Ethernet cards listen to the traffic on the cable and transmit only if there is
a break in the traffic when the channel is quiet. This technique is called
Carrier-Sense Multiple Access With Collision detection (CSMA/CD). With
collision detection, if two cards start transmitting at the same time, they see
the collision, stop, and resume some time later. Token Ring networks use a much
more complex process called token passing. Token Ring cards wait for permission
in order to transmit into the cable that forms an electrical loop. The cards
use their serial numbers in order to find the master interface card. This card
starts a message called a token. When a card with information to send receives
the token, it sends the data across the network. After the addressed interface
card receives the information and returns it to the originating card, the token
is given back to the master to be passed onto the next card. The ARCnet network
uses a very similar system to that of the Token Ring. Instead of using a token,
the master card keeps a table of all active cards and polls each one in turn,
giving permission to transmit.
4.4. Tied In A Knot
Various types of cabling are used to connect the LAN adapters to the
servers. Unshielded twisted pair wires offer rather slow speed, are very
inexpensive, are small, and can only span very short distances. These cables
use the RJ-45 connector. Coaxial cable offers fast speed, is rather expensive,
has a medium sized diameter, and can span medium distances. Coaxial cable uses
BNC connectors. The shielded twisted pair cable offers fast speed, is more
expensive than the coaxial cable, has a large diameter, and can only span short
distances. These cables use the IBM data connector. The fiber optic cable is
the fastest possible type of data transfer, costs astronomical amounts of money,
has a tiny diameter, and can span very long distances. This cable uses the ST
fiber optic connector. Wiring hubs are used as central points for the cables
from the network interface cards.
5.5. Loves Me, Loves Me Not, Server Based, Peer To Peer…
There are two general types of LANs. The Server-based networks rely on
one major server to store data, offer access to perhiperals, handle the printing
and accomplish all the work associated with network management. The Server-
based networks have a high start up cost, but offer high security as well as
ease of operation. These networks become more economical as more computers are
added to the network. In Peer to peer networks the network responsibilities are
divided among many computers. Some act as file servers, others as print servers,
some as CD-ROM servers, tape drive servers, etc. The Startup cost of these
networks is much cheaper, but when more computers are added to the network, some
of the servers may not be able to handle the extra activity.
5. Links Between LANs
5.1. She Just Won’t Send Sysop!
Most networks have very short information transfer ranges. But, in an
ever shrinking world the need for links between LANs has never been higher.
This section will explain the components and information needed to link LANs.
When an electric current travels over a long length, it’s charge decreases, and
it is susceptible to electromagnetic interference. To combat the length problem
a component has been devised. A repeater is a little box that is inserted
between a cable. It’s primary function is to amplify the weakening pulse and
send it on it’s way. Bridges are used to analyze the station address of each
Ethernet packet and determine the destination of the message. The Routers strip
the outer Ethernet packets of a data packet in order to get the data. This data
is sent to other routers in other places of the world and then repackaged by
those routers. The removal of the excess data packets by the routers decreases
the time required to transfer that data. If networks use the same addressing
protocol, bridges can be used to link them, however, if they use different
addressing protocols, only routers may be used. During these times MANs
(Metropolitan Area Networks) are in use and development today. These use
routers that are connected preferably via a fiber optic cable, to create one
large network.
5.2. Pluto Calling Earth!
Any networks larger than 1000m typically rely on telephone digital lines
for data transfer. These networks are called Circuit Switched Digital Networks .
Circuit Switched Digital Networks utilize a switching matrix at the central
office of a telephone company that connects local calls to long distance
services. The Telephone companies now offer dial up circuits with signaling
rates of 56, 64, and 384 kilobits per second as well as 1.544 megabits per
second. Another type of LAN to LAN connections are packet switching networks.
These are services that a network router calls up on a digital line. They
consist of a group of packet switches that are connected via intraswitch trunks
(usually fiber optic) that relay addressed packets of information between them.
Once the packet reaches the destination packet switch, it sends it via another
digital connection to the receiving router.