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Tuesday, August 5, 2008

Mac frame format

Medium access control is very important to prevent collisions and delays. The data link layer is responsible for medium access control and several protocols are used for this purpose. Carrier sense multiple access is one way of controlling access to the medium. In CSMA/CD the system first senses the carrier before transmitting when the carrier is idle the system sends the data. When the carrier is busy the system backs of for a certain period of time before sensing the carrier again.

let us take a closer look at the Mac frame format

The Preamble is a seven byte field made up of alternating 0's and 1's which is very essential for synchronization between the transmitter and the receiver.

Start frame delimiter has a fixed sequence 10101011 which indicates the start of a frame and identifies the first bit for the receiver.

The Destination address is either 2byte or 6byte which refers to the address of the receiving system.

The Source address is either 2byte or 6byte which refers to the address of the sending system.

The length field provides the length of the data field of the logical link control layer.

The data from the LLC layer is encapsulated in to the Mac frame.

There are some extra bits needed for padding to ensure that the frame is matching the size perfectly.

A 32 bit CRC is provided at the end of the frame for error detection.

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