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Different types of switching techniques are employed to provide communication between two computers. These are : circuit switching, message switching and packet switching.
Circuit Switching
In this technique, first the complete physical connection between two computers is established and then data are transmitted from the source computer to the destination computer. That is, when a computer places a telephone call, the switching equipment within the telephone system seeks out a physical copper path all the way from sender telephone to the receiver’s telephone. The important property of this switching technique is to setup an end-to-end path (connection) between computer before any data can be sent.
Message Switching
In this technique, the source computer sends data or the message to the switching office first, which stores the data in its buffer. It then looks for a free link to another switching office and then sends the data to this office. This process is continued until the data are delivered to the destination computers. Owing to its working principle, it is also known as store and forward. That is, store first (in switching office), forward later, one jump at a time.
Packet Switching
With message switching, there is no limit on block size, in contrast, packet switching places a tight upper limit on block size. A fixed size of packet which can be transmitted across the network is specified. Another point of its difference from message switching is that data packets are stored on the disk in message switching whereas in packet switching, all the packets of fixed size are stored in main memory. This improves the performance as the access time (time taken to access a data packet) is reduced, thus, the throughput (measure of performance) of the network is improved.

Optical fibers consist of thin strands of glass or glass like material which are so constructed that they carry light from a source at one end of the fiber to a detector at the other end. The light sources used are either light emitting diodes (LEDs) or laser diodes (LDs). The data to be transmitted is modulated onto the light beam using frequency modulation techniques. The signals can then be picked up at the receiving end and demodulated. The band width of the medium is potentially very high. For LEDs, this ranges between 20 and 150 mbps and higher rates are possible using LDs.
The major problems with optical fibers are associated with installation. They are quite fragile and may need special care to make them sufficiently robust for an office environment. Connecting either two fibers together or a light source to a fiber is a difficult process.
One of the major advantages of optical fibers over other media is their complete immunity to noise, because the information is travelling on a modulated light beam.
A side effect of this noise immunity is that optical fibers are virtually impossible to tap. In order to incept the signal, the fiber must be cut and a detector inserted.
Despite its shortcomings, optical fiber is an important technology and will be a very attractive transmission indeed.

Computer networks have made a major impact on the society as a whole but we will discuss only of the important ones.
1. Sharing. The potential advantage of network is to provide an easy and flexible means of
sharing. There are three distinct types of sharing.
(a) Peripherals. These are often expensive. It is impractical for each computer on the network to have both its laser printer (for quality printing) and dotmatrix printer (for all general printouts). A main frame may have one of each connected to it allowing all users con¬trolled access in a cost effective manner.
(b) Users of a multiuser system can share and exchange information in a number of ways. Examples are sending electronic mail or having controlled access to the same files or data base.
(c) In a traditional time sharing system, all control is performed centrally ; if the processor fails then the entire system fills. In a network system, this need not be the case. The failure of one node should not have a ‘domain’ effect on the rest. This is called distributed control and is a very lively area of research at present.
2. Access to remote database. Another major area of network use is access to remote database.
It is easy for the average person sitting at his PC to make reservation for airplanes, trains, hotels and so
on anywhere in the world with instant confirmation.
3. Communication facilities. A third category of potential widespread network use is as a communication medium. It is possible for everyone, not just people in the computer business, to send and receive electronic mail. This mail is also able to contain digitized voice, still pictures and even moving television and video images.
Using computer network as a sophisticated communication system may reduce the amount of travelling done, thus saving energy. The information revolution is expected to change society as much as the Industrial Revolution did.

The advent of the microprocessor chip marked the beginning of the fourth generation computers. Medium scale integrated (MSI) circuits yielded to Large and Very Large Scale Integrated (VLSI) circuits packing about 50000 transistors in a chip. (Today even VVLSI circuits are being developed.) Semiconductor memories replaced magnetic core memories. The emergence of the microprocessor (CPU on a single chip) led to the emergence of extremely powerful personal computers. Computer costs came down so rapidly that these found places in most offices and homes. The faster accessing and processing speeds and increased memory capacity helped in development of much more powerful operating systems.
The second decade (1986-present) of the fourth generation observed a great increase in the speed of microprocessors and the size of main memory. The speed of microprocessors and the size of main memory and hard disk went up by a factor of 4 every 3 years. Many of the mainframe CPU features became part of the microprocessor architecture in 90s. In 1995 the most popular CPUs were Pentium, Power PC etc. Also RISC (Reduced Instruction Set Computers) microprocessors are preferred in powerful servers for numeric computing and file services.

The third generation computers replaced transistors with ‘Integrated Circuits’ known popularly as chips. The ‘Integrated Circuit’ or I.C. was inverted by Jack Kilby at Texas Instruments in 1958.
An I.C. is wafer thin slice of extremely purified silicon crystals. A single I.C. has many transis-j ; tors, resistors and capacitors along with the associated circuitry encapsulated in a small pack-  age with many leads.
From small scale integrated (SSI) circuits which had about 10 transistors per chip, technology developed to medium scale integrated, (MSI) circuits with 100 transistors per chip. The size of main memories reached about 4 megabytes. Magnetic disk technology also improved and it became feasible to have drive having capacity upto 100 MBs. The CPUs because much more powerful with the capacity of carrying out 1 million instructions per second (MIPS).
The third generation computers using integrated circuits proved to be highly reliable, relatively inexpensive, and faster. Less human labour was required at assembly stage. Examples of some mainframe computers developed during this generation are : IBM-360 series, ICL-1900 series, IBM-370/168, ICL-2900, Honeywell Model 316, Honeywell-6000 series. Some mini computers developed during this phase are : ICL-2903 manufactured by International Computers Limited, CDC-1700 manufactured by Control Data Corporation and PDP-11/45 (Personal Data Processor -11/45)
Computers these days found place in other areas also like education, survey, small businesses, estimation, analysis etc. along with their previous usage areas i.e. scientific and engineering.