This European Telecommunication Standard (ETS) contains the Abstract Test Suite (ATS) specification for the Network Layer (NL)@ Circuit Mode Basic Services (CMBS) of the Inter-exchange signalling protocol@ for Private Integrated Services Networks (PISN). The objective of this ATS specification is to provide conformance tests which give a high probability of inter-operability of the NL. The ATS specification covers the procedures described in ETS 300 172 [1]. The ISO standard for the methodology of conformance testing (ISO/IEC 9646-1 [3]@ ISO/IEC 9646-2 [4] is used as basis for the test methodology. This ATS specification is applicable to implementations which support either a Basic Rate or a Primary Rate access interface@ or both@ operating over a leased line. It is applicable to various PINX roles@ i.e. Originating@ Terminating@ Transit@ Incoming/Outgoing Gateway. Annex A provides the Partial Protocol Implementation eXtra Information for Testing (PIXIT) proforma of this specification. Annex B provides the Protocol Conformance Test Report (PCTR) proforma of this specification. Annex C provides the Tree and Tabular Combined Notation (TTCN) part of this specification.