Integrated Services Digital Network (ISDN) - Diversion supplementary services; 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 300207-4 V 3.1.1 (2001-11) as German Standard)