scope:

The present document specifies the data link message and parameter codings for the PSTN subscriber line protocol to support display and related services sent by the Local Exchange (LE). The data transmission in the subscriber line protocol is accomplished by using asynchronous voice-band Frequency-Shift Keying (FSK) signalling.

The requirements imposed on the FSK signaling-based subscriber line protocol deal with data encoding, data transmission requirements and the three layers of the protocol at the network side of the interface: presentation layer, data link layer and physical layer.

Terminal Equipment (TE) can be connected by analogue access directly to the LE or via an Access Network (AN). In the latter case, data transmission can be applied from the LE or from elsewhere in the network hence a transmission path needs to exist from the LE to the TE before data transmission. It is the network operator's responsibility to ensure transmission path establishment. Transmission path establishment procedures are outside the scope of the document.