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
Computer Networks may be classified on the basis of geographical area in two
1. Local Area Network (LAN)
2. Wide Area Network (WAN)
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
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
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
Digital voice transmission and storage
Metropolitan Area Network
A Metropolitan Area Network (MAN) is similar to a Wide Area Network (WAN)
except that it is localized to a given area such as a city. They were uses primarily
for fire and police in a city but with the advent of wifi, almost anyone can go on a
MAN network. As with wifi, range is limited.
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.
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 Lahore and want to see some data of
a computer located at Islamabad, 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.
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.
What is Network Cabling?
Cable is the medium through which information usually moves from one network
device to another. There are several types of cable which are commonly used with
LANs. In some cases, a network will utilize only one type of cable, other networks
will use a variety of cable types. The type of cable chosen for a network is related
to the network's topology, protocol, and size. Understanding the characteristics of
different types of cable and how they relate to other aspects of a network is
necessary for the development of a successful network.
The following sections discuss the types of cables used in networks and other
Unshielded Twisted Pair (UTP) Cable
Shielded Twisted Pair (STP) Cable
Fiber Optic Cable
Coaxial cabling has a single copper conductor at its center. A plastic layer provides
insulation between the center conductor and a braided metal shield (See fig. 3).
The metal shield helps to block any outside interference from
Fig. 3. Coaxial cable
Although coaxial cabling is difficult to install, it is highly resistant to signal
interference. In addition, it can support greater cable lengths between network
devices than twisted pair cable. The two types of coaxial
Thin coaxial cable is also referred to as thinnet. 10Base2 refers to the
specifications for thin coaxial cable carrying Ethernet signals. The 2 refers to the
approximate maximum segment length being 200 meters. In actual fact the
maximum segment length is 185 meters. Thin coaxial cable has been popular in
Thick coaxial cable is also referred to as thicknet. 10Base5 refers to the
specifications for thick coaxial cable carrying Ethernet signals. The 5 refers to the
maximum segment length being 500 meters. Thick coaxial cable has an extra
protective plastic cover that helps keep moisture away from the center conductor.
This makes thick coaxial a great choice when running longer lengths in a linear bus
network. One disadvantage of thick coaxial is that it does
Coaxial Cable Connectors
The most common type of connector used with coaxial cables is the Bayone-Neill-
Concelman (BNC) connector (See fig. 4). Different types of adapters are available
for BNC connectors, including a T-connector, barrel connector, and terminator.
Connectors on the cable are the weakest points in any network. To help avoid
problems with your network, always use the BNC connectors that crimp, rather
Fig. 4. BNC connector
Unshielded Twisted Pair (UTP) Cable
Twisted pair cabling comes in two varieties: shielded and unshielded. Unshielded
twisted pair (UTP) is the most popular and is generally the best option for school
networks (See fig. 1).
Fig.1. Unshielded twisted pair
The quality of UTP may vary from telephone-grade wire to extremely high-speed
cable. The cable has four pairs of wires inside the jacket. Each pair is twisted with
a different number of twists per inch to help eliminate interference from adjacent
pairs and other electrical devices. The tighter the twisting, the higher the supported
transmission rate and the greater the cost per foot. The EIA/TIA (Electronic
Industry Association/Telecommunication Industry Association) has established
standards of UTP and rated six categories of wire (additional categories are
Unshielded Twisted Pair Connector
The standard connector for unshielded twisted pair cabling is an RJ-45 connector.
This is a plastic connector that looks like a large telephone-style connector (See fig.
2). A slot allows the RJ-45 to be inserted only one way. RJ stands for Registered
Jack, implying that the connector follows a standard borrowed from the telephone
industry. This standard designates which wire goes with each pin inside the
Fig. 2. RJ-45 connector
Shielded Twisted Pair (STP) Cable
Although UTP cable is the least expensive cable, it may be susceptible to radio and
electrical frequency interference (it should not be too close to electric motors,
fluorescent lights, etc.). If you must place cable in environments with lots of
potential interference, or if you must place cable in extremely sensitive
environments that may be susceptible to the electrical current in the UTP, shielded
twisted pair may be the solution. Shielded cables can also help to extend the
maximum distance of the cables.
Shielded twisted pair cable is available in three different configurations:
1. Each pair of wires is individually shielded with foil.
2. There is a foil or braid shield inside the jacket covering all wires (as a
3. There is a shield around each individual pair, as well as around the
entire group of wires (referred to as double shield twisted pair).
