IEEE 802.11eQoS on WLANs

Speaker : Min-Hua YangAdvisor : Ho-Ting WuDate: 10/25/05

Overview 802.11 background information

DCFPCFQos limitations of 802.11

802.11e (QoS support)EDCAHCCA

Simulation result

IEEE 802.11 background information

WLAN( Wireless Local Area Network )A LAN to which mobile users (clients) can con

nect and communicate by means of high-frequency radio waves rather than wires.

WLAN Standard IEEE 802.11 (IEEE) HiperLAN (European Telecommunications Standards

Institute, ETSI)

IEEE 802.11

IEEE 802.11 Topology

Basic Service set networks (BSS) Independent BSS networks Infrastructure BSS Extended Service Set (ESS) networks

The Basic Service Set (BSS) The BSS consists of a group of any number of stations. The basic building block of IEEE 802.11 LAN

Independent BSS (IBSS) network

stations communicate directly

no connection to a wired network

Ad hoc network

Infrastructure BSS (BSS)

Include an access point (AP) The AP may have connecti

on to an wired network All stations communicate w

ith the AP

Communication between stations must go through AP consume twice the bandwi

dth

The Extended Service Set(ESS)

Distribution system (DS) provides logical services necessary to handle address-to-destination mapping and seamless integration of multiple BSSs

An AP is a STA that provides access to the DS

AccessPoint

WirelessClient

WirelessClient

WirelessClient

Basic Service Set (BSS)

WirelessClient

AccessPoint

Distribution System

Basic Service Set (BSS)Access

Point

WirelessClient

WirelessClient

WirelessClient

Basic Service Set (BSS)

WirelessClient

AccessPoint

Distribution System

Basic Service Set (BSS)

Terminology- Interframe Space

Time interval between frames.SIFS – Short Interframe Space PIFS – PCF Interframe SpaceDIFS – DCF Interframe Space DIFS > PIFS > SIFS

Fixed for each PHY Provide priority levels

IEEE 802.11

Two coordination functions are defined the mandatory Distributed Coordination Funct

ion ( DCF) based on CSMA/CA optional Point Coordination Function (PCF) ba

sed on poll-and-response mechanism. Most of today’s 802.11 devices operate in

the DCF mode only.

Distributed Coordination Function ( DCF)

Possible in both Infrastructure and Ad hoc mode

Known as Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA)

Used During Contention period (CP) DCF can support best-effort services , not

any QoS guarantees.

Basic Access Mechanism-CSMA/CA

Sense the medium before transmitting. If the medium is not busy, the transmission may

proceed. If the medium is busy, invoke backoff procedure. Transmit after the backoff procedure If the transmission is not successful, invoke back

off procedure.

Virtual Carrier Sense Mechanism

Network Allocation Vector (NAV) Set NAV based on duration information (a

vailable in MAC header, RTS, CTS) NAV counts down to zero at a uniform rate. IF NAV=0, medium is idle; otherwise, it is

busy.

Basic Access Mechanism-CSMA/CA

Example of backoff intervals

IEEE 802.11 DCF (cont.)

BackoffTime = Random() × aSlotTime where Random( ) = [0, CW]

CWmin≤ CW ≤CWmax.

aSlotTime fixed for each PHY CWnew= (CWold+ 1) ×PF –1 (where PF=2) PF : Persistence Factor

A example of CW

Other Collision Avoidance Mechanism – RTS/CTS Hidden Terminal Problem:

Hidden Terminal are STAs that the receiver can hear but that cannot be detected by other senders.

In order to solve the problem,a optional RTS/CTS is introduced.

The source sends a short RTS frame before each data transmission,and the receiver replies with a CTS frame if it is ready to receive.

Basic access scheme v.s RTS/CTS access scheme

Point Coordination Function (PCF) [Optional in Standard] Used during Contention-Free Period (CFP) A single AP controls access to the medium, and

a Point Coordinator (PC) Agent resides in the AP.

AP polls each station for data, and after a given time interval moves to the next station.

No stations are allowed to transmit unless it is polled.

AP could have a priority scheme for stations. PCF is useful for time-sensitive applications.

PCF(cont.)

IEEE Legacy 802.11 MAC SuperFrame consists of Contention Period (C

P) and Contention Free Period (CFP). PCF used during CFP and DCF used during C

P.

QoS Limitations of 802.11

DCF (Distributed Coordination Function) Only support best-effort services No guarantee in bandwidth, packet delay and jitter Throughput degradation in the heavy load

PCF (Point Coordination Function) Inefficient central polling scheme Unpredictable beacon frame delay due to incompatible

cooperation between CP and CFP modes Transmission time of the polled stations is difficult to control

IEEE 802.11e – Overview

New terminology QAP – QoS Access Point QSTA – QoS Station HC – Hybrid Coordinator

In order to support QoS in 802.11 WLAN , 802.11e has defined a new mechanism , namely, Hybrid Coordination Function (HCF).

