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Local Area Networks
(LAN)
by
Ansi-Christ
A local area network connects devices that are within a
short range. Most LANs are unable to connect to public
communications services. LANs were developed because most
communications within a business either occur in the very
same building or within a short distance of each other.
This allows for the sharing of information and equipment and
thus a large savings for the company.
This files is the first in a series of files. It is not
meant to be a complete systems manual for all LANs, but a basic
reference manual on the terminology and general set up. In later
files there will be more specific information.
This file can be distributed freely as long as none of the
information is changed. If I have made a mistake, please remember
that we are all human and this does happen. Contact me on one of
the two boards listed at the end to point out an error.
Topologies:
Ring -
A ring type set up goes in one direction around a group
of devices until it reaches the device that is meant as the
destination. The ring will be connected by a cable to form
the ring. The ring topology uses a method known as token
passing. A token is like a bucket in which information can be
dumped. If a certain device does not contain this "bucket"
then it can not send out the data until it gets it. This will
prevent data collision from occurring.
A LAN will usually transmit data in packets (buckets =
packets) and add the necessary routing information to the
packet. (this tells the packet where it's destination is)
Because a ring system has a circular shape the information is
routed in one direction and it will still reach its
destination.
One note - I have altered the description of a token
slightly to avoid confusion. A token is usually sent before a
packet and will tell the next device if the following packet
is full or is capable of receiving data.
Diagram of a ring topology:
O---------O
/ \ O = nodes
| |
O O (I apologize for the shitty diagram)
\ |
\ |
O-----O----O
Tree -
The tree set up links components to the network via
connectors. The network can be tapped at any available
point. Data can pass from one device to another and does not
have to route through a centralized point. Just the way a
tree network is set up should make it much faster than a ring
type network with the same computers and devices connected to
it. This type of network is also very useful because you
could add any additional devices as they were need via a
connector without having to go through major modifications to
the network itself.
A tree network does occasionally, just because of the
nature of it's set up, have certain problems with controlling
the passing of data. There is a safety factor involved with a
tree network though. If one of the systems along the network
go down it will not adversely affect the network as a whole,
just that certain part.
Diagram of a tree network:
again I apologize for the shitty diagram.
O
/ \
O \
| O
| O |
| | O
O | /
| /
O
Star -
The star network is connected through a central
processing device. This device usually takes the form of a
PBX or a host computer system. All lines in this network are
connected to this central unit. When information must go to
another node on this network it must first pass through the
center. The star topology is very common and is probably the
most common set up for a network.
There is a problem with the star though. If the central
unit goes down then the whole network is useless.
Diagram of a star topology:
O
O | O
\ | /
\ | /
\ | /
O--------O--------O
/ |\
/ | \
/ | \
O | O
O
Transmission media -
Twisted pair wire is commonly used to connect all kinds of
equipment. This is used in older buildings where it may be
very expensive to convert to newer types of cable. Problems
with this include distortion of signals especially at higher
transmission speeds.
Coaxial cable consists of a single conductor surrounded by a
flexible metallic shielding to minimize signal loss and
interference. Coaxial permits a high transmission speed.
Twin - Axial cable uses two Coaxial cables, one receiving and
one transmitting. These cables are better than twisted pair
cable because of the higher transmission speed and longer
distance.
Baseband and Broadband Systems -
A baseband LAN assigns its entire capacity to a single
user for a brief time. Only one device can use its pathway.
These networks usually use 3/8 inch coaxial cable.
Advantages are low cost, easy installation, and considerable
capacity. Baseband does, though, not lend itself to voice
quality communication.
A broadband LAN is sophisticated and uses common
television cable and a modem. Although it is expensive it
will allow the network to handle voice, video,
teleconferencing, and graphics transmission. It does
this by using different bandwidths so one signal does not
interfere with another signal. It is good for
high volume office communication.
Fiber Optics are the next step although very expensive
it will eventually replace the other cables. Fiber Optics
use light to transmit large amounts of information in a very
short time. A single cable can carry 240 thousand calls at
once.
Interconnecting LANs -
Bridges are one way of connecting similar networks.
Gateways are a way of connecting dissimilar networks.
It acts as a translator between these networks. It
understands different file structures, data types, and access
methods.
Vendors -
Ungermann-Bass is a large independent vendor. Offering both
base and broadband services and coaxial cable and fiber optic
LANs.
Systek is the OEM supplier of IBM's broadband network.
3Com is a major supplier of PC LANs.
Corvus sells Omninet. They offer a system based on twisted
pair wiring.
Interlan Inc. provides Ethernet based products.
Wangnet by Wang laboratories.
Codex 4000 series by Codex corp.
Information systems network by none other than AT&T
information systems. (commonly abbreviated ISN)
Novell network
LAN terms and what they mean - (note - originally printed in LAN
magazine)
Access method: a way to determine which workstation or PC
will be next to use the LAN. A set of rules by network
hardware and software that direct the traffic over the
network. Examples include Token passing and Carrier Sense
Multiple Access for Collision Detection (CSMA/CD)
Collision: The result of two workstations trying to use a
shared transmission medium (cable) at the same time. The
electronic signals collide and ruin both signals. The whole
process takes seconds.
Collision detection: The process of detecting when a
collision has occurred. Workstations know a collision has
occurred if the other station sends no reply that the signal
was received.
Disk Server: a device equipped with disks and a program that
allows users to make and store files on the disks. Allows
each user to have an increase in storage space normally not
accessible at their PC.
Drop cable: The cable which allows connection and access to
the trunk cables of a network.
Locking: prevents people from changing the same data at the
same time.
Polling: used in star networks. allows the central "hub"
system to know when a node wants to transmit.
LAN Magazine Call Anarchia at 518-869-6035
12 West 21 Street TSD at 518-377-6487
New York, NY 10010
1-800-LIBRARY or boards for those who take their
212-691-8251 telecom seriously.