Introduction to Local Area Networks 1 The use of local area networks in business organizations has seen incredible growth since 1988. LANs can be found in almost every size and type of organization, and they provide the computing power for everything from occasional printer sharing to running business-crucial operational and financial systems. 2 At least part of the reason for this growth is that the task of inter- connecting computers has become far easier. In the past, installing a network and getting it

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Nov 8, 2013 (3 years and 5 months ago)



Introduction to Local Area Networks

The use of local area networks in business organizations has seen incredible growth
since 1988. LANs can be found in almost every size and type of organization, and
they provide the computing power for everything from

occasional printer sharing to
running business
crucial operational and financial systems.

2 At least part of the reason for this growth is that the task of inter

computers has become far easier. In the past, installing a network and getting i
to work successfully required two parts good luck and one part magic. Today, network
interfaces are much easier to install and configure, (that is, to se
t up so that
they function cor
rectly with their host computer and its software). Network
interfaces a
lso are more cooperative about "t
alking" to each other, even be
hardware from different manufacturers. As a result, setting up a LAN is now a largely
straightforward task (although a little good luck still helps).

3 Lower network costs also have cont
ributed to LAN growth. The previous high cost
of network components limited the availability of connections to those with very
specific business needs (for example, access to an expensive disk storage device
or high
speed printer). However, the price of ne
twork com
ponents has dropped
ly, while the number and level of services they enable has greatly in

creased. This makes it far more affordable, and therefore, more cost
to connect most or all of the personal computers within a depart
ment, or an office.
Once connected, users can take advantage of simple functions like sharing disk space
or laser printers. Connection to LAN also allows them to share files, exchange
electronic mail, arrange scheduling, and collaborate on group projects.

4 But one of the biggest reasons for the growth of LANs is that they can make good
business sense. ALAN provides impressive levels of computing capacity at lower costs
and with greater flexibility, when compared to traditional mainframe computing
s. A centralized mainframe computer is typically very expensive, both for
initial acquisition and for capacity upgrades. And upgrades usually come in large,
expensive chunks. That means you hold off on in

stalling the upgrade until you
absolutely need it,

then you probably get more capacity than you need.
On the other
hand, personal com
puters and the related network hardware are very cost
You can build processing power and storage capacity rivaling many main

frames. You
can add more file server
s, increase disk capacity, add

more printers, or just reconfigure the whole network more easily and

cheaply than changing a mainframe. In many circumstances LANs,

really do provide the most "bang for the buck. "

What Is a Local Area Network ?

5 The t
erm network refers to interactions taking place between de fined units.
For our purposes, a network refers to a collection of microcomputers linked by some
type of medium. The link can be for one specific purpose, such as to share l
disks or fast print
ers. In

variably, once the lines of communication are opened,
other uses for the network appear. When computers can communicate, they can

perform any useful tasks such as exchange electronic mail for their users, accessing


host computer through a network
gateway, or even

competing in games and simulations
with other network users. But the key feature of any data network is the
interconnection of two or more computers.

6 The connection between computers requires a medium that is used to transport data.

most common way to connect computers is through the use of electrical or optical
cables which are known as bounded media. The term "bounded" is used since, for all
practical purposes, the energy used to exchang
e data remains within the medi
um itself.
st bounded media do permit energy leakage to a greater or lesser extent, but this
is no
t key to their proper function

7 Bounded media such as cables contrast with unbounded media like radio frequency
links, microwaves, and infrared technologies. Un

bounded media depend on the
radiation of energy in order to exchange data. Devices using an unbounded medium
are designed to transmit and receive data without the use of cables, in the same
way a radio transceiver (li
ke a "walkie
talkie" or "handie

is used to ex

change voice without the use of wires.
For the most part, the choice of bounded or
unbounded media is merely an implementation decision.
The choice specifies how
computers will be connected, not what someone actually does with the connectio

