Media Access Control


Media Access Control (MAC) is a sub-layer defined by IEEE 802 within the OSI-Model Data-link Layer

Media Access Control sublayer that determines who is allowed to access the media at any one time (e.g. CSMA/CD).

Media Access Control, generally, has two forms of media access control:

  • distributed which is often called "Broadcast"
  • centralized -
Both of these may be compared to communication between people. In a network made up of people speaking, i.e. a conversation, we look for clues from our fellow talkers to see if any of them appear to be about to speak. If two people speak at the same time, they will each pause a random amount of time and then attempt to speak again, effectively establishing a long and elaborate game of saying "no, you first".

Media Access Control sublayer also determines where one frame of data ends and the next one starts – frame synchronization. There are four means of frame synchronization:

  • time based - simply puts a specified amount of time between frames.
  • character counting - simply notes the count of remaining characters in the frame's header.
  • byte stuffing - precedes the frame with a special byte sequence such as DLE STX and succeeds it with DLE ETX. Appearances of DLE (byte value 0x10) have to be escaped with another DLE. The start and stop marks are detected at the receiver and removed as well as the inserted DLE characters.
  • bit stuffing - replaces these start and end marks with flag consisting of a special bit pattern (e.g. a 0, six 1 bits and a 0).

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