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 is networking. A computer network is a group of computers that are connected together and that communicate with one another for a common purpose.
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 The old model of single computer serving all of the organizations computational needs has been replaced by one in which a large number of separate but interconnected computers. These systems are called computer networks.
Two computers are said to be interconnected if they are able to exchange information. The connection need not be via a copper wire, fiber optics, microwaves, and communication satellites can also be used.
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.
Local Area Networks (LAN):
Local area network, generally called LANs, is privately-owned networks within 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:
- Their size,
- Their transmission technology,
- 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 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. 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) move bits between machines.
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.
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.
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.
Broadly speaking, there are two types of transmission technologies:
- Broadcast networks.
- Point-to-point networks.
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:
- 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.
- 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.
- 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.
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
- Bluetooth protocols
- Routing protocol
- SSH Secure shell
- telnet telephone network
- HTTP hyper text transfer protocol
- TCP Transmission control protocol
- POP Post office protocol
- PPP Point to point protocols
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