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A QoS Guaranteed Multipolling Scheme for Voice Traffic in IEEE 802.11 Wireless LANs

Der-Jiunn Deng 、 Chong-Shuo Fan 、 Chao-Yang Lin

Speaker: Chong-Shuo FanDate:2006/06/26

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Outline

1. Introduction2. Improved Approach3. Simulations4. Conclusions

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1. Introduction In order to reach a higher Quality of Service (QoS) i

n network applications, the 802.11e Task Group has deployed a hybrid coordination function (HCF) to improve the original IEEE 802.11 Medium Access Control (MAC) protocol.

The HCF defines two medium access mechanisms, one of which is channel access control.

Nevertheless, choosing the right MAC parameters and QoS mechanism so as to achieve predictable performance remains an unsolved problem

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HCF in Controlled Access Mode HCF operation is similar to the operati

on of PCF. HCF can operate in two modes.

Coexisting with EDCF. Using a contention-free period (CFP).

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2. Improved Approach For each real-time station S, we Use tw

o variable: rc: the packet transfer rate : the maximum amount of jitter (i.e. packe

t delay variation)

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In the BSA of IEEE 802.11 our AP reserves some of its memory

to create token buckets each representing a real time session

that connects two stations, say A and B and generated when A or B enters the WTT state

A packet with a relatively smaller amount of jitter has lower priority

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Theorem 1 (1/2)

Let *1 p2 SIFS CFPoll t ACK

*i p2 SIFS CFpoll t ACK

i 1ck

pk 1 ci

r(2 SIFS CFPoll t ACK)

r

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Theorem 1 (2/2) If and , i = 2, …,n, then all voice packe

ts of each session can be transmitted within their jitter constraints.

If a packet of the ith session though handoff, satisfies and , where represents the time needed for handoff, this packet will also meet its jitter constraint.

cii r

1

ii

ciii r

1

iii i

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Proof (1/3)

Handoff part Assume the maximum waiting time of

the token, produced by the ith voice source, after handoff from the other BSA is

Our goal actual waiting time of the packet, say

, is less than its required and tolerable jitter , i.e. .,

1

i ii

i

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Proof (2/3)

When i=1 , The waiting time of the first packet equal

s its own transmission time (2*SIFS + CFPoll + tp + ACK), therefore, when i =1 this establishes the induction basis.

Assume that our induction hypotheses stands for the (i-1)th voice source, ie. ,

_

1 1 p 1 1 12 SIFS CFPoll t ACK

jj 1-ij1

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Proof (3/3) Assume , which means at the time point ,

all voice sources, from 1 to i-1, will have been multi-polled. Hence, the amount of packets generated between (0, )is , which means the total transmission time will be

From the already known fact , we can derive the

following formula:

Since this contradicts our hypothesis, which states that , we obtain , which also stands for the ith voice source.

iii ii

ii

1-i

1kiick )(r

i 1*

ck i i pk 1

( r ( ) 1) (2 SIFS CFPoll t ACK)

ciii r

1

i 1*

ck i i pk 1

( r ( ) 1) (2 SIFS CFPoll t ACK)

i 1

* *ckp i i i

k 1 ci

r( 1) (2 SIFS CFPoll t ACK)

r

iii

iiii

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Theorem 2

Suppose n voice sources are scheduled in the given priority order. The average waiting time is minimized for voice packets if for all i < j

cjci rr

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2.1 Proposed Scheme

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Improvement (1/3)

1. If accepting the request of a new voice source P in the previous DCF mode, AP will build a new token bucket in its buffer for P, and assign a priority based on P’s tolerated jitter

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Improvement (2/3)

2. Under the PCF mode the station when polled must wait a period of ti

me, SIFS, before transferring its packet. When piggyback indicates that the underlying s

ession has not terminated, AP produces a new token every .

However, AP needs SIFS + CFPoll to poll the stations. A station needs SIFS + ACK to respond.

Therefore, in the same connection, the time duration from the removal of T to the production of the next token is - (2*SIFS + CFPoll+ tp +ACK).

cr

1

cr

1

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Improvement (3/3)

3. When the underlying session is ready to close, the piggybacking bit = 1, i.e., End-of-file and AP removes the corresponding bucket.

4. When all buckets are temporarily empty, AP checks if there is enough time to run DCF mode before the next token T arrives. If yes, it sends a CF-End frame to end CFP and enters CP mode. If not, it waits for T

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Theorem 3 Several voice sources with and ,

i=1,2,3,…,n, are given. There exists a cycle LCT =L.C.M. (The

Least Common Multiple) within which the amount of packets transmitted is .

If two or more packets of different sessions arrive at the same time point, based on Theorem 2 , a session with lower jitter has lower priority. This ensures a minimum total waiting time.

cir i

cnc2c1 r

1....,,.........

r

1,

r

1

n

1ici LCT)(r

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Simulations (1/3)

Fig.4.1 Channel Utilization for Sessions

0

0.2

0.4

0.6

0.8

1

1 3 5 7 9 15 25 35 45

Number of Sessions

Cha

nnel

Util

izat

ion

QPSM

QPMM

IEEE 802.11b

IEEE 802.11e

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Simulations (2/3)

Fig.4.2 Throughput for Sessions

00.20.40.60.8

11.2

1 3 5 7 9 15 25 35 45

Number of Sessions

Thr

ough

put

IEEE 802.11b, e

QPSM, QPMM

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Simulations (3/3)

Fig.4.3 Dropping Rate for Sessions

0

0.2

0.4

0.6

0.8

1

1 4 7 10 25 40

Number of Sessions

Dro

ppin

g Rat

e

IEEE 802.11 b, e

QPSM, QPMM

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Conclusions We record the scheduling results in a queue,

within which an AP (Access Point) can poll and then enable mobile users to communicate with their opposite sites.

This occurrence can solves the problem that some voice packets do not suit QoS in IEEE 802.11e standard with multi-polling.

During the time-gap in which no voice packets are transmitted, the scheme changes to DCF mode to transfer data packets.