HDLC - High-Level Data Link Control
The HDLC protocol was developed from SDLC and is actually a superset of SDLC.
HDLC is used for both point-to-point and multipoint connections like SDLC.
HDLC is the default WAN protocol for Cisco routers.
HDLC supports full-duplex communications which means information can be transmitted and received at the same time.
It supports synchronous and asynchronous communications.
The original HDLC did not support multiple protocols but Cisco's proprietary HDLC frame allows for indicating multiple protocols at higher layers.