HDLC Frame Classes

In the HDLC protocol, three classes of frames are used:

1. Unnumbered frames are used for link management, for example they are used to set up the logical between the primary station and a secondary station and to inform the secondary station about the mode of operation which is used.
2. Information frames are those who carry the actual data. The information frames can be used to piggyback acknowledgment information relating to the flow of Information frames in the reverse direction when the link is being operated in ABM or ARM.
3. Asynchronous Balanced Mode (ABM) is used mainly on full duplex point-to-point links for computer to computer communications and for connections between a computer and a packed switched data network, in this case each station has an equal status and performs the role of both primary and secondary functions. This mode is used in the protocol set known as X.25
4. Asynchronous Response Mode (ARM) is used in unbalanced configurations. It allows a secondary station to initiate a transmission without receiving permission from the primary station. This mode is normally used with point-to-point configurations and full duplex links and allows the secondary station to send frames asynchronously with respect to the primary station.
5. Supervisory frames are use for error and flow control. They contain, send and receive sequence numbers.