LOCAL AREA NETWORKS (LANS)

The Institute of Electrical and Electronics Engineers, which establishes
network standards, defines a local area network (LAN) as a data
communications system that enables a number of independent devices to
communicate with each other in a limited geographic area. In other words,
a LAN is a network of

computers

linked together via cable within a limited area. LANs are proprietary
systems limited to a finite number of users. Consequently, LANs are not
connected with public telephone and cable systems. They also generally
serve an area of less than one mile, and are usually confined to a single
building. Nevertheless, a

computer network

spanning a university campus or a large industrial site with multiple
buildings also can be classified as a LAN. LANs also permit
workers—isolated in separate offices—to operate off the same
system, as if they were all sitting around a single computer. In addition,
they allow audio, video, and data communication and have higher bandwidth
connections than traditional

wide area networks

Besides linking computers, a LAN also may include one or more servers as
well as several printers and other computer equipment, depending on the
number of network users.

One of the great attributes of a LAN is that it may be installed simply,
upgraded or expanded with little difficulty, and moved or rearranged
without disruption. Perhaps most importantly, anyone initiated in the use
of a personal computer can be trained to communicate or perform work over
a LAN. Moreover, LANs can improve productivity by enabling workers to
share information and databases and can save companies money by allowing
them to purchase fewer computer peripherals such as printers and plotters,
which can be shared via a LAN.

Local area networks have their genesis in distributed computing systems
that were introduced during the 1960s. Initially, they consisted of
“dumb” terminals connected to a single mainframe processor
via a wiring system. Instructions entered on the terminal keyboard were
registered at the mainframe, where they were processed, and a visual
representation of these instructions was sent back to the terminal for
display on a screen.

The first protocols for these distributed computer workstations were
proprietary, meaning that they were designed specifically for equipment
designed by a certain company. As a result, IBM equipment could not be
mixed with Digital Equipment Corp. (DEC), Xerox Corp., Wang, or any other
manufacturer’s machinery.

By the late 1970s, however, several companies proposed
“open” standards under which equipment from one manufacturer
could be made to emulate the operating system of another. This allowed
manufacturers to compete for business in systems that had previously been
closed to all but the system designer.

The first of these standards was Ethernet, a
“listen-and-transmit” protocol developed by DEC, Intel
Corp., and Xerox. Ethernet identified the type of instructions being
generated and, if necessary, conditioned them so they could be read by the
mainframe on a common terminal, or bus.

In 1980 the Institute of Electrical and Electronics Engineers determined
that continued reluctance to open protocols would seriously retard the
growth of distributed computer systems. It established a group called the
802 committee to establish networking standards for the entire industry.
This would ensure that customers could migrate from one vendor to another
without sacrificing their considerable investments in existing systems.
The standards also compelled manufacturers to follow the standards, or
risk being dealt out of the market.

Ethernet was offered to the 802 committee as a standard. Several
manufacturers balked because Ethernet did not work well under heavy
traffic. Instead, the Ethernet standard was adapted into three versions,
corresponding to network designs. Still, these standards could support
1,024 workstations over an end-to-end distance of two kilometers.

Processor manufacturing technology, however, had progressed so far that
entire computers could be condensed into a single desktop unit. The first
of these “personal computers” (PCs) was introduced by IBM in
1981.

IBM’s PC featured an open bus architecture, meaning that IBM
provided design specifications to other manufacturers in the hope that
they would design compatible equipment and

software

for the system. IBM could impose this architecture on the market because
it had very high market penetration and was the leading manufacturer in
the industry.

The PC changed the type of information sent over office computer networks.
Terminals were no longer “dumb,” but contained the power to
perform their own instructions and maintain their own memories. This took
considerable pressure off the mainframe device, whose energies could now
be devoted to more complex tasks.

An analysis of common office tasks revealed that as much as 80 percent of
the work performed by the average employee never left the room in which it
was produced. This factor established demand characteristics for
individual, worker-specific PCs.

The remaining 20 percent of office tasks required transmission of data for
access by other workers. LANs enabled this data to be directed to a common
printer, serving a dozen or more workers. This eliminated the need for
each worker to have a printer and ensured that the one printer provided
was not underutilized.

In addition, LANs allowed data to be called up directly on other
workers’ computers, providing immediate communication and
eliminating the need for paper. The most common application was in
interoffice communications, or

electronic mail

(e-mail). Messages could be directed to one or several people and copied
to several more over the LAN.

As a result, an e-mail system became something of an official record of
communications between workers. Addressees became obligated to respond to
e-mail messages in a timely manner because their failure to answer could
be documented for supervisors.

