" WHAT A LAN IS "

A LAN is a data communications network spanning a limited geographical area, 
a few miles across at most, but more commonly a single office, floor or 
building.  It allows users to communicate by electronic mail and to share 
information and such computer resources as mass st0orage, backup facilities,
software, printers or pen plotters for blueprints.
Typically, a LAN is made up of hardware, software and connections.  The 
connections and the software are the distinctive part: network interface 
card(circuit boards) that fit inside the connected computers; cable to link
these computers together; driver and protocol software to move data bet. 
computers; user interface software to connect users to network services; and
operating system software to actually process the applications'requirements 
for the item such as a files and printers.
Through a LAN, an individual personal computer can link up with other 
personal computers in a workgroup or department.  Other network tools can
broaden the possible interconnections bet. computers; they can even connect
personal comp. with midrange computers and mainframe.  Wide area networks,
or WANs, can connect personal computers located in different buildings or 
cities.  Often, the LAN is linked to larger LANs or WANs that form a 
company-wide information system.  LANs generally are brought into two types 
of offices; those with many seperate personal computers and those with 
mainframes.  We will discuss both situations in this chapter going into the 
distinctive ways that LANs affect each one.
 

MAJOR MAINFRAME-TYPE BENEFITS OF LANs
*Communication.  Once a LAN connects the people in your company, every
        possible form of discourse is possible, from electronic notes to 
        spreadsheets to graphics files and video.  People like to communi-
        cate, and they can do that the network quickly and easily, whether
        the other user is at the next desk or through a gateway-in another 
        state.  Most network users say the benefits of electronic mail alone 
        justify the investment.
*Shared Hardware.  A network gives each user access to any peripheral,
	from modems and the printers to such expensive devices as high-
	resolution film printers or plotters.  Using this equiptment at
	maximum efficiency can save your company a great deal.
*Shared Information.  The concept is simple; Networks connect each
	personal computer so that everyone can use the same tools and 
	share the same data and update common data simultaneously.
	Networks are especially useful when many users are constantly
	updating the same files.  Data produced jointly within a dept.
	can be shared with other parts of the company or even the public
	in secure database. 
*Security.  Even though a network connects many user together
	and can make access to data more open, it also offer security
	safeguards for protecting users and their date.  Passwords and
	access codes ensure that only authorize people can read or write
	to sensitive files.  And network offer another type of security.
	While single-user system can lose valuable data when a hard disk
	mailfunctions, network software can protect against data loss by
	using more than one hard disk drive on the network for data storage.
	Since duplicae records are being kept, data integrity is virtually
	assured.
*Productivity.  With a local area network to share computer resources
	and information, people can do their jobs more quickly, more
	efficiently and with less hassle.  The LAN will quickly become the
	heart of your business.

TOPOLOGIES
"Topology" signifies the way LANs are physically put together how the 
various nodes are linked.  There are a variety of LANs topologies.
*Bus Networks
A bus network, often called a linear bus network, is the simplest type of 
LAN.  A common cable runs the length of the network and workstations,
servers, printers and other hardware are connected to it to form the LAN. 
Coiaxial Ethernet is strung as a bus network.
Advatages:  Low cost, simple and easy to install.  Use a minimum amt. of
	    cabling.
Disadvatages:  A break in the common cable disable all nodes farther down
	       the line.  It can be difficult to locate cable failure in 
	       a large bus LAN.
*Star Networks
The name of this network type gives an idea of its configuration, although
the term "pinwheel"might be more appropriate.
Advantages:  A cable break disables only one spoke if the network limiting
	     the discruption.  Easy to locate cable breaks.
Disadvantages:  Higher cost due to price of hub; although quite able, hubs
are single-point-of-failure.  Star LANs use of cables.
*Ring Networks
Ring networks link computers and other nodes in a ringlike loop.  In a bus
network, information passes from nodes to node.  Unlike bus LAN each node 
features a repeater that amplifies the signal allowing ring networks to 
cover very long geographical distances.  A number of different fault-
tolerance technologies allow ring LAN to keep functioning as usual in the 
event a node or a cable failure FDDI and tken ring networks follow the 
ring topology.
Disadvantages:  The LAN slow down as more nodes are added.
*Hybrids
	Many users opt for hydrid LANs that take advantage of two or more
	topologies.  This approach allows the LAN to meet the specific 
	physical requirements of a particular office while providing the
	best returns in cost and performance.

 HOW LANs WORK IN REAL LIFE
Different people have different concerns about what a LAN can do for them,
depending on the specific job they need to do.  Here are some examples from
the broad variety of potential LAN user.
A telemarking department head can have online access to customer account 
and inventory information stored in a multiuse database.  Since entries in
the databse are updated instantaneously, the phone sales staff won't promise
items that are out of stock, and customers won't exceed their credit limits 
by replacing multiple orders before they are posted.
The project leader of a regularly revised training manual, by storing the
draft manual on a fiel server, can make orderly revisions and ensure that 
each team member works only on the most current version.  Workflow software
allows the project leader to ensure that each team member receives the 
document in the order, for either rewriting or approval.
A regional sales manager can use electronic mail to communicate with sales 
people who are always on the phone or on the road, and to collect information
needed to prepare reports.
A lead programmer can give each team access to common library modusle stored 
on a file server and control the proliferation of multiple versions.
A publisher can standardize style specifications, spelling and hyphenation
dictionaries to ensure consistency in the company's publication can go from 
the writer to the printer with new system of electronic text management.
An executive assistant can maintain the boss's schedule of meetings or 
coordinate meeting with other members of the staff by using groupware.
A customer service manager can keep track of returned items as well the types
of problems reported for each product by monitoring a shared database.

LANs AND THE MAINFRAME OFFICE
"Downsizing in the computer world signifies getting rid of old hardware and 
software in favor of more efficient technologies.  Many owners of mainframes 
are replacing them with more cost-effective distributed LAN System, which can 
be far less expensive to configure, operate and maintain.  As of this writing,
a basic, no-frills LAN server (the computer at the heart of most LANs) can be 
acquired for the less than $3,000, and user workstations, with independent
processing and the storage capabilities, cost less than $1,000 a piece.  
Compare these figures with the price of a $100,000, mainframe and "dumb" terminal
that the sell for about the same price as PC workstations.  Furthermore, 
maintenance and service costs for LAN hardware are usually a fraction of similar
mainframe expenses.  Employees with less technical skill, moreover, can do a lot 
of maintenance and troubleshooting.
Beyond cost, however, LANs offer great operational advantages over mainframe that
mean new effectiveness in business and human terms.
LANs, of course, and perhaps never will be.  For example, if a company or office
routinely deals with large files of 100 megabytes or more, a big computer is still
the way to go.  But even airlines and insurance companies two firms that must deal 
with vast volumes of data are recognizing the advantages of linking their mainframe 
and minicomputers to LANs for routine work.  A LAN lets access minicomputers or 
mainframes not by making direct connections but by distributing mainframe sessions
through a device called a gateway, thus reducing the need to buy terminals in 
addition to the terminal-emulating PC workstation.
Other LAN software lets users work within their PC interface to access and analyze
data residing on a mainframe.
For the average company or office, a LAN offers the most flexible way of storing 
information and allowing groups of users to reliably input,access, manipulate and
store data.  Where as mainframes and minicomputers were built for the computer 
specialists to use, LANs are designed with ordinary office workers in mind.  Ways
have been devised to make the big multiuser system more human-oriented.  But such
technological after thoughts don't change the fundamental structure of the systems, 
which discourage casual communication between users and the local processing of 
favorite applications and other PC features that LAN users take for granted.