The “telephony service” provides speech transmission at an audio bandwidth of 3.1 kHz. The communication is bidirectional,with both directions continuously and simultaneously active during the speech phase.The network may use processing techniques appropriate for speech such as analogue transmission, echo cancellation and low bit-rate encoding.The digital signal at the S/T reference point follows the encoding laws for speech (according to Recommendation G.711),A-law or μ-law and the network may use digital signal processing techniques.It may also be necessary to use echo cancellation techniques in particular when interworking with other networks such as the PSTN.User information is provided over a B-channel, signalling is provided over the D-channel. Tones and announcements are provided by the network, encoded according to Recommendation G.711,although terminals can generate tones or other indications based on the messages received.