PCs transformed LANs from mere shared processors to fully integrated
communication devices. In fact, developments in processing technology
endowed some PCs with even greater capacity than the mainframe computers
to which they were attached. For some applications, the need for a
mainframe was completely eliminated. With processing power distributed
among PCs, the mainframe’s main role was eclipsed. While still
useful for complex processing, administrative functions and data file
storage became the job of a new device, the file server.

A local area network generally requires three principal components besides
the computers being connected: network cards, cable or wire, and software.
While software-driven, the physical properties of a LAN include
interfaces, called network access units, which connect computers to
networks. These units are actually network cards installed on computer
motherboards. Their job is to provide a connection, monitor availability
of access, set or buffer the data transmission speed, ensure against
transmission errors and collisions, and assemble data from the LAN into
usable form for the PC. A LAN consisting of two to four computers,
however, can be created without a network card and this kind of network is
called a slotless system. In a slotless system, the computers’
serial and parallel ports are connected to each other. Such LANs are very
inexpensive and businesses use them largely for sharing hard-drive space
and printers, but they cannot support high-speed data transmission.

The next part of a LAN is the wiring, which provides the physical
connection from one PC to another, and to servers and printers and other
peripherals. The properties of the wiring determine transmission speeds.

The first LANs were connected with coaxial cable, a variety of the type
used to deliver cable television. Certain kinds of coaxial cable are
relatively inexpensive and coaxial cable is simple to attach. More
importantly, these cables provide great bandwidth (the system’s
rate of data transfer), enabling transmission speeds up to 20 megabits per
second.

During the 1980s, however, AT&T introduced a LAN wiring system
using ordinary twisted wire pair of the type used for telephones. The
primary advantages of twisted wire pair are that it is very cheap, simpler
to splice than coaxial, and is already installed in many buildings as
obsolete or redundant wiring. In fact, many buildings were left with
stranded 25-pair wiring once used for key telephone systems.

But the downside of this simplicity is that its bandwidth is more limited,
meaning that twisted pair, designed for voice communication, transmits
data at a slow rate. For example, AT&T’s first LAN product,
StarLAN, had a capacity of only one megabit per second. Subsequent
improvements expanded this capacity tenfold and eliminated the need for
shielded, or conditioned, wiring.

A more recent development in LAN wiring is fiber distributed data
interface (FDDI) or fiber-optic cable. This type of wiring uses thin
strands of glass to transmit pulses of light between terminals. Its
advantages are that it provides tremendous bandwidth and thus allows very
high transmission speeds: data transmission at a rate of up to 100
megabits per second. And, because it is optical rather than electronic, it
is impervious to electromagnetic interference. Fiber optics also supports
a network of up to 1,000 computers and can transmit signals up to 50
miles. Its main drawbacks, however, are that splicing is difficult and
requires a high degree of skill and that it costs far more than its
counterparts.

The primary application of fiber is not between terminals, but between LAN
buses (terminals) located on different floors. As a result, FDDI is used
mainly in building risers. Within individual floors, LAN facilities remain
coaxial or twisted wire pair.

Where a physical connection cannot be made, such as across a street or
between buildings where easements for wiring cannot be secured, microwave
radio may be used. It is often difficult, however, to secure frequencies
for this medium.

Another alternative in this application is light transceivers, which
project a beam of light similar to fiber-optic cable, but through the air,
rather than over cable. These systems do not have the frequency allocation
or radiation problems associated with microwave, but they are susceptible
to interference from fog and other obstructions.

The software needed for a LAN depends on the kind of network being
created: whether slotless, peer-to-peer, or server-based. Slotless system
software usually enables users to perform the rudimentary tasks associated
with slotless systems: sharing hard-drive space and printers. In addition,
this software may provide e-mail and security features. Basic slotless
system utilities are included in standard operating systems such as
Windows 95 and Windows 98.

Peer-to-peer software facilitates peer-to-peer networks, which allow all
users to access and use the resources of all computers attached to the
network, including hard drives and printers. Peer-to-peer LANs, however,
generally lack security and administration capabilities of server-based
LANs, as well as the capacity for large data transmissions. Nevertheless,
they are less expensive than their server-based counterparts.

Client/server software is designed for networks that designate a computer
as the hub or server of the LAN. The server computer is linked to all the
other computers of the network—the client computers—and it
carries out the majority of the server duties, such as user access control
and coordinating user tasks. The server cannot be used as a workstation,
however, without special software allowing it to function as one. The
client computers use the programs and data stored on the server.
Server-based networks are best for companies with heavy network traffic.

