Integrated Services Digital Network (ISDN) - User-to-User Signalling (UUS) supplementary service; Digital Subscriber Signalling System No. one (DSS1) protocol - Part 4: Abstract Test Suite (ATS) and partial Protocol Implementation eXtra Information for Testing (PIXIT) proforma specification for the user (Endorsement of the English version EN 300286-4 V 1.3.3 (2000-05) as German Standard)