For any HDLC communications session, one station is designated primary and the other secondary. A session can use one of the following connection modes, which determine how the primary and secondary stations interact.

The Link Access Procedure-Balanced (LAP-B) and Link Access Procedure D-channel (LAP-D) protocols are subsets of HDLC.

HDLC LAPB Level 2 protocol
Describes HDLC Link Access Procedure Balanced (LAPB).

HDLC Overview
Provides an overall description of the HDLC protocol including details on the basic framing structure.

SDLC protocol overview
Describes the HDLC implementation of SDLC (HDLC Normal Response Mode).