Transmission Modes

Transmission Modes

The transmission mode enables the receiving computer to know where one byte ends and the next byte begins on the transmission medium. The two transmission modes are: -

. Asynchronous Transmission
. Synchronous Transmission

Asynchronous Transmission

Asynchronous Transmission is also called as start and stop transmission. In this case a sender can send a character at any time convenient and the receiver will accept it. In this case data is transmitted character by character at irregular intervals. Each character is preceded with a start bit and succeeded with a stop bit.

This kind of transmission is well suited for keyboard type terminals and tape devices. The advantage of using this kind of transmission is that it does not require any local storage at the computer. Therefore it is economical to implement.

The main disadvantage of using asynchronous transmission is the idle time during character transmission.

Synchronous Transmission

In case of Synchronous Transmission, it involves a group of characters in the form of a block and each block is framed by a header and trailer information. The header contains synchronizing information used by the receiving device to set the clock time in synchronize with the clock time of sender. The trailer consists of information about sender and receiver, this kind of transmission is well suited for remote communication between a computer and devices such as buffered card reader and printers. Here the block length is about 80-130 characters long. It is also used with buffered terminals for communication in a network. The primary advantage of using synchronous transmission is efficiency because it offers higher data rate and lesser time difference between the blocks as compared to asynchronous transmission.

Communication Systems

Communication Systems

In case of sending and receiving messages or data from one place to other place, we have many elements working together. All these elements put together to work efficiently is known as system. The communication system has the sole purpose of passing data or information in the most effective manner.

Communication system is the combination of the hardware, software and data transfer links that make up a communication facility of transferring data in a cost effective manner.

A communication system can either be analog or digital or a combination of the two. That is, the information can be transmitted in either analog or digital form with in the communication network. E.g. computer generated data are to be sent over analog telephone lines, the digital signals must be converted to analog form. The technique by which a digital signal is converted to its analog form is known as modulation. The reserve process that is conversion to its analog signal to its digital form at the destination device called demodulation. The process of modulation and demodulation that is the conversion of digital data to analog from and vice versa is carried out by a special device called modem (modulation/demodulation).

The analog form of message is sent via telephone line and then it reaches the destination where they are again transformed into digital pulses by the other modem connected with the receiver PC. Thus the message or data is transmitted and received by the two PC’s.

As the analog signal passes through the telephone lines, its strength starts decreasing with the distance, so amplifiers are used to boost the strength of the signal. These amplifiers add noise to the signal which sometimes causes disruption of the message. But good quality of the modem at receiving end as well as at the transmitting end is able to cut down the effect of the noise.