Integrated Services Digital Network (ISDN) - Signalling System No. 7 (SS7) - ISDN User Part (ISUP) version 4 for the international interface - Part 33: Abstract Test Suite (ATS) and partial Protocol Implementation eXtra Information for Testing (PIXIT) proforma specification for basic services (Endorsement of the English version EN 300356-33 V4.1.1 (2003-07) as German standard)