Networking

Networking

Introduction

A collection of two or more computers interconnected by the telephone lines, co-axial cable, satellite links, radio and microwave transmission and some other communication techniques.  A computer network is a group of computers that are connected together and that communicate with one another for a common purpose.

Although the computer industry is young compared to anther industries (e.g., automobiles air transportation), computer have made spectacular progress in a short time.  During the first two decades of their extrinsic, computer system highly centralized, usually a single large room.  A medium size company or university might have had one or two computers, while large instructions had at most a few dozen. The idea that with in 20 years equally powerful computers smaller than postage stamps would be mass produced by the millions was pure science fiction.

The merging of computers & communications has had a profound influence on the way computer systems are organized. The concept of the computer center as a room with a large computer to which users bring their work for processing is now totally obsolete. The old model of single computer serving all of the organizations computational needs has been replaced by one in which a large number of separate but interconnected computers do the job. These systems are called computer networks.

Two computers are said to be interconnected if they are able to exchange information’s. The connection need not be via a copper wire; fiber optics, microwaves, and communication satellites can also be used. By requiring the computer to be autonomous, we which to execute from our definitions systems in which there is a clear master/slave relation. If one computer can forcibly starts , stop, or control another one, the computers are not autonomous. A system with one control unit and many slaves is not a network; nor is a large computer with remote printers and terminals.

There is considerable confusion in the literature between a computer network and a distributed system. The key distinction is that in a distributed system, the extence of multiple autonomous computers is transparent to the user. He can type a command to run a program. And it runs. It is up to the operating system to select the best processor, find and transport all the input files to the processor, and put the result in the appropriate place. In other words, the user of a distributed system is not aware that there are multiple processors; it looks like a virtual uni-processer. A location of jobs to processors and files to disks, moment of files between where they are stored and where they are needed, and all other system functions must be automatic.

With a network, user must explicitly log on to one machine, explicitly submit jobs remotely, explicitly move files around and generally handle all the network management personally. With the distributed system, nothing has to be done explicitly it is all automatically done by the system without the users knowledge.

In effect, a distributed system is a software system built on top of a network. The software gives it a high degree of cohesiveness and transparency. Those distinction between a network and a distributed system lies with the software rather than with the hardware. Nevertheless, there is considerable our lap between the two subjects. E.g., both distributed system and computers networks need to move file around. The difference lies in whom in vokes the movement, the system or the user.

Local Area Networks (LAN):

Local area network, generally called LANs, is privately-owned networks with in a single building or campus of up to a few KM in size. They are widely used to connect personal computers and workstation in company offices and factories to share resources (e.g., printers) and exchange information. LANs are distinguished from other kinds of networks by three characteristics:

1.    Their size,

2.    Their transmission technology,

3.    Their topology.

LANs are restricted in size, which means that the worst-case transmission time is bounded and known in advance. Knowing this bound makes it possible to use certain kinds of designs that would not otherwise be possible. It also simplifies network management.

Metropolitan Area Networks (MAN):

A metropolitan area network, or MAN (plural: MANs, not MEN) is basically a bigger version of a LAN and normally uses similar technology. It might cover a group of nearby corporate offices or a city and might be either private or public. A MAN can support both data and voice, and might even be related to the local cable television network. A MAN just has one or two cables and does not contain switching elements, which shunt packets over one of several potential output lines.

Wide Area Networks (WAN):

A wide area network, or WAN, spans a large geographical area, often a country or continent. It contains of machines intended for running user (i.e., application) programs. We will follow traditional usage and call these machines hosts. The term end system is sometimes also used in the literature. The hosts are connected by a communication subnet, or just subnet for short. The job of the subnet is to carry messages from host to host, just as the telephone system carries words from speaker to listener. By separating the pure communication aspects of the network (the subnet) from the application aspects (the hosts), the complete network design is greatly simplified.

In most wide area networks, the subnet consists of two distinct components: transmission lines & switching elements. Transmission lines (also called circuits, channels, or trunks) move bits between machines.

The switching elements are specialized computers used to connect two or more transmission lines. When data arrive on an incoming line, the switching element must choose an outgoing line to forward them on.

In most WANs, the network contains numerous cables or telephone lines, each one connecting a pair of routers. If two routers that do not share a cable nevertheless wish to communicate, they must do this indirectly, via other routers.  When a packet is send from one router to another via one or more intermediate routers, the packet is received at each intermediate router in its entirety, stored there until the required output line is free, and the forwarded.  A subnet using this principle is called a Point to Point, store and forward or packet switched subnet.  Nearly all wide area networks (except those using satellite) have store and forward subnet.  When the packet are small and all the same size, they often called cells.

Wireless network:

Mobile computers, such as notebook computers and personal digital assistants (PDA) are the fastest growing segment of the computer industries.  Many of the owners of these computers have desktop machines on LAN’s and WAN’s back at the office and want to be connected to their home base even when away from home or en route.  Since having a wired connection is impossible in cars and airplanes, there is a lot of interest in wireless network.

Wireless networks come in many forms. Some universities are already installing antennas all over campus to allow students to sit under the trees and consult the libraries card catalogs.  Here the computers communicate directly with the wireless LAN in a digital form.  Another possibility is using a cellular (i.e. portable) telephone with a traditional analog modem.  Direct digital cellular service, called CDPD (Cellular Digital Packet Data) is becoming available in many cities.

Network Hardware

It is now time to turn our attention from the application and social aspects of networking to the technical issues involved in network design. There is no generally excepted taxonomy into which all computer network fit, but two dimensions stand out as important: transmission technologies and scale. We will now examine each of these in turns.

Broadly speaking, there are two types of transmission technologies:

1. Broadcast networks.
2. Point-to-point networks.

Broadcast networks: It have a single communication channel that is shared by all the machines on the network. Short messages, called packets in certain contexts, sent by any machine are received by all the others. An address field within the packet specifies for whom it’s intended. Upon receiving a packet, a machine checks the address field. If the packet is intended for itself, it process the packet, if the packet is intended for the other machine, it is just ignored.

Broadcast systems generally also allow the possibility of addressing a packet to all destinations by using a special code in the address field. When a packet with this code is transmitted, it is received and processed by every machine on the network. This mode of operation is called broadcasting. Some broadcast system also support transmission to a subset of a machines, something now has multicasting.

Point-to-point networks: It consists of many connections between individual pairs of machines. To go from the source to the destination, a packet on this type of network may have to first visit one or more intermediate machines. Often multiple routes, of different lengths are possible, so routing algorithm play an important role in point-to-point networks. As a general rule (although there are many exceptions), smaller, geographically localized networks tend to use broadcasting, where larger usually are point-to-point.

Whenever we want two devices – transmitting and receiving device to communicate with each other, we need hardware’s to achieve that.  We would be discussing about the various hardware such as:

1.    Sender and Receiver hardware

2.    Communication devices

3.    Communication channels

 

Sender and Receiver Hardware:

Following hardware are used for handling communication messages, data transfer etc.

Nodes and workstations:

Data communication is done using various communication devices and softwares interconnected for information exchange.  The devices used to communicate a data in communication network are called Workstations.  These workstations may be computer, terminal, printer, telephones and other communication devices.  Each workstation is connected to something called a Data Communication Network Mode.

Multiplexer:

As the name suggests, multiplexing is a form of data transferring which are communication channels and is used for several transmission.  For e.g. the telephone lines that we used for our daily conversation can carry 100’s and even 1000’s of conversations using multiplexing.  In other words multiplexing is a type of network which allows a number of simple, low cost terminals to share each communication lines introduces almost no delay and requires no special computer softwares.

Multiplexing is used in two major ways:

FDM (Frequency Division Multiplexing)

TDM (Time Division Multiplexing)

FDM: By dividing a communication channel into various smaller segments of different frequencies.

TDM: By taking groups of bytes from each sender and send or transmit them over the channel one after another.  Each group of data bytes are tagged at the beginning and end with start and stop bytes.  These by control bytes are then separated at the receiving end and send to their respective places.  This type of multiplexing occurs so fast that the transmission seems continuous.

Communication devices:

There are several types of communication devices or interface used in data communication.  These interfaces are connection between receiver and sender hardware involved in data communication and the communication network.  Some of these interface or communication devices are discussed below:

1.     Modems: Modulator and Demodulators are devices which converts digital signals in to analog for transmission over the analog transmission facilities such as telephones.  At the receiving end, a modem performs the reverse function and converts analog signals into digital form.

2.     Codec performs the opposite function of a modem.  A communication port / adapter is a connector on the computer, which is used as an entry to departure point for data involved in data communication.  A common type of communication port is RS-232c.  This adapter is used for transfer of data between the computer and the telephone lines.

3.     Line Drivers are simple devices used to transmit digital signals over short distances.  These devices do not modulate or demodulate signals, but use filters to reduce the high frequency components and the modified signal is transmitted directly over the media.  These devices are used for the distances up to 1 KM and can achieve data transfer rates of up to 19200 BPS.  It is mainly used for connecting VDU terminals with a computer at a distance of more than 100 feet or so.

Communication channels:

The most basic hardware required for communication is the media through which data is transferred.  There are several types of media, and the choice of the right media depends on many factors such as cost of transmission media, efficiency of data transmission and the transfer rate.

Some of the following transmission Medias is as follows:

1.    Two wire open line:  This is the simplest of all the transmission media.  It consists of a simple pair of metallic wires made of copper or some times aluminums of between 0.4 and 1mm diameter, and each wire is insulated from the other.   There are variations to this simplest form with several pairs of wire enclosed in a single protected cable called a multi core cable or molded in the form of a flat ribbon.

This type of media is used for communication within a short distance, up to about 50 M, and can achieve a transfer rate of up to 19200 bits per second.

2.    Twisted Pair cable: A twisted pair consists of a pair of insulated conductors that are twisted together.  The advantages of a Twisted Pair cable over the Two Wire Open Lines are; it provides better immunity from spurious noise signals.  As the Two Wires are closed to each other, both pick equal interferences caused by extraneous signal sources and this reduces the differential signal added by the noise.

Twisted Pair cable is used for communication up a distance of 1 KM and can achieve a transfer rate of up to 1-2 MBPS.  But as the speed increased the maximum transmission distances reduced, and may require repeaters.

 

Twisted pair cables are widely used in telephone network and are increasingly being used for data transmission.

3.    Co-axial Cable: A co-axial cable consists of a solid conductor running co-axial inside a solid or braided our annular conductor.  The space between the two conductors is filled with a dielectric insulating material.  Larger the cable diameter, lower is the transmission loss, and higher transfer speeds can be achieved.  A co-axial cable can be used over a distance of about 1 KM and can achieve a transfer rate of up to 100 MBPS.

 

A co-axial cable is of two types- a 75 Ohm cable which is used by the cable TV operators and 50 Ohm cable which is used in high speed broad band networks and is low loss cables.

4.    Fiber Optic Cables:

 

A fiber optic cable carries signals in the form of fluctuating light in a glass or plastic cable. An optical fiber cable consists of a glass or plastic core surrounded by a cladding of a similar material but with a lower refractive index. The core transmits the light while the change in refractive index. The core transmit the light while the change in refractive index  between the core and the cladding causes total internal reflection, thus minimizing the loss of light  from fiber.

As light waves gave a much wider wand width then the electrical then the electrical signal and are immune from electromagnetic interferences, this leads to high data transfer rate of about 1000 mega bites per second & can be used for long & medium distance transmission links.

5.    Radio, Microwaves & Satellite Channels 

Radio, Microwaves & Satellite Channels use electromagnetic propagation in open space. The advantage of these channels lie in their capability to cove large geographical areas & being inexpensive than the wired installation.

The demarcation between radio, Microwave & satellite channels lie in the frequencies in which they operate. Frequency bellow 1000 MHZ are radio frequencies & higher the Microwave frequencies.

The radio frequency transmission may be bellowing 30 MHZ above 30 MHZ & thus the techniques of transmission are different. Owing to the characteristics of the ionosphere, frequencies bellow 30MHZ are reflected back towards the surface of the earth. Above 30Mhz propagation is on line of sight paths. Antennas are placed in between the line-of- sight paths to increase the distance. Radio frequencies are prone to attenuation and, thus, they require repeats along the path to enhance the signal. Radio frequencies can achieve data transfer rate of 100 Kbps to 400 Kbps.

Microwave links use line- of- sight transmission with repeaters placed every 100-200 Kms. Microwave links can achieve data transfer rates of about 1000 Mbps.

Satellite links use microwave frequencies is the order of 4-12 GHz with the satellite as a repeater. They can achieve data transfer rates of about 1000 Mbps.

Network Concept and Classifications:

Communication using computer has brought a revolution in the world of computer technology, particular in the field of computers.  We have always heard of networking or the term network, a network is a way or means of transmitting or receiving information one or more sources.  As an e.g. car salesman after years in the business, have developed a network of associates. When the car salesman needs a car to make a sale the car salesman calls out to his network to retrieve information on the location of the car. Employment agents also develop a network. Their customers become their network. Employment agents will frequently keep in touch with their clientele for possible openings or to locate a candidate for an opening. Without the capability of networking, these two people would have a difficult time. It is the same in computing. Networks provide the means for locating transporting information.

  

In computing networks, the origin of the information request utilized the service of a network to locate & return the information. This is done with addresses. In the two previous examples of the car sales man & the employment agent, a telephone number can be considered the address of their associate or client. Addresses in computer networking are used in the same manner. These addresses identify the network resource. There are two popular architectures for networking – hierarchical & peer.

Hierarchical addressing is defined in a master slave relationship. In hierarchical network, the master controls the network & therefore assigns addresses to the network resource. This architecture has the maniframe as the master & all network resources as slave. The bases of this is that if the master does not know before hand of a network resource existence through a pre- defined address then that resource can not participate in the network.

Peer networking does not need pre- defined networking addressing. Instead, each resource on the network is seen as a peer. Each network resource is a peer to the other network resource. When a new network resource joins the network it introduced itself & notifies its peer to any other network resources that it knows about peer networks are share network information.

The entire computer network can be classified into two board categories. They are (a) LAN (Local Area Network) (b) WAN (Wide Area Network)   

LAN (Local Area Network)

As numbers of system grow within an organization, a need is felt for sharing expensive resource and exchanging data and information between systems. This need of information exchange and resource and sharing with in a organization has given birth to a local area network or LAN.

A LAN is a data communication network, which connects many computers or workstation ( Computer’s Terminals, Printer etc.) and permits exchange of data & information among themselves, with in a localized area, typically confined to a building, or a cluster of buildings. The distance between two communications prints connected on the same LAN channels is usually up to 02 to 05 kms.

LANs are not rigidly defined but tend to share most of all of the following characteristics.

§  The transmission media is shared by all the connected devices in the network.

§  Each device is connected in the network can either operate stand alone or in the network.

§  Area covered is small.

§  Data transfer rates are high, usually 1 Mbps- 100 Mbps (Million of bits per second).

§  Each device connected in the network can communicate with any other device in network.

§  Cost of setting up the network is usually low.

LAN Topology

A network topology refers to the physical lay out of the network in which all the devices are connected. This includes all the hardware that makes up the network. The points of the connection to the network by the station are calls Nodes or Link stations. There are several types of topographical design & strategies are used to implement LAN. The majority of these are based on three types of topologies.

           

a) Star            b) Bus            c) Ring

Each topology has its advantages & disadvantages.

Star Topology

Star topology is shown bellow. In this topology. A number of stations are connected directly to a central station or controller. Communication on the connecting links between the stations & the central station of the star topology can be bi- directional and are point to point. A station on this type of network passes an information frame to the central controller, which then forwards the information to the destination station. The central controller manage and controls all communication between stations on the network.

 

                                                                                   

Star Network

Failure of a station on a star network is easy to detect and can be remove from the network. However, failure of the central controller will disable communication through out the whole network.

Bus Topology

 A bus topology is shown bellow all stations are connected to a single communication line. This single communication line is referred to a bus. Information frames originating at a station are propagated away from the station in both directions on the bus. Each station on the bus interrogates the information frame destination address failed for its on addresses. If the destination failed does not mach the station address, the station discards the information frame back on to the bus. If the destination address matches the station addresses, it accepts the information frame & processes the frame.

An extension to the bus topology is tree topology is in the following figure. Tree topology extends the branches of the bus topology allowing more stations to access the bus.

 

Bus Topology

On a bus or tree network there is no central point for management & control.  These function ions are distributed to each station on the bus. A brake in the bus can be difficult to locate but limits the outage to communications between stations that traverse the broken point.

Ring Topology

A ring topology is shown bellow. Local area network that have each station attached to an adjacent station using point to point links from a physical ring. Each station attached and active to the ring regenerate the information frame, then retransmits the information frame on the ring. The ring itself is logically circle and the information travels in one direction.

		Ring Topology

 

Failure of a station in a ring topology disrupts the ring because the information frame is not regenerated. Additions or deletions of stations of the ring can be disruptive, if the changes are not managed properly.

LAN Hardware and Software

As we have seen so far, to realize a LAN process, several functions are to be performed. These are so specialized in nature the require hardware specially built for such purpose. Here we will discuss briefly the basic hardware components of LAN, these are:

(A)  Transfermission channel

(B)  Network Interface Unit (NIU)

(C)  Servers

(D)  Work Station

(A) Transfermission Channels

Generally following four types of channels are used for data communication in a LAN. They are:

(i) Twisted Pair Cable

(ii) Coaxial Cable

(iii) Fiber Optic Cable

(iv) Radio waves

(B)  Network Interface Units (NIU)

Network interface units connect each device in the LAN network to shared transmission device. It contains the rules or logic to access the LAN. NIU is also used for to implement LAN protocols and for device attachments. Its functions depend on the type of topology used in LAN.

( C) Servers

One of the major benefits of implementation of LAN is sharing expensive resources such as storage device, printer etc. this is achieved through providing servers on the LAN. It is a dedicated computer that controls on or more resources. This contains both hardware & software for LAN. Three major categories of servers used in LANs are

(i)            File Server

(ii)          Printer Server

(iii)         Modem Server

In a networking file server is used to share storage space for files. Beside providing storage space for files in a LAN environment, it is used for talking periodical backup, and also to provide gateway to other servers with in & between LANs.

Similarly printer server is use to handle printing works of all workstations connected in the network.

In the LAN environment also modem is required to get connected to other network or simply to use a telephone. A modem server is used to share these expensive resources by all connected workstation in a networking ring.

LAN Software/ Operating System

As the name suggest LAN operating system is required to operate on the LAN system, manage the tremendous work load with a number of various types of server attached to it. It has basically two aspects (i) Server Software (ii) Work station Software. As case of other multi user operating system, LAN operating system also facilitate the sharing of expensive resources such as printer, storage space etc. among all LAN users, provides security of data permits connections to other network.

There are various types of LAN operating systems for example Novel Netware, LAN server, omni met, PC Net, IBM PC LAN, Etherlik plus etc.

WIDE Area Network

As the name suggest, WAN spread across countries and continents satellites being one of the transmission media.

A wide area network WAN is a network that links separate geographical location. A WAN can be a public system such as the Public Switched Telephone Network (PSTN) or one of the various packet switched services provided by the public telecommunication authorities. WANs can also use most other types of other types of circuit including satellite networks, ISDN, Value Added Networks (VANs/VADs).

The network can be a private system made up from the local telephone company or set up using public systems as virtual private network. A virtual private network is one which operates in the same way as a private network but which uses public switched services for the transmission of information.

The main distinguishing feature between a LAN and WAN is that, the LAN is under the complete control of the owner, whereas the WAN needs the involvement of another authority like the telephone company. LANs are also able to handle very high data transfer rates at low cost because of the limited area covered. LANs have a lower error rate then WANs.

Communication Switching Techniques

In a WAN, two computing devices are not connected directly. A network of switching nodes provides a transfer path between the two devices. The process of transferring data blocks from one node to another is called data switching.

There are three switching techniques commonly employed and these are:

1.   Circuit Switching

In circuit switching there is a dedicated communication path between the sending and receiving devices. The dedicated path is a connected sequence of links switching nodes. A conventional telephone network, where a dedicated path is set between the called party for the duration of a telephone call is an example of circuit switching.

Communication viz. circuit switching involves three steps-circuit establishment; data transfer; and circuit termination. Circuit switching is mainly used for voice telephone network, but is not that effective for data communication network, as channel capacities are not fully utilized, as data communication equipments do not generate data continuously.

2.        Massage Switching

Massage switching is an alternative switching techni8que, where it is not necessary to establish a dedicated path between the sending and receiving devices. In massage switching, the sending device appends the destination address to the massage and passes to the network; the massage is then passed through the network from one node to another till it reaches the intended destination. Each switching are electronic mails, computer files, telegrams and transaction queries and responses. A complete exchange may consist of several messages.

The basic disadvantage of massage switching is the variable delay at intermediate switching nodes.

           

3.        Packet Switching

Packet switching combines the advantages of message & circuiting switching. Packet switching is functionally similar to message switching, in which data is transmitted in blocks, stored by the first switching node it meets in the network and it forwarded to the next and subsequent downstream nodes until it reaches the destination. The length of data block is limited in packet switching network. Typical maximum length of packets is between 128 bytes to 4096 bytes. There are two approaches to packet switching:

§  Datagram

§  Virtual circuit

In datagram approach, each packet is treated independently and may follow a different path through the network. Packets may be reordered, dropped or delivered in wrong sequence. The communication protocols provide the error recovery sequencing of packets at the receiving device.

In virtual circuit approach, a fixed logical path through the network from the sender to the receiver is established before any packets are sent. This path remains unchanged for the duration of the session. This is quite like circuit switching, but no resources are reserved along the path. Packets are buffered at intermediate nodes awaiting transmission.

WAN Devices / Hardware

The switching techniques utilized the routine technology of data transfer. Routing is responsible for searching a path between two computing devices that wish to communicate and for following the data packets on this path. Devices such as bridges, router and gateway provide this routing function.

1.    Bridges

Bridges are used to connect two LANs that use identical LAN protocols over a wide area. The bridge acts as an address filter which picks up packets from one LAN that are identical for a destination on the another LAN and passes these packets on the network. Bridges operate at the data link layer (layer 2) of the OSI model. As all devices use the same protocols, the amount of processing required at the bridge is minimal. If the distance between the two LANs is large, the user would require two additional bridges at either end of the communication link.

Besides a point-to-point link, the intervening communication facility can be a network such as a wide area packet switching network in such case the bridges need to add X. 25 link layer header and trailer.

2.   Routers

Routers can be used to connect networks that may not be similar. Routers provide connectivity between two LANs or two WANs over large geographical distance. Routers operate at the network layer (layer 3) of the OSI model. All routers participate in routing protocols to access the network topology, and based on this information routers computes the best route from a sender to the receiver,

For large wide area network spanning thousands of kilometers, the normal practice is to put network routers at suitable locations to minimize link cost for leased link and provide adequate reliability from link failures. Networks and other system are then connected to the nearest router.

3.   Gateways

Gateways are used to connect to dissimilar LANs. The term gateway and routers are used interchangeably, though there is a subtle difference between the two. A router operates at the network layer (layer 3) of the OSI model, whereas a gateway operates on the application layer (layer 7) of the OSI model. A gateway is required to convert data packets from one protocol format to another before forwarding it, as it connects two dissimilar networks.

While discussing the WAN devices we referred to X.25 and protocols. We will now see what x.25 is.

§  What is X.25

Towards the end of the 1960s, the conman telecommunication carriers around the world recognized the need for data communication service that would be distinct from voice circuits. The plan was to put together a series of recommendation during the 1970s using ITTCC (International telegraph & telephone consultative committee) as the total standards body. This ITTCC recommendation series to define the new service was doubled the X series. During the 1970s sum 60 to 70 recommendations had involved. The most famous of these is X.25, which define an interface in to a particular regular data network that of a packet switched network.

ITTCC developed X.25 as the standard interface between the Data Terminal Equipment (DTE, computer system) and Data Circuit Terminating Equipment (DCET- the network node to which the DTE is connected) the recommendation has been widely accepted as the industry standard for public packet switched networks.

X.25 is a protocol for interfacing to a Public Packet Switched Network. It is not a protocol for implementing a network. Two systems that support X.25 cannot necessary be connected back – to – back. They can only be connected through a DCE in a public packet switched.

Type of WIDE Area Network

The essential purpose of WIDE Area network, regardless of the size or technology used is to link separate locations in order remove data around. A WAN allows these locations to access shared computer resources and provides the asocial infrastructure for developing wide spread distributed computing system.

We will now discuss the different types of WAN which are communally used.

1.    Public Networks

Public networks are those networks which are installed and run by the telecommunication authorities and are made available to any organization or individual who subscribe. Examples include Public Switched Telephone Networks (PSTN), Public Switched Data Network (PSDN), Value Added Service (VANs/ VADs) and the Integrated Service Digital Networks (ISDN). We would be discussing the main features of there services:

§  Public Switched Telephone Network (PSTN)

The features of the PSTN are its low speed, the analog nature of transmission, restricted bandwidth & its wide spread availability. As PSTN is designate for telephones, modems are required when it is used for data communication.

§  Public Switched Data Network (PSDN)

The term PSDN covers a number of technologies; all through currently it is limited to Public Packet Switch Networks available to the public. The main features of all PSDNs are their high label reliability and the high quality of the connection provided. The can support both high & low speeds at appropriate costs.

§  Value Added Services (VANs/ VADs)

In value added services, the provider of such services must process, store and manipulate the data that is carried on the network, that add value to it. The technique can be used in specific types of business in which it is advantageous to be able to share information with other companies in the same line.

§  Integrated Services Digital Network (ISDN)

                  The ISDN is the networking concept providing for the integration of voice, video and data servicing using data service using digital transmission combining both circuits and packet switching techniques. The motivating force behind ISDN is that telephone networks around the world have been making a transition towards utilizing digital transmission facilities for many years.

2.    Private Networks

The basic technique used in all forms of private WAN is to use private (or more usually leased) circuits to link the locations to be served by the network. Between these fixed points the owner of the network has complete freedom to use the circuits in any way they want. They can use the circuit to carry large quantities of data or for high speed transmission.

Private WIDE area network can be built using what ever standard technology is available. The way private networks have generally been set up has to specify