HDLC and PPP Compared
HDLC and PPP are both methods for accessing the services of WAN providers by using protocols that connect peer devices.
HDLC is the default protocol for WAN links on Cisco devices.
PPP is the most common protocol used on dial-up, leased lines, and with ISDN services.
Cisco's HDLC has a proprietary field that allows for a protocol type but PPP further extends the typical HDLC frame by providing a protocol field.
PPP uses the link control protocol (LCP) to establish, configure, maintain, and terminate point-to-point connections.