1- Message
The message is the information or data that is to be communicated. It may consist of text, numbers, pictures, sounds, videos or any combination of these.
2- Sender
A device that is used for sending messages (or data) is called sender. It is also called transmitter or source. The sender can be a computer, telephone, or a video camera etc. Usually, a computer is used as sender in data communication system.
3- Receiver
A device that is used for receiving messages is called receiver. It is also known as sink. The receiver can be a computer, telephone set, printer, or a fax machine etc. Usually, a computer is also used as receiver in data communication system.
4- Medium
The path through which data is transmitted (or sent) from one location to another is called transmission medium. It is also called communication channel. It may be a wire, or fiber optic cable, or telephone line etc. If the sender and receiver are within a building, a wire is used as the medium. If they are located at different locations, the medium may be telephone line, fiber optics, microwave or satellite system.
5- Encoder and Decoder
In communication systems, computers are used for senders and receivers. A computer works with digital signals. The communication channels usually use analog signals. The encoder and decoder are used in communication systems to convert signals from one from to another.