HDLC

HDLC was imported by the ITU into the X.25 protocol stack. It was modified by IBM to become the SDLC protocol that became the layer 2 protocol for IBM's System Network Architecture (SNA). HDLC is now the basis for synchronous PPP (point to point protocol) used by many servers to connect to a wide area network, most commonly the Internet.

The frame begins and ends with the bit pattern 01111110 or hex 7E, called the frame delimiter.

Then there is an address and a control field followed by a data field that may be 0 to 5000 octets long. Then a frame sequence check (FSC) is added and then the frame delimiter is added to the end. The frame delimiter continues to be sent until another frame is sent. This generates a continuous bit pattern:
0111110011111100111111001111110
_____ ______ ______ ______
_ __ __ __ _

This is used by the modems to train and synchronize their clocks via phase-locked loops.

To avoid transmission failures special care must be taken if this special bit pattern (h7E) occurs in data. Therefore bitstuffing is introduced sender sided, which will insert a 0-bit after 5 1-bits, to avoid the frame delimiter pattern. The reciever deletes 1 0-bit after 5 1-bits to restore the original data.

See also PPP, SDLC, SLIP, EAP.