HCF is implemented by all QAPs and QSTAs HCF has two access mechanisms

Contention based Enhanced distributed channel access (EDCA)

Controlled channel access HCF Controlled Channel Access (HCCA)

IEEE 802.11e – MAC frame format

IEEE 802.11e Overview - Enhanced distributed channel access (EDCA)

EDCA defines four Access Categories (AC) Background Best Effort Voice Video

EDCA supports 8 User Priority (UP) values Priority values (0 to 7) identical to the IEEE 802.1D priorities

Rules One UP belongs to one AC Each AC may contains more than one UP Traffic of higher UP will be transmitted first in one AC

IEEE 802.11e Overview-User Priority (UP)

8 User Priority Identical to IEEE 802.1D priority tags.

Basic concepts – Transmission opportunity (TXOP)

Time interval permitted for a particular STA to transmit packets.

During the TXOP, there can be a series of frames transmitted by an STA separated by SIFS.

TXOP types EDCA TXOP initiation

Obtained by winning a successful EDCF contention

Polled TXOP (HCCA TXOP) Obtained by receiving a QoS CF-poll frame from the QAP

802.11e EDCF – Access Category

EDCF defines access category (AC) mechanism to support the priority mechanism at the non-AP QSTAs.

Each QSTA has four ACs. An AC is an enhanced variant of the DCF which contend

s for transmission opportunity (TXOP) using the set of parameters such as CWmin[AC], CWmax[AC], AIFS[AC], etc.

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

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

EDCA – Accessing the medium

EDCA use different IFSs SIFS – ACKs, between multiple frames with the cont

inuation EDCA TXOP DIFS / AIFS – Used by DCF and EDCA for different

access categories

EDCF - Arbitration Interframe Space (AIFS)

QSTA use AIFS to defer the contention window or transmission for each AC

AIFS[AC] = AIFSN[AC]x aSlotTime+ aSIFTimeAIFSN for each AC is broadcast via beacon fr

ame containing ‘EDCA Parameter Set’element

DIFS = 2 x aSlotTime+ aSIFTime

EDCF – Some elements of the Beacon frame

Default EDCF Parameter Set

EDCA details

Each AC has own Interframe space – AIFS Backoff Counter (BO) CWmin, CWmax, CW TXOP limit QSTA listens beacon frames t

o receive this information Each QSTA implements own q

ueues for each AC traffic From the queues the frame wit

h the highest priority is sent if internal collision happens

Legacy 802.11 station and 802.11e station with four ACs within one station.

802.11e HCCA Overview - Hybrid Coordination Function(HCF)

Designed to increase efficiency by reducing the contention on the medium

Uses ”polling” Like PCF Thus, HCCA can send ”polling” both under CFP and

CP PCF – Only polling in CP

Specifically assigned transmit times for every frame

Enables QoS guarantees

802.11e HCCA (cont.)

Has higher priority than EDCA .Under HCCA, HC(Hybrid Coordinator) has full controll over the wireless medium

If HC needs it, it could take over the controll of the medium by sending a QoS CF-Poll

802.11e HCCA (cont.)

Differences between hybrid coordinator (HC) and point coordinator (PC):HC can poll QSTAs in both CP and CFPHC grants a polled TXOP to one QSTA, which

restricts the duration of the QSTA’s access to the medium.

An example of an 802.11e superframe

Improve Efficiency – 802.11e

Block Acknowledgment Send multiple MSDUs without a bunch of ACKs Group ACK

Direct Link Protocol (LDP) No support for DLP in legacy MAC – Needs to talk through the

AP Less use of the channel The STAs is enable to talk directly to each other Sounds easy, lots of things to think about:

Power saving Security

Simulation-QoS , does it work?

EDCF Parameters for three queues

Throughput and delay performance for EDCF

Throughput(K

B/

s)

EDCF maintains the throughput of high-priority audio and video flows by punishing the background traffic.

Comparison of total throughput between EDCF and DCF

The total throughput of EDCF is lower than that of DCF when the traffic load is larger than 48%:EDCF reduces the throughput of low-priority flows considerable and therefore results in decreasing the total throughput.

Mean delay of audio,CBR video versus channel load for EDCF and HCF

The result show that the HCF controlled channel access mechanism can guarantee the minimum delay requierment(50ms) for the admitted flows in different load rate

EDCF works very well under low load conditions but suffers from delay degradation in high-load condition

Reference

Qiang Ni, Lamia Romdhani, and Thierry Turletti. "A Survey of QoS Enhancements for IEEE 802.11 Wireless LAN". Wiley Journal of Wireless Communication and Mobile Computing (JWCMC), John Wiley and Sons Ltd., 2004; Volume 4, Issue 5: 547-566.

“Analysis of IEEE 802.11e for QoS support in wireless LANs”Mangold, S.; Sunghyun Choi; Hiertz, G.R.; Klein, O.; Walke, B.;Wireless Communications, IEEE Volume 10,  Issue 6,  Dec. 2003 Page(s):40 - 50

Yu-Sun Liu Ph. D. WLAN上課講義 Google.com