8 The "local area network" also can be defined in many ways, each one subject to
an amount of
pseudo religious

fervor. Our meaning of LAN is a network whose components
are within the complete control of your own organization.
We also focus on LANs made

up mainly of personal computers and their related hardware, although our definition
of a LAN can include any type of computer resource. We also in

elude campus area
networks (CANs) involving links between buildings at one company location.

of LANs in general

and wireless LANs in particular, will be within this

9 This definition is designed to exclude networks made up of segments that are
under the control of others, such as a data communica

tion line or satellite link
rented fro
m a third party in order to connect offices in Toronto and San Francisco.
These are usually thought of as wide area networks (WANs), and while they might
involve wireless technology, such links are outside the scope of this book. Similarly,
links within a
city (known as metropolitan area networks or MANs) involving
party carriers such as your local telephone
company is

not included in our
definition of a LAN.

10 Any LAN consists of the following major components:

Network nodes (also called


A, connecting medium

Various levels of network communication protocols

(Optionally) internetworking devices (Which are really just a

special type of node).

11 Local

Area Network Nodes. A node on a LAN is simpl
y a device

attached to the network at a given point. For wired LANs, the at


is a physical connection between the network and the de

vice, while for a wireless LAN, attachment can refer to a radio or in


link. In either case, the devi
ce communicates through an in



to the Chosen medium, whether that medium is bounded or


12 The attached device is normally a computer, but it can also be an


device used as a go
between for different portions of

a net
work or between different types of networks.

13 The Network Interface Card. Between the computer and the

connecting medium sits the netw
ork interface card (NIC). The N

serves as the gateway between the computer's internal components and the network
One side of the NIC connects to the computer's internal bus, giving it
access to the memory and the processor.
The other side acts as the interface to the
connecting medium.

14 Network Roles. Computers attached to the network usually fill

one of two main
roles. They function either as a:

~ N Server computer, providing services to the network such as

storing files, printing data, or connecting to host computer

or other network

~ N Client computer, using the
available services from the net work

15 Some data networks emphasize clear and distinct relationships

between nodes. In these cases, referred to as client/server networks,

a node is either a server or a client, but not both. A server node,

usually a
capacity, high
performance computer, will offer ser

vices such as making its disk drive available to store files on. behalf of

client computers. The client nodes, as you might expect, eagerly

take advantage of the server's offerings.

16 However,

on other networks, a node can act as either a client or

as a server, or even both at the same time. These are known as peer

peer networks. On a peer
peer network, each
~ has equal

status and importance. Each computer can offer access to any
resources connected
to it (for example, disks or printers) as well as access other nodes offering their
services. Peer
peer networks are of

ten used for informal sharing arrangements (for example, between

members of a department or work group).


Services Offered by Nodes. Offering network services involves

accepting requests from client computers on the network, performing

some type of processing to service the request, and delivering the results back to
the client across the network.
A progra
m on the server listens for network requests
and knows how to deal with them.
h then makes use of the network to return the results
to the client.

18 Server computers can provide many different types of network services. For
example, general
purpose compu
ters are computers on a network that allow users to
access them in order to perform a variety of tasks. Tasks include logging on,
executing specific commands or procedures, transferring files to and
from another
computer, or send
ing and receiving electroni
c mail.
Many types of computers using
operating systems with names like MVS, VM, OS/400, VMS, and UNIX provide general


services to the network.

19 Aside from the general
purpose computer, there are also many special
These computers are de
signed to perform one or two main tasks very
efficiently and effectively. Some examples of servers include: 1) file servers;
2) print servers; 3) database servers; and 4) other specialty servers.

20 Client Nodes. Each client node consists of a computer w
ith the appropriate
software that can make use of network services offered by server nodes. Software
running on the client computer must be able to intercept and interpret local requests
for remote service, formulate the appropriate message, and send the r
equests across
the network to the correct server node. Once sent, the client node waits for a re

sponse. When received, the details are provided to the program or user making the

21 Almost any type of computer can be a client. This includes the
computers mentioned as being servers, plus a full

Connecting Media

22 You now know about network nodes and the types of roles they play on the network.
Now it's time t
o review how the nodes are con
nected together using connecting media.
e connecting medium, which might be electrical wires, fiber
optic cable, or
wireless, carries the data between nodes on the network.

