QoS in IEEE 802.11 Networks

Resources

Introduction

IEEE 802.11

Simple, EffectiveDesigned for Best Effort Service

Real Time Services: Throughput and DelaySensitive

End-to-End QoS Guarantees, IEEE 802.11eEnd-to-End QoS Guarantees, IEEE 802.11e

IEEE 802.11 Architecture

STASTA

STA STA

STASTASTA STA

APAP

ESS

BSS

BSSBSS

BSS

Existing Wired LAN

Infrastructure Network

Ad Hoc Network

IEEE 802.11e

- QAP = QoS AP- QSTA = QoS Station

TCP

802.11 PHY

802.11 MAC

IP

802.3 MAC

802.3 PHY

TCP

802.3 PHY

802.3 MAC

IP

802.11 MAC

802.11 PHY

LLCLLC LLC

Station

AP

server

infrastructure network

Layers

PHY Layer

-HR/DSSS: High Rate Direct Sequence Spread Spectrum

- FHSS : Frequency Hopping Spread Spectrum

MAC Sublayer

PCF (Point Coordination Function)DCF (Distributed Coordination Function)

PCF-Point Coordinator (PC)-Only in infrastructure networks-Designed for delay-bounded services

PIFS

EIFS: Extended IFS

PCF- Centralized: location-dependent errors- Stations must wait for polling: Delay at low

load - AP needs to contend for the channel using

DCF to begin a CFP: variable CFP- Managing large number of stations using

polling affects the applications that use DCF

- No admission control

DCF• Distributed, Contention-based• CSMA/CA• Binary Exponential Back-Off

(CW: Contention Window)• Physical channel sensing• Virtual channel sensing (NAV:

Network Allocation Vector)• Hidden Terminal problem:

RTS/CTS• Timers• Retry limits• Fragmentation

Fragmentation

PriorityNAV: Network Allocation Vector

DIFS: DCF Inter Frame Space

SIFS: Short IFS

PIFS: PCF IFS

EIFS: Extended IFS

time

medium busy SIFSPIFSDIFSDIFS

next framecontention

Tunable Parameters

QoS Mechanisms

Service Differentiation

Priority: classification of traffic

Fair Scheduling: partitioning the bandwidth fairly by regulating the wait times of traffic classes according to given weights

EDCF (Enhanced DCF)

Virtual Collision Handler ~ Internal Collisions

Priority AC For1 0 Best Effort

2 0 Best Effort

0 0 Best Effort

3 1 Video Probe

4 2 Video

5 2 Video

6 3 Voice

7 3 Voice

AC: Access Category

AIFS: Arbitrary IFS

Persistent Factor DCF (P-DCF)

- A persistent factor P is selected; small P means higher priority traffic class

- A uniform random number r is generated every slot in back-off stages.

- A flow stops the back-off and starts transmission only if r > p in the current slot given no transmission occurs in previous slots The back-off interval is a geometric distributed random variable with P

Geometric random variable is the number of trials required to obtain the first failure

Distributed Weighted Fair Queue2 schemes are proposed:• CW for a flow = Difference between actual and expected throughput

A station decreases the CW to get higher priority Lower CW when the actual throughput is lower than theexpected one

• Li’ = Ri/Wi Ri = the actual throughput Wi = the weight Each station adjust its CW by comparing others Li’

SelfishnessMore stations will have small value of CW

Distributed Fair Scheduling (DFS)The back-off interval is based on the packet length and traffic class

For flow i, BIi proportional to:

1. The weight (higher for higher throughput classes)

2. The packet length 3. A scaling factor (to min the probability of

collisions in case different stations have same back off interval)

Distributed Deficit Round Robin (DDRR)1. Each throughput class i at station j is given a service

quantum rate (Qij) equal to its required throughput

2. A deficit counter (DCij) is advanced at the rate Qij in a round robin fashion

3. Once a DCij becomes positive, the ith queue is allowed to send one packet

4. After transmission, DCij will be decreased by packet length each time a packet is transmitted

DCij is used to calculate IFSij (time before transmit or back-off): larger DCij, smaller IFSij

DDRR

Polling in a round robin way

Queues of different throughput classes

Admission Control and Bandwidth Reservation

• Service differentiation does not perform well under high traffic loads

• There is a need to protect existing streams

• A wireless node has no knowledge of exact condition of the network

• With CSMA/CA, bandwidth provision is quite difficult

Measurement-Based Admission Control

- The decision is made on measurement of existing network status (delay, throughput, …)

- Different methods used:- Virtual MAC: the use of virtual MAC frames,

and using a virtual source algorithm to tune the virtual MAC.

- Probe packet: the use of probe packet for ad hoc

- Data probe: the use of data packets

Calculation-Based Admission Control

• Performance metrics or criteria for evaluating the network status

• Permissible throughput propagation• Saturation-based

Scheduling and Reservation-Based Schemes

• ARME (ASSURED RATE MAC EXTENSION

• ): - Extension of DCF - Uses token bucket-based algorithm to detect

overloading condition - improvements mad by adjusting CW

Scheduling/Reservation• AACA:

- RTS/CTS used for reservation - Mainly was for solving hidden terminal problem

Link Adaptation

Dynamically change the transmit rate , specified in the PLCP header of the PHY layer, that depend on channel conditions

Link Adaptation• Received Signal Strength (RSS)

– Each station maintains 12 RSS thresholds and corresponding transmission rate

– Measure RSS and adjust the transmission rate• PER-Prediction

– Decisions are based on Packet Error Rate-Prediction• MPDU-Based

• Success/Fail Thresholds• Code Adapts To Enhance Reliability

Direct Link Protocol (DLP)

• QSTA transmits directly to another QSTA• Set up with the QAP is needed• STAs cannot go into power saving mode

for active duration of the direct stream.• DLP is not applied in Ad Hoc networks• DLP messages can include security

information

Group ACK• Send a group of frames before any ACK to

reduce overhead• GroupAckReq• GroupAck frame with an ACK bitmap• Sender retry unacknowledged frames with

a retry limit• Receiver should keep the state of burst

data received (sender address, bit map, sequence numbers)

Challenges

• IEEE 802.11e and DiffServ• IEEE 802.11e and IntServ• Integration of WLAN and MANET• Integration of WLAN and Bluetooth• Integration of WLAN and 3G wireless

networks

Resources1. “A SURVEY OF QUALITY OF SERVICE IN IEEE 802.11 NETWORKS” By: HUA ZHU, MING LI, IMRICH CHLAMTAC, AND B. PRABHAKARAN THE

UNIVERSITY OF TEXAS AT DALLAS2. www.eecs.berkeley.edu/~ergen/docs/IEEE-802.11overview.ppt 3. www.cs.ucla.edu/classes/ winter04/cs117/chap7wlanRvsd.ppt 4. http://www.it.iitb.ac.in/~kirang/academic/MTP/Firststage/slides.pdf

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