Media Access Control
| layer | originally | expanded |
|---|---|---|
| 7th | application | application |
| 6th | presentation | presentation |
| 5 | meeting | meeting |
| 4th | transport | transport |
| 3 | Mediation | Mediation |
| 2 | Fuse | Logical Link Control |
| Media Access Control | ||
| 1 | Bit transfer | Bit transfer |
Media Access Control [ ˈmiːdja ˈækses kənˈtɹəʊl ] or Medium Access Control [ ˈmiːdjəm ] ( MAC , " Media Access Control ") is an extension of the OSI model designed by the Institute of Electrical and Electronics Engineers (IEEE) . The security layer (layer 2) of the OSI model is divided into the sub-layers Media Access Control (2a) and Logical Link Control (2b), the MAC being the lower of the two.
The OSI model arranges the hardware and software parts required in a computer network in a total of seven layers of increasing complexity. The higher a layer, the less interesting it is for the technical process of data transmission and the more it is concerned with the actual content of the data.
The MAC is the second lowest layer and comprises network protocols and components that regulate how multiple computers share the shared physical transmission medium . It is required because a common medium cannot be used by several computers at the same time without data collisions and thus communication disruptions or data loss occurring. In the original OSI model, such competition for the communication medium was not provided, which is why the MAC is not included there.
Access types
Depending on how the MAC is implemented, access to the medium is either controlled or competitive .
Controlled access
Controlled access ( collision avoidance ) means that access to the medium is regulated in such a way that no collisions can occur.
A common example is school lessons: many students want to talk; but if they do that at the same time, nothing is understood. That's why the students get in touch and the teacher decides who is allowed to speak. In this case, the MAC is implemented through an additional communication channel , because in addition to the acoustic data transmission medium sound (the speaking of the students and the teacher), there is the visual synchronization medium light (the teacher sees which students are reporting).
Sophisticated network protocols make additional communication channels superfluous.
Competitive access
Competitive access ( collision resolution ) means that everyone can access the medium and that there are rules for handling collisions without complications - CSMA / CD is such a protocol.
This type of access is also used intuitively for normal telephone calls: if the partners start talking at the same time, they stop immediately, everyone waits for a random period of time, and whoever starts talking again first has the floor.