Originally Published in Computer Jagat Magazine


As the world’s economy grows, more and more data is produced. This has created an ever increasing demand for faster data communications locally, nationally and globally. Data communication thus emerged as a natural result of the development of sophisticated computer systems— it has become the milestone in the today’s information explosion age.


Computers are used to generate and process information. But generated information is not useful in itself. Sometimes it is necessary to transfer this information from one place to another. Data communication is the transfer of data or information between computer devices.

In the middle of nineteenth century data communication used to take place mostly through telegraph cables using Morse Code. However, with the beginning of computer era, the need to send data from one point to another with greater speed and efficiency arose. Also as the computer industry began to grow faster, the better and speedier were the computers. Under this condition, data communications have continued to change dramatically.

A direct link between the computers could send data between them, but problem arises when the sending and receiving computers are placed at remote distances. Because digital signals, when transmitted over long distances, suffer severe distortion. Also it is an impractical plus very expensive idea to join all sending and receiving computers, via cables. Data maybe transferred from one computer to another at remote distances over telephone lines, coaxial cables, wave guides, or over satellite communications. Since telephone lines already have a wide network, the logical choice of transmission medium for data communication becomes the telephone lines. However, data processed by computers are in the parallel form to reduce time and increase speed, whereas, the signal through a telephone line is in serial form. Moreover, computers process data as digital signals that are either on or off, i.e. in binary form at a speed more than 1MHz. On the contrary, telephone signals are continuously rising and falling, i.e., truly are analog signals in the frequency range of 300 to 3500 Hz. Therefore, some sort of conversion must take place for data to be effectively passed from the computer over the telephone lines and this conversion is performed by two devices— Asynchronous Communication Interface Adapter (ACIA) and ‘Modem’.

The ACIA, connected to the outside world via a D-shaped 25-pin connector and built into every computer, takes parallel data from the computer’s bus and turns it into serial bit-stream and vice-versa. And modem modulates the digital signals at the transmitter, thereby rendering them suitable for transmission over telephone lines and recovers the original digital signals at the receiving end by demodulation.


The name “Modem” has been derived from two words Modulator and DEModulator. Ampdem performs the function of modulation and demodulation as the name implies. That is the function of the modem is to convert digital data signals to suitable audio signals compatible with transmission lines on the transmitting end, a process referred to as modulation. Also it converts the audio signals back into digital data at the receiving end by demodulation.

If data is to be transmitted over telephone lines, the signals must be converted to reside with in the audio frequency spectrum from 300, to 3500 Hz. Modems have been developed to provide this function.

When two computers are communicating, they send information back and forth. If these two computers are within a reasonable distance, they can send and receive information through a direct cable connection, which is sometimes called a null modem connection. The further the distance the signal has to travel, the greater the chance that some information will be lost. For this reason, in a direct cable connection, it is generally recommended that the computers be within 50 meters of each other.

Computers can exchange information over telephone lines by using two modems—one on each side, a calling computer (or a terminal) contacts a receiving computer through a telephone number, and a communication link is established after control signals have been exchanged between computers and modems.


A data communication system, providing electronic distribution of information from computer to computer consists of five basic components. These basic components are:

(a)    The sending or originating computer: The originating computer or terminal has data to transmit. The data may consist of a file on a disk or may be entered into a keyboard, transmitted as it is typed.

(b)    Data Communication device Attached to the sending computer:

The data communication device attached to the sending computer converts the data into a form that can be transmitted.

(c)    Communication Channel:

The communication channel (also called a communication link) carries the data being carried from place to place. There are many possible communication channels, including telephone lines, coaxial cables, optical fibre systems or microwave relay systems.

(d)    Data Communication Device Attached to the Receiving Device:

The data communication device attached to the receiving computer converts the transmitted data into a form so that the receiving computer can understand.

(e)    Receiving Computer:

The receiving computer or terminal receives the data and displays them on a screen, prints them, or stores them in a file.