Fiber Optic Cable
Fiber optic cabling consists of a center glass core surrounded by several layers of
protective materials (See fig. 5). It transmits light rather than electronic signals
eliminating the problem of electrical interference. This makes it ideal for certain
environments that contain a large amount of electrical interference. It has also
made it the standard for connecting networks between
Fiber optic cable has the ability to transmit signals over much longer distances than
coaxial and twisted pair. It also has the capability to carry information at vastly
greater speeds. This capacity broadens communication possibilities to include
services such as video conferencing and interactive services. The cost of fiber optic
cabling is comparable to copper cabling; however, it is
The center core of fiber cables is made from glass or plastic fibers (see fig 5). A
plastic coating then cushions the fiber center, and kevlar fibers help to strengthen
the cables and prevent breakage. The outer insulating jacket made of teflon or PVC.
Fig. 5. Fiber optic cable
There are two common types of fiber cables -- single mode and multimode.
Multimode cable has a larger diameter; however, both cables provide high
bandwidth at high speeds. Single mode can provide more distance, but it is more
More and more networks are operating without cables, in the wireless mode.
Wireless LANs use high frequency radio signals, infrared light beams, or lasers to
communicate between the workstations and the file server or hubs. Each
workstation and file server on a wireless network has some sort of
transceiver/antenna to send and receive the data. Information is relayed between
transceivers as if they were physically connected. For longer distance, wireless
communications can also take place through cellular telephone technology,
microwave transmission, or by satellite.
Wireless networks are great for allowing laptop computers or remote computers to
connect to the LAN. Wireless networks are also beneficial in older buildings where
it may be difficult or impossible to install cables.
The two most common types of infrared communications used in schools are line-
of-sight and scattered broadcast. Line-of-sight communication means that there
must be an unblocked direct line between the workstation and the transceiver. If a
person walks within the line-of-sight while there is a transmission, the information
would need to be sent again. This kind of obstruction can slow down the wireless
network. Scattered infrared communication is a broadcast of infrared transmissions
sent out in multiple directions that bounces off walls and ceilings until it eventually
hits the receiver. Networking communications with laser are virtually the same as
line-of-sight infrared networks.
Advantages of wireless networks:
Mobility - With a laptop computer or mobile device, access can be
available throughout a school, at the mall, on an airplane, etc. More an
more businesses are also offering free WiFi access.
Fast setup - If your computer has a wireless adapter, locating a
wireless network can be as simple as clicking "Connect to a Network"
-- in some cases, you will connect automatically to networks within
Cost - Setting up a wireless network can be much more cost effective
than buying and installing cables.
Expandability - Adding new computers to a wireless network is as
easy as turning the computer on (as long as you do not exceed the
maximum number of devices).
Disadvantages of wireless networks:
Security - Wireless networks are much more susceptible to
unauthorized use. If you set up a wireless network, be sure to include
maximum security. You should always enable WEP (Wired
Equivalent Privacy) or WPA (Wi-Fi Protected Access), which will
improve security and help to prevent virtual intruders and freeloaders.
Interference - Because wireless networks use radio signals and similar
techniques for transmission, they are susceptible to interference from
lights and electronic devices.
Inconsistent connections - How many times have you hears "Wait a
minute, I just lost my connection?" Because of the interference caused
by electrical devices and/or items blocking the path of transmission,
wireless connections are not nearly as stable as those through a
Power consumption - The wireless transmitter in a laptop requires a
significant amount of power; therefore, the battery life of laptops can
be adversely impacted. If you are planning a laptop project in your
classroom, be sure to have power plugs and/or additional batteries
Speed - The transmission speed of wireless networks is improving;
however, faster options (such as gigabit Ethernet) are available via
cables. In addition, if set up a wireless network at home, and you are
connecting to the Internet via a DSL modem (at perhaps 3 Mbps),
your wireless access to the Internet will have a maximum of 3 Mbps
The standard model for networking protocols and distributed applications is the
International Standard Organization's Open System Interconnect (ISO/OSI) model.
It defines seven network layers.
Layer 1 - Physical
This layer defines the cable or physical medium itself, e.g. unshielded
twisted pairs (UTP). All media of transmission are functionally equivalent
in this layer and the main difference is in convenience and cost of
installation and maintenance.
Layer 2 - Data Link
Data Link layer defines the format of data on the network ( a network data
frame, packet and destination address). The Maximum Transmission Unit
(MTU) is defined by the largest packet that can be sent through a data link
Layer 3 - Network
This layer defines the protocols that are responsible for data delivery at the
required destination, and requires.
