In bit-synchronous protocols of the link layer, such as High Level Data Link Control (HDLC) or Synchronous Data Link Control (SDLC), the primary station controls and monitors the transmission activities of the secondary stations.
In addition to the above functions, the primary station performs management functions, such as error correction. The primary stations send commands to the secondary stations and receive appropriate responses from them.
In addition to the secondary station and the primary station, there is also the combined station. In HDLC protocols, the combined station can be a station that is able to assume the role of a primary station or a secondary station.