Broadband Integrated Services Digital Network (B-ISDN) - Signalling ATM Adaptation Layer (SAAL); Service Specific Co-ordination Function (SSCF) for support of signalling at the Network Node Interface (NNI) - Part 1: Specification of SSCF at NNI (ITU-T Q.2140 (1995), modified); English version ETS 300438-1:1995