Transcript Document

Compiled by :
S. Agarwal
Lecturer & Systems Incharge
St. Xavier’s Computer Centre
St. Xavier’s College, Kolkata.
Introduction
Communication - means to an exchange of
information between two or more parties and
it can be exchange in a variety of way, that
might be in the form of word, letters,
messages drawing, body movement or any
other symbols that represents the idea which
make to other understandable.
Data communication : entails electronically
exchanging data or information. It is the
movement of computer information from one
point to another by means of electrical or
optical transmission system. This system
often is called data communication
networks.
The key technology of the information age is
computer communications.
•The
value
of
high-speed
data
communication network is that it brings the
message sender and receiver closer together
in time.
•It facilitates more efficient use of computers
and improves the day to day control of
business by providing faster information
flow.
A computer network is a system in which a set of
computers and/or other communication facilities
are connected to each other. It is a collection of
computers which are in some way connected
such that they can exchange data between
themselves and other computers on the network.
It might involve physical wire, infrared or radio
frequency communication, and can be usually
used to share files, devices and connections to
other computers. In short, a computer network
allows your computers to talk to each other.
Reasons for networking:
To allow users to communicate with
each other.
•Pass data between users without the use of
floppy disks — Files can be copied and
accessed across the network, eliminating the
time wasted and inconvenience caused by using
floppy disks to transfer files. There is also less
restriction on the size of file that can be
transferred over the network.
To share hardware.
•Share expensive peripherals, such as
printers — All of the computers can access
the same printer, modems or CD-ROM
tower.
To share software and information.
•Centralize key computer programs, such as finance
and accounting programs, word processing, time and
billing — It is often important that all users have access to
the same program (and not copies of it) so that they can
work on it simultaneously. Networking allows offices to have
such a central program that all users can access.
•Automate unattended backup of valuable — It is always
essential to keep backups of any important files. You can
automate this procedure by having a computer program that
backs up the files for you. Without a network, you would
have to manually copy files, which is time consuming.
Basic Components
of a Network
Server or host computer: A server is a machine that provides
clients with service. Servers share a specific resource with other
computers.
Examples of the servers are the database server, that provides
the large database and the communication server that provides
connection to another network, to commercial database, or to a
powerful processor. In larger LANs, the server is dedicated to
being a server. In a peer-to-peer LAN, the server may be both a
server and a client computer. The server stores data or software
that can be accessed by the client. There are file, database,
network, access, modem, facsimile, printer, and geteway
server. Servers are usually microcomputers (often more
powerful than the other microcomputers on the network), but
they may be minicomputers or mainframes.
Client: A client is a computer such as a PC or a workstation
attached to the network, which is used to access shared
network resources. Client computer uses the resources shared
by server computer. The client is the input/output hardware
device at the other end of communication circuit. It typically
provides users with access to the network and data and
software on the server. There are three major categories of
clients: terminals, minicomputers/workstation, and special
purpose terminals.
Circuit: The circuit is the pathway through which the message
passes. Circuits can be twisted pair cable, coaxial cable, fiber
optic cable, microwave transmission, and so forth. There are
many devices in the circuit that perform special functions, such
as hubs, switches, routers and gateways.
•Cables & Connectors – Cables carry electronic signals
from one computer to another & Connectors attach cables
to network interface cards.
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.
Unshielded Twisted Pair (UTP) Cable
Twisted pair cabling comes in two varieties: shielded and
unshielded.
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.
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 telephonestyle connector. 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 connector.
Shielded Twisted Pair (STP) Cable
A disadvantage of UTP is that it may be susceptible
to radio and electrical frequency interference.
Shielded twisted pair (STP) is suitable for
environments with electrical interference; however,
the extra shielding can make the cables quite bulky.
Coaxial 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. The metal shield helps to block any
outside interference from fluorescent lights, motors, and other
computers.
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.
Coaxial Cable Connectors
The most common type of connector used with coaxial cables
is the Bayone-Neill-Concelman (BNC) connector.
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 than screw, onto the cable.
Fiber Optic Cable
Fiber optic cabling consists of a center glass core surrounded by several
layers of protective materials 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 buildings, due
to its immunity to the effects of moisture and lighting.
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 more
