bitstuffing

Bit stuffing is a technique used in data communications to insert non-information bits into a data stream to ensure synchronization and delimitation of frames. It is employed in certain synchronous protocols and some bus standards.

Mechanism: After a run of five consecutive bits of the same value (either five 1s or five 0s), the transmitter inserts a bit of the opposite value. The result is that no more than five identical bits occur in succession in the transmitted stream. The receiver monitors the incoming bits, and when it detects five consecutive bits of the same value, it discards the next bit (the stuffed one) and continues processing from the following bit.

This method helps maintain clock recovery and prevents false frame boundaries, especially with fixed flag sequences used in framing schemes such as the HDLC flag 01111110. By breaking long runs of identical bits, bit stuffing reduces the chance that data appears to begin or end a frame at the wrong point.

Common implementations: bit stuffing is used in HDLC and PPP frame formats, and in CAN bus communications, among others.

Limitations and notes: bit stuffing adds overhead, since stuffed bits are not part of the user payload; it requires careful handling by both transmitter and receiver; the rules are defined per protocol, and errors in stuffing can complicate synchronization or error detection.