LANs: Media Access Control

Local Area Networks (LANs) differ, depending on which media access control (MAC) technique is used, which describes how devices share access to the LAN. A MAC technique is necessary, because every LAN uses broadcasting, which means that independent devices can start sending data at the same time. This would corrupt the data and therefore has to be prevented. There exists a set of standards to this issue laid down in the IEEE standard 802 for Local Area Networks. The standards differ at the physical layer and MAC sublayer of the OSI model, but they are compatible at the remainder of the data link layer. In standard 802.2 this part of the data link layer (Logical Link Control, provides addressing and controlling mechanisms) is described; 802.1 contains an introduction to the standards. The three major techniques for media access control are described in standard 802.3 to 802.5: CSMA/CD (Carrier Sense Multiple Access with Collision Detection), token bus and token ring.

CSMA/CD: when a device wants to send data, it first checks the channel if anyone else is sending. If it is busy, the device waits until it goes idle, otherwise it transmits immediately. If a collision happens (two or more devices begin to transmit at the same time), all involved devices stop their transmission, wait a random time and repeat the process again.
Ethernet is an example for a CSMA/CD network.

Token bus and token ring: the main disadvantages of the CSMA/CD standard are its indeterministic behavior and the missing opportunitity to attach priorities to the frames sent. Therefore token-passing was invented. Every device within the LAN knows about the address of the device to its "right" or "left". Then a special control data frame is passed from one device to the other. Whichever device holds this token is allowed to send data, afterwards it gives the token to its neighbour. Collisions can not occur. The difference between token bus and token ring is the physical layout of the network; the token is always passed around in a (virtual) circle.

Again, which MAC technique is used is not independent form the choice of transmission media and topology.

In 1992 approximatly 59% of all sold LANs used CSMA/CD technology and 33% Token Ring. This relations shifts more and more to Token Ring because it is a more common MAC technology for High-speed LANs.