Integrated Services Digital Network (ISDN) - Advice of Charge (AOC) 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 300182-4 V 1.2.4 (1998-06) as German standard)