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This article 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 in this article includes:

- Media access methods including contention-based, token passing, and polling

- Network technologies including Token Ring and FDDI

You may notice Ethernet missing from the list above. Given the importance of a thorough understanding of Ethernet for the CCNA exam, it will be covered in detail in the next article in the series.

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 over the course of the series.

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 the next article.

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 the upcoming Ethernet article.

Token Passing

By far a more orderly way of transmitting data, token passing technologies are referred to as being deterministic. That means that a system gains access to the media is a predictable and regular fashion. In a token passing environment, systems cannot transmit data unless they have the token. A token is a special frame that continuously circles the network, passed from system to system. When a system has data to send, it waits for the token. Once it has the token, it can then send data to other systems. Once the data has been sent successfully, it releases the token back onto the network, giving other systems the ability to transmit. Token passing is the media access method used in both Token Ring and Fiber Distributed Data Interface (FDDI) networks.

Polling

Polling is another media access method, but one that is not terribly popular is LAN environments. Like token passing, polling is deterministic. Unlike token passing, where a special frame is passed from system to system, polling relies on what is known as a master device to coordinate which system has the ability to transmit at any given point in time. The master device queries (or polls) network systems to see if they have any data to transmit. If they do, they send data. If not, the next system is queried, and so on. Polling media access is commonly found in mainframe environments. One major disadvantage of polling is that should the master device fail, all nodes become incapable of sending data, thus representing a single point of failure.

LAN Technologies

Networks today generally use one of three network technologies. These technologies generally consist of elements found at both the Data Link and Physical layers of the OSI model. The most popular by far is Ethernet, but that wasn't always the case. In fact (though you may not remember it) there was a time when it looked as though Token Ring would rule the networking world. However, given advances in Ethernet technology, Token Ring has steadily lost ground in recent years. The third most popular LAN technology is FDDI, which uses fiber optic connections to make high-speed data transfer a reality. While FDDI was touted as a solution for desktop connectivity many years ago, it is now most commonly used to link campus and Metropolitan Area Networks (MANs). Quite simply, the cost of implementing FDDI infrastructure proved prohibitive.

Token Ring

Token Ring was developed by IBM in the 1970's and gained widespread popularity through the 80's and early 90's. As described earlier, Token Ring uses token passing as its media access method. The media most commonly used for Token Ring networks is Shielded Twisted Pair (STP) with network speeds of 4 or 16 Mbps. Higher speed versions of Token Ring do exist, but the 16 Mbps version is by far the most popular.

In 1982 the IEEE standardized a version of Token Ring in its 802.5 standard. While not identical to the IBM version, it was the basis for the standards and the two are generally compatible. In reality, almost all Token Ring networks deployed today are based on the IBM version.

A Token Ring network is comprised of a ring to which all systems interconnect. Many people wrongly believe that the ring consists of a long run of cable to which systems are attached. More correctly, Token Ring networks are configured in what appears to be a star, with stations connecting to what appear to be hubs. These hubs are properly referred to as MultiStation Access Units, or MSAUs. An MSAU actually forms the circuitry of the ring within the device. In this way, Token Ring networks are often described as being a physical star / logical ring.

To extend the network to include other systems, MSAUs have two additional ports - one marked Ring In (RI) and the other marked Ring Out (RO). Many MSAUs can be interconnected by attached the Ring Out interface from one MSAU to the Ring In interface of another, as long as the ring is completed.

The token that stations pass in a Token Ring network is three bytes in length. Data frames can be anywhere from 54 bytes up to as large as 17K, although frame sizes that large aren't often used.

On any Token Ring network, one system is designated as the Active Monitor. The Active Monitor watches the ring for errors and irregularities, such as a data frame that continues to circle the ring. In cases where this happens, it is the responsibility of the Active Monitor to remove the frame from the network. Fault Tolerance is also achieved using a method called beaconing. In it, a system that detects a fault will send a special frame, and the network (via the MSAUs) will attempt to bypass the error station or port.

Fiber Distributed Data Interface

As described in the media access section, FDDI also uses token passing as its way of passing data over the network. FDDI networks use fiber optic cabling, which allows them to span greater distances, in some cases up to 2 kilometers between devices. A copper-based version of FDDI also exists and is referred to as CDDI - Copper Distributed Data Interface.

FDDI is best known for the fault tolerance it provides. Instead of a single ring, a FDDI network consists of two rings, on which data flows in different directions. The outer ring is referred to as the primary ring, and is where data is normally passed. The inner ring sits idle until a network error occurs. In cases where an error does occur, the network will reconfigure itself around the error.

Four types of systems can be connected to a FDDI network. The main devices are hubs referred to as concentrators. When a concentrator attaches to both rings, it is referred to as a Dual Attachment Concentrator. When is connects to only the primary ring, it is called a Single Attachment Concentrator. End stations that connect to both rings are Dual Attachment Stations, while those only connecting to the primary ring are Single Attachment Stations. Note that single attachment devices can be powered down without affecting the network, but dual-attached devices cannot.

Dan DiNicolo is a technical trainer, consultant, author, and the managing editor of the free IT learning web site 2000Trainers.com. When he's not busy traveling the world as an IT volunteer with organizations like Geekcorps, Dan makes his home in the snowy northern backwoods of Canada.