Layer 4 - Transport
This layer subdivides user-buffer into network-buffer sized datagrams and
enforces desired transmission control. Two transport protocols,
Transmission Control Protocol (TCP) and User Datagram Protocol (UDP),
sits at the transport layer. Reliability and speed are the primary difference
between these two protocols.
Layer 5 - Session
This leyer defines the format of the data sent over the connections.
Layer 6 - Presentation
This layer converts local representation of data to its canonical form and
vice versa. The canonical uses a standard byte ordering and structure
packing convention, independent of the host.
Layer 7 - Application
Provides network services to the end-users. e.g Mail.
Introduction to peer-to-peer architecture
In contrast to client-server networks there is no dedicated server in peer-to-peer
architecture . Thus each computer in such a network is part server and part client.
This means that each computer on the network is free to share its own resources. A
computer which is connected to a printer may even share the printer so that all
other computers may access it over the network.
The advantages and disadvantages of a peer-to-peer network are as follows:
It is easy to install.
Configuration of computers is easy.
Users can control their shared resources.
The cost and operation of this network is less.
It is ideal for small businesses having ten or fewer computers.
It needs an operating system and a few cables to get connected.
A full time network administrator is not required.
A computer can be accessed anytime.
Network security has to be applied to each computer separately.
Backup has to be performed on each computer separately.
No centralized server is available to manage and control the access of data.
Users have to use separate passwords on each computer in the network.
Client Server Setup
A computer network in which one centralized, powerful computer (called
the server) is a hub to which many less powerful personal computers or
workstations (called clients) are connected. The clients run programs and
access data that are stored on the server
Centralized - Resources and data security are controlled through the
Scalability - Any or all elements can be replaced individually as needs
increase. Flexibility - New technology can be easily integrated into
Interoperability - All components (client/network/server) work together.
Accessibility - Server can be accessed remotely and across multiple
Speed- network will run far better as data and resources are handled by a
dedicated machine. Also currently the user of the machine experiences
poor performance when everyone accesses it
Backup -as all data is stored centrally it is easy to backup
Support and management -as the server controls the majority of settings
on the network etc the job of support is far easier as the main element of
support is provided to the server and not individual machines. Global
changes are easy to make from one location.
Can have a single point of failure.
Server can get overloaded.
Generally more expensive and difficult to set up initially.
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). 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.
1) It is easy to modify and add new computers without disturbing the rest of the
2) The center of the star network is a good place to diagnose the faults.
3) Single computer failure does not necessarily bring down the whole star network.
1) If the central device fails the whole network fails to operate.
2) Star networking is expensive because all network cables must be pulled to one
central point, requires more cable than other network topologies.
Fig. 4.3: Star Topology
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
1) Easy to use and to understand.
2) Requires least amount of cable to connect the computers together. It is therefore
less expensive than other cabling arrangements.
3) It is easy to extend a bus; two cables can be joined into 1 longer cable with a
BNC, Barrel connector making a longer cable and allowing more computers to join
1) Heavy network traffic can slow a bus considerably as only 1 computer can send
a message at a time.
2) It is difficult to troubleshoot the bus. A cable break or loose connector causes
reflection and stops all the activity.
Fig. 4.4: Bus Topology
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.
1) Each node has equal access.
2) Capable of high speed data transfer.
1) Failure of one computer on the ring can affect the whole network.
2) Difficult to troubleshoot the network.
Topologies remain an essential part of network design speculation. But
understanding these can help you to get the deeper knowledge of the elements like
hub, switch etc.
A network setup where each of the computers and network devices are
interconnected with one another, allowing for most transmissions to be distributed,
even if one of the connections go down. This topology is not commonly used for
most computer networks as it is difficult and expensive to have redundant
connection to every computer. However, this topology is commonly used for
wireless networks. Below is a visual example of a simple computer setup on a
network using a mesh topology.
No traffic problem as there are dedicated links.
Robust as failure of one link does not affect the entire system.
Security as data travels along a dedicated line.
Points to point links make fault identification easy.
The hardware is expansive as there is dedicated link for any two nodes and
There is mesh of wiring which can be difficult to manage.
Installation is complex as each node is connected to every node.
Repeaters, Bridges, Routers, and Gateways
In a word: intelligence.