LANs are designed in several different topologies or physical patterns of
connecting terminals. The most common topology is the bus, where several
terminals are connected directly to each other over a single transmission
path. Its layout is linear and it resembles a street with several
driveways. The bus network requires cables that allow signals to flow in
either direction, called a full duplex medium. Each terminal on the bus
LAN contends with other terminals for access to the system. When it has
secured access to the system, it broadcasts its message to all the
terminals at once. The message is picked up by the one terminal or group
of terminal stations for which it is intended. The bus network’s
lack of routing and central control make it very reliable, because failure
of one of the network’s computers generally will not impede the
flow of other network traffic.

A second topology, the star network, also works like a bus in terms of
contention and broadcast. But in the star, stations are connected to a
single, central node that administers access. The central node knows the
path to all the other nodes, which makes routing easy. The central node
also enables access control and establishing a priority status for users.
Several of these nodes may be connected to one another. For example, a bus
serving 6 stations may be connected to another bus serving 10 stations and
a third bus connecting 12 stations. The star topology is most often used
where the connecting facilities are coaxial or twisted wire pair.

The ring topology connects each station to its own node, and these nodes
are connected in a circular fashion. Node I is connected to node 2, which
is connected to node 3, and so on, and the final node is connected back to
node 1. Messages sent over the LAN are regenerated by each node, but
retained only by the addressees. Eventually, the message circulates back
to the sending node, which removes it from the stream. Consequently, this
configuration does not require routing.

LANs are effective because their transmission capacity is greater than any
single terminal on the system. As a result, each station terminal can be
offered a certain amount of time on the LAN, like a time-sharing
arrangement. To take advantage of this window of opportunity, stations
organize their messages into compact packets that can be quickly
disseminated.

In contending for access, a station with something to send stores its data
packet in a buffer until the LAN is clear. At that point the message is
sent out. Sometimes, two stations may detect the opening at the same time
and send their messages simultaneously. Unaware that another message has
been sent out, the two signals will collide on the LAN. When this happens
it is up to the software to determine who should go first and ask both
machines to try again.

In busy LANs, collisions would occur all the time, slowing the system down
considerably. To solve the problem, the LAN software circulates a token.
This works like a ticket that is distributed only to one station at a
time. Instead of waiting for the LAN to clear, the station waits to
receive the token.

When it has the token, the station sends its packet out over the LAN. When
it is done, it returns the token to the stream for the next user. Tokens,
used in ring and bus topologies, virtually eliminate the problem of
collisions by providing orderly, noncontention access.

The transmission methods used on LANs are either baseband or broadband.
The baseband medium uses a high-speed digital signal consisting of square
wave DC voltage. While it is fast, it can accommodate only one message at
a time. As a result it is suitable for smaller networks where contention
is low. It also is very simple, requiring no tuning or frequency
discretion circuits. As a result, the transmission medium may be connected
directly to the network access unit and is suitable for use over twisted
wire pair facilities.

In contrast, the broadband medium tunes signals to special frequencies,
much like cable television. Stations are instructed by signaling
information to tune to a specific channel to receive information. The
information within each channel on a broadband medium may also be digital,
but they are separated from other messages by frequency. As a result, the
medium generally requires higher capacity cables, such as coaxial cable.
Suited for busier LANs, broadband systems require the use of tuning
devices in the network access unit that can filter out all but the single
channel it needs.

File and printer servers provided the initial impetus for companies to
develop LANs so that they could share databases and expensive peripherals
such as printers. Furthermore, the heart of the LAN, the administrative
software, generally resides either in a dedicated file server (which
functions as a server only) or, in a smaller, less busy LAN, in a computer
acting as a file server (which also can function as a workstation). In
addition to acting as a kind of traffic cop by controlling and regulating
user access, this server holds files for shared use in its hard drives,
administers applications such as operating systems, and coordinates tasks
such as printing.

Where a single computer is used both as a workstation and a file server,
response times may lag because its processors are forced to perform
several instructions at once. In addition, the system will store certain
files on different computers connected with the LAN. Consequently, if one
machine is down, the entire system may be crippled. Moreover, if the
system were to crash due to undercapacity, some data may be lost or
corrupted.

The addition of a dedicated file server may be costly, but it provides
several advantages over a distributed system. In addition to ensuring
access even when some machines are down, it is unencumbered by multiple
duties. Its only jobs are to hold files and provide access.

Since 1990 one of the most notable developments in LANs has been the
growth of communication servers that allow LANs to communicate with
networks outside of the LANs themselves. Communication servers enable
remote LAN access, e-mail, fax, and other communication services. Like
other servers, this one controls access and facilitates use of
communications software and hardware. As with file and printer servers, a
separate computer may be designated as a dedicated server to enhance
reliability.

