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.
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
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:
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.
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.