Overview
High-level Data Link Control (HDLC) is a bit-oriented link layer protocol. Its most prominent feature is that it can transmit any type of bit stream transparently.
HDLC supports point-to-point link only and does not support point-to-multipoint link.
HDLC supports neither IP address negotiation nor authentication. It uses keepalive messages to check link status.
HDLC works only on synchronous interfaces or synchronous/asynchronous interfaces in synchronous mode. This protocol applies to serial interfaces operating in synchronous mode and POS interfaces.