Company

LOGO

Provision of Multimedia Services in 802.11-based Networks

Colin Roby

CMSC 681 Fall 2007

Overview

1. Multimedia Streaming in 802.11 Networks1. Multimedia Streaming in 802.11 Networks

2. 802.11e Mac Layer Enhancement 2. 802.11e Mac Layer Enhancement

3.Sustain Quality of Service in WLAN3.Sustain Quality of Service in WLAN

4. Challenges for Quality of Service Provision 4. Challenges for Quality of Service Provision

1.Multimedia Streaming in 802.11 Networks

Background Info on 802.11 Network

Contention-based network:1) System can only transmit when the sharing media is free and clear of signals.2) If multiple systems find the media is free and attempt to transmit, data collision will result.3) Busy network will induce higher collision rate and reduce performance.

MAC – Media Access Control ProtocolResponsible for sharing the physical connection to the network among several computers.

1.Multimedia Streaming in 802.11 Networks

Common problems in sustaining quality of service in streaming real-time multimedia in 802.11 networks

Technology limitations:1) Random resources sharing nature of the networks affects QoS(quality of service)2) Enforce common application level QoS control at MAC level - provide some priority mechanism in the MAC protocol.

Flaws in the default ip-level differentiated service:1) Use a single MAC queue to manage ip traffic, and service is at the same level.2) High priority flow sometimes is treated same as non-priority (best effort) flow 3) The differentiated services is difficult to use variable MAC factor (parameters) which determines the network condition.

Problems in the basic distributed coordination function (DCF) algorithm:1) when network contention increases will result frequent collision, which introduces disparity between services received by different hosts on the network 2) when different channel competing for the medium, the access order is not the same as the backlogging order, creates the random nature of the 802.11 MAC protocol.

802.11e Mac Layer Enhancement

HCF (Hybrid coordination function)

HCCAEDCA

Enhanced Distributed Channel Access

HCF controlled Channel Access

802.11e Mac Layer Enhancement

802.11e’s new MAC layer function HCF(hybrid coordination function)

EDCA – Enhanced Distributed Channel Access:Goal: high priority traffic class receives more transmission time than low priority class.It combines three different MAC parameters to achieve this goal:1) Arbitration interface space (AIFS) parameter set2) Minimum/maximum contention window parameter set3) Transmission duration (optional)* MAC parameter are customized towards each traffic class.* Drawback: non-multimedia application will experience degraded performance, can not be universally applied to all network configuration.

HCCA – HCF Controlled Channel Access:Goal: reduce randomness and increase service guarantees in contention-based network access.1) Channel pools each station from a predefined, continuously updated polling list2) Each station provides a traffic specification that characterize the burstiness (TSPEC), each station explicitly targets a particular scheduling method.Most significant parameter: Mean data rate, delay bound, maximum service interval, nominal MSDU size.

802.11e Mac Layer Enhancement

EDCA – Different class parameter value:

802.11e Mac Layer Enhancement

HCCA frame for traffic specification:

802.11e Mac Layer Enhancement

802.11e’s new MAC layer function HCF(hybrid coordination function)(Continued)

ETSI HiperLan (European Technical Standard Institution)European standard – an alternate method for media access – Elimination Yield-Non Preemptive Multiple Access(EYNPMA)Hiperlan breaks into three phases:1) Prioritization phase – burst duration, jaming sequence2) Elimination phase – same priority3) Sensing phase – backoffed process detects idle networkDrawback: maintains the characteristic of high randomness, make it hard to guarantee fairness.

Service differentiation for distributed IEEE 802.11 networksService differentiation at EDCA is achieved through setting static parameters for different traffic class. Each parameter influences a particular medium access priority.

1) AIFS – smaller value allows earlier start, limits flow collisions between different traffic class,But increases contention between flow from same traffic class.2) Frame Size/TXOP – changes the amount data to be send, results in higher or lower throughput.3) Persistent Factor – scale up and down contention window4) Backoff – various algorithms can be used to improve QoS performance – determines the spacingbetween two successive transmission attempts.

802.11e Mac Layer Enhancement

802.11e’s new MAC layer function HCF(hybrid coordination function)(Continued)

ECDA parameter effects:1) AIF – minimize intra-class collision in heavily loaded networks 2) Frame Size – increase burst rate of data, but does not guarantee transmission time3) Persistent Factor – large persistent factor will cause colliding traffic to rapidly (multiplicative) Increase contention window.4) Backoff Algorithm – adaptive in nature, monitoring network conditions to dynamically adjustMAC parameters.

Sustain Quality of Service in WLAN

DynamicReservation

DBASECARC

DAC

VirtualMAC

Resource Control Method

Bandwidth provision: Estimating

the achievable QoS performance

Bandwidth provision method:1) DBASE – (Distributed Bandwidth Allocation/Sharing/Extension) protocol : Split contentioninto two subperiods, one for real-time traffic, one for non-real-time traffic.2) CARC – (Call Admission and Rate Control) : control each station’s arrival rate to achievedesired minimum throughput, maximum delay, jitter or loss rate in the networks.3) Combination of DAC (Distributed admission control) and two-level protection and guaranteemechanisms – DAC uses statically allocated network bandwidth, each traffic class receives a fixedshare of bandwidth.4) Virtual MAC and virtual source algorithms – a virtual MAC operates in parallel to the real MAC Algorithm. VMAC estimates the collision probability. But not actually transmit any data. VMAC usesTime stamped virtual packet, and calculate total delays to estimate the channel capacity to supportnew demands.5) Dynamic multiple-threshold reservation algorithm – use cellular infrastructure networks and assign different priority to different traffic class. Cellular network assign each flow a fixed transmission slots.

Sustain Quality of Service in WLAN

MAC Fairness

Inter-TCInter-TC Intra-TCIntra-TC

Traffic flow between same

traffic class

Traffic flow between different

traffic class

Balance

DFS - Distributed fair scheduling1) Determines which station should access the medium first2) Initiate backoff process before transmitting each frame3) Backoff process is proportional to packet size and inversely proportional to flow priority

Advantage: cause lower priority station to generate longer backoff intervalsDrawback: can not guarantee QoS to flows of the same priority, fairness is limited at throughput

MAC Fairness

Challenges for Qos Provision

A B C D

Trade off betweenachieved network throughputand delay guarantees-Increasing flow’sthroughput beyondcertain thresholdresults enqueuingdelays

Find optimal network point – maximizing the number of Qos-enabled service regardless network configuration

Construct admission control mechanism enable all possible per-class load distribution within the limits of Qos metrics

Admission decision should be made at different active stations rather than admitting station.

Reference:1) Provisioning of Multimedia Services IN 802.11-Based Networks: Facts and Challenges(IEEE Wireless Communications, October 2007)

Reference