4.4 COMPUTER
NETWORK
A computer network is an
interconnection of various computer systems located at different places. In
computer network two or more computers are linked together with a medium and
data communication devices for the purpose of communicating data and sharing
resources. The computer that provides resources to other computers on a network
is known as server. In the network
the individual computers, which access shared network resources, are known as workstations or nodes.
Computer
Networks may be classified on the basis of geographical area in two broad
categories.
1. Local Area Network (LAN)
2. Wide Area Network (WAN)
4.4.1 Local Area Network
Networks
used to interconnect computers in a single room, rooms within a building or
buildings on one site are called Local Area Network (LAN). LAN transmits data
with a speed of several megabits per second (106 bits per second).
The transmission medium is normally coaxial
cables.
LAN
links computers, i.e., software and hardware, in the same area for the purpose
of sharing information. Usually LAN links computers within a limited
geographical area because they must be connected by a cable, which is quite
expensive. People working in LAN get more capabilities in data processing, work
processing and other information exchange compared to stand-alone computers. Because of this information exchange most of
the business and government organisations are using LAN.
Major Characteristics of LAN
·
every computer has the potential to communicate with
any other computers of the network
·
high degree of interconnection between computers
·
easy physical connection of computers in a network
·
inexpensive medium of data transmission
·
high data transmission rate
Advantages
·
The reliability of network is high because the failure
of one computer in the network does not effect the functioning for other
computers.
·
Addition of new computer to network is easy.
·
High rate of data transmission is possible.
·
Peripheral devices like magnetic disk and printer can
be shared by other computers.
Disadvantages
If
the communication line fails, the entire network system breaks down.
Use of LAN
Followings
are the major areas where LAN is normally used
·
File transfers and Access
·
Word and text processing
·
Electronic message handling
·
Remote database access
·
Personal computing
·
Digital voice transmission and storage
4.4.2 Wide Area Network
The
term Wide Area Network (WAN) is used to describe a computer network spanning a
regional, national or global area. For example, for a large company the head quarters
might be at Delhi and regional branches at Bombay, Madras, Bangalore and Calcutta.
Here regional centers are connected to head quarters through WAN. The distance
between computers connected to WAN is larger. Therefore the transmission medium
used are normally telephone lines, microwaves and satellite links.
4.4.3 Characteristics of WAN
Followings
are the major characteristics of WAN.
1.
Communication
Facility: For a big company spanning over different parts of the
country the employees can save long distance phone calls and it overcomes the
time lag in overseas communications. Computer conferencing is another use of
WAN where users communicate with each other through their computer system.
2.
Remote Data
Entry: Remote data entry is possible in WAN. It means sitting at any
location you can enter data, update data and query other information of any
computer attached to the WAN but located in other cities. For example, suppose
you are sitting at Madras and want to see some data of a computer located at
Delhi, you can do it through WAN.
3.
Centralised
Information: In modern computerised environment you will find that big
organisations go for centralised data storage. This means if the organisation
is spread over many cities, they keep their important business data in a single
place. As the data are generated at different sites, WAN permits collection of
this data from different sites and save at a single site.
4.4.4 Examples of WAN
1. Ethernet: Ethernet developed by Xerox
Corporation is a famous example of WAN. This network uses coaxial cables for
data transmission. Special integrated circuit chips called controllers are used to connect equipment to the cable.
2. Aparnet: The Aparnet is another example of
WAN. It was developed at Advanced
Research Projects Agency of U. S. Department. This Network connects more than
40 universities and institutions throughout USA
and Europe.
Difference between LAN and WAN
·
LAN is restricted to limited geographical area of few
kilometers. But WAN covers great
distance and operate nationwide or even worldwide.
·
In LAN, the computer terminals and peripheral devices
are connected with wires and coaxial cables. In WAN there is no physical
connection. Communication is done
through telephone lines and satellite links.
·
Cost of data transmission in LAN is less because the
transmission medium is owned by a single organisation. In case of WAN the cost of data transmission
is very high because the transmission medium used are hired, either telephone
lines or satellite links.
·
The speed of data transmission is much higher in LAN
than in WAN. The transmission speed in LAN varies from 0.1 to 100 megabits per
second. In case of WAN the speed ranges from 1800 to 9600 bits per second
(bps).
·
Few data transmission errors occur in LAN compared to
WAN. It is because in LAN the distance
covered is negligible.
4.5 NETWORK TOPOLOGY
The
term topology in the context of communication network refers to the way the
computers or workstations in the network are linked together. According to the
physical arrangements of workstations and nature of work, there are three major
types of network topology. They are star topology, bus topology and ring
topology.
4.5.1 Star topology
In
star topology a number of workstations (or nodes) are directly linked to a
central node (see, Fig. 4.3). Any communication between stations on a star LAN
must pass through the central node. There is bi-directional communication between various nodes. The central
node controls all the activities of the nodes. The advantages of the star topology
are:
·
It offers flexibility of adding or deleting of
workstations from the network.
·
Breakdown of one station does not affect any other
device on the network.
The
major disadvantage of star topology is that failure of the central node
disables communication throughout the whole network.
4.5.2 Bus Topology
In bus topology
all workstations are connected to a single communication line called bus. In this type of network topology
there is no central node as in star topology. Transmission from any station
travels the length of the bus in both directions and can be received by all
workstations. The advantage of the bus topology is that
·
It is quite easy to set up.
·
If one station of the topology fails it does not
affect the entire system.
The disadvantage
of bus topology is that any break in the bus is difficult to identify.
4.5.3 Ring
Topology
In ring topology
each station is attached nearby stations on a point to point basis so that the
entire system is in the form of a ring. In this topology data is transmitted in
one direction only. Thus the data packets circulate along the ring in either
clockwise or anti-clockwise direction. The advantage of this topology is that
any signal transmitted on the network passes through all the LAN stations. The
disadvantage of ring network is that the breakdown of any one station on the
ring can disable the entire system.
IN-TEXT
QUESTIONS 2
1. Differentiate
between LAN and WAN.
2. What are
the different types of network topology?
3. State True
or False
(a) Networks used to interconnect computers in
a single room, rooms within a building or buildings on one site are called Wide
Area Network (WAN).
(b) The term Wide Area
Network (WAN) is used to describe a computer network spanning a regional,
national or global area.
(c) The speed of data transmission is much
higher in WAN than in LAN.
(d) In star topology a number of workstations
(or nodes) are directly linked to a central node.
(e) The
advantage of the bus topology is that, If one station of the topology fails it
does not affect the entire system.
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