23 Network Topologies. Nodes can be connected in a variety of physical
configurations. The layout of the connecting media

between nodes on the network is
referred to as the network topology. Each of the common methods is known by a
geometric shape. These shapes include the layouts of a bus2, ring3, star4, trees,
or mesh6, which are easy to visualize when thinking about conne
ctions using bounded
media such as electrical cable.

24 In the simplest case, two node
s might have a link running di
rectly between them.
This is referre
d to as a point
point connec
tion and is useful when only two de
need to exchange information (f
or example, a note
computer copying files to
and from a desktop computer).

25 However, the more general network case involves two or more devices sharing the
connecting medium. Every node on the network is capable of receiving data from the

g medium and of plac
ing data on the medium for delivery to another node.
This occurs, for example, when all the personal computers (PCs) in a department are
connected to one common cable.

26 Controlling Access to the Medium. Sharing access to a common med
ium requires
that rules be put in place. Whether the participants are two friends at a restaurant,
eight guests at a dinner party, or hundreds of people in a town meeting, only one
person can speak

effectively at one time, while everyone else listens. With
out these
rules for the discussion, messages and meaning will be lost.

27 Computer networks are very similar. Whenever two or more nodes share one
physical medium, there must be rules governing which node can transmit while the
others listen. Every node
must be prepared to follow these rules of conversation,
otherwise data might be lost or distorted. And lost data means messages must be
repeated(the computer equivalent of shouting louder), clogging the network and


reducing efficiency.

28 ~ The rules for u
se of a computer network revolve around when a node is allowed
to access the shared connecting medium so that it can send a message on the network.
wo main approaches of media ac
cess control are used, and these are
known as
contention7 and deterministic

etwork Communication Protocols

29 The network nodes and connecti
ng media form the physical por
tions of a local
area network. Like a network of telephones and wires, or two
way radios and radio

waves, we now have an infras
tructure of computers that can exc
hange information.
But what do they

say to each other?

30 The answer lies in more rul
es, known as network communica
tions Protocols. People
can use many different languages and proto

cols to communicate. Unfortunately,
unless two people can speak the same

language, they will have a great

deal of
difficulty in communi
cating. The same applies to computers. If they don't use the
same language and protocols for networ
k communications, then they can

not exchange
data. There are many different network communicat
ion protocols available for use,
and like human languages, it's not always

possible to translate directly between

31 In an attempt to bring a level of standardization between protocols, the
International Standards Organization (ISO) developed the O
pen Systems Interconnect
Model (OSI Model). The model is intended to illustrate the separable f
unctions needed
for data commu
nication among computers. The main benefit of the model is that it
helps people to understand the data communication process in ter
ms of functions
operating at separate layers. Each layer plays a role in getting the data between
the source and destination nodes. The OSI Model involves seven separate laye
rs, but
not many network commu
nication protocols comply with this view. Instead, e
protocol has its own view on the number of layers and the function of each.
Internetworking Devices

32 A simple LAN consists of a
number of network nodes, a con
necting medium, and
a network communication protocol used to ex

change data between nodes.

But it is
also possible to combine small, simple LANs into large, complex networks. Larger
networks can help companies get a better return out of their investments in applica

tions such as electronic mail and corporate databases. To do this, we use vario
types of internetworking devices.

33 Interconnecting devices pass
messages from one LAN to anoth
er. They offer
different levels of functionality and sophistication like repeaters, (which simply
extend the range of one LAN,) bridges, (which can extend t
he range of a L
AN or
partition one busy LAN in
to two separate, more

efficient sections, ) routers,
( which
connect LANs of different hardware types, )
gateways, (which connect differ
network architectures by performing a convers
ion at the applica
tion lev
el,) etc.
In addition, the functions formerly associated with one device are now being combined
into hybrid units. Devices with

names such as brouters allow both bridging and
routing functions to take

place on a selectable basis, depending on the protocol
and ad

dresses involved. How Does Wireless Fit in ?