All communications between computers are managed by data communication software. The précised procedures used for communication depend on the particular software; some general procedures are listed below:

I. Communication between two Microcomputers:

The procedure used to communicate between two microcomputers is as follows:

(A)    Both the microcomputer user start the communication program, which gives them a menu of options.

(B)    Both the user choose the same options, including the number of start and stop bits, the type of parity checking, the communication rate, full-duplex or half-duplex transmission, and so on.

(C)    Before establishing the communication link, the users decide who will originate the telephone call. The originator enters the telephone number. The communication program then instructs the modem to dial the number, and the program reports the users whether the connection has been established.

(D) After the connection has established, the users can have an on-line conversation. The can type messages back and forth to each other and also record their conversation.

(E)    When a user is ready to transfer files, he or she sets his or her computer to send; the called user sets his or her computer to receive. The communication program takes over by reading the information from the disk and sends it through the communication link, the receiving computer stores the incoming information on a disk and also can display on its screen.

(F)    When the transfer has been completed, the users say “goodbye” and terminate the connection.

2:         Communication between a microcomputer and a Mainframe Computer:

The procedure for communication between a microcomputer and a minicomputer or a mainframe computer is similar. One difference is that the big computers have safe guards that limit access to files and facilities to authorized users. The most common safeguards are user identification number and password.

After the communication link has been established, the receiving computer asks for the user’s identification number. The microcomputer user sends the identification number, after which the receiving computer checks whether the identification number is valid. It then asks for the user’s password.

A password is a confidential sequence of characters that allows access to the system. It is required for the user to obtain access to the mainframe computer. It also determines exactly which files and facilities the microcomputer user is allowed to read.


Once a communication link has been established between two points, it can be used in one of three communication modes. The modes are Simplex, Half Duplex, and Full Duplex.

Simplex transmission is that transmission which occurs in one direction only. In a simplex transmission mode, one device is always a transmitter and the other device is a receiver.

Half-duplex transmission permits trans­mission in either direction; however, trans­mission can occur in only one direction at a time. Thus at any instant, if one device acts as a transmitter, the other acts as a receiver and vice-versa.

Full-duplex allows data transfer in devices simultaneously. Thus one device may be transmitting and receiving simultaneously while the other does the same. The two simultaneous transmissions may or may not be related and occur on two separate and distinct communication channels.

The communication mode to be chosen may be the result of communication medium limitations, hardware usage or programming conventions.

Most of the present day modems offer half-duplex, full duplex or both facilities. Simplex is not used with modems.


Data generated by computer is normally character oriented and is outputted in serial form as a series of equal duration bits. This generated data is transmitted between different communicating points. However, some sort of information is usually required as to how the data will be transmitted. There are two methods of doing this and these are referred to as synchronous and asynchronous transmission.

Synchronous Transmission:

In synchronous transmission, characters are transmitted as groups, preceded and followed by control characters. In synchronous communication, data bytes are sent one after another at regular intervals. The data form as continuous stream of bits spaced at regular intervals, with no space between consecutive bytes. A timing mechanism causes the receiving modem to read the stream at precisely the correct frequency. When the receiving modem has read the required number of bits to makeup a character, it sends the character to the receiving computer.

In this type of transmission, since data is sent as a block of characters, both transmitter and receiver need to have buffers for storing block of characters, a major advantage of using this transmission technique is high speed. Since fewer bits are needed to identify the beginning and end of character coding. Again, as there is no gap between characters, precise data space is not wasted. Its chief drawback is inaccuracy; when a receiver goes out of synchronization, losing track of where individual character begin and end, correction of errors takes additional time. Synchronous transmission is used when data rate over 2400 bauds is required.

