This document provides an overview of computer networks and their basic components. It discusses how networks connect devices like computers, printers and servers to allow sharing of resources. Local area networks (LANs) connect devices within a building, while wide area networks (WANs) span greater distances. The two main types of networks are peer-to-peer and client-server. Client-server networks have centralized data storage on servers for easier management. Network topologies like star, bus and ring determine how data flows. Key network components include network interface cards, hubs, switches, bridges, routers and gateways. The document also describes the CSMA/CD and token ring access methods for transmitting data over the network.

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NETWORKS
AN OVERVIEW OF NETWORKS
Introduction
A network is an interconnection of two or more computers for the purpose of sharing
information and resources.
Apart from computers a network may also include other devices like printers, tape drives, hubs,
switches, routers etc. Each of these devices is generally referred to as a Node.
When a device is turned on and able to access the network it is called On-line, otherwise it is
called Off-line.
The various devices are connected together using communication lines like telephone lines,
coaxial cables, optical fibers etc. and use some communication software to communicate. The
extent of the network can range from a few devices inside a building to millions of devices
distributed all over the world.
Basic Terminology
Local Area Network (LAN) : A communication network consisting of computers distributed
over a relatively small geographical area like a building or a group of buildings within a campus.
Wide Area Network (WAN) : Similar to a LAN but spanning a much greater area like a country
or even the entire globe.
Node : A general term for any device on the network.
Bandwidth : The amount of data which can be transmitted over a particular segment of a network
within a specified amount of time e.g. Mbps
Load : Denotes the total number of users using the resources of the network at a time.
Determines the overall network activity and performance speeds.
Network Administrator : A person responsible for managing the activities on a network and
maintain its efficiency under various conditions.

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Types of Networks
A network can very broadly be classified into two types:
Peer-to-peer: All the computers on the networks act as peers i.e. they have equal rights and
privileges, and share information equally. Applications run on each system independently and no
single system controls the network.
Client-Server: Here the network consists of two classes of computers. The Servers, which are the
more powerful machines control the activities of the network by providing services to other
systems. They usually contain a central repository of data and resources, and often a NOS to
manage these. The other class of machines called Clients are less powerful machines which take
resources from the servers. Programs running on the clients are called front-end applications and
often retrieve data from the back-end servers using SQL. Each client has a specific level of
access that enables them to use, view or manipulate data on the servers.
Advantages of a Client-Server network
These include the following:
 A central location of data enables easier management and maintenance.
 Security measures like password protection and periodic backup of data are easier to
implement.
 Since the clients share the resources of the server, only one copy of the resources are
necessary instead of individual copies on all the clients thereby avoiding duplication.
 Data being located at a single location, can be updated periodically thereby providing
consistency, instead of different versions of the data on various clients.
 The total computing load is distributed among the clients and servers thus allowing the
machines to run more efficiently.

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Network Topologies
A network topology denotes the physical pattern to which a network conforms. The topology
determines how data flows, the level of fault tolerance, the management and trouble-shooting
considerations. Multiple topologies may be combined within a single network.
Star: Connects all network devices through a central point, usually a server. The network
provides fault tolerance if one or more clients fail, but stops functioning if the central server fails.
Bus: Simplest of the configurations. Each of the devices attached to a single trunk cable. A break
in the cabling anywhere causes the entire network to fail.
Ring: Devices are connected to each other in a circle. If a single device fails the rest of the
network continues to function properly.

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NETWORK COMPONENTS
Network Interface Card (NIC)
A Network Interface Card is an expansion card that is added to a computer to enable it to
communicate on a network. The NIC physically connects a computer on a network to the
network cable. It is also responsible for converting the data from the computer into a form which
can be sent out over the network and vice versa.
A network card can have various types of connectors to accommodate various types of cables.
The NIC usually has a memory buffer where information is temporarily stored and transmitted to
or from the network. The manufacturer of a NIC encodes an unique physical address into the
hardware.
The physical address is also called hardware address or media access control (MAC) address.
It is a 48-bit address of the form 00-40-95-e0-5d-c7. The first 24 bits (00-40-95) is the vendor’s
unique code while the remaining bits (e0-5d-c7) is the unique device ID.

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NETWORK EQUIPMENT
Repeaters
These are devices connected to two cable segments. A signal appearing on one of the segments,
gets amplified and put on the other. Repeaters do not understand packets, they only understand
volts. Hence repeaters are said to be operating in the physical layer.
Hub
A hub has a number of input lines that it joins electrically. Frames appearing on any of the lines
are sent out on all the others. If two frames arrive at the same time, they will collide just as on a
coaxial cable. Thus the entire hub forms a single collision domain. Thus a hub also only
recognizes electrical volts and therefore operates on the physical layer.

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Bridge
A bridge connects two or more LANs as shown. When a frame arrives, the bridge extracts the
destination hardware address from the frame header. Since a bridge recognizes frame addresses it
is said to operate in the data-link layer.
Switch
Switches are similar to bridges in that both use frame addresses. However a switch connects
individual computers while a bridge connects LANs. Each port corresponds to a different card
connected to a high-speed backplane (typically running at many Gbps). Since each port is its
own collision domain switches never lose frames to collisions. However if frames come in faster
than switches can retransmit them, the switch may run out of buffer space and may start
discarding frames. To alleviate this problem modern switches start forwarding frames as soon as
the destination folder field has come in.
Router
Routers do not read in the frame information but the IP information in the packet header. Hence
routers operate in the network layer.
Gateway
Gateways operate at the transport or application layers. A gateway can copy information from
one protocol like TCP/IP to another protocol like X.25.

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ACCESS METHODS
Network hosts access other hosts using the network medium using two methods.
Carrier Sense Multiple Access / Collision Detect (CSMS/CD)
CSMA/CD is a “probabilistic” mode of access since there is no guarantee that a particular host
will be able to access the network within a specific interval of time.
When a host is ready to transmit, it first checks (listens) for a signal already on the cable from
other hosts.
 If a signal is present, then the host delays transmission until the cable is free, periodically
listening to the network to find the status.
 If no signal is present then the host starts transmission and holds exclusive right of the
cable until the transmission is completed.
If two hosts detect an idle cable and begin transmission at the same time, then a collision occurs.
On detecting a collision the hosts stop their transmission and each wait for a random interval of
time before again beginning transmission. This method is usually used for networks with light
and medium traffic, because for high traffic there may be too many collision for proper
functioning.
Since hosts transmit as soon as they find the cable to be free, unnecessary delays are avoided
resulting in optimum use of the medium. Since hosts may have to wait a long time before getting
a chance to transmit on heavily loaded networks, this method may not be used to send critical or
urgent data.
Token Ring Passing
The second access method is called Token Ring Passing method, which is called “deterministic”
model as all hosts get a chance to transmit.
In this method, a token, consisting of a string of bits, is passed around all the hosts one after
another. Each host can hold the token for a pre-defined amount of time before handing it over to
the next host. The host which holds the token at a point of time has exclusive rights to transmit
over the cable while the other hosts can only listen.
This method is usually used for heavily loaded networks or where critical or urgent data needs to
be transmitted as there is a guarantee that each host will be able to transmit after a specific
amount of time. This is not used for lightly loaded networks because hosts which do not need to
transmit may be holding the token causing unnecessary delays.