An Improved Mesh Coordinated Channel Access Mechanism for IEEE 802 An Improved Mesh Coordinated Channel Access Mechanism for IEEE 802.11s Wireless Mesh Networks Md. Shariful Islam, Muhammad Mahbub Alam and Choong Seon Hong Networking Lab, Department of Computer Engineering Kyung Hee University, Korea. The third AsiaFI Winter School, 24-27 February, 2010

Presentation Outline Introduction MCCA- an overview Problem description Proposed improved mesh coordinated channel access mechanism Performance Analysis Conclusions

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.

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.

MCCA – an overview Reservation Phase Each node identifies the free slots in its two-hop neighborhood by examining the MCCAOP advertisement (MADV) messages of adjacent nodes. MADV includes TX-RX-all slots for which the MP is either a transmitter of receiver Interfering 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.

MCCA – an overview (cont’d) Access Phase MCCA-owner Needs to contend for the channel during its MCCAOP using IEEE 802.11e EDCA contention parameters Transmission must finish within the reserved MCCAOP MCCA-aware Refrain from transmission during known MCCAOP by setting their NAV values Non-MCCA Do not honor MCCA reservation and allowed to contend for the channel during a MCCAOP

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

Improved mesh coordinated channel access mechanism Proposed Access Mechanism Introduce a new Inter Frame Space (IFS) for MCCA referred to as MCCA inter frame space (MIFS) as MIFS = 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.

iMCCA- an example 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 guaranteed Block ACK is used to further increase channel efficiency

Performance Evaluation 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

Performance Evaluation (cont’d) 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.

Conclusions 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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