1.4 DATA TRANSMISSION MODE

 There are three basic elements in every communication system; namely the transmitter, the channel and the receiver. The transmitter and the receiver are isolated from with other and is it the channel that connects them together as shown in Figure 3:


Let’s understand the communication system with help of an example:

When you speak to your friend on telephone, you are the sender. The telephone line through which your voice is transmitted is the medium and your friend is the receiver. This is a simple example of voice communication. The same concept holds good for the data communication also. Data communication is the function of transporting data from one point to another. In this case, the sender and the receiver are normally machines, in particular, computer devices and the transmission medium may be telephone lines, microwave links, satellite links, etc. however, the messages that are transmitted are, data not voice conversations. Thus, the electronic system that transfers data from one point to another are called data communication systems. Unlike computers that process and rearrange data, data communication systems transmit data from one point to another without any change.

The transmitting equipment converts the data of the sending system into a form that can be sent over the communications channel, accepted by the receiving equipment, and converted back into usable data by the receiving system. Data sent over a communications system is in one of the following two forms: Analog or Digital.

An analog signal used in data communications varies continuously (as a sine wave) between a minimum and a maximum value along with time. As the signal varies, it assumes an infinite number of specific values between the two limits. The signal can be varied in amplitude (amplitude modulation), frequency (frequency modulation), or phase (phase modulation) to convey the data.

A digital signal has a limited set of values (1 or 0, true or false, etc.) i.e., it will have only two values either 0 or 1. A limited number of discrete pulses can be transmitted in a fixed period. The unique sequence of the bits represents the data. Digital equipments (computers and peripherals) within a system normally communicate with each other in pure digital pulses (serial and parallel). Transmitting digital Communication information over a distance requires the use of special equipment to convert digital data pulses into a form acceptable to the various types of communications channels.

There are two major dataformatting methods which are used to make sure the transmitting computer and the receiving computer(s) are synchronized:

Asynchronous (characterframed)

and Synchronous (messageframed).

Both methods are used to identify intelligence transmitted in the form of serial bit streams.

1.4.1 Asynchronous Transmission

Asynchronous transmission of data is commonly found in landline communications systems and some forms of teletype communications. Generally, asynchronous, or characterframed, transmission is used to transmit sevenor eightbit data, usually in ASCII (American Standard Code for International Interchange) character format. Each character has a specific start and end sequence—usually one start bit and one or two end (stop) bits. Figure 4 illustrates the transmission format of an asynchronous data stream. A parity bit (even or odd) maybe included ensuring the accuracy of the transmitted data. Asynchronous characters may be transmitted one at a time or as a string of characters; however, each character transmitted will have start and end bits. When data signals are transmitted in this format, synchronization occurs on a characterbycharacter basis between the transmitting and receiving devices and provides some allowance for timing inaccuracies. Any inaccuracy in timing is corrected with the arrival of the next character.


1.4.2 Synchronous Transmission

Most tactical digital information links communicate using synchronous messages. Synchronous transmission is a more sophisticated method of data transmission. It sends data in long uninterrupted streams, with a predefine start and stop sequence. The start sequence is generally referred to as the preamble. The principal function of the preamble is to alert the receiver of incoming data and provide a reference to synchronize the receiver with the transmitted signal. Following the preamble is a start code that informs the receiving equipment of the beginning of the message data. The basic format of the synchronous data message is shown in Figure 5. The incoming bit stream is then used to synchronize the receiver timing. A stop code follows the message data to indicate the end of transmission.



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