1.7. Modes and methods of information transfer

For correct data exchange between the local computer network nodes, certain modes of information transfer are used:

1) simplex (unidirectional) transmission;

2) half-duplex transmission, in which the source and the receiver receive and transmit information alternately;

3) duplex transmission, in which simultaneous simultaneous transmission occurs, that is, each station simultaneously transmits and receives data.

In information systems, duplex or serial data transmission is very often used. Allocate synchronous and asynchronous methods of serial data transmission.

The synchronous method is different in that data is transmitted in blocks. To synchronize the receiver and transmitter at the beginning of the block send the synchronization bits. After that, the data, the error detection code and the symbol indicating the end of the transfer are transmitted. This sequence forms the standard data transfer scheme for the synchronous method. In the case of synchronous transmission, data is transmitted both in the form of characters and as a stream of bits. The error detection code is often the cyclic redundancy error detection code (CRC), which is determined by the contents of the data field. With it, you can unambiguously determine the accuracy of the received information.

The advantages of the method of synchronous data transfer include:

• high efficiency;

• reliable built-in error detection mechanism;

• high data transfer rate.

The main disadvantage of this method is expensive interface equipment.

The asynchronous method is different in that each character is transmitted in a separate package. Start bits warn the receiver to begin transmitting, after which the character itself is transmitted. The parity bit is used to determine the validity of the transmission. The parity bit is one when the number of ones in the symbol is odd, and zero when their number is even. The last bit, which is called the "stop bit", signals the end of the transfer. This sequence forms the standard data transfer scheme for the asynchronous method.

The advantages of the asynchronous transfer method are:

• inexpensive (compared to synchronous) interface equipment;

• simple waste transmission system.

The disadvantages of this method include:

• loss of the third part of the bandwidth for the transmission of service bits;

• low transmission rate compared with the synchronous method;

• the inability to determine the reliability of the information obtained using a parity bit with a multiple error.

The asynchronous transfer method is used in systems in which data is exchanged from time to time and does not require a high transfer rate.