IEEE802.11e Enhanced MAC for QoS and

Efficiency What is QoS and why do we need it? Overview of 802.11e

EDCA, TXOP, Traffic classes, burst ACKsDirect Link Protocol

WLAN EDCA

QoS (Quality of Service)

QoS parametersDelay/latency, available bandwidth, error correction, acknowledgement scheme

All 802 MAC schemes are Best Effort. But,

Voice traffic : rather loss than delayData traffic : no loss, less stringent delay

How do we provide QoS?1. Categorize the traffic2. Define a scheme to each category

Priority (e.g. CW, DIFS), specific transmission slots

WLAN EDCA

Features of IEEE802.11eFully backwards-compatible

Stations without 802.11e will be able to operate in an 802.11e environment.

Two means of QoS provisioningPrioritizing traffic : CW, DIFS etc.Allocating specific transmission times for traffic

Three optional means for increasing efficiency (throughput) of the network

Burst acknowledgement : many at a chanceDirect link protocol : between STAs without the APNo acknowledgement : e.g.) temperature monitoring

WLAN EDCA

Hybrid CF (HCF)

Two featuresImplemented at every station using 802.11eIn an Infrastructure BSS, a centralized scheduling function called Hybrid Coordinator (HC) that operates at the AP

Coexists with both DCF and PCF.

WLAN EDCA

Differentiated Traffic ClassesEach packet is allocated either to

One of the Traffic Streams (TS) cf) per-flowOr one of the Traffic Classes (TC) cf) per-class

Each station has, simultaneously in use,Upto 8 Traffic Streams And 8 Traffic Classes

Layers above the MAC specify through the MAC SAP of the TS or TC that each frame belongs to

WLAN EDCA

PHY

e.g. 802.1D

MAC

SAPs for 8 TSs SAPs for 8 TCs

Default EDCA Parameter Sets for 802.11a and 802.11b

AC 802.11a(aCWmin=15, aCWmax=1023)

802.11b(aCWmin=15, aCWmax=1023)

Non-AP Station AP TXOP

Limit

Non-AP Station AP TXOP

Limit

CW

min

CW max

AIF

SN

CW

min

CW max

AIF

SN

CW

min

CW max

AIF SN

CW

min

CW max

AIF

SN

BK 15 1023 7 15 1023 7 0 31 1023 7 31 1023 7 0

BE 15 1023 3 15 63 3 0 31 1023 3 31 127 3 0

VI 7 15 2 7 15 1 3.008 ms

15 31 2 15 31 1 6.016 ms

VO 3 7 2 3 7 1 1.504 ms

3 15 2 7 15 1 3.264 ms

WLAN EDCA

Traffic Categories 802.1D(1993)

TC Acronym Types

1 BKBackground

2 -Spare

0 (default) BEBest Effort

Prior to 1 & 2

3 EEExcellent

effort

4 CLControlled

load

5 VIVideo

6 VOVoice

7 NCNetwork control

# of queues

Traffic types

1 BE

2 BE VO

3 BE CL VO

4 BK BE CL VO

5 BK BE CL

VI VO

6 BK BE

EE

CL

VI VO

7 BK BE

EE

CL

VI VO

BE

8 BK

- BE

EE

CL

VI VO

NCWLAN EDCA

User Priority and AC in IEEE 802.11e

Priority User Priority

(UP)

Access Category (AC)

Designation (informative)

Lowest

.

.

.

Highest

1 AC_BK Background

2 AC_BK Background

0 AC_BE Best Effort

3 AC_VI Video

4 AC_VI Video

5 AC_VI Video

6 AC_V0 Voice

7 AC_V0 Voice

WLAN EDCA

IEEE 802.11e -EDCA

AC_VO[0]

AC_VI[1] AC_BE[2]

AC_BK[3]

AIFS 2 2 3 7

CWmin 3 7 15 15

CWmax 7 15 1023 1023WLAN EDCA

AIFS[j]

AIFS[i]

DIFS

PIFS

SIFSBackoff WindowBusy medium

Contention window

Next frame

DIFS/AIFS

Slot time

Defer access Select slot and decrement backoff

as long as medium is idle

Immediate access when

Medium is free DIFS/AIFS[i]

Traffic streams

Periodic traffic TS TXOP on HCTXOP Transmission Opportunities

TS Specifications (Tspec) for negotiation even not guaranteed by HC

ACK policy (no ACK, ACK, Burst ACK)PriorityInter-arrival time of MSDUsMin and mean data rate, maximum burst size Delay and jitter (delay variation) bounds

WLAN EDCA

Enhanced Distributed Channel Access (EDCA)

For packets not assigned to any TSDifferent access categories (ACs)

AIFS Arbitration IFS (CWmin, CWmax)Each Access Categories (ACs) runs the DCF protocol independently as a separate station.Up to 4 ACs for a station

