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Networking Basics:
Common LAN Types

Explains speed measures of computer networks and gives example speed-requirements

Over the years, many technological changes have taken place. There have been technologies that have come and gone. There are also many who have stood the test of time and evolved with the constant hardware, software and application changes.  Many of these technologies are still around but it is only a matter of time before they are dinosaurs. Some examples of LAN technologies are: FDDI, Token Ring, Ethernet and the always promised but never delivered, ATM.

FDDI:
Fiber Distributed Data Interface. Used primarily as a backbone connection due to its high speed (100Mb) it was the precursor to Fast Ethernet. It was based on a ring topology allowing the ring to "self heal" in the event of a failure. A good technology as different media (Ethernet and Token ring) could attach to the same ring but not a feasible one for the desktop. Also has a different frame format than Ethernet or Token Ring frames. FDDI uses the same topology as the IBM Token Ring.  But instead of one ring, FDDI uses a dual ring topology, which comprised of two counter-rotating rings.  The traffic of these rings travels in a opposite  directions.

One of the two rings is a primary ring, and the other is called the secondary ring.  Primary ring is used for data transmission while the secondary ringis mostly used as a backup ring. There are two different stations that can be attached to the FDDI ring.  A class A or single-attachment stations (SAS) attach to only one ring (the primary ring).  A class B or dual-attachment stations (DAS) attach to both rings.  Each DAS has at least two ports - an A port: where the data from the primary ring comes in and the data from the secondary ring exits (if secondary ring was in use), and a B port, where the data from the primary ring exits and the data from the secondary ring comes in (if secondary ring was in use).

Here is an example of a basic FDDI Ring:


Token Ring:
Based on the token passing method developed by IBM. Token Ring controls the medium by passing a control frame (token) from one device to the next. Only the computer possessing the token may transmit. Its topology is a logical ring but usually a physical star.

Ethernet:
Developed at the PARC in Palo Alto CA in the early 70's by Robert Metcalf. It is a Carrier Sense Multiple Access transmission method. This means if the channel is available then any device  connected to the medium can transmit. It is the most common LAN technology today weel suited for any and all types of applications.

We will focus on Ethernet and its associated advances as it is by far the technology of choice. it is also the standard by which we base all  other technologies in regards to performance, price and delivery.

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