Synchronous communication relies on the presence of a clocking system at both ends of the transmission. These clocks must be synchronized at the beginning of the session so that the timing of the transmission—not the use of start and stop bits, as in asynchronous communication—defines where data begins and ends.
When data is transmitted in synchronous fashion, a unique 8-bit pattern is used to define the start of the data stream (Figure 110). This special bit pattern is embedded in the digital signal to assist in maintaining the timing between the sender and receiver.
Synchronous communication is used over digital lines and can achieve much higher data rates than asynchronous communication, which is typically used over analog telephone lines. For example, the synchronous method of communication is used on ISDN lines because it handles data more efficiently than the typical modem’s asynchronous technique.
While asynchronous communication sends small blocks of data with many control bits for error correction, synchronous techniques use large blocks of data with control bits only at the start and end of the transmission. The higher quality of digital lines permits minimal error checking. A modem used over an analog line does not handle synchronous communication well because noise on the line disrupts synchronization.
An example of synchronous communication is Ethernet for local-area networking. On the wide-area network, ISDN is an example of a service that is based on synchronous communication.
Last Word
Synchronous communication is usually much more efficient in its use of bandwidth than asynchronous communication because more data can be transmitted without the use of control and error bits. Another advantage of synchronous communication is that the frame structure allows for easy handling of control information that may be required by data link layer protocols. There is a natural position—usually at the start of the frame—for any special codes needed by the communication protocol.
Figure 110
In synchronous communication, a unique 8-bit pattern indicates the start of the clock.
For example, the High-Level Data Link Control (HDLC) ensures that data passed up to the next layer has been received exactly as transmitted— error free, without loss, and in the correct order. HDLC also performs flow control, which ensures that data is transmitted only as fast as the receiver can receive it.