WLAN EDCA

MAC SAP

AC 1 AC 2 AC 3 AC 4

EDCF MAC

IEEE 802.11e Access Category

WLAN EDCA

IEEE 802.11e station with four backoff entities

Eight priorities 0-7 according to 802.1D are

Mapped to four access actegories (Acs)

7 6 5 4 3 0 2 1

Four access categories (Acs) representing four priority to four access actegories (Acs)

High Priority Low Priority

Backoff :AIFS[AC_VO]CWmin[AC_VO]Cwmax[AC_VO]

Backoff :AIFS[AC_VI]CWmin[AC_VI]Cwmax[AC_VI]

Backoff :AIFS[AC_BE]CWmin[AC_BE]Cwmax[AC_BE]

Backoff :AIFS[AC_BK]CWmin[AC_BK]Cwmax[AC_BK]

Upon parallel access at the same slot, the higher-priority ACBackoff entity transmits; the other backoff entity entities act as if Collision occurred

transmission

Backoff :DIFS15

1023

One

priorityBackoff

entity

transmission

AIFS = 2,3 ….(for station

AIFS= SIFS+aSlotTime x AIFSN

Transmission Opportunities (1)

Acquired in two waysQoS-Polled TXOP by the HC Or, an AC can successfully contend on the medium.

A specified period time is allowed to a station or AC.All frames within a TXOP are separated by SIFS.Multiple MPDUs may be transmitted within a TXOP.It may fragment MSDU or MMPDUs.

WLAN EDCA

Transmission Opportunities (2)

TXOP can start during either the CFP or CP, but must finish within that period.Controlled Access Period (CAP)

For the HC, to satisfy TSPECs and deliver data it has been queued, A CAP may be used by the HC to transmit data or to allocate TXOPs to other stations

WLAN EDCA

Block ACKs

Acknowledge multiple MPDUs by a Block ACKs to reduce the overhead

WLAN EDCA

Originator

Recipient

data

Block ACK request

Block ACK

SIFS

Direct Link Protocol

Within an Infrastructure BSSWithin transmitting range of the sourceNot in power save mode

Before DLP handshake via the APExchange capability (security)Tear down via the AP

WLAN EDCA

AP

STA

STA

STA

BSS

DLP

Normal path

Use-case : video conferencing and data traffic over 802.11g

WLAN using DCF and EDCA

WLAN EDCA

AP

Video conferencing stations

Web browsing

File transfer

Wired Network (e.g. The Internet) IEEE 802.11

Network Source : Sony Shimakawa and Stanford Tobagi

Issues

Good QoS : voice, video, lip sync (<133ms)Video conferencing is a demanding application.

High bandwidthUser-perceived quality sensitive to loss and delay

Impact of delayDCF vs EDCA with prioritized packetsRealistic simulation

Protocols and wireless channel (path loss, fading)Realistic traffic and quality metrics

WLAN EDCA

V/C Quality Requirements

User-perceived quality requirementsVideo : 384kbps

Image quality : PSNR>20dBFrame rate : > 5fps (encoded at 15fps)

Voice : 64kbpsMean opinion score (MOS)>3.6Playout deadline of 150ms

Voice/video synchronizationVideo may lag voice by < 133ms

TCP condition RTT 1~60ms, RWin=16~64kBytes

WLAN EDCA

Capacity of V/C w.r.t. Cell Size

The larger cell, the poorer channelLimiting factor is voice delay.

WLAN EDCA

20

10

4

15m

20m

25m

30m

Vid

eo c

onf.

capaci

ty

54Mbps

12Mbps

6Mbps

EDCA MAC protocol

EDCA : Prioritized MAC protocolEach device includes 4 Channel Access Functions (CAF).

WLAN EDCA

DCF

All traffic

DCF MAC

CAF-VO

Voice

EDCA MAC

CAF-VO

Video

CAF-VO

Web

CAF-VO

FTP

EDCA vs. DCF

Low priority CWmin=15, AIFS>DIFS

High priority CWmin=3, AIFS=DIFS more collisions, less overhead

Contention-free burstsreduces overheadmay increase delay for low priority traffic

WLAN EDCA

EDCA vs DCF with FTP TrafficDCF allows higher capacity when 0 or 1 FTP usersEDCA improves FTP Performance

Extra delay due to CAF is not an issue.

WLAN EDCA

15

10

5

2 4 6 8

Vid

eo c

onf.

capaci

ty

EDCA

DCF

7

4

2

2 4 6 8

EDCA

DCF

FTP users Video conf. users

Avera

ge F

TP b

itra

te

[Mbps]

2 FTPs

4 FTPsSmall cell

(r=10m)

24Mbps data rate

Conclusions

In good channel conditions, up to 19 simultaneous video conferencing sessions can be supported by a single AP.TCP-based traffic (FTP, Web) reduces V/C capacity when DCF is used.EDCA effectively priorities V/C over TCP while improving TCP application performance.EDCA supports fewer V/C calls when few or no TCP applications present, due to high collision rate

WLAN EDCA