Integrated Services Digital Network (ISDN); User-to-User Signalling (UUS) supplementary service - Digital Subscriber Signalling System No. one (DSS1) protocol - Part 6: Abstract Test Suite (ATS) and partial Protocol Implementation eXtra Information for Testing (PIXIT) proforma specification for the network (Endorsement of the English version EN 300286-6 V 1.4.1 (2003-01) as German standard)