Data communications are the exchange of data between two devices via some form of transmission medium such as a wire cable, wireless or optical fiber etc. For data communications to occur, the communication devices must be part of a communication system to made up of a combination of hardware (physical equipment) and software (programs).
Characteristics of data communication
- Delivery: Must deliver data to the correct destination.
- Accuracy: Must deliver the data accurately.
- Timeliness: Must deliver data in a timely manner. (For video and audio, timely deliver means delivering data as they are produced, in the same order as they produced and without significant delay. This kind of delivery is called real-time transmission.
- Jitter: Jitter refers to the variation in the packet arrival time. (Example: video packets are sent every 30-ms if some of the packets arrive with 30-ms delay and others with 40-ms delay, an uneven quality in the video is the result.)
Components
There are five components of data communications.
- Message: text, numbers, pictures, audio, and video.
- Sender: person, computer, telephone, video camera etc.
- Receiver: same as sender and television. etc.
- Transmission medium: cable (twisted-pair, coaxial), radio waves, optical fiber.
- Protocol: Protocol is a set of rules that govern the data communications. Actually, it represents an agreement between the communicating devices (sender and receiver).
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| Fig: 5 components of data communication |
Data Representation
Text
Texts are represented as a bit pattern, a sequence of bits (0s or 1s). Different sets of bit patterns have been designed to represent text symbols. Each set is called code and the process of representing symbols is called coding. Today, the prevalent coding system is called Unicode, which uses 32-bits to represent a symbol or character used in any language in the world.
Number
Numbers are also represented by bit patterns. The number directly converted to a binary number to simplify mathematical operations.
Image
Images are also represented by bit patterns. First, an image divided into pixels then each pixel is assigned a bit pattern.
Audio
Audio is by nature different from text, numbers, or images. It is continuous, not discrete.
Video
Video can either be produced as a continuous entity (TV camera), or it can be a combination of images, each a discrete entity, arranged to convey the idea of motion.
Data Flow
Communication between two devices can be simplex, half-duplex, or full-duplex.
Simplex Mode
In simplex mode, the communication is unidirectional (one-way street). Only one of the two devices on a link can transmit and the other can only receive.
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| Fig: Simplex |
Half-Duplex Mode
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.
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| Fig: Half-duplex |
Full-Duplex
In full-duplex mode, both stations can transmit and receive simultaneously.
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| Fig: Full-duplex |
