1.1 Introduction of telecommunication and data communication  

The sending or receiving messages, information our data is communication. Communications in the same locality are usually face to face but communications in different localities use some type of technologies. Telephone, mobile, e-mail, text messaging, viewing pages on the web, downloading files on the Internet, etc. use telecommunication systems.  

The telecommunication system is the collection of nodes and links to enable telecommunication. 

Telecommunication (also called telecom) is communication (i.e. exchanging of data, information) at a distance using electric signals or electromagnetic waves. The telephone network, mobile network, radio broadcasting system, computer network, etc. Are some of the examples of telecommunication systems. 

The devices such as telephones, mobiles, computers, etc. that we use in the telecommunication system for communication are known as nodes. 

1.2 Data communication 

A computer processes data to produce information. The data and information produced by a computer may need to be transferred to another computer. The transferring of data/information is only possible when computers are interconnected through transmission media in communication technologies. Such interconnected group of computers is known as computer network. 

Data communication is telecommunication in which digital data or information is transferred from one computer to another computer on the computer network. 

The components of data communication system are message, sender, receiver, medium and protocol. 

Message is the data/ information that to communicated. 

Sender (i.e. source) is the computer/ computing device that generates in sends the message. 

Receiver is the computer/ computing device that receives the message. 

Medium is the channel or path through which the message is transferred from the sender to the receiver. The medium may be wired or wireless. 

Protocol is a set of programs that links computing devices logically and transfers the message from a sender to a receiver through the medium. 

1.3 Data transmission modes 

Data transmission mode (i.e. Communication mode) defines the direction of the flow of data between two communication devices connected on the network. 

Data transmission mode is the flow of signals from one communication device to another communication device on the network. 

There are three modes for transmitting data from one device to another. They are: 

1. a)    SIMPLEX. 

1. b)    HALF DUPLEX. 

1. c)    FULL DUPLEX. 

Simplex communication mode is a mode in which data can be received or transmitted in only one direction. transmission of data from radio station to radio is one of the examples of simplex communication mode. 

Half duplex communication mode is a communication mode in which data can be transmitted in both directions but only one at a time. Transmission of data through a walkie-talkie is one of the examples of half duplex communication mode. 

Full duplex communication mode is a communication mode in which data can be transmitted in both directions at a time. Transmission of data through telephone, mobile and computer is full duplex. 

FULL FORMS: 

Telecom: Telecommunication. 

E-mail: Electronic Mail. 

IRC: Internet Relay Chat.