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What is a Local
Area Network?
A Local Area Network (commonly
referred to as a LAN) refers to a communication network with the
following characteristics:
- The network is confined to a
small area typically a single building or a cluster of buildings.
- The network consists of a shared
transmission medium.
- The data rate on the network
is high, anywhere from 1Mbps to 100Mbps.
- The devices on the network are
peers. That is any device can initiate data exchange with any
other device.
A LAN is an increasingly popular
way for organizations large and small to get additional productivity
gains from the use of personal computers. Large businesses, government
agencies and universities have long realized the need for "collaborative
computing" or people using their computers as a group. Today,
more and more small businesses, local governments and schools
are using the power of LANs to increase productivity and efficiency.
It is well beyond the scope of this document to discuss the "enterprise
wide" local and wide-area networks typically found in large
organizations. Rather, this document will concentrate on the designs
(referred to as topologies), hardware and software typically found
in smaller Workgroup LANs. The purpose here is to provide a base
level of understanding of the terminology commonly used by networking
professionals and how they interrelate.
LAN Hardware
The most basic elements of workgroup LANs are personal computers.
They may either be used by individuals or connected as supporting
hardware. LAN hardware and software is designed to connect all
types of PCs including DOS and Windows PCs, Apple Macintoshes,
UNIX workstations, desktop and portable systems. When any one
of these computers are connected to a LAN they are referred to
as a workstation whether they are PCs, Macs or true UNIX workstations.
This is only the first example of confusing LAN terminology.
One of the great benefits of a LAN
is the ability to share computing devices that are used only occasionally.
Devices that are commonly shared on a LAN include printers, modems,
scanners and the like. One can save considerable computing expense
by sharing these types of devices over a LAN as opposed to dedicating
individual devices to individual users.
All computing devices on a network;
workstations, printers, etc., are referred to as nodes.
The next group of hardware to be
considered is the hardware that is required for the physical network
itself. For any node to be connected to the LAN, interface hardware
is required. This interface hardware does not typically come standard
with the computing device and must be purchased separately
as either an option from the original manufacturer or from the
hundreds of manufacturers that specialize in networking hardware.
When connecting a workstation, the interface hardware is referred
to as an adapter. An adapter is typically connected to the workstation
via an expansion slot. On a desktop system, these expansion slots
are in the back of the system and are the same bus expansion slots
that are used to add other hardware adapters that provide for
or enhance the system's sound, video and multimedia capabilities.
Most newer portable systems and notebook computers have slots
dedicated to expansion devices that are referred to as PC Card
slots. Different types of computers, portables or desktops, usually
accept only adapters that are designed specifically for them.
For example, a desktop PC will not accept the same type of adapter
as a notebook PC or a desktop Mac. Networking adapters are also
commonly referred to as Network Interface Cards or NICs. The LAN
connection for a printer, however, is referred to as a print server
and not a NIC.
An
Ethernet Adapter
- Media Interfaces: RJ-45 (10BASE-T)
at top right, BNC (10BASE2) at bottom right
- Bus Interface: ISA connector
is along bottom edge of adapter
 Notes: This adapter supports both 10BASE-T with an
RJ-45 and 10BASE2 with a BNC connector. Other adapters may have
a single interface and therefore support only one Ethernet cable
implementation while others can have three connectors (RJ-45,
BNC and an AUI for 10BASE5) and thereby support all three implementations.
Media is a term that largely refers
to the cable or wires connecting together the various computing
devices that make up a LAN. There are many different media types
in use today in LANs.
Twisted Pair is the most common
type of LAN media. It consists of strands of copper wire twisted
together to make up a single cable. Most typically eight wires
make up a twisted pair cable. This twisted pair cable is then
either Shielded (and referred to as Shielded Twisted Pair or
STP) or unshielded and referred to as Unshielded Twisted Pair
or UTP. Of the two, UTP is far more common and is similar to
the type of wire that is used for telephones in the United States
and is therefore often referred to as telephone cable.
The next most common type of LAN
media is coaxial cable or coax. Two different versions of coax
are used in LANs: Thin coaxial and thick coaxial. Of the two,
thin coax is far more popular and is similar in appearance to
the cable used in cable television systems in the United States.
Other media types include Fiber
Optic cable, which allows for high speed and long distance data
transmission and is principally found in only larger LANs. Another
type of media is not a cable at all. This type of media is either
infrared light (like a television remote control) or radio waves.
Networks that have no physical media are referred to as wireless
and are typically found only in niche applications where stringing
cable is impossible or impractical.
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