Integrated Services Digital Network (ISDN) - Digital Subscriber Signalling System No. one (DSS1) protocol; Basic call control; enhancement at the "b" service entry point for Virtual Private Network (VPN) applications - 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 301060-6 V 1.1.4 (1999-11) as German standard)