1. An Improved Mesh Coordinated Channel Access Mechanism for IEEE 802
An Improved Mesh Coordinated Channel Access Mechanism for IEEE s 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, February, 2010

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

3. 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 s comes up with an optional access mechanism
Mesh Coordinated Channel Access (MCCA)- allows to reserve future slots for collision free transmission of QoS data.

4. 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.

5. 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.

6. MCCA – an overview (cont’d)
Access Phase
MCCA-owner
Needs to contend for the channel during its MCCAOP using
IEEE e 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

7. 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

8. 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.

9. 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

10. 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

11. 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.

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

13. Thank You..
Q&A