Admission Control for IEEE 802.11e Wireless LANs

Student: Conroy Smith

Supervisor: Neco Ventura

Department of Electrical Engineering

University of Cape Town

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Overview

Introduction IEEE 802.11e QoS Enhancement Proposed Problem Proposed Admission Control solution Performance Evaluation and Preliminary Results Conclusions Future Work

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Introduction

Wireless LANs are expected to have a major impact on people’s daily life styles Provides Cheaper Internet Connectivity

Relatively high throughput

Ease of Implementation

Being endowed with roaming capabilities

Voice enabled devices are now being equipped with WiFi capabilities

This allows WiFi to compete directly with 3G Cellular Networks

The Internet

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Introduction

Original 802.11 Standard Lacks QoS SupportWLANS was traditionally a “Best Effort” Wireless Access

IEEE 802.11e provides a QoS enhancementModifications made to MAC layerProvides service differentiation

QoS can only be achieved at when the network load is not too heavyChannel overloading decreases network throughput

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IEEE 802.11e QoS enhancement

MAC Layer Modification of the IEEE 802.11e enhancement

IEEE 802.11e Specifies are new Hybrid Co-ordination Function (HCF)

The HCF Specifies 2 Access modes:

HCF Controlled Channel Access (HCCA)

Enhanced Distributed Channel Access (EDCA)

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IEEE 802.11e EDCA

The EDCA allow service differentiation, by supporting 8 different priorities

further mapped to 4 Access Classes (ACs) Each AC behaves as a single enhanced DCF contending

entity with dedicated queues

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IEEE 802.11e EDCA

Differentiation achieved by:AIFSCWminCWmaxTXOP

Queuing Architecture of EDCA

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IEEE 802.11e EDCA

Contention in the EDCA:

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Problem of overloading 802.11 channel in the EDCA

Traffic congestion and can lead to severe overall network degradation.

New flows will may not be able to achieve their QoS

They may also degrade the QoS of other admitted flows

The need for Admission control has become apparent

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Proposed Admission Control For IEEE 802.11e

A measurement/model-based Admission control

WLAN channels are modeled using the a modified Bianchi Model [4]

This is used to Bandwidth Estimations for each Virtual Station

• Each AC queue act as a Virtual Station

Queue utilization and collision statistics are measured and used by the model

Uses TSPEC to negotiate Admission control

Flows must state their throughput requirements

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Admission Negotiation

Typical TSPEC Negotiation

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Admission Control Decisions

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P(C) - Probability of a collision in a slot

P(I) - Probability of an idle slot

P(S) - Probability of a successful Tx in a slot

P(S|VS=i) - Probability of a successful Tx on Virtual Station i

A new request will demand more Throughput for its “Virtual Station” New flows will be Accepted ONLY if:

– Achievable Bandwidth (Si) ≥ Requested Bandwidth For All Virtual Stations

Achievable Bandwidth:

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Admission Control Decisions: Calculating the Probabilities

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Calculating Transmission Probability for each Virtual Station

A modified Bianchi model is used to calculate the Transmission Probabilities for each Virtual Station

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- Measured Collision Probability

- Percentage of time that the Virtual station is considered to be Saturatedip

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Accuracy of Bandwidth Estimations

Bandwidth Estimation Framework Integrated in the NS-2 Contributed Model from [5]

Simulation Consists of:1 AP6 Stations, 3 have unlimited data to send and 3 are unsaturatedAll Stations transmit at 18 Mbps (802.11a)

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Performance Evaluation of proposed admission control solution

Accuracy of the Bandwidth Estimations

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Performance Evaluation: Simulation Set up

At time t = 3 sec, each station has a TCP session and a voice and video flow

A new voice and video requests are added every 2 seconds• Voice flows: 64 Kbps• Video flows: 750 Kbps

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Preliminary Results

Performance without Admission Control(TCP session + Voice flow + Video flow)

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Preliminary Results

Performance with Admission Control(TCP session + Voice flow + Video flow)

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Preliminary Results

Performance without Admission Control(Total Throughput)

Flow 16 admitted

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Preliminary Results

Performance with Admission Control(Total Throughput)

Flow 16 rejected

Flow 18 rejected

Admitted Flows:

•9 Voice

•7 Video

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Conclusions

IEEE 802.11e WLANs provides QoS Support

EDCA Allows differentiation of 4 ACs Differentiated by, CW, TXOP and AIFS

Channel overloading can lead to severe degradation of QoS for EDCA flows

A measurement aided model-based admission control solution is proposed to protect QoS for EDCA flows

The accuracy of the bandwidth estimation indicates that effective admission control decisions can be made

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Future Work

Extend the Admission Control scheme to be compatible with TXOP Bursting and RTS/CTS handshake mechanisms

Investigate whether acceptable delays are achieved

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References

[1] Y. Xiao and H. Li, “Evaluation of Distributed Admission Control for the IEEE 802.11e EDCA“, IEEE Communications Magazine, vol. 42, no. 9, pp. S20–S24 2004

[2] D. Gu and J. Zhang, “A New Measurement-based Admission Control Method for IEEE 802.11 Wireless Local Area Networks”, Mitsubishi Elec. Research Lab, Tech. rep. TR-2003-122, Oct. 2003.

[3] D. Pong and T. Moors, “Call Admission Control for IEEE 802.11 ContentionAccess Mechanism”, Proc. IEEE GLOBECOM’03, vol. 1, San Francisco, CA, Dec. 2003, pp. 174–78.

[4] G. Bianchi, “Performance Analysis of the IEEE 802.11 Distributed Coordination Function”, IEEE JSAC, 18(3): 535-47, Mar. 2000.

[5] http://yans.inria.fr/ns-2-80211/

[6] H. Wu et. al. “IEEE 802.11e Enhanced Distributed Channel Access (EDCA) Throughput Analysis”

[7] Z Kong et. al. “Performance Analysis of IEEE 802.11e Contention-Based Channel Access”

[8] J. F. Robinson and T. S. Randhawa, “Saturation Throughput Analysis of IEEE 802.11e Enhanced Distributed Coordination Function”

Questions

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