This section describes the protocols handled by the system and the points where these protocols change. The figure is a pictorial guide through intra-port and inter-port data transmission state changes. The pictorial guide illustrates the flow of data from DTE equipment, through DCE equipment, into a communications port on the system. The data flow is shown by solid lines. Below these lines are the protocols used at particular points in the data stream.

Not shown in the figure is the treatment of D-channels in ISDN-PRI and ISDN-BRI transmissions. PRI and BRI D-channels transport information elements contain call-signaling and caller information. These elements conform to ISDN level-3 protocol. In case of BRI, the elements are created by the terminal or data module. In case of PRI, the elements are created by the system. The system inserts the elements into the D channel at the DS1 port.

Therefore, for ISDN transmissions, BRI terminals and data modules, and DS1 ports insert, interpret, and strip both Layer-2 DCE information and Layer-3 elements. Also, the DS1 port passes Layer-3 elements to the system for processing. For more information about Layer 2 or 3, see Open Systems Interconnection model.