Furthermore, the LANs of the late 1990s began to include servers devoted
other applications such as those for decision support, transaction
processing, and data warehousing. The number of application servers is
forecast to increase significantly as more companies add dedicated
application servers to their LANs.

The speed of the LAN is measured in terms of throughput, a figure
different from transmission speed because it takes into account the
capacity of the wiring and the distance between stations. The data rate,
which most directly represents response time, is determined by throughput
and other factors such as overhead bits and other signals, error and
collision recovery, software and hardware efficiency, and the memory
capacity of disk drives.

As mentioned earlier, LANs are generally limited in size because of the
physical properties of the network: distance, impedance (a kind of
electrical resistance), and load. Some equipment, such as repeaters, can
extend the range of a LAN. Repeaters have no processing ability, but
simply regenerate signals that are weakened by impedance.

Other types of LAN equipment with processing ability include gateways,
which refer to the hardware and software necessary to enable
technologically different networks to communicate with each other. A
gateway, for example, can compensate for dissimilar protocols to pass
information by translating them into a simpler code, such as ASCII. A
bridge works like a gateway, but instead of connecting technologically
different networks, it connects networks employing the same kind of
technology. Similar to a bridge, a router is the hardware and software
connection between two (or more) networks or subnetworks that routes
traffic from one network or subnetwork to another. But routers primarily
control the transmission of packets to their destinations.

Gateways, bridges, and routers can act as repeaters, boosting signals over
greater distances. They also enable separate LANs located in different
buildings to communicate with each other.

In some cases, separate LANs located in different cities—and even
separate countries—may be linked over a public network. Whether
these are “nailed up” dedicated links or switched services,
the connection of two or more such LANs in separate geographic locations
is referred to as a wide area network (WAN).

WANs require the use of special software programs in the operating system
to enable dial-up connections that may be performed by a router. Unless
limited to modem speeds, these connections may require special services,
such as integrated services digital network (ISDN), to ensure efficient
transmission, particularly of large data files. Increasingly, companies
employing LANs in separate locations also operate WANs.

Another device, which can be used to create LANs, is the private branch
exchange. Private branch exchanges (PBXs) are telephone switching systems
that generally serve one company or network and route data and information
to specific servers, rather than broadcasting to all stations. PBXs are
oblivious to operating systems and use only twisted pair. As a result, PBX
networks are somewhat slower and their applications are more limited than
other kinds of LANs.

LANs are susceptible to many kinds of transmission errors. Electromagnetic
interference from motors, power lines, and sources of static, as well as
shorts from corrosion, can corrupt data. In addition, different kinds of
cables are more susceptible to these problems than others. Software bugs
and hardware failures can also introduce errors, as can irregularities in
wiring and connections.

LANs generally compensate for these errors by working off an
uninterruptible power source, such as batteries, and using backup software
to recall most recent activity and hold unsaved material. Some systems may
be designed for redundancy, such as keeping two file servers and alternate
wiring to route around failures.

In addition, as computer software evolves requiring faster processors and
faster rates of transmission, LAN technology also must evolve. Multimedia
and video applications in particular force companies to upgrade their LANs
in order to use such applications in a network environment. Consequently,
LANs increasingly need to transmit data at gigabit, not megabit, speeds
and hence older technology must be upgraded or replaced.

When purchasing a LAN, or even investigating the possibility of installing
one, several considerations must be kept in mind. The

costs

involved and the administrative support needed often far exceed
reasonable predictions.

Three general concerns when considering a LAN include administration,
security, and productivity. Administration utilities regulate and
coordinate file, application, peripheral, and resource use, while security
utilities control access to the network. Productivity refers to the tasks
a company wants to perform via a LAN, which may include file, database,
and printer sharing. Moreover, thorough consideration of potential costs
should include such factors as purchase price of equipment, spare parts
and

taxes,

installation costs, labor and building modifications, and permits.
Operating costs include forecasted public network traffic, diagnostics,
and routine maintenance. In addition, the buyer should seek a schedule of
potential costs associated with upgrades and expansion of the network,
since company LANs tend to require new technology and to expand
periodically.

The vendor should agree to a

contract

expressly detailing the degree of support that will be provided in
installing and turning on the system. In addition, the vendor should
provide a maintenance contract that binds the company to make immediate,
free repairs when performance of the system exceeds prescribed standards.
All of these factors should be addressed in the buyer’s

request for proposal,

which is distributed to potential vendors.

[

John

Simley

,

updated by

Karl

Heil

]