Data Communication
Data communication refers to the exchange of data between a source and a receiver. Data communication is said to be local if communicating devices are in the same building or a similarly restricted geographical area.
The meanings of source and receiver are very simple. The device that transmits the data is known as source and the device that receives the transmitted data is known as receiver. Data communication aims at the transfer of data and maintenance of the data during the process but not the actual generation of the information at the source and receiver.
Datum means the facts information statistics or the like derived by calculation or experimentation. The facts and information so gathered are processed in accordance with defined systems of procedure. Data can exist in a variety of forms such as numbers, text, bits and bytes. The Figure is an is an illustration of a simple data communication system.
A data communication system may collect data from remote locations through data transmission circuits, and then outputs processed results to remote locations. Figure provides a broader view of data communication networks. The different data communication techniques which are presently in widespread use evolved gradually either to improve the data communication techniques already existing or to replace the same with better options and features. Then, there are data communication jargons to contend with such as baud rate, modems, routers, LAN, WAN, TCP/IP, ISDN, during the selection of communication systems. Hence, it becomes necessary to review and understand these terms and gradual development of data communication methods.
Necessary to review and understand these terms and gradual development of data communication methods.
Components of data communication system
A Communication system has following components:
- 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.
- 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.
- 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.
- 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, and microwave or satellite system.
- 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.
- Encoder: The encoder is an electronic device. It receives data from sender in the form of digital signals. It converts digital signals into a form that can be transmitted through transmission medium.
- Decoder: The decoder is an electronic device. It receives data from transmission medium. It converts encoded signals (i.e. analog signals) into digital form.
Data Transmission
Data may be transfer from one device to another by means of some communication media. The electromagnetic or light waves that transfer data from one device to another device in encoded form are called signals. Data transmissions across the network can occur in two forms i.e.: Analog signal and Digital signal.
- Analog Signal. The transfer of data in the form of electrical signals or continuous waves is called analog signal or analog data transmission. An analog signal is measured in volts and its frequency is in hertz (Hz).
- Advantages of Analog Signalling
- Allows multiple transmissions across the cable.
- Suffers less from attenuation.
- Disadvantages of Analog Signalling
- Suffers from EMI.
- Can only be transmitted in one direction without sophisticated equipment.
- Digital Signal - The transfer of data in the form of digit is called digital signal or digital data transmission. Digital signals consist of binary digits 0 & 1. Electrical pulses are used to represent binary digits. Data transmission between computers is in the form of digital signals.
- Advantages of Digital Signalling
- Equipment is cheaper and simpler than analog equipment.
- Signals can be transmitted on a cable bidirectional.
- Digital signals suffer less from EMI.
- Disadvantages Digital Signalling
- Only one signal can be sent at a time.
- Digital signals suffer from attenuation.
Techniques of Data Communication
There are two possible techniques of sending data from the sender to receiver, i.e.: - Parallel transmission and Serial transmission.
- Parallel Transmission. In parallel transmission each bit of character / data has a separate channel and all bits of a character are transmitted simultaneously. Here the transmission is parallel character by character.
- Serial Transmission. In serial transmission, the data is sent as one bit at a time having a signal channel for all the bit, i.e.: Sender Receiver
- Types of Serial Transmission - On serial transmission it is very essential to know exactly where one-character ends and the next begins. The necessary synchronization that helps determine which bit is the first bit of the incoming character may be synchronous or asynchronous.
- Asynchronous Serial Transmission - Computer communication that occurs one bit at a time with start and stop bits at the beginning and the end of each character is called Asynchronous Serial Transmission. In this type of transmission, there is no fixed time relationship with one character.
- Advantages of Asynchronous Serial Transmission
- This type of transmission is very simple.
- This type of transmission is cheaper.
- Disadvantages of Asynchronous Serial Transmission
- This type of transmission is slow.
- Synchronous Transmission - In this method a clock signal is used and the sending as well as the receiving devices is synchronized with this clock signals. It doesn’t use start and stop bits but the character is sent in character groups called block
- Advantages of Synchronous Transmission - It is very fast as compared to Asynchronous Series Transmission.
- Disadvantage of Synchronous Transmission - It uses more expensive and complex equipment.
Modes of Data Communication
The manner in which data is transmitted from one location to another location is called data transmission mode. There are three ways or modes for transmitting data from one location to another.
These are: Simplex; Half duplex and Full duplex.
- Half Duplex - In half duplex mode, data can be transmitted in both directions but only in one direction at a time. During any transmission, one is the transmitter and the other is receiver. So each time for sending or receiving data, direction of data communication is reversed, this slow down data transmission rate. In half duplex modes, transmission of data can be confirmed.
- Half Duplex Mode - Wireless communication is an example of half duplex.
- Advantages of Half Duplex
- Costs less than full duplex.
- Enables for two-way communications.
- Disadvantages of Half Duplex
- Costs more than simplex.
- Only one device can transmit at a time.
- Simplex Mode - In simplex mode, data is transmitted in only one direction. A terminal can only send data and cannot receive it or it can only receive data but cannot send it. Simplex mode is usually used for a remote device that is meant only to receive data. It is not possible to confirm successful transmission of data in simplex mode. This mode is not widely used. Speaker, radio and television broadcasting are examples of simplex transmission, on which the signal is send from the transmission to your TV antenna. There is no return signal.
- Advantages of Simplex - Cheapest communication method.
- Disadvantage of Simplex - Only allows for communication in one direction.
- Full Duplex - In full mode, data can be transmitted in both directions simultaneously. It is a faster mode for transmitting data because no time wastes in switching directions. Full Duplex Mode - Example of full duplex is telephone set in which both the users can talk and listen at the same time.
- Advantage of Full Duplex - Enables two-way communication simultaneously.
- Disadvantage of Full Duplex - The most expensive method in terms of equipment because of two bandwidth channels is required.
A protocol performs the following functions:
- Data sequencing. It refers to breaking a long message into smaller packets of fixed size. Data sequencing rules define the method of numbering packets to detect loss or duplication of packets, and to correctly identify packets, which belong to same message.
- Data routing. Data routing defines the most efficient path between the source and destination.
- Data formatting. Data formatting rules define which group of bits or characters within packet constitute data, control, addressing, or other information.
- Flow control. A communication protocol also prevents a fast sender from overwhelming a slow receiver. It ensures resource sharing and protection against traffic congestion by regulating the flow of data on communication lines.
- Error control. These rules are designed to detect errors in messages and to ensure transmission of correct messages. The most common method is to retransmit erroneous message block. In such a case, a block having error is discarded by the receiver and is retransmitted by the sender.
- Precedence and order of transmission. These rules ensure that all the nodes get a chance to use the communication lines and other resources of the network based on the priorities assigned to them.
- Connection establishment and termination. These rules define how connections are established, maintained and terminated when two nodes of a network want to communicate with each other.
- Data security. Providing data security and privacy is also built into most communication software packages. It prevents access of data by unauthorized users.
- Log information. Several communication software is designed to develop log information, which consists of all jobs and data communications tasks that have taken place. Such information may be used for charging the users of the network based on their usage of the network resources.
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