Broadband integrated Services Digital Network (B-ISDN) - Digital subscriber signalling systems No. two (DSS2) protocol; Connection characteristics; ATM traffic descriptor modification with negotiation by the connection owner - 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 301486-4 V 1.1.1 (2002-02) as German Standard)