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