A New MAC Protocol for Wi-Fi Mesh Networks Tzu-Jane Tsai, Hsueh-Wen Tseng, and Ai-Chun Pang IEEE AINA’06

Outline Introduction Problems and challenges MASH (Medium Access with ScHeduling) Protocol Simulation Conclusions

Introduction (cont.)

This paper proposed a new MASH protocol to schedule the packet transmission between Mesh APs. add a new control packet, pseudo CTS (CTSp), to avoid the collisions in this wireless multi-hop environment.

Problems and challenges For multihop wireless networks, we point out two important issues: Backhaul Networking Multimedia Transport

Backhaul Networking Using TCP as the transport layer protocol to evaluate the throughput performance. The instability problem of throughput occurred in the TCP connections. A serious unfairness existed among TCP connections.

Multimedia Transport The IEEE e is a QoS protocol designed for WLAN. The collision rate will increase very fast in ad hoc networks

Problems and challenges (cont.) Analyzing the solutions of the performance in wireless multihop environment:

Problems and challenges (cont.) Consider the hidden terminal problem: multiple IEEE channels. Since the control of resources is more efficient, the hidden terminal problem can be lessen.

Problems and challenges (cont.) Consider the exposed terminal problem: the use of power control algorithms directional antennas

Problems and challenges (cont.) Consider the QoS-guaranteed multimedia transmission: IFS (Ideal Flow Scheduling) Centralized mechanism Support per-flow fairness model Data-Driven Cut-Through Medium Access (DCMA) protocol + MPLS Provide a distributed per-flow scheduling Packets forwarding performance is significantly improved

MASH (Medium Access with ScHeduling) Protocol One channel is dedicated to transfer control packets, and the others are used to transfer normal data packets. Using control packets to have a pre- negotiation for the transmission schedule in data channels.

MASH Protocol (cont.) The MASH protocol is primary based on the DCF (Distributed Coordination Function) architecture. Not only the collisions in data channels can be avoided, but also the spatial-reuse efficiency can be enhanced.

Network topology illustrating the packet collision Transmission schedule on control/data channel

Idle Set Tx time Tx Data Download Send RTS Wait for CTS Wait for CTS/CTS P Tx timer expires RTS sent Time out Data sent CTS received CTSp received CTS received Has data to send or relay The state diagram of a transmitting node in MASH

Wait Put NAV & Priority into the queue by scheduling Send CTS Idle Send CTS Set Tx timer Save priority & NAV of the data into a buffer Send CTS P CTS sent Timer expires and data channel busy Timer expires and data channel idle RTS received CTS sent & queue is empty Save finished CTSp Sent Scheduling finished & at duration time T CTS before data channel idle At duration time T CTS before data channel idle CTS sent & queue is not empty The state diagram of a receiving node in MASH

ABCEDF Common channel Data channel Example for hidden terminal problem in MASH R A -> B CRCRCPCP C B -> C ABCBCDE D -> E

ABCEDF Common channel Data channel Example for exposed terminal problem in MASH R A -> B CRCRC B -> C ABCBED E -> D

Simulation Simulation parametersValue Poisson Traffic with a mean arrival rate10~250 packets/sec Simulation duration200 sec RTS/CTS frame transmission rate1 Mbps Data Transmission Rate2 Mbps

Simulation (cont.) The cross topology in simulation

Simulation (cont.) The relationship between throughput and traffic load under the cross topology

Simulation (cont.) The relationship between end-to-end delay and traffic load under the cross topology

Simulation (cont.) The grid topology in simulation

Simulation (cont.) The relationship between throughput and traffic load under the grid topology

Simulation (cont.) The relationship between end-to-end delay and traffic load under the grid topology

Conclusions MASH solves not only hidden problem but also exposed terminal problems. In this paper, the authors propose the MASH protocol to provide a low latency and high throughput backhaul mechanism in WMNs.