Medium Access Control Protocols

All LANs and MANs consist of collections of devices that must share the network’s transmission capacity. Some means of controlling access to the transmission medium is needed to provide for an orderly and efficient use of that capacity. This is the function of a medium access control (MAC) protocol. We begin this section with an overview of medium access control techniques, then examine the most important MAC protocols.

Medium Access Control Techniques

The key parameters in any medium access control technique are where and how. Where refers to whether control is exercised in a centralized or distributed fashion.

In a centralized scheme, a controller is designated that has the authority to grant access to the network. A station wishing to transmit must wait until it receives permission from the controller.

In a decentralized network, the stations collectively perform a medium access control function to dynamically determine the order in which stations transmit.

A centralized scheme has certain advantages, including

• It may afford greater control over access for providing such things as priorities, overrides, and guaranteed capacity.
• It enables the use of relatively simple access logic at each station.
• It avoids problems of coordination.

The principal disadvantages of a centralized scheme are

• It creates a single point of failure.
• It may act as a bottleneck, reducing performance.

The pros and cons of distributed schemes are mirror images of the points made above.

The second parameter, how, is constrained by the topology and is a trade-off among competing factors, including cost, performance, and complexity. In general, we can categorize access control techniques as being either synchronous or asynchronous.

With synchronous techniques, a specific capacity is dedicated to a connection. This is the same approach used in circuit switching, frequency-division multiplexing (FDM), and synchronous time-division multiplexing (TDM). Such techniques are generally not optimal in LANs and MANs because the needs of the stations are unpredictable. It is preferable to be able to allocate capacity in an asynchronous (dynamic) fashion, more or less in response to immediate demand. The asynchronous approach can be further subdivided into three categories: round robin, reservation, and contention.

Round Robin

With round robin, each station in turn is given the opportunity to transmit. During that opportunity, the station may decline to transmit or may transmit subject to a specified upper bound, usually expressed as a maximum amount of data transmitted or time for this opportunity. In any case, the station, when it is finished, relinquishes its turn, and the right to transmit passes to the next station in logical sequence. Control of sequence may be centralized or distributed.

When many stations have data to transmit over an extended period of time, round robin techniques can be very efficient. If only a few stations have data to transmit over an extended period of time, then there is a considerable overhead in passing the turn from station to station, since most of the stations will not transmit but simply pass their turns. Under such circumstances other techniques may be preferable, largely depending on whether the data traffic has a stream or bursty characteristic. Stream traffic is characterized by lengthy and fairly continuous transmissions; examples are voice communication and bulk file transfer. Bursty traffic is characterized by short, sporadic transmission; interactive terminal-host traffic fits this description.

Reservation

For stream traffic, reservation techniques are well suited. In general, for these techniques, time on the medium is divided into slots, much as with synchronous TDM. A station wishing to transmit reserves future slots for its use. Again, reservations may be made in a centralized or distributed fashion.

Contention

For bursty traffic, contention techniques are usually appropriate. With these techniques, no control is exercised to determine whose turn it is; all stations contend for time in a way that can be, as we shall see, rather rough and tumble. These techniques are distributed in nature. Their principal advantage is that they are simple to implement and, under light to moderate load, efficient. For some of these techniques, however, performance tends to collapse under heavy load.

The discussion above has been somewhat abstract and should become clearer as specific techniques are discussed further.

Some of the standard medium access control techniques are: