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UNIT 6
6.1 Introduction and necessity of computer networking
Computer network is defined as interconnection of two or more than two computers
with some transmission media for the sake of sharing information, resources and communication. A computer
network can be expanded within a room, within a building, in a city or throughout the world. A computer
network consists of transmission media, networking devices and protocol to interconnect and communicate. It
can be a wired or wireless network depending on the components used to connect.
There are various purposes for building computer network which should meet various requirements. One of the
main goals of a computer network is to enable its users to share resources, to provide low cost facilities and easy
addition of new processing services. The computer network thus, creates a global environment for its users and
computers.
Some of the basic goals that a Computer network should satisfy are:
• Cost reduction by sharing hardware and software resources.
• Provide high reliability by having multiple sources of supply.
• Provide an efficient means of transport for large volumes of data among various locations (High throughput).
• Provide inter-process communication among users and processors.
• Reduction in delay driving data transport.
• Increase productivity by making it easier to share data amongst users.
• Repairs, upgrades, expansions, and changes to the network should be performed with minimal impact on the
majority of network users.
• Standards and protocols should be supported to allow many types of equipment from different vendors to
share the network (Interoperatability).
• Provide centralized/distributed management and allocation of network resources like host processors,
transmission facilities etc.
There are numerous applications of computer networks some of them are:
• Share resources and information.
• Access to remote information.
• Person-to-person communication.
• Interactive entertainment.
Share Resources and Information
Using a Computer network we can share expensive resources such as laser printers, CD-ROM Drives, Fax
machines etc. We can share information and many persons can work together on projects and tasks that require
co-ordination and communication, even though these users may not be physically close.
Access to Remote Information
Access to remote information involves interaction between a person and a remote database. Financial
Institutions allow access to their information so that people can pay their bills, handle their investments and
manage their bank accounts electronically. Online shopping also allows people, access to product information
before purchasing the product.
These days, many newspapers and digital libraries are available online and allow users to access news and
information which is of interest to them. Another application is the World Wide Web, which contains
information about a wide variety of subjects like health, sports, science, recreation, history, government etc.
Person-to-person Communication
Person-to-person communication is mostly carried out using e-mail (Electronic mail)
Electronic mail is a simple but more valuable and useful than the telephone because by using e-mail we can
convey information that is difficult or impossible to read over the telephone, like reports, tables, charts, images
etc.
Using a computer network, it is also possible to organise virtual meeting among people who are far away from
each other and this is called video conferencing. Virtual meetings have other applications such as in Distance
education, getting medical opinions from distant specialists (remote diagnosis) etc. Multimedia communication
can also be used for tele-training.
Interactive Entertainment
Computer Networks such as Internet offer a wide variety of entertainment; there are many companies online,
which offer video-on-demand. A large variety of multi-person real-time simulation games, like hide-and seek in a
virtual dungeon and flight simulators with the players in one team trying to shoot down the players in the
opposite team and many such games are available on-line.
6.2 Different types of multiplexing: Simplex, Full Duplex, Half Duplex
In a computer network, there are 3 operating modes of data transfer.
Simplex Operation
In simplex operation, a network cable or communications channel can only send information in one direction;
which is like a “one-way street”. In fact, there are at least two different places where simplex operation is
encountered in modern networking.
Simplex operation is also used in special types of technologies, especially ones that are asymmetric. For
example, one type of satellite Internet access sends data over the satellite only for downloads, while a regular
dial-up modem is used for upload to the service provider. In this case, both the satellite link and the dial-up
connection are operating in a simplex mode.
Half-Duplex Operation
In Half duplex operation, a network cable or communication channels are capable of sending information in
both directions between two nodes but only one direction can be used at a time.
This is a fairly common mode of operation when there is only a single network medium (cable, radio frequency
and so on) between devices. While this term is often used to describe the behavior of a pair of devices, it can
more generally refer to any number of connected devices that take turns transmitting. For example, in
conventional Ethernet networks, any device can transmit, but only one may do so at a time. For this reason,
regular Ethernet networks are often said to be “half-duplex”, even though it may seem strange to describe a
LAN that way.
Full-Duplex Operation
In full-duplex operation, a connection between two devices is capable of sending data in both directions
simultaneously. Full-duplex channels can be constructed either as a pair of simplex links or using one channel
designed to permit bidirectional simultaneous transmissions. A full-duplex link can only connect two devices, so
many such links are required if multiple devices are to be connected together.
For example, router connects between two networks and can sends as well as receive data packets from both
networks at same time.
In simplex transmission, data flow only in one direction -- from the sending device to the receiving device.
Simplex transmission is used only when the sending device does not require a response from the receiving
device.
Security systems and fire alarms that contain a sensor use simplex transmission.
In half-duplex transmission, data can flow in either direction -- from the sending device to the receiving device,
and back -- but only in one direction at a time.
Many fax machines, credit card verification systems, and automatic teller machines use half-duplex
transmission.
In full-duplex transmission, data can flow in both directions at the same time.
A regular telephone line supports full-duplex transmission so that both parties can talk at the same time.