Data communication system 


In a computer network, computing devices are connected in various ways, to communicate and share resources. Data communication is the exchange of digital data between any two devices through a medium of transmission.

Message : It is the information to be communicated. Major forms of information include text, picture, audio, video, etc. 

Sender : The computer or device that is used for sending messages is called the sender, source or transmitter. 

Receiver : The computer or device that receives the messages is called the receiver. 

Medium : It is the physical path through which a message travels from the sender to the receiver. It refers to the way in which nodes are connected.

 Protocol : The rules under which message transmission takes place between the sender and the receiver is called a protocol.

Communication medium 

Data communication is possible only if there is a medium through which data can travel from one device to another. The medium for data transmission over a computer network is called communication channel or communication medium. 

The communication medium between computers in a network are of two types: guided and unguided. In guided or wired medium, physical wires or cables are used and in unguided or wireless medium radio waves, microwaves or infrared signals are used for data transmission

1 Guided medium (Wired) 

The coaxial cable, twisted pair cable (Ethernet cable) and optical fibre cable are the different types of cables used to transfer data through computer networks.

a. Twisted pair cable (Ethernet cable)

b. Coaxial cable

c. Optical fibre cable

 Optical fibres are long thin glass fibres through which data is transmitted as light signals. Data travels as fast as light and can be transmitted to far off distances. 

Parts of Optical fibre

• Core - the thin glass rod at the centre through which the light travels. 

• Cladding - the outer optical material surrounding the core that reflects the light back into the core.

• Coating - the plastic coating that protects the cable from damage and moisture.

These optical fibres are arranged in bundles of hundreds and thousands and are protected by the outer covering of the cable called jacket. At the source end, the optical transmitter converts electrical signals into optical signals (modulation) using semiconductor devices such as light-emitting diodes (LEDs) or laser diodes. At the destination end, the optical receiver, consisting of a photo detector, converts light back to electric signals (demodulation) using the photoelectric effect. The speed of transmission and the distance of signals are higher for laser diodes than for LEDs.

 Characteristics of optical fibre cable 

• High bandwidth for voice, video and data applications 

• Carries data over a very long distance at a stretch. 

• Not susceptible to electromagnetic fields, as it uses light for data transmission. 

• The most expensive and the most efficient communication media available for computer networks. 

• Installation and maintenance are difficult and complex.

2 Unguided medium (Wireless)

a. Radio waves 

Radio waves have a frequency range of 3 KHz to 3 GHz. Radio waves can be used for short and long distance communication. These waves are easy to generate and can go over the walls of a building easily. That is why radio waves are widely used for communication-both indoors and outdoors. Cordless phones, AM and FM radio broadcast and mobile phones make use of radio wave transmission.

b. Micro waves 

Micro waves have a frequency range of 300 MHz (0.3 GHz) to 300 GHz. Microwaves travel in straight lines and cannot penetrate any solid object. Therefore, high towers are built and microwave antennas are fixed on their top for long distance microwave communication. As these waves travel in straight lines the antennas used for transmitting and receiving messages have to be aligned with each other. 

c. Infrared waves 

Infrared waves have a frequency range of 300 GHz to 400 THz. These waves are used for short range communication (approx. 5 m) in a variety of wireless communications, monitoring and control applications

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