HDLC is the protocol which is now considered an umbrella under which many Wide Area protocols sit. ITU-T developed HDLC in 1979, and within HDLC there are three types of stations defined:
- Primary Station – this completely controls all data link operations issuing commands from secondary stations and has the ability to hold separate sessions with different stations.
- Secondary Station – this can only send responses to one primary station. Secondary stations only talk to each other via a Primary station.
- Combined Station – this can transmit and receive commands and responses from one other station.
When transferring data, stations are in one of three modes:
- Normal Response Mode (NRM) where the secondary station needs permission from the primary station before it can transmit data. Mainly used on multi-point lines.
- Asynchronous Response Mode (ARM) where the secondary station can send data without receiving permission from the primary station. This is hardly ever used.
- Asynchronous Balanced Mode (ABM) where either station can initiate transmission without permission from the other. This is the most common mode used on point-to-point links.
There are three types of HDLC frame types defined by the control field:
- Information Frames are used for the data transfer between stations. The send sequence, or next send N(S), and the receive sequence, or next receive N(R), hold the frame sequence numbers. The Poll/Final bit is called Poll when used by the primary station to obtain a response from a secondary station, and Final when used by the secondary station to indicate a response or the end of transmission.
- Supervisory Frames are used to acknowledge frames, request for retransmissions or to ask for suspension of transmission. The Supervisory code denotes the type of supervisory frame being sent.
- Unnumbered Frames are used for link initialisation or link disconnection. The Unnumbered bits indicate the type of Unnumbered frame being used.