difficult to install and modify.
Wireless LANs
Not all networks are connected with cabling; some
networks are wireless. These 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.
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.
Wireless LANs have several disadvantages. They are very
expensive, provide poor security, and are susceptible to
interference from lights and electronic devices. They are also
slower than LANs using cabling.
Bits per second
In data communications, bits per second (abbreviated bps
and, by some, bit/sec) is a common measure of data speed
for computer modems and transmission carriers. As the term
implies, the speed in bps is equal to the number of bits
transmitted or received each second.
Larger units are sometimes used to denote high data
speeds. One kilobit per second (abbreviated Kbps) is equal
to 1,000 bps. One megabit per second (Mbps) is equal to
1,000,000 bps or 1,000 Kbps.
Protocol - Protocol can be defined as an agreed-upon
format for transmitting data between two devices. The
protocol determines the following:
•The type of error checking to be used
•Data compression method, if any
•How the sending device will indicate that it has
finished sending a message
•How the receiving device will indicate that it has
received a message
There are many standard protocols, among them:
AppleTalk, Ethernet, NetBEUI, and TCP/IP
Ethernet
The Ethernet protocol is by far the most widely used. Ethernet uses an
access method called CSMA/CD (Carrier Sense Multiple Access/Collision
Detection). This is a system where each computer listens to the cable
before sending anything through the network. If the network is clear, the
computer will transmit. If some other node is already transmitting on the
cable, the computer will wait and try again when the line is clear.
Sometimes, two computers attempt to transmit at the same instant. When
this happens a collision occurs. Each computer then backs off and waits a
random amount of time before attempting to retransmit. With this access
method, it is normal to have collisions. However, the delay caused by
collisions and retransmitting is very small and does not normally effect the
speed of transmission on the network.
The Ethernet protocol allows for linear bus, star, or tree topologies. Data
can be transmitted over twisted pair, coaxial, or fiber optic cable at a
speed of 10 Mbps.
LocalTalk / Appletalk
LocalTalk is a network protocol that was developed by Apple Computer,
Inc. for Macintosh computers. The method used by LocalTalk is called
CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance). It is
similar to CSMA/CD except that a computer signals its intent to transmit
before it actually does so. LocalTalk adapters and special twisted pair
cable can be used to connect a series of computers through the serial
port. The Macintosh operating system allows the establishment of a peerto-peer network without the need for additional software. With the addition
of the server version of AppleShare software, a client/server network can
be established.
The LocalTalk protocol allows for linear bus, star, or tree topologies using
twisted pair cable. A primary disadvantage of LocalTalk is speed. Its
speed of transmission is only 230 Kbps.
Token Ring
The Token Ring protocol was developed by IBM in the mid-1980s. The
access method used involves token-passing. In Token Ring, the
computers are connected so that the signal travels around the network
from one computer to another in a logical ring. A single electronic token
moves around the ring from one computer to the next. If a computer does
not have information to transmit, it simply passes the token on to the next
workstation. If a computer wishes to transmit and receives an empty
token, it attaches data to the token. The token then proceeds around the
ring until it comes to the computer for which the data is meant. At this
point, the data is captured by the receiving computer. The Token Ring
protocol requires a star-wired ring using twisted pair or fiber optic cable. It
can operate at transmission speeds of 4 Mbps or 16 Mbps. Due to the
increasing popularity of Ethernet, the use of Token Ring in school
environments has decreased.
FDDI
Fiber Distributed Data Interface (FDDI) is a network protocol that is
used primarily to interconnect two or more local area networks, often
over large distances. The access method used by FDDI involves
token-passing. FDDI uses a dual ring physical topology.
Transmission normally occurs on one of the rings; however, if a
break occurs, the system keeps information moving by automatically
using portions of the second ring to create a new complete ring. A
major advantage of FDDI is speed. It operates over fiber optic cable
at 100 Mbps.
ATM
Asynchronous Transfer Mode (ATM) is a network protocol that
transmits data at a speed of 155 Mbps and higher. ATM works by
transmitting all data in small packets of a fixed size; whereas, other
protocols transfer variable length packets. ATM supports a variety of
media such as video, CD-quality audio, and imaging. ATM employs a
star topology, which can work with fiber optic as well as twisted pair
cable.
ATM is most often used to interconnect two or more local area
networks. It is also frequently used by Internet Service Providers to
utilize high-speed access to the Internet for their clients. As ATM
technology becomes more cost-effective, it will provide another
solution for constructing faster local area networks.
Protocol
Cable
Speed
Topology
Ethernet
Twisted Pair, Coaxial, Fiber
10 Mbps
Linear Bus, Star, Tree
Fast Ethernet
Twisted Pair, Fiber
100 Mbps
Star
LocalTalk
Twisted Pair
.23 Mbps
Linear Bus or Star
Token Ring
Twisted Pair
4 Mbps - 16 Mbps
Star-Wired Ring
FDDI
Fiber
100 Mbps
Dual ring
ATM
Twisted Pair, Fiber
155-2488 Mbps
