While receiving a stream of framed data, frame synchronization is the process by which incoming frame alignment signals, i.e., distinctive bit sequences (a syncword), are identified, i.e., distinguished from data bits, permitting the data bits within the frame to be extracted for decoding or retransmission. This is sometimes referred to as "framing".
A common practice in telecommunications, for example in T-carrier, is to insert, in a dedicated time slot within the frame, a noninformation bit or framing bit that is used for the actual synchronization of the incoming data with the receiver. In a bit stream, framing bits are used in determining the beginning or end of a frame. They occur at specified positions in the frame, do not carry information, and are usually repetitive.
If the transmission is temporarily interrupted, or a bit slip event occurs, the receiver must re-synchronize.
Source: includes material from Federal Standard 1037C in support of MIL-STD-188