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High Level Data Link Control (HDLC) is a bit oriented Layer 2 protocol used by older computer systems to connect to each other. 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 format of the frame is that it 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. An important point is that the frame delimiter continues to be sent until another frame is sent. This generates a continuous bit pattern that looks like:

 _____  ______  ______  ______ 
_     __      __      __      _

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