The present document specifies the subscriber line protocol for the support of PSTN display services at Local Exchange
(LE) in "on-hook" state. The subscriber line protocol is accomplished by using asynchronous voice-band
Frequency-Shift Keying (FSK) signalling. The data transmission specified in the present document is provided only in
the direction from the LE to the TE.
In addition, annex B is provided in order to allow the continuation of use in those networks that already implemented a
Dual Tone Multi-Frequency (DTMF) based subscriber line protocol.
The requirements imposed on the FSK signalling-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.
The requirements imposed on the DTMF-based subscriber line protocol deal with the transfer of the DTMF coded
display information. The procedures and the encoding arrangements are specified in annex B of the present document.
Terminal Equipment (TE) can be connected by analogue access directly to the LE or through 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 operators responsibility to ensure
transmission path establishment. Transmission path establishment procedures are outside the scope of the present
document.