A computer network can be simply defined as the interconnection of two or more independent computers. Applications of computer networks are found everywhere.
Advantages of using computer networks:
a) Resource sharing:
A network is needed because of the desire to share the sharable programs, data and equipment available to anyone on the network without regard to the physical location of the resource and the user.
b) High reliability:
A network may have alternative sources of supply (e.g., replicated files, multiple CPUs, etc.). In case of one resource failure, the others could be used and the system continues to operate at reduced performance. This is a very important property for military, banking, air traffic control and many other applications.
c) Cost-benefit advantage:
A network may consist of many powerful small computers, one per user. You can keep the data and applications on one or more shared and powerful file server machines. This is called the client-server model.
Such model offers a much better price/ performance ratio than old mainframes. At present many server services have been moved to internet based resources set up by a third party and shared by many (called cloud).
d) Scalability:
The ability to increase system performance gradually by adding more processors (incremental upgrade).
e) Powerful communication medium:
Networks make cooperation among far-flung groups of people easy where it previously had been impossible.
Data transmission channels:
The data transmission has to be done over a transmission channel or media. It can be classified as:
1. Guided channels
2. Unguided channels
Guided media:
Guided media provide a physical connection between two devices. A signal travelling through guided media is directed and contained within the physical limits of the medium.
a. Twisted pair cable:
Twisted pair cable is still the most common transmission media. A twisted pair cable consists of two conductors which are normally made of copper.
Twisting is done to reduce crosstalk. Twisted Pairs are very effective for relatively short distances (a few hundred feet),
Twisted pairs are used for long distance connections
e.g. telephone lines which are usually organized as larger cable containing numerous twisted pairs.
b. Optical fiber:
An optical fiber consists of two concentric cylinders. an inner core surrounded by a cladding. Both the core and the cladding are made of transparent plastic or glass material, which transmit signals in the form of light.
Optical fiber use reflections to guide light through a channel.
Optical fibers can provide bandwidth to distance ratios in order of 100 s of MHz per kilometer.
Advantages:
Higher Band width – it can support higher band width and hence can transfer data at a higher rate.
These cables are much lighter than the copper cables
These cables are more immune to tapping than the copper cables.
Less signal attenuation – its transmission distance is greater than the twisted pair and it can run for 50 kms without regeneration.
Disadvantages:
Installation or maintenance – it needs expertise which is not available everywhere.
Unidirectional – Propagation of light is unidirectional and we need two fibers for bidirectional communication.
Costly – the cables and interfaces used are relatively expensive.
Unguided media
Unguided media is used for transmitting the signal without any physical media. It transports electromagnetic waves and is often called wireless communication. Signals are broadcast through air and received by all who have devices to receive them.
a. Radio waves:
Electromagnetic waves ranging in frequencies between 3 Kilo-Hertz and 1 Giga-Hertz are normally called radio waves.
Radio waves are easy to generate and can travel long distances and can penetrate buildings easily, therefore widely used for communication.
These are omni-directional which implies that these travel in all directions from the source, so the transmitter and receiver do not have to be carefully aligned physically.
Radio waves are very useful in multicasting and hence used in AM and FM radios.
Bluetooth is a very popular application of short wave length radio transmission in the frequency band of 2400 to 2480 MHz.
b. Microwaves:
Electromagnetic waves ranging from 1 to 300 GigaHertz are called microwaves. Microwaves are unidirectional that is the sending and receiving antennas need to be aligned.
Microwave is by far the most widely used form of radio transmission.
Telecommunication carriers and TV stations are the primary users of microwave transmission.
An important form of microwave system is a satellite system, which is essentially a microwave system plus a large repeater in the sky.
Unidirectional property of microwave helps in avoiding interference by a pair of aligned antenna to another.
High frequency micro waves cannot be received inside the building.
c. Infrared:
Infrared signals range between 300 Giga- Hertz to 400 Tera-Hertz. These can be used for short range communication.
High range infrared rays cannot be used for long range communication as it cannot penetrate walls.
Infrared signals are generated and received using optical transceivers.
Infrared systems represent a cheap alternative to most other methods, because there is no cabling involved and the necessary equipment is relatively cheap.
It cannot be used outside building as rays of sun contain infrared which leads to interference in communication.
Infrared having wide bandwidth can be used to transmit digital data with a very high data rate.
Infrared signals can be used for communication between keyboards, mouse and printers.
