Integrated Services Digital Network (ISDN) - Digital Subscriber Signalling System No. one (DSS1) protocol; Completion of Calls on No Reply (CCNR) supplementary service - 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 301065-4 V 1.2.1 (2002-04) as German Standard)