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IEEE 802.11 MAC Enhancements for Quality of Service

IEEE Std 802.11e

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Characteristics of IEEE 802.11e

• The major enhancement of 802.11e– Traffic differentiation

– Enhanced DCF (contention-based)

– EDCF

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MAC Architecture

• DCF : A contention-base access for 802.11. • PCF : An option to support contention-free access in 802.11.• Hybrid Coordination Function (HCF): IEEE 802.11 Task Group E (TGe)

proposes HCF to provide QoS for real-time applications.

IEEE 802.11 MAC Architecture IEEE 802.11e MAC Architecture

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HCF - Introduction

• HCF combines functions from the DCF and PCF with enhanced QoS-specific mechanisms.

• HCF consists of – Enhance DCF (EDCF) for contention-based access

– Controlled Access (HCCA) for contention-free access

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EDCF – Traffic Category

• The EDCF provides differentiated access to the WM for 8 priorities, identical to IEEE 802.1D priority tag, for non-AP STAs.– Priorities are numbered from 0 (the lowest priority) to 7

(the highest priority).

• The set of MSDUs with the same priority is refer to a Traffic Category (TC).

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EDCF – Access Category (1/5)

• EDCF defines access category (AC) mechanism to support the priority mechanism at QSTAs.

• An AC is an enhanced variant of the DCF which contends for transmission opportunity (TXOP) using the set of parameters such as CWmin[AC], CWmax[AC], AIFS[AC], etc.

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EDCF – Access Category (2/5)IFS (Inter Frame Spacing)

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EDCAS

IFS

2 slots

2 slots

3 slots

7 slots

0 ~ 3 slots

0 ~ 7 slots

0 ~ 15 slots

0 ~ 15 slots

Voice

Video

BestEffort

Back-ground

Random BackoffWait

MinimumWait

Assumptions: WME Default Parameters Backoff values shown are for

initial CW equal to CWmin = 15

AIFSN

AIFSN

AIFSN

AIFSN

SIF

SS

IFS

SIF

S

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EDCF – Access Category (3/5)

• The parameter set is specified in the “EDCA parameter set element” of beacon frames.

• Default QoS parameter set:

video

voice

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EDCF – Access Category (4/5)Queues

• An QSTA has four ACs. • Collision between ACs within a QSTA is called

internal collision. Collisions will be resolved internally (giving to higher-priority queues).

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EDCF – Access Category (5/5)

• The mapping from 8 priories to 4 ACs is:

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Summary

• A new EDCF block• Use of different IFS in traffic categories for QoS

support