34 Now that you understand the basic structure of any LAN, you might wonder how


wireless LANs differ from their wired counter

parts. As far as the host computer
is concerned, the network inter

face ca
rd for a wireless LAN will appear similar
o any other NIC. Of course, the


itself will be very different in that it will
be designed to communicate over a wireless connecting medium such as radio waves
or infrared. The most visible, or perhaps invisibl
e, difference for a wireless LAN
is that there are no

copper wire or fiber
optic ca
bles used to connect network nodes.
The protocols that are most closely tied to the physical components of the network
are different for wireless LANs. However, for the most

part, the higher
level network
protocols used on a wireless LAN are similar or identical to those on a wired LAN.
Wireless LANs are usually tied to wired LANs. Therefore, a connection is made th
an internetworking device

like a bridge, which is there
fore part wireless, part

I. Notes:

1. This reading text is adapted from Chapter 2 of the book entitled

Wireless Local Area Networks : Technology, Issues, and Strategies by Peter T.
Davis and Craig R. McGuffin, published by Mc
Hill, Inc.,

New York, 1995. So
far little information is found of the authors.

2. Bus: One of the network topologies, which is formed by connect

ing each node to a single medium, with each being able to see all

activities on the connecting medium, with t
he help of a T


II. How well did you read ?

3. What are the major components of a LAN?

a. Network nodes

b. A connecting medium

c. Network communication protocols

d. Internetwo
rking devices (optional)

e. a, b, c, and d

4. Which of the following is not true about local area network nodes?

a. A node on a LAN is simply a device attached

to the network at

a given point.

b. A node is but a computer.

c. A node can also be an internetworking device as a go

for different portions of a network, or between different types

of networks

d. A node is connected to the networ
k through the network inter
face card.

5. What

is not true with the network interface card (NIC)?

a. NIC is located between the network and a node.

b. The NIC serves as the gateway between the network and the

computer's internal components.


Both sides of
the NIC connec
t to the

computer s bus

d. One side of the NIC connects to the comput


, the other side acts as

the interface to thec0nnectingmedium .


6. What is the fun
ction of a computer attached to


al area net work node?

a. It serves as a server providing servic
es to

the network like storing files,

printing data or connecting to

host computer.

b. It serves as a connector to other networks.

'c. 'It serves as client computer using the servic
es from the network.

d. a, b, and


7. What services are offered by server nodes?

a. They act as file servers.

b. They act as printer servers.

c. They act as database servers and other specialty servers.

d. a, b, and


8. What is not true

with client nodes?

a. Each client node consists of a computer with the appropriate

software to make use of network services.

b. All types of computer can be a client.

c. A client node is different from a server node in the fact that

the former uses the available services from the network while

the latter provides the services to the network.

d. Almost any type of computer can be a client.

9. Which of the following is not true about connecting media?

a. The co
ng medium carries the data b
een nodes
n the


b. The layout of connecting media is called topology.

c. Topologies can be divided into the bus, ring, star, tree, or


d. Not every node on the network is
capable of receiving dater

from the connecting medium.

11. What is true about network communication protocols?

a. A network communication protocol is a formally specified set

of conventions governing the format and control of inp
uts and

outputs between two communicating systems.

b. There are many different network communication protocols

available for use.

e. One of the important protocols is the Open Systems Interconnect Model
created by ISO.

. a, b, and


12. What are the functions of internetworking devices?

a. lnternetworking devices are also nodes in a sense but serve to

combine small, simple LANs into larger, complex networks.

b. Internetworking devices pass messages fro
m one LAN to an


c. lnternetworking devices offer different levels of functionality

like repeaters, bridges, routers and gateways.

d. a, b, andc13. In what way is a wired local area network different from a



s one?

a. A wireless connecting medium is used in the latter.

b. The latter has no copper wire or fiber
optic cables used to

connect network nodes.

c. The latter's internetworking devices are part wireless and part


d. a, b, and