Editing High-Level Data Link Control (section)

=== Synchronous framing ===
Because a flag sequence consists of six consecutive 1-bits, other data is coded to ensure that it never contains more than five 1-bits in a row.  This is done by [[bit stuffing]]: any time that five consecutive 1-bits appear in the transmitted data, the data is paused and a 0-bit is transmitted.

The receiving device knows that this is being done, and after seeing five 1-bits in a row, a following 0-bit is stripped out of the received data.  If instead the sixth bit is 1, this is either a flag (if the seventh bit is 0), or an error (if the seventh bit is 1).   In the latter case, the frame receive procedure is aborted, to be restarted when a flag is next seen.

This bit-stuffing serves a second purpose, that of ensuring a sufficient number of signal transitions.  On synchronous links, the data is [[NRZI#Non-return-to-zero inverted|NRZI]] encoded, so that a 0-bit is transmitted as a change in the signal on the line, and a 1-bit is sent as no change.  Thus, each 0 bit provides an opportunity for a receiving [[modem]] to synchronize its clock via a [[phase-locked loop]].  If there are too many 1-bits in a row, the receiver can lose count.  Bit-stuffing provides a minimum of one transition per six bit times during transmission of data, and one transition per seven bit times during transmission of a flag.

When no frames are being transmitted on a simplex or full-duplex synchronous link, a frame delimiter is continuously transmitted on the link.  This generates one of two continuous waveforms, depending on the initial state:

[[File:NrziEncodedFlags.png]]

The HDLC specification allows the 0-bit at the end of a frame delimiter to be shared with the start of the next frame delimiter, i.e. "011111101111110". Some hardware does not support this.

For half-duplex or multi-drop communication, where several transmitters share a line, a receiver on the line will see continuous idling 1-bits in the inter-frame period when no transmitter is active.

HDLC transmits bytes of data with the least significant bit first (not to be confused with [[little-endian]] order, which refers to byte ordering within a multi-byte field).