Media access control

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Communication in a LAN is physically performed by the LAN adapters that attach the computers to the medium. Once a computer has generated a message and passed the message to the LAN adapter for transmission, it is the task of the LAN adapter to conduct this transmission. It should be fairly evident that any form of communication requires a language and rules of etiquette.

Without a language, there is no way to frame a message so it can be understood. People use a number of languages, including English, French, and German. In a LAN the language can be thought of as the frame, or sequence of bits, that the LAN adapter transmits. Somewhere there must be a definition of how the frame is structured. Just as there are many human languages, there are many different frame structures. All of the LAN adapters must use the same structure to successfully communicate.

Rules of etiquette also govern who gets to talk when. As human communicators, we have a number of different ways of communicating and we apply the correct rules to the environment. For example, in a large classroom an instructor might require participants to raise their hands before speaking. In a social gathering, less formal rules are used (e.g., “don’t interrupt” and “give others a chance to talk”). Like humans, LAN adapters implement rules that govern which device gets to communicate next.

The language and etiquette implemented by LAN adapters are called the Media access control (MAC) scheme. The role of the MAC scheme is critical when the underlying network is broadcast. This would be true for logical bus or ring topologies, regardless of the physical topology on which they are implemented. When the topology is point-to-point and full duplex, however, access control ceases to be critical or even relevant.

In general, MAC schemes have historically been categorized as contention-based or polling. Contention schemes are MAC schemes in which the participants literally contend for access to the medium. Polling schemes involve no contention. Access to the medium is granted in an orderly fashion with every device taking its turn.

There are several different schemes that have been used throughout the history of LANs. Some of the more common contention-based schemes include carrier sense multiple access with collision detection (CSMA/CD) and carrier sense multiple access with collision avoidance (CSMA/CA). CSMA/CD is most like a social gathering. The basic rules are, “Don’t interrupt” and “Make sure you do not accidentally collide with another transmitter once you start communicating.” This is the approach used by shared-bandwidth Ethernet LAN adapters, which have become increasingly rare with the advent of Ethernet switching. CSMA/CA is a variation that adds a bit of random delay in some circumstances to further reduce the probability of collisions occurring. It is still widely used in Wi-Fi networks.

The predominant polling scheme was token passing, which is actually considered a distributed polling scheme more like a baton passing scheme. An electronic token passes from device to device, and only the LAN adapter possessing the token is permitted to transmit. Token Ring and Fiber Distributed Data Interface (FDDI) used this approach, but are both considered obsolete technologies, not worthy of further discussion.

PodSnacks

<mp3>http://podcast.hill-vt.com/podsnacks/2008q3/mac.mp3%7Cdownload</mp3> | Media access control (MAC)