Asynchronous Transmission:

In asynchronous transmission, each character is transmitted separately, that is, one character at a time. The character is preceded by a start bit, which is always a ‘O’ (zero) that tells the receiving device where the character coding begins, then five to eight bits-representing the actual information being transmitted, an optional parity bit (even or odd parity) for error detection capability and is followed by 1, 1F(1,2), or 2 stop bits, which tells the receiving device where the character coding ends, after which is an interval of time on the channel. Then the next character is sent, start bit first, character’s bits next, stop bit(s) lactates, allow the receiving and sending computers to synchronize the transmission. This is the most common technique of data transmission worldwide. Its principal advantage is accuracy, while its main drawback is slow transmission speed caused by the great number of start and stop bits. Asynchronous communication is typically used at communication rates lower than 2400 bauds.


There are several parameters governing how serial communications data is formatted. The most common serial communications parameters that will encounter are the baud rate, the number of data bits per word, parity, and the number of stop bits.

Baud rate: “Baud rate” is the speed at which data is transmitted. This parameter, also referred to as bits per second, bit rate, or data rate, typically ranges from 300 to 9600 baud.

Number of Data Bits: The “number of data bits” refers to the number of bits that constitutes one data word. In serial communications, each data word is transmitted as a sequence of bits. This word may be immediately preceded or followed by other bits (parity or start and stop bits) that are used for controlling the communication. The number of stop bits is variable; the default is one.

Parity: Parity is a method for detecting errors in data communications. The parity bit is added at the end of data word, the value of this bit is a function of the rest of the data word. “Even parity” means that the parity bit is set so that the sum of all the bits in the data word (including the parity bit) is even and “odd parity” means that the parity bit is so set that the sum of all bits is two stop bits are added to the end of a data word, these bits tell the receiver where the end of each data word is number. “Mark parity” means that the parity bit is always a 1; “Space parity” means that the parity bit is always a 0. “No parity” means that no parity bit is added to the end of a data word.

Stop Bits: In asynchronous serial com­munications, either one, one and a half, or two stop bits are added to the end of a data word, these bits tell the receiver where the end of each data word is.


In the world of data communications, equipment that includes terminal and computer ports is referred to as Data Terminal Equipment or DTE. In comparison, modems and other communication devices are referred to as Data Communications Equipment or DCE.

The physical, electrical and logical rules for the exchange of data between DTEs and DCEs are specified by interface standards; the most commonly used is the EIA RS-232C standard. The RS-232C specifies the use of D-shaped 25-pin interface connector.


When one computer communicates with another over long distances, the data are transmitted serially. Internally, computers almost universally use data in parallel form. Therefore, this parallel data must be transferred into a serial form before being send through telephone lines and after reception the data string must be recomposed into parallel form. Most personal computers, however, has fitted with them a serial interface through which modems are used to send serial data.

At present, three types of standard are followed in case of modems. These are:

Bell Standard, EIA (Electronics Industries Association) standard and CCITT standard. Of these, the RS-232C standard, issued by EIA, is perhaps of the most interest to the hobbyist or microcomputer users. It defines all the features of communication, that is, the number of pins in the connection, the dimension and construction of the connector, the signal levels on the pins, the interrelationship between the signals and the procedure for exchanging information. This so-called DB-25 connector is, there­fore, widely used for communication pur­poses. RS-232C is a recommended stand­ard (RS) published by the Electronics In­dustries Association (EIA) in 1969, with the number 232 referencing the identification number of one particular communication standard and the suffix C identifies the current revision level. It specifies (among other things) that the marks and spaces that make up the code must be of certain amplitudes.

The RS-232C voltage levels are defined as follows: Logic 1 (mark) = less negative than -3V Logic 0 (space) = more positive than + 3V Any voltage between -3V and + 3V is undefined. Typically, an RS-232C system uses nominal voltages of -12V and +12V for a ’1′ or ’0′, respectively, the more positive voltage than +15V and the more negative voltage than – 12V is also undefined.