Linear Bus, Star, Tree
Network Architectures
Network architecture guides a network builder to make a plan or
structure by which the component parts are related. Network
architecture is a framework of rules to ease the operation, maintenance,
and growth of a communication network by isolating the user and the
application programs from the details of the network.
The work done by any application program can be separated in to four
general functions. The first one is data storage, almost all application
programs requires data to be stored and retrieved. The section function
is data access logic, when data is stored than it need to be accessed.
The third one is application and the fourth function is presentation logic.
These four functions are the basic building block of any application.
There are three fundamental network architecture:
1. Host Based Architecture
2. Clint Based Architecture
3. Clint-Server Architecture
Host-based architecture: Host-based architecture performs virtually all
the network. It was the very first data communication network architecture
and the host computer performs all four function of application program.
This is the simple architecture and works well.
In host based network all data are flow from the one central host. Client
computers enable users to send and receive the message from the host
computer. Client computers send the message to the host computer for
processing and accepted the instruction from the host on what to display.
In host based network all work must be done by the host computer
because of these host computers become overloaded and can not process
quickly for the all user's demand. Response time become slower and
network managers are required to spend more money to upgrade the host
computers.
Client based architecture: The clients are the microcomputer on a LAN
and host computers are servers on the same network.
In client based architecture all process must be done by the client
computer or the application software on the client computer is
responsible for the all four application program functions; the server
simply stores the data. As host based architecture, this architecture also
have overload problem, since all data are process from the client so if
more users are in active in the same network than logically the network
circuit can not perform well and be overloaded. The fundamental
problem of this architecture is all the data from the server must travel to
the clients for processing which makes slow the entire network.
Client server architecture: In client-server computing, several servers
may work together over the network to support the business application.
Client-server architecture makes a balance between client computer and
host computer.
In client server architecture, they split the application program function in
to two groups, client handles the presentation logic, and server handle
the data access logic and data storage while application logic may be on
both or can reside either on client or on the server. Because of its nature,
the network is not overloaded with entire files being transferred back and
forth through the network circuit for processing at each remote
terminal. For example; if the insurance company wants to request the list
of all the customer and their insurance police, the client would accept the
request than format the request to be understood by the server and
transmit it to the server. On the other side, upon receiving the request
form the client, the server search the database for all the requested and
than transmit only the matching request to the client.
Network Software - computer applications used to operate
the network and its many components.
What is a Network Operating System?
Unlike operating systems, such as DOS and Windows, that
are designed for single users to control one computer,
network operating systems (NOS) coordinate the activities of
multiple computers across a network. The network operating
system acts as a director to keep the network running
smoothly.
The two major types of network operating systems are:
Peer-to-Peer & Client/Server
Peer-to-Peer
Peer-to-peer network operating systems allow users to share resources and files located
on their computers and to access shared resources found on other computers. However,
they do not have a file server or a centralized management source. In a peer-to-peer
network, all computers are considered equal; they all have the same abilities to use the
resources available on the network. Peer-to-peer networks are designed primarily for
small to medium local area networks. AppleShare and Windows for Workgroups are
examples of programs that can function as peer-to-peer network operating systems.
Advantages of a peer-to-peer network:
Less initial expense - No need for a dedicated server.
Setup - An operating system (such as Windows) already in
place may only need to be reconfigured for peer-to-peer
operations.
Disadvantages of a peer-to-peer network:
Decentralized - No central repository for files and applications.
Security - Does not provide the security available on a
client/server network.
Client/Server
Client/server network operating
systems allow the network to
centralize
functions
and
applications in one or more
dedicated file servers. The file
servers become the heart of the
system, providing access to
resources
and
providing
security.
Individual workstations (clients) have access to the resources available
on the file servers. The network operating system provides the
mechanism to integrate all the components of the network and allow
multiple users to simultaneously share the same resources irrespective of
physical location. Novell Netware and Windows NT/2000 Server are
examples of client/server network operating systems.
Advantages of a client/server network:

