25-06-2012, 01:22 PM
MEdIUM ACCESS PROTOCOLS
MEdIUM ACCESS PROTOCOLS.ppt (Size: 997.5 KB / Downloads: 25)
What is a MAC protocol?
A MAC protocol in a multi-access medium is essentially a distributed scheduling algorithm that allocates the channel to requesting nodes.
Two commonly used access principles in wireless networks are fixed-assignment channel access and random access methods.
Coordinates signal transmissions over shared channel.
Determines the communication schedules between many pair of users.
Determines the wireless-channel-occupancy durations .
Design Issues
The main issues need to be addressed while designing a MAC protocol for ad hoc wireless networks:
Bandwidth efficiency is defined at the ratio of the bandwidth used for actual data transmission to the total available bandwidth. The MAC protocol for ad-hoc networks should maximize it.
Quality of service support is essential for time-critical applications. The MAC protocol for ad-hoc networks should consider the constraint of ad-hoc networks.
Synchronization can be achieved by exchange of control packets.
RADIO LINK VULNERABILITY
Wireless channel capacity is limited due to high bit-error rate.
The radio link vulnerability may tremendously impact the utilization of the radio channel(s) and the service fairness among different mobile nodes.
Solutions such as forward error correction (FEC) and automatic repeat request (ARQ) results in inefficient bandwidth utilization.
Increase in transmission power to can broaden interference region, thereby resulting in the reduction of spatial reuse.
DESIGN GOALS
The operation of the protocol should be distributed.
The protocol should provide QoS support for real-time traffic.
The access delay, which refers to the average delay experienced by any packet to get transmitted, must be kept low.
The available bandwidth must be utilized efficiently.
The protocol should ensure fair allocation of bandwidth to nodes.
Control overhead must be kept as low as possible.
The protocol should minimize the effects of hidden and exposed terminal problems.
The protocol must be scalable to large networks.
It should have power control mechanisms.
The protocol should have mechanisms for adaptive data rate control.
It should try to use directional antennas.
The protocol should provide synchronization among nodes.
HRMA
Hop reservation multiple access protocol (HRMA)
a multichannel MAC protocol which is based on half-duplex, very slow frequency-hopping spread spectrum (FHSS) radios
uses a reservation and handshake mechanism to enable a pair of communicating nodes to reserve a frequency hop, thereby guaranteeing collision-free data transmission.
can be viewed as a time slot reservation protocol where each time slot is assigned a separate frequency channel.