V interfaces at the digital Service Node (SN) - Interfaces at the VB5.1 reference point for the support of broadband or combined narrowband and broadband Access Networks (ANs) - Part 4: Abstract Suite (ATS) and partial Protocol Implementation eXtra Information for Testing (PIXIT) proforma specification (Endorsement of the English version EN 301005-4 V 1.1.2 (2000-05) as German Standard)