9.1 The Synchronization Problem

Synchronization between the transmitter and the receiver is a main requirement for correct data transmission.

In serial communication of a stream of bits, the receiver must recognize the following for the communication to be successful:

		The beginning and end of data blocks with each block composed of a number of bits representing a unit of data, e.g. a character.
		The duration of each bit in the signal received.

This is essential so that the receiver knows the rate at which data is sent be the transmitter.

9.1 The Synchronization Problem

To illustrate the importance of synchronization, consider the example shown in Figure 9.1 where the transmitter sends a stream of bits to the receiver.

A bit is sent every P seconds.

The receiver samples the received signal to detect each bit every P seconds.

The receiver samples the signal at the middle of each bit period as illustrated using the blue lines on the signal received.

The receiver's clock in this example is slower than the transmitter's clock.

Thus, the sampling of the first bit at the receiver side will be slightly drifted away from the bit center.

This drift is accumulated in subsequent bits until the signal is wrongly sampled at the ninth bit interpreting it as 0 although the ninth bit sent by the transmitter is 1.

Subsequent bits are also wrongly interpreted as illustrated in Figure 9.1 and shown below:

9.1 The Synchronization Problem

Figure 9.1: Errors Caused by the Lack of Synchronization between the Transmitter and Receiver Synchronization between the transmitter and the receiver can be achieved using synchronous and asynchronous techniques that will be described next.