An Improved Mesh Coordinated Channel Access Mechanism for IEEE 802.11s Wireless Mesh Networks

Md. Shariful Islam, Muhammad Mahbub Alam and Choong Seon HongNetworking Lab, Department of Computer EngineeringKyung Hee University, Korea.

The third AsiaFI Winter School, 24-27 February, 2010

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Presentation Outline

• Introduction• MCCA- an overview• Problem description• Proposed improved mesh coordinated channel

access mechanism •Performance Analysis•Conclusions

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Introduction

• Wireless Mesh Networks (WMNS) provide internet access to end-users

with low cost.• Act as a wireless backbone• Most of the traffic is to/from Internet

and thus, gateway centric.• Nodes can act both as an access point

and relay node.• High volume of traffic makes traditional

EDCA less effective and it can not provide QoS guarantee.

• Upcoming IEEE 802.11s comes up with an optional access mechanism▫ Mesh Coordinated Channel Access

(MCCA)- allows to reserve future slots for collision free transmission of QoS data.

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InternetMesh Point Portal (MPP)

Mesh Point (MP)

Mesh Access Point (MAP)

Legacy Station (STA)

Mesh Link

Legacy Link

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Contributions

• Identify the problems with current MCCA mechanism in the presence of contention from non-MCCA nodes.

• Introduce an improved channel access mechanism for MCCA.

• Through simulation we show that our proposed iMCCA mechanism performs better than the existing schemes.

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MCCA – an overview

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Each node identifies the free slots in its two-hop neighborhood by examining the MCCAOP advertisement (MADV) messages of adjacent nodes.

MADV includesTX-RX-all slots for which the MP is either a transmitter of receiverInterfering Times report (IR)- all slots where the MP is neither tra-

nsmitter nor receiver, but slots are busy because of transmission / reception of its neighbor.

Sender initiates MCCAOP request with duration, offset and periodicity.Receiver replies with Setup Reply if slots do not overlap and its

neighbor MPs’ MCCA Access Fraction (MAF) limit do not exceed.

Each node identifies the free slots in its two-hop neighborhood by examining the MCCAOP advertisement (MADV) messages of adjacent nodes.

MADV includesTX-RX-all slots for which the MP is either a transmitter of receiverInterfering Times report (IR)- all slots where the MP is neither tra-

nsmitter nor receiver, but slots are busy because of transmission / reception of its neighbor.

Sender initiates MCCAOP request with duration, offset and periodicity.Receiver replies with Setup Reply if slots do not overlap and its

neighbor MPs’ MCCA Access Fraction (MAF) limit do not exceed.

Reservation Phase

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MCCA – an overview (cont’d)

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MCCA-ownerNeeds to contend for the channel during its MCCAOP using IEEE 802.11e EDCA contention parametersTransmission must finish within the reserved MCCAOP

MCCA-awareRefrain from transmission during known MCCAOP by setting their NAV values

Non-MCCADo not honor MCCA reservation and allowed to contend for the

channel during a MCCAOP

MCCA-ownerNeeds to contend for the channel during its MCCAOP using IEEE 802.11e EDCA contention parametersTransmission must finish within the reserved MCCAOP

MCCA-awareRefrain from transmission during known MCCAOP by setting their NAV values

Non-MCCADo not honor MCCA reservation and allowed to contend for the

channel during a MCCAOP

Access Phase

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Problem description

• Performance of MCCA is affected due the presence on non-MCCA node.▫ Probability of collision increases as number of contending

non-MCCA node increases in the neighborhood▫ MCCAOP may be foreshortened if a non-MCCA node wins the

backoff

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Improved mesh coordinated channel access mechanism

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• Proposed Access Mechanism▫ Introduce a new Inter Frame Space (IFS) for MCCA referred to as

MCCA inter frame space (MIFS) asMIFS = SIFS + aSlotTime

▫ Non-MCCA nodes using EDCA waits for an Arbitration Inter Frame Space (AIFS) time before initiating backoff process.

AIFS = SIFS + AIFSN [AC] x aSlotTime

▫ If the MCCA-owner finds that the medium is idle after MIFS time, it initiates the transmission without invoking any backoff process.

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iMCCA- an example

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MCCA-owner: node x, non-MCCA: node y and MCCA-aware: node z Node x starts transmitting after sensing the channel idle for MIFS

period.Non-MCCA node y senses the channel as busy after its AIFS period.MCCA-aware node z already set its NAV and refrain from transmission Thus, MCCA-owner’s access during MCCAOP is guaranteedBlock ACK is used to further increase channel efficiency

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Performance Evaluation

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Simulation Parameters

Tool Network Simulator-2

Topology 3x3 grid

Area 150m x 150m

Packet Size 80 Byte

Interface 802.11a

MAF limit .43

Performance metrics

1. Cumulative throughput 2. MCCA utilization

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Performance Evaluation (cont’d)

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• iMCCA allows to complete transmission during MCCAOP• Non-MCCA STAs do not starve• MCCA utilization becomes high with EMDA as non-MCCA STAs

can not gain access during MCCAOP.

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Conclusions

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• We propose an improved channel access mechanism to enhance the effectiveness of MCCA mechanism introduced in IEEE 802.11s.

• Explicitly identify the problems with current access mechanism provide simple and straight forward solutions.

• Finally, simulation results demonstrate the effectiveness of the proposed scheme.

• Still a delayed start due to an ongoing transmission from non-MCCA node can affect the performance of MCCA.

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Thank You..

Q&A

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