What is Data Communication System

 What is Data Communication System.?

 In the connected world, a computer does not work as a standalone system but as part of a communication system. Besides computers, most of the large/complex systems like the navigation systems for ships or aircraft or rockets, the satellites and many other systems rely on the communication system. In the most fundamental sense, communication involves implicitly the transmission of data or information (the information is derived from data) from one point to another through a succession of processes. Data is transmitted over any communications medium as either digital or analog form. The most important factors affecting the transfer of a signal over a medium are noise and attenuation. Noise is the external disturbances whereas attenuation is defined as degeneration of the signal. A simple communication system can be represented by the block diagram shown in Figure

The communication system essentially consists of five parts: Source: Source produces a message or sequence of messages to be communicated to the receiver. The source output may be in many different forms such as a waveform, a sequence of binary digits, and a set of output from sensors in a space probe, or many other similar forms. An Encoder: Encoder represents any processing of the source messages/ signals prior to transmission. The processing might include, for example, any combination of modulation (discussed in later section), data reduction and insertion of redundancy to combat the channel noise. The Channel: Channel is the medium for transmitting signals from transmitter to receiver. It may be a telephone line, a high frequency radio link, a space communication link or a storage medium. A channel is usually subject to various types of noise disturbances, which on telephone line, for example, might take the form of a time-varying frequency response, crosstalk from other lines, thermal noise, and impulsive switching noise. A channel subject to noise is called noisy channel. An error-correcting code corrects errors due to noise. The Decoder: A decoder represents the processing of a channel output received from the channel to produce an accepted replica of the input at the destination. The Destination: Destination is the receiver. It may be the person or object for whom the message is intended. An example of communication system: Suppose a student computer is connected through a modem to a telephone line. If she/he wants to send a file to his/her friend over a communication system, his/her computer is the source, the modem converts his digital file into analog signal that can be transmitted over the telephone line to the receiver's modem which at its end converts the signal back to the digital signal. The digital data then is accepted by the destination computer. Some standard data transmission concepts are:

·         The data in a communication system may be transmitted as analog or digital data over a single path serially or number of parallel paths.

·         The data can be sent asynchronously when both the source and receiver are not following timing or synchronously when both sender and receiver agree on the sequence of arrival of data.

·         Modes of Data Transmission: There are 3 modes of data communication:

 

Half Duplex Mode: Half duplex communications occurs when data flows in both directions; although in only one direction at a time. An example of a half-duplex system is a Walkie-Talkie system used a two-way radio normally by Police. You may use the word "Over" to indicate the end of transmission, and ensure that only one party transmits at a time. In such systems sender and receiver both transmit on the same frequency. Full Duplex Mode: In full duplex mode of data transmission, data is transmitted in both the direction simultaneously. This means that both the devices in a network can send and receive the data at the same time. It is like a two lane road with traffic moving in both directions at the same time. In this mode signals going in either direction share the capacity. Half of the bandwidth is used for sending data in one direction, while the other half is used for receiving data from other direction. An example is a telephone conversation.

·         Speed of transmission: Speed of data transmission plays a major role in data communication. How fast data can be transmitted from place to place is sometimes called bandwidth. Bandwidth is a data transmission rate that tells the maximum amount of information (bits/second) that can be transmitted along a channel. It is measured in kilobits, kbps, 1,000 of bits per second, or megabits (Mbps), millions of bits per second. Actual transfers are considerably lower because of software and protocol overheads.

·         Some Sample Transmission Speed: Dial-up modems are generally capable of a maximum bit rate of 56 kb/s (kilobits per second) and require the full use of a telephone line—whereas broadband technologies support at least double this bandwidth. Broadband usually has a high rate of data transmission. In general, any connection to the customer of 256 kb/s (0.256 Mb/s) or greater is more concisely considered broadband Internet.

·         Packet, and Circuit Switching: This terminology has started from telephone network, where switching offices were places having switches that were used to create connection from one source to destination. Circuit switching involves creating a switched path for entire communication, for example, when you make a telephone call the connection is established by switching and is available for the whole communication. Whereas in packet switching a message is broken in small packet which are handed over from a source to destination through many small steps.

A Computer Network although works on the basic communication system, but is much more than that. It is characterized by a number of tasks that are mostly implemented with the help of networking software that takes care of addressing, routing and reliable delivery of messages. These software are implemented as a number of protocols. 

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