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Essential Network Concepts - Page 3
This Study Guide carries on with our review of basic network concepts, including a look at network media access methods and an introduction to network technologies. The material to be covered includes:- Media access methods including contention-based, token passing, and polling
- Network technologies including Token Ring and FDDI
Media Access Methods
The term media access method refers to the ways in which different technologies gain access to the physical network. While this section takes a generic look at some of the different methods, they will ultimately be applied to the different technologies that we'll look at.
Contention
Contention-based media access methods are those in which systems literally compete (or contend) for access to the media. The key word used to describe a contention-based system is "shared". All systems share access to the media and can only transmit data when the wire is clear of signals. In that way, networks using contention methods are pretty much "every system for themselves." No method exists to ensure that systems get deterministic access to the media. Ethernet is an example of a contention-based system that we'll look at in more detail in other Study Guides.
Since systems share the media, any system that has data to transmit must wait until the line is clear. If two systems attempt to communicate at the same time, a collision occurs, corrupting the data. When this happens, systems have to back off from transmitting for a random period of time. If collisions continue to occur, the backoff time continues to increase. On networks experiencing high rates of collision, this leads to even poorer performance.
A great example of contention in action is when you have multiple systems plugged into an Ethernet hub. Notice the collision light blinking repeatedly? That's a good sign that you network isn't performing nearly as well as it could be. In the past, it was felt that Ethernet would never become a viable enterprise technology, mainly because of the fact that it is contention based. However, with the introduction of bridging and especially switching technologies, this has become less of an issue. We'll explore that line of thinking more in Ethernet.