UNE-ETS 300661-1ED1

Broadband Integrated Services Digital Network (B-ISDN);Digital Subscriber Signalling System No. two (DSS2) protocol;Direct Dialling In (DDI) supplementary service;Part 1: Protocol specification [ITU-T Recommendation Q.2951, clause 1 (1995), modified] (Endorsed by Asociación Espanola de Normalización in December of 2022.)