Data transmission is the movement of information using some standard methods. These methods include electrical signals carried along a conductor, optical signals along an optical fibers and electromagnetic areas.
Basic Elements of a Communication System
The following are the basic requirements for working of a communication system:
- The sender (source) who creates the message to be transmitted
- A medium that carries the message
- The receiver (sink) who receives the message
In data communication four basic terms are frequently used. They are:
- Data: A collection of facts in raw forms that become information after processing.
- Signals: Electric or electromagnetic encoding of data.
- Signaling: Propagation of signals across a communication medium.
- Transmission: Communication of data achieved by the processing of signals.
Data communication software enables to communicate with other systems. The data communication software instructs computer systems and devices as to how exactly data is to be transferred from one place to another. The procedure of data transformation in the form of software is commonly known as protocol. The data transmission software or protocols perform the following functions for the efficient and error free transmission of data.
1. Data sequencing: A long message to be transmitted is broken into smaller packets of fixed size for error free data transmission.
2. Data Routing: It is the process of finding the most efficient route between source and destination before sending the data.
3. Flow control: All machines are not equally efficient in terms of speed. Hence, the flow control regulates the process of sending data between fast sender and slow receiver.
4. Error Control: Error detecting and recovering is the one of the main functions of communication software. It ensures that data are transmitted without any error.
Data Transmission Modes
There are three ways for transmitting data from one point to another.
1. Simplex: In simplex mode, the communication can take place in one direction. The receiver receives the signal from the transmitting device. In this mode the flow of information is Uni-directional. Hence, it is rarely used for data communication.
2. Half-duplex: In half-duplex mode, the communication channel is used in both directions, but only in one direction at a time. Thus a half-duplex line can alternately send and receive data.
3. Full-duplex: In full duplex, the communication channel is used in both directions at the same time. Use of full-duplex line improves the efficiency as the line turnaround time required in half-duplex arrangement is eliminated. Example of this mode of transmission is the telephone line.
Digital and Analog Transmission
Data is transmitted from one point to another point by means of electrical signals that may be in digital and analog form. In analog signal the transmission power varies over a continuous range with respect to sound, light and radio waves. On the other hand, a digital signal may assume only discrete set of values within a given range.
Analog signal is measured in Volts and its frequency is in Hertz (Hz). A digital signal is a sequence of voltage represented in binary form. When digital data are to be sent over an analog form the digital signal must be converted to analog form. So the technique by which a digital signal is converted to analog form is known as modulation. And the reverse process, that is the conversion of analog signal to its digital form, is known as demodulation. The device, which converts digital signal into analog, and the reverse, is known as modem.
Asynchronous and Synchronous Transmission
Data transmission through a medium can be either asynchronous or synchronous. In asynchronous transmission data is transmitted character by character as you go on typing on a keyboard. Hence there is irregular gaps between characters. However, it is cheaper to implement, as you do not have to save the data before sending.
On the other hand, in the synchronous mode, the saved data is transmitted block by block. Each block can contain many characters. Synchronous transmission is well suited for remote communication between a computer and related devices like card reader and printers.
Types of Communication Services
A term used to describe the data-handling capacity of a communication service is bandwidth. Bandwidth is the range of frequencies that is available for the transmission of data. A narrow range of frequencies in a communication system is analogous to a garden hose with a small diameter. The flow of information in such a system its data rate is restricted, just as is the flow of water in the narrow hose. Wider bandwidths permit more rapid information flow. The communication data transfer rate is measured in a unit called baud. Baud is identical to bits per second. Therefore, a rate of 300 baud is 300 bits per second.
Communication companies such as American Telephone and Telegraph (AT&T) and Western Union are called common carriers, and they provide three general classes of service for both voice and data communication:
1. Narrowband handles low data volumes. Data transmission rates are from 45 to 300 baud. The low-speed devices might use narrow band communications.
2. Voiceband handles moderate data transmission volumes between 300 and 9600 baud. They are used for applications ranging from operating a CRT to running a line printer. Their major application is for telephone voice communication hence, the term voiceband.
3. Broadband handles very large volumes of data. These systems provide data transmission rates of 1 million baud or more. High-speed data analysis and satellite communications are examples of broadband communication systems.
Following are the major communication devices which are frequently used:
1. Wire Pairs: Wire pairs are commonly used in local telephone communication and for short distance digital data communication. They are usually made up of copper and the pair of wires is twisted together. Data transmission speed is normally 9600 bits per second in a distance of 100 meter.
2. Coaxial Cables: Coaxial cable is groups of specially wrapped and insulated wires that are able to transfer data at higher rate. They consist of a central copper wire surrounded by an insulation over which copper mesh is placed. They are used for long distance telephone lines and local area network for their noise immunity and faster data transfer.
3. Microwave: Microwave system uses very high frequency radio signals to transmit data through space. The transmitter and receiver of a microwave system should be in line-of-sight because the radio signal cannot bend. With microwave very long distance transmission is not possible. In order to overcome the problems of line of sight and power amplification of weak signal, repeaters are used at intervals of 25 to 30 kilometers between the transmitting and receiving end.
4. Communication Satellite: The problem of line-sight and repeaters are overcome by using satellites which are the most widely used data transmission media in modern days. A communication satellite is a microwave relay station placed in outer space. In satellite communication, microwave signal is transmitted from a transmitter on earth to the satellite at space. The satellite amplifies the weak signal and transmits it back to the receiver.
The main advantage of satellite communication is that it is a single microwave relay station visible from any point of a very large area. In microwave the data transmission rate is 16 giga bits per second. They are mostly used to link big metropolitan cities.