Basically, a serial interface consists of a transmitted signal (pin 2), a received signal (pin 3), and a ground connection (pin 7). This is the barest minimum for bi-directional link. However, handshaking lines are used in addition, but are not essential. In this case, the Request to Send (pin 4), Carrier to send (pin 5), Data Terminal Ready (pin 20), Data Set Ready (pin 6), Carrier Detect (pin 8), and Ring Indicator (pin 22) lines are required, and the remainders are seldom encountered in normal practice.


Data communication systems are designed to provide information flow among computers. There are variety of reasons for interlinking computers and peripherals. Some of which are outlined below:

The ability to share and exchange data between systems is a compelling reason for interconnection. Users from different locations can easily transfer information in the preparation of a document or for an analysis.

There are many applications centered on remote accessing of data bases. Simple examples are the information services and financial services available to personal computer user. More sophisticated examples, requiring many interactions between the remote site and the data base and its associated program include remote computerized medical diagnosis and remote computer aided education.

Communication, the transfer of information, is the basis of office automation. Airline reservation system, automated banking systems, inventory control systems etc. provide a number of examples.

When one reserves seat on an airplane, the agent enters the reservation on a terminal connected to the airline’s computer. Since the computer is usually located far from the agent (sometimes several thousand miles away), data communication must be used to relay data from the terminal to the computer and back from the computer to the terminal.

Most banks now provide a wide range of banking services through automatic teller machines (ATM‘s). Users can make deposits and withdrawals, cheek balances, and even pay utility bills through the machines. An automatic teller machine is connected to the bank’s main computer, which may be located at another end of the city or even in another state. The transaction request is sent to the computer using a data communication system.

Many retail stores use point of sale terminals instead of cash register. These terminals send records of sales to a central computer, which maintains accounting and inventory records.

Another popular application is the electronic mail. Such a mail can be read, filed, forwarded to other individuals, with added comments or read by the addresses at different locations. Obviously, such a service has many advantages over postal mail in terms of delivery speed and flexibility. It has also advantages over voice telephone service in terms of providing a record, reducing cost (for long distance calls), and eliminating the need for both users to communicate at the same time.


The proliferation of computers has caused an “information explosion”. As time goes by we hear more and more about public services that will make computers and giant data bases available to every home. This information explosion has instigated the computer communication to become one of the most widely expanding field of research area. With technological innovations different types of signals (e.g. computer data, human voice, audio, video, information) are now transmitted through the same communication channel, and computer communications are extensively used for this purpose.

With the wide spread use of computers in Bangladesh, the necessity of communicating and transmitting data from place to place becomes inevitable day by day. There are some Electronics and Computer firms communicating with U.S.A. England, Singapore and Japan, have installed modems with their personal computers. But these modems, however, are very expensive. To overcome this difficulty, modems should be designed and communication software be developed here which could be used in scientific, industrial, educational institutions or in office applications in our country for realizing greater potential of computer usage.


Lecturer, Dept. of Electronics &

Computer Science,

University of Jahangimagar.

Reference :

1.   MD. Al-Amin Bhuyan. “Implementation of Data Communication over Telephone Lines” M.Sc (Thesis). Dept. of Applied Physics & Electronics. Dhaka University. 1992.

2.   MD. Al-Amin Bhuyan & Jalalur Rahaman, “Design and Development of an FSK modem for Data Communication”, Vol. 2, No. 1 1992. Journal of Bangladesh Electronics Society.

3.   C. Mohen. “Telecomputing the rightway,” Electronics foryou, Vol. 23, No.4, ApriU991.

4.   S.  K.  Basandra.  “Data Communication”. Electronics foryou, Vol. 23. No.4. April 1991.

5.   A. S. B. Raj, “Modem,” Practical Electronics. Vol. 21. No. 6, June. 1985.

6.   M. Tooley and D. Whitfield, “Modems”, Practical Electronics Vol.21. No. 6, June 1985.

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