High-Level Data Link Control (HDLC) is a bit-oriented synchronous data link Layer 2 protocol developed by the International Organization for Standardization (ISO).
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.
