Synchronous Transmission

Synchronous Transmission

Synchronous transmission eliminates the need for start and stop bits by synchronizing the clocks on the transmitting and receiving devices. This synchronization is accomplished in two ways:

By transmitting synchronization signals with data. Some data encoding techniques, by guaranteeing a signal transition with each bit transmitted, are inherently self-clocking.
By using a separate communication channel to carry clock signals. This technique can function with any signal-encoding technique.

Figure 6.2 illustrates the two possible structures of messages associated with synchronous transmission.

Both synchronous transmission methods begin with a series of synch signals, which notify the receiver of the beginning of a frame. Synch signals generally utilize a bit pattern that cannot appear elsewhere in messages, ensuring that the signals always are distinct and easily recognizable by the receiver.

A wide variety of data types can be transmitted. Figure 6.2 illustrates both character-oriented and bit-oriented data. Notice that under synchronous transmission, multiple characters or long series of bits can be transmitted in a single data frame. Because the transmitter and receiver remain in synchronization for the duration of the transmission, frames may be very long.

When frames are long, parity is no longer a suitable method for detecting errors. If errors occur, multiple bits are more likely to be affected, and parity techniques are less likely to report an error. A more appropriate error-control technique for synchronous transmission is the cyclic redundancy check (CRC). In this technique, the transmitter uses an algorithm to calculate a CRC value that summarizes the entire value of the data bits. This value is then appended to the data frame. The receiver uses the same algorithm, recalculates the CRC, and compares the CRC in the frame to the CRC value it has calculated. If the values match, the frame almost definitely was transmitted without error.

When synchronous transmission links are idle, communicating devices generally send fill bits to the devices synchronized.

Synchronous transmission offers many advantages over asynchronous transmission. The overhead bits (synch, CRC, and end) comprise a smaller portion of the overall data frame, which provides for more efficient use of available bandwidth. Synchronization improves error detection and enables the devices to operate at higher speeds.

The disadvantage of synchronous transmission is that the more complex circuitry necessary for synchronous communication is more expensive.