Primary
Primary stations manage the connection to all secondary sessions by sending commands. Primary stations open and close connections and poll remote stations for data, or availability. Primary stations are used primarilly in multi-point networks or with mainframe applications where the mainframe is the primary station. The primary station uses unbalanced mode to communicate with the secondary stations. The primary station is responsible for all error detection, line control and data flow control.
Secondary
Secondary machines respond to commands sent from the Primary. Secondary stations are usually terminals attatched to mainframes. Communication between secondary stations and primary stations is in unbalanced mode. The secondary station can only respond to the primary station and only when signaled to do so by the primary station.
Combined
Combined stations can establish and tear down their own connections. A combined station has the capabilities of both primary and secondary stations. These station types are usually used in point-to-point serial links such as a V.35 link to a CSU/DSU or between routers connected by a T1 or frame relay. Communication between two combined stations is in balanced mode.

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