Nodes within a network running HDLC are called stations. HDLC
supports three types of stations for data link control:
Primary stations—Responsible for controlling the
secondary and combined other stations on the link. Depending on the
HDLC mode, the primary station is responsible for issuing acknowledgement
packets to allow data transmission from secondary stations.
Secondary stations—Controlled by the primary station.
Under normal circumstances, secondary stations cannot control data
transmission across the link with the primary station, are active
only when requested by the primary station, and can respond to the
primary station only (not to other secondary stations). All secondary
station frames are response frames.
Combined stations—A combination of primary and
secondary stations. On an HDLC link, all combined stations can send
and receive commands and responses without any permission from any
other stations on the link and cannot be controlled by any other station.
