Inserting bits in data in order to break up a bit pattern that may cause the transmission to go out of synchronization. For example, in T1 lines, timing is maintained by detecting a change from 0 to 1. If too many zero bits are transmitted consecutively, the receiving end may lose synchronization because too much time has passed without sensing voltage. Therefore, in long strings of zeros, a set of bits that begins with a 1 and functions as a timing signal is "stuffed" into the stream of zeros at certain intervals.