Speaker ： Fu-Yuan ChuangAdvisor ： Ho-Ting WuDate ： 2007.01.02

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Outline

Introduction to IEEE 802.11e Tuning of 802.11e Network Parameters Adaptive Contention-Window MAC

Algorithms for QoS-Enabled Wireless LANs

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Introduction to IEEE 802.11e

New terminologyQAP – QoS Access PointQSTA – QoS StationHC – Hybrid Coordinator

A new mechanism defined in IEEE 802.11e -- Hybrid Coordination Function (HCF)

HCF is implemented by all QAPs and QSTAs

HCF has two access mechanismsContention based

○ Enhanced distributed channel access (EDCA)Controlled channel access

○ HCF Controlled Channel Access (HCCA)3

Architecture

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Comparison of 802.11 and 802.11e

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CAP controlled access phase :

A time period when the HC maintains control of the medium after gaining medium access by sensing the channel to be idle for a PIFS duration

Transmission Opportunity (TXOP) Under HCF, TXOP is basic unit of transmission TXOP types

EDCA TXOP○ Obtained by a QSTA winning an instance of EDCA

contention during the CPHCCA TXOP

○ Obtained using the controlled channel accessPolled TXOP

○ by a non-AP QSTA receiving a QoS (+)CF-Poll frame during the CP or CFP

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Enhanced Distributed Channel Access (EDCA) EDCA defines four Access Categories (AC)

VoiceVideoBest EffortBackground

EDCA supports 8 User Priority (UP) valuesPriority values (0 to 7) identical to the IEEE

802.11D priorities Rules

One UP belongs to one AC (Access Category)Each AC may contains more than one UPTraffic of higher UP will be transmitted first within one

AC7

EDCA-Access Category (AC)

Four access categories (ACs) that support the delivery of traffic with differentiated UPs

An AC is an enhanced variant of the DCF which contends for TXOP using the following parameters: CWmin[AC], CWmax[AC], AIFS[AC].

Each AC queue functions as an independent DCF STA and uses its own backoff parameter.

In EDCA, the Contention-Window (CW) size and the Inter-frame space (IFS) is AC dependent

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EDCA Details Each AC has its own

Interframe space – AIFS

Back off Counter CWmin, CWmax, CWTXOP limit

QSTAs obtains these info from beacon frames

Each QSTA implements own queues for each AC

If internal collision happens, the frame with higher priority will be sent

8 UPs mapping to 4 Access Category(AC)

8 User Prioritys per QSTA

Higher PriorityLower Priority

AC [0] AC [1] AC [2] AC [3]

BackoffAIFS[0]

CWmin[0]CWmax[0]

BackoffAIFS[1]

CWmin[1]CWmax[1]

BackoffAIFS[2]

CWmin[2]CWmax[2]

BackoffAIFS[3]

CWmin[3]CWmax[3]

Virtual Collision Handler

Transmissoin Attemp

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EDCA Parameter Set element

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ACM : admission control mandatory

ACI : Access category identify

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Contentions among Different ACs in

EDCA Contention among EDCAFs (AC, AIFS,

CWmin , CWmax ) to win a TXOP

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Default Values for Each ACAIFS[AC] = AIFSN[AC] × aSlotTime + aSIFSTime.

DIFS=2*aSlotTime +aSIFSTime

AC AIFSN CWmin/CWmax

AC_VO 2 3-7

AC_VI 2 7-15

AC_BE 3 15-1023

AC_BK 7 15-1023

Juliana Freitag, Nelson L. S. da Fonseca, and Jos´e F. de Rezende, “Tuning of 802.11e Network Parameters,” IEEE Communications Letters , Volume 10,  Issue 8,  Aug. 2006 Page(s):611 - 613

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Introduction

A novel mechanism for tuning the access parameters of 802.11e QAP and QSTAsTo solve the asymmetry problemTo produce balanced uplink and downlink

delaysThe network can operate under much higher

loads

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Introduction - asymmetry problem

the existing asymmetry between the uplink and downlink delays which occurs when using the 802.11e contention method

The QAP is responsible for forwarding all traffic to/from QSTAs

Since both QAP and QSTAs have the same probability of accessing the medium, the queues in the QAP can rapidly build up, increasing the downlink delay

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The Proposed Approach

The adjustment of TXOP value is used to improve the throughput of the classes at the QAP leading to more balanced delay

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Idea - TXOP

假設每個 QSTA在每個 Class中有一個 flow，第 i 個 Class 的 TXOP允許 q 個 frames被傳送

則在 QAP中第 i 個 Class 的 TXOP的值調整為可以傳送 k*q 個 frames (k為第 i 個 class 中downlink flow的數目 )

可讓 downlink throughput接近 uplink throughput

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Mechanism

Load最小的 Class將其 TXOP設為 0只可傳送一個 frame

其他 class根據一個 ratio調整 TXOPThe ratio between the load that has arrived at

their queues and the load that has arrived at the queue of the class with the lowest load

可讓 load大的 queue獲得較大的傳送時間

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CW

當 STA數目不多時， low CWmin 可以減少idle的時間並增進 channel utilization

當 STA數目過多時， high CWmin 可以避免collision

As the number of stations with active flows of a certain class increases, the CWmin value of this class should increase as well as that of all classes with lower access priority

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Mechanism

若 STA( 在 class i中有 active flows)數目大於CWmin(i)，則 CWmin(i) is increased to the next power of 2 minus 1

反之，則 CWmin(i) value is reduced to the immediate lower power of 2，縮短 idel的時間

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Samer El Housseini, Hussein Alnuweiri, “Adaptive Contention-Window MAC Algorithms for QoS-Enabled Wireless LANs,” wireless Networks, Communications and Mobile Computing, 2005 International Conference on , Vol.1,  pp. 368- 374, June 2005

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Saturation Throughput

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The Throughput Derivative Algorithm

after each CW change, the TD Algorithm employs measurements of the throughput taken in the AP

CWmax is also changed while keeping the same ratio (CWmax / CWmin)

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The Throughput Derivative Algorithm

The throughput derivative is taken over the present and a few past measurements of the throughput

If the derivative is positiveThe AP continues to increase the CW

If the derivative is negativeThe AP decrease the CW

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The Throughput Derivative Algorithm

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References IEEE-802.11WG, “IEEE Standard for Information technology -

Telecommunications and information exchange between systems - Local and metropolitan area networks - Specific requirements Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications Amendment 8: Medium Access Control (MAC) Quality of Service Enhancements,” November 2005

Juliana Freitag, Nelson L. S. da Fonseca, and Jos´e F. de Rezende, “Tuning of 802.11e Network Parameters,” IEEE Communications Letters , Volume 10,  Issue 8,  Aug. 2006 Page(s):611 – 613

Samer El Housseini, Hussein Alnuweiri, “Adaptive Contention-Window MAC Algorithms for QoS-Enabled Wireless LANs,” wireless Networks, Communications and Mobile Computing, 2005 International Conference on , Vol.1,  pp. 368- 374, June 2005

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