Broadband Integrated Services Digital Network (B-ISDN); signalling ATM Adaptation Layer (SAAL); service Specific Co-ordination Function (SSCF) for support of signalling at the User-Network Interface (UNI) - Part 1: Specification of SSCF at UNI (ITU-T Q.2130 (1995), modified); English version ETS 300437-1:1995