4.3 High-Level Data Link Control Protocol
High-level Data Link Control (HDLC) protocol provides error detection and data stream control. HDLC is governed by the following standards: ISO 3309, ISO 4335, ISO 7776, ISO 7809, ISO 8471, and ISO 8885.
HDLC was derived from the SDLC protocol created by IBM. SDLC was used for synchronous transfer. Today, SDLC is mostly understood as a subset of HDLC, even though not all of the options of the SDLC protocol were included in HDLC.
Later, the HDLC standard was extended for asynchronous transfer as well. The asynchronous variant of HDLC is usually used by the PPP protocol that is described in Section 4.4. From now on in this chapter, we will assume synchronized, bit-oriented transfer on a physical layer.