Hubs, switches, and routers are all devices that let you connect one or more
computers to other computers, networked devices, or to other networks. Each has
two or more connectors called ports into which you plug in the cables to make the
connection. Varying degrees of magic happen inside the device, and therein lies
the difference. I often see the terms misused so let's clarify what each one really
A hub is typically the least expensive, least intelligent, and least complicated of
the three. Its job is very simple: anything that comes in one port is sent out to the
others. That's it. Every computer connected to the hub "sees" everything that every
other computer on the hub sees. The hub itself is blissfully ignorant of the data
being transmitted. For years, simple hubs have been quick and easy ways to
connect computers in small networks.
A bridge goes one step up on a hub in that it looks at the destination of the packet
before sending. If the destination address is not on the other side of the bridge it
will not transmit the data.
A bridge only has one incoming and one outgoing port.
To build on the email analogy above, the bridge is allowed to decide if the message
should continue on. It reads the address email@example.com and decides if there is a
firstname.lastname@example.org on the other side. If there isn’t, the message will not be
Bridges are typically used to separate parts of a network that do not need to
communicate regularly, but still need to be connected.
A repeater connects two segments of your network cable. It retimes and
regenerates the signals to proper amplitudes and sends them to the other segments.
When talking about, ethernet topology, you are probably talking about using a hub
as a repeater. Repeaters require a small amount of time to regenerate the signal.
A switch does essentially what a hub does but more efficiently. By paying
attention to the traffic that comes across it, it can "learn" where particular addresses
are. For example, if it sees traffic from machine A coming in on port 2, it now
knows that machine A is connected to that port and that traffic to machine A needs
to only be sent to that port and not any of the others. The net result of using a
switch over a hub is that most of the network traffic only goes where it needs to
rather than to every port. On busy networks this can make the network
A router is the smartest and most complicated of the bunch. Routers come in all
shapes and sizes from the small four-port broadband routers that are very popular
right now to the large industrial strength devices that drive the internet itself. A
simple way to think of a router is as a computer that can be programmed to
understand, possibly manipulate, and route the data its being asked to handle. For
example, broadband routers include the ability to "hide" computers behind a type
of firewall which involves slightly modifying the packets of network traffic as they
traverse the device. All routers include some kind of user interface for configuring
how the router will treat traffic. The really large routers include the equivalent of a
full-blown programming language to describe how they should operate as well as
the ability to communicate with other routers to describe or determine the best way
to get network traffic from point A to point B.
A gateway is either a server with a gateway application installed or a device that
connects a network of computers to another network. If Network A wants to
connect to Network B and vice versa, both networks must have gateways that
provide exit and entry points for computers from the two networks to communicate.
Gateways are important. They define the boundaries of your network.
An Internet Protocol address (IP address) is a usually numerical label assigned to each device
(e.g., computer, printer) participating in a computer network that uses the Internet Protocol for
communication. An IP address serves two principal functions: host or network interface
identification and location addressing. Its role has been characterized as follows: "A name
indicates what we seek. An address indicates where it is. A route indicates how to get there."[
Network and Host ID Fields
The four octets that make up an IP address are conventionally represented by a, b, c, and d
respectively. The following table shows how the octets are distributed in classes A, B, and C.
IP Network Host
Address ID ID
A a.b.c.d a b.c.d
B a.b.c.d a.b c.d
C a.b.c.d a.b.c
Class A: Class A addresses are specified to networks with large number of total hosts. Class A
allows for 126 networks by using the first octet for the network ID. The first bit in this octet, is
always set and fixed to zero. And next seven bits in the octet is all set to one, which then
complete network ID. The 24 bits in the remaining octets represent the hosts ID, allowing 126
networks and approximately 17 million hosts per network. Class A network number values begin
at 1 and end at 127.
Class B: Class B addresses are specified to medium to large sized of networks. Class B allows
for 16,384 networks by using the first two octets for the network ID. The two bits in the first
octet are always set and fixed to 1 0. The remaining 6 bits, together with the next octet, complete
network ID. The 16 bits in the third and fourth octet represent host ID, allowing for
approximately 65,000 hosts per network. Class B network number values begin at 128 and end at
Class C: Class C addresses are used in small local area networks (LANs). Class C allows for
approximately 2 million networks by using the first three octets for the network ID. In class C
address three bits are always set and fixed to 1 1 0. And in the first three octets 21 bits complete
the total network ID. The 8 bits of the last octet represent the host ID allowing for 254 hosts per
one network. Class C network number values begin at 192 and end at 223.
Class D and E: Classes D and E are not allocated to hosts. Class D addresses are used for
multicasting, and class E addresses are not available for general use: they are reserved for future
Straight Through Cable
Crossover - Regular