The principle difference between the synchronous and asynchronous modes of transmission is that in the synchronous case, the receiver uses a clock which is synchronised to the transmitter clock.
Synchronous transmission has the advantage that the timing information is accurately aligned to the received data, allowing operation at much higher data rates. It also has the advantage that the receiver tracks any clock drift which may arise (for instance due to temperature variation).
In Synchronous Communication the clock at the receiver is synchronised with the received data.
The clock signl can be transferred from the transmitter to the receiver by:
Figure showing a synchronous link, where the clock is encoded at the sender, a single wire carries the clock and data. At the receiver, the clock is extracted from the recieved signal using a digital phase locked loop. Ths clock is then used to drive a decoder which reconstructs the original data bits.
A problem arises when using NRZ to encode a synchronous link which may have long runs of consecutive bits with the same value. The figure below illustrates the problem that would arise if NRZ encoding were used with a DPLL recovered clock signal, as in synchronous CAN bus. If the encoded data contains long 'runs' of logic 1's or 0's, this does not result in any bit transitions. A lack of transitions prevents the receiver DPLL from reliably regenerating the clock making it impossible to detect the boundaries of the received bits at the receiver, thus in CAN bus a bit-stuffing method is used to eliminate consecutive runs' of logic 1's or 0's.
A long run of bits with the same value results in no transitions on the cable when NRZ encoding is used
Prof. Gorry Fairhurst, School of Engineering, University of Aberdeen, Scotland. 2020