Abstract: Conformance with established standards and interoperability between products in an integrated services digital network (ISDN) system defined by a layered hierarchy of interrelated protocols is ascertained by remotely testing the protocols between a tester in a local testing centre and a system under test located at a geographically removed site. The protocols tested relate to the first three layers of the system which are concerned mainly with the establishment, holding and release of a telecommunications path. Cost effectiveness in product testing is achieved by accessing both the tester and the system under test and communicating test procedures between the local and remote sites over a packet switching network to support testing at the remote site. Communications over a virtual circuit in the network are established between a pair of remote ISDN test access interfaces, one located at each site.