2)

AssumptionsAssumptionsN nodes within the transmission range of each N nodes within the transmission range of each

other ( N <= 100 )other ( N <= 100 )Packet collisions are the only source of packePacket collisions are the only source of packe

t errort errorTransmission can be overheard by other nodeTransmission can be overheard by other node

ssPackets should be transmitted in turnPackets should be transmitted in turnAll nodes have the same CWAll nodes have the same CWminmin and CW and CWmaxmax

SBA(3)SBA(3)

TTff : duration of faild period : duration of faild period II : idle period : idle periodTTss : success busy period : success busy periodUU : utilization period : utilization period

SBA(4)SBA(4)

Within , the mean interarrival time at eaWithin , the mean interarrival time at each node is B/2, where B is a fixed backoff inch node is B/2, where B is a fixed backoff intervalterval

Prob{ A node starts transmission in } = Prob{ A node starts transmission in } = 2 / B * 2 / B *

PPss = Prob{ None of the other nodes transmit = Prob{ None of the other nodes transmit

in γ} = in γ} = 12

1

N

B

SBA(5)SBA(5)

By approximating the arrivals of all nodes By approximating the arrivals of all nodes by a Poisson arrival process, the total by a Poisson arrival process, the total arrival rate is N * 2 / B, so the average idle arrival rate is N * 2 / B, so the average idle time istime is

SBA(6)SBA(6)

The average failed period can be expressed The average failed period can be expressed asas

SBA(7)SBA(7)

Applying the above 3 equations into Applying the above 3 equations into channel model, the channel throughput as channel model, the channel throughput as a function of a function of NN and and BB can be obtains can be obtains

SBA(8)SBA(8)

To find the optimum B(BTo find the optimum B(Boptopt),),

must be solved numericallymust be solved numericallyBBoptopt(N) = 4Nγ (N) = 4Nγ

SBA(9)SBA(9)

SBA(10)SBA(10)

SBA(11)SBA(11)

SBA(12)SBA(12)

SBA(13)SBA(13)

Based on the above calculation of the optiBased on the above calculation of the optimum backoff intervals, we can find the optimum backoff intervals, we can find the optimum values of (α, β, and γ)mum values of (α, β, and γ)

(α, β, γ) = (1.2, 0.8, 0.93)(α, β, γ) = (1.2, 0.8, 0.93)

Performance Evaluation(1)Performance Evaluation(1)

Performance Evaluation(2)Performance Evaluation(2)

Performance Evaluation(3)Performance Evaluation(3)

Performance Evaluation(4)Performance Evaluation(4)

Performance Evaluation(5)Performance Evaluation(5)

ConclusionsConclusions

SBA operates close to the optimum, maxiSBA operates close to the optimum, maximizing the network throughput with mizing the network throughput with fair accfair accessess from active nodes, without the from active nodes, without the precise precise knowledgeknowledge of the number of active nodes of the number of active nodes

SBA can also be extended to other types oSBA can also be extended to other types of MAC schemesf MAC schemes

Thank youThank you