There are a variety of methods by which data is merged onto a network, a concept referred to as the media access method. The media access method used depends on the way in which a particular technology such as Ethernet or Token Ring communicates. This section will look at the three most popular methods – contention-based, token passing, and polling.
Contention-based media access describes a way of getting data on to the network whereby systems ‘contend for’ or share the media. On a contention-based network, systems can only transmit when the media is free and clear of signals. In this way, devices listen to the media, and if no other system is transmitting, they can go ahead and send data. In cases where more than one system finds the network free and attempts to transmit, a data collision will occur, and systems will need to retransmit. On busy networks, the number of collisions can quickly get very high, adversely affecting performance. Remember that in this scenario, only a single system truly has access to the media at any given time, even though multiple systems may have data to send.
The best example of a contention-based network technology is Ethernet, which uses a scheme called Carrier Sense Multiple Access with Collision Detection (CSMA/CD). The fact that Ethernet is contention-based is a reason why many people thought that the technology would never be a good solution for large networks. As time passed, different techniques were developed to provide a way for contention-based networks to scale to larger sizes. A great example is the use of switches to segment a network, thus significantly reducing (or even eliminating) collisions.
A more orderly scheme for moving data between network systems is found when token passing is used. In token-passing media access environments, a special frame referred to as a token repeatedly circles the network, passed from system to system. If a system has control of the token, it can transmit data. If it doesn’t, it must wait for the token to become available again.
While this might sound like a very slow way to go about passing data, it’s important to understand that the token moves around the network at incredibly high speeds. Understand also that because this method isn’t contention based, there won’t be any collisions, further increasing performance
Examples of technologies that use token-passing media access include Token Ring and Fiber Distributed Data Interface (FDDI), both of which will be described in detail later in this chapter.
While contention and token-passing methods are by far the most popular ways in which PCs access LAN media, some technologies rely on a technique called polling. Polling is a deterministic way of allowing systems access to the network while also avoiding collisions. When used, a device referred to as the master polls systems to see if they have data to transmit. In this way, polling is similar to token passing, except that the central device controls the order in which systems are contacted. The downside of polling is that when the master device fails, the network fails. Most popular in mainframe and minicomputer environments, polling is a technique used in protocols such as Synchronous Data Link Control (SDLC).