In consequence, asynchronous transmission is inappropriate for high data rates, and synchronous transmission must be used. When using synchronous transmission, special procedures must be adopted to permit the unique identification of the frame. This is achieved by setting the frame header to a predetermined pattern. On transmission, any repeat of this pattern in the data is destroyed by the addition of binary zeroes, that are removed again on reception. This process is known as bit-stuffing .
Synchronous receivers require a timing signal from the transmitter.
An additional channel may be used in the system to transmit the clock signal. This is wasteful of bandwidth, and it is more customary to embed the timing signal within the transmitted data stream by use of suitable encoding. This transition is used to generate the clock signal used by the receiver. In this scheme, a transition in voltage level is used to represent a 1 or zero, rather than a voltage level itself, so only a two level line is needed. A transition occurs in the middle of each bit.