1. Message. The message is the information (data) to be communicated. Popular forms of
information include text, numbers, pictures, audio, and video.
2. Sender. The sender is the device that sends the data message. It can be a computer,
workstation, telephone handset, video camera, and so on.
3. Receiver. The receiver is the device that receives the message. It can be a computer,
workstation, telephone handset, television, and so on.
4. Transmission medium. The transmission medium is the physical path by which a message
travels from sender to receiver. Some examples of transmission media include twisted-pair wire,
coaxial cable, fiber-optic cable, and radio waves
5. Protocol. A protocol is a set of rules that govern data communications. It represents an
agreement between the communicating devices.
DATA TRANSMISSION MODES
Communication between two devices can be simplex, half-duplex, or full-duplex.
In simplex mode, the communication is unidirectional, as on a one-way street. Only one of the
two devices on a link can transmit; the other can only receive. Keyboards and traditional
monitors are examples of simplex devices. The keyboard can only introduce input; the monitor
can only accept output. The simplex mode can use the entire capacity of the channel to send data
in one direction.
In half-duplex mode, each station can both transmit and receive, but not at the same time. When
one device is sending, the other can only receive, and vice versa. In a half-duplex transmission,
the entire capacity of a channel is taken over by whichever of the two devices is transmitting at
the time. Walkie-talkies and CB (citizens band) radios are both half-duplex systems. The halfduplex mode is used in cases where there is no need for communication in both directions at the
same time; the entire capacity of the channel can be utilized for each direction.
Data Communication and Computer Networks