Centralized - Resources and data security are controlled through the server.

Scalability - Any or all elements can be replaced individually as needs increase.
 Flexibility - New technology can be easily integrated into system.

Interoperability - All components (client/network/server) work together.

Accessibility - Server can be accessed remotely and across multiple platforms.
Disadvantages of a client/server network:

Expense - Requires initial investment in dedicated server.

Maintenance - Large networks will require a staff to ensure efficient operation.

Dependence - When server goes down, operations will cease across the network.
LAN TOPOLOGIES :
What is a Topology?
The physical topology of a network refers to the
configuration of cables, computers, and other peripherals.
Physical topology should not be confused with logical
topology which is the method used to pass information
between workstations.
Main Types of Physical Topologies
Linear Bus
A linear bus topology consists of a main run of cable with a terminator at
each end (See fig. 1). All nodes (file server, workstations, and peripherals)
are connected to the linear cable. Ethernet and LocalTalk networks use a
linear bus topology.
Advantages of a Linear Bus Topology

Easy to connect a computer or peripheral to a linear bus.

Requires less cable length than a star topology.
Disadvantages of a Linear Bus Topology

Entire network shuts down if there is a break in the main cable.

Terminators are required at both ends of the backbone cable.

Difficult to identify the problem if the entire network shuts down.

Not meant to be used as a stand-alone solution in a large building.
Star
A star topology is designed with
each
node
(file
server,
workstations, and peripherals)
connected directly to a central
network hub or concentrator.
Data on a star network passes through the hub or concentrator before
continuing to its destination. The hub or concentrator manages and
controls all functions of the network. It also acts as a repeater for the data
flow. This configuration is common with twisted pair cable; however, it can
also be used with coaxial cable or fiber optic cable.
Advantages of a Star Topology

Easy to install and wire.

No disruptions to the network then connecting or removing devices.

Easy to detect faults and to remove parts.
Disadvantages of a Star Topology

Requires more cable length than a linear topology.

If the hub or concentrator fails, nodes attached are disabled.

More expensive than linear bus topologies because of the cost of the
concentrators.
Ring Topology
Ring topologies are used on token ring networks. Each device processes and retransmits the
signal, so it is capable of supporting many devices in a somewhat slow but very orderly fashion. A
token, or small data packet, is continuously passed around the network. When a device needs to
transmit, it reserves the token for the next trip around, then attaches its data packet to it. The
receiving device sends back the packet with an acknowledgment of receipt, then the sending
device puts the token back out on the network. The most common type of cabling used for token
ring networks is twisted pair, although there are nine different types that can be used. With IBM
Type 1 Shielded cable, you can have up to 33 network segments with 260 devices on each.
Transmission rates are at either 4 or 16 megabits per second.
Advantages
•Very orderly network where every device has access to the token
and the opportunity to transmit
•Performs better than a star topology under heavy network load
•Can create much larger network using Token Ring
Disadvantages
•One malfunctioning workstation or bad port in the MAU can
create problems for the entire network
•Moves, adds and changes of devices can affect the network
•Network adapter cards and MAU's are much more expensive
than Ethernet cards and hubs
•Much slower than an Ethernet network under normal load
Tree
A tree topology combines
characteristics of linear
bus and star topologies. It
consists of groups of starconfigured workstations
connected to a linear bus
backbone cable. Tree
topologies allow for the
expansion of an existing
network, and enable
schools to configure a
network to meet their
needs.
Advantages of a Tree Topology

Point-to-point wiring for individual segments.

Supported by several hardware and software venders.
Disadvantages of a Tree Topology

Overall length of each segment is limited by the type of cabling used.

If the backbone line breaks, the entire segment goes down.

More difficult to configure and wire than other topologies.
Mesh Topology
A Mesh topology consists of a network where every device on the network is physically
connected to every other device on the network. This provides a great deal of
performance and reliability, however the complexity and difficulty of creating one
increases as the number of nodes on the network increases. For example, a three or four
node mesh network is relatively easy to create, whereas it is impractical to set up a mesh
network of 100 nodes. Mesh networks are not used much in local area networks (LANs)
but are used in Wide Area Networks (WANs) where reliability is important and the number
of sites being connected together is fairly small.
Considerations When Choosing a Topology:
Money. A linear bus network may be the least
expensive way to install a network; you do not have to
purchase concentrators.
Length of cable needed. The linear bus network uses
shorter lengths of cable.
Future growth. With a star topology, expanding a
network is easily done by adding another concentrator.
Cable type. The most common cable in schools is
unshielded twisted pair, which is most often used with
star topologies.
Topology
Configuration
Advantage
Disadvantage
Star
All
nodes
are
connected to the
central computer
Relatively efficient;
Nodes are free to join
or quit from the
network
Dependent on the
central computer
Ring
All
nodes
are
connected in a
continuous loop
High speed
Whole network will
break down if one
node fails
Bus
All
nodes
are
connected to a single
cable and share it
Easy to install and
cheap;
No need to restart
the network if a node
fails or joins the
network
Comparatively
speed
low