Media Access Control

Media Access Control [mi ː dja ækses kəntɹəʊl ] or Medium Access Control [mi ː djəm ] (MAC, engl. " Media access control ") is a by the Institute of Electrical and Electronics Engineers ( IEEE) designed extension of the OSI model. IEEE divided the data link layer (layer 2) of the OSI model in the media access control sublayers (2a) and Logical Link Control (2b), wherein the MAC is the lower of the two.

The OSI model assigns to the needed in a computer network hardware and software components in a total of seven layers of increasing complexity. The higher a layer is, the less interested in the technical process of data transmission and the more they engaged in the actual content of the data. The MAC is the second lowest layer and includes network protocols and components that govern how multiple computers to share the commonly used physical transmission medium. It is needed, because a common medium can not be used simultaneously by multiple users without causing data collisions and thereby to communication errors or loss of data. In the original OSI model such competition was not provided to the communication medium, which is why the MAC is not included there.

Types of access

Depending on the implementation of the MAC is the access to the medium rather than controlled or competitive.

Controlled access ( engl. collision avoidance ) means that access is controlled to the media so that no collisions can occur. An everyday example is the school teaching: Many students want to talk; if they do the same, but you know nothing. Therefore, notify the student and the teacher determines who can talk. In this case, the MAC is implemented by an additional communication channel, because in addition to the acoustic data transmission medium of sound ( speaking of the students and the teacher ) here comes the visual synchronization medium light (the teacher sees which students log in). Sophisticated network protocols make additional communication channels superfluous.

Concurrent Access ( engl. collision resolution) means that everyone has access to the medium and that there are rules, such as collisions are treated without complications - CSMA / CD is one such protocol. This type of access is used intuitively even with ordinary telephone conversations: talking at the same time start the partners, so they hear immediately, everyone is waiting for one random amount of time, and who first begins to talk again, has the floor.

List of known MAC protocols

  • Concurrent Access ALOHA
  • CSMA / CD
  • CSMA / CA
  • Token-Ring
  • Token bus
Carrier sense multiple access with collision detection Reservation ALOHA Carrier sense multiple access with collision avoidance
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