ISO/IEC ISP 12067 is applicable to interworking units concerned with operating in the Open Systems Interconnection (OSI) environment. It specifies a combination of OSI base standards that collectively provide a relay function for the connection-mode Network Service.

This part of ISO/IEC ISP 12067 specifics subnetwork-dependent media-dependent requirements applicable to an interworking unit using virtual calls over a permanent access to a PSDN. The operation of an interworking unit may involve relaying from one subnctwork to another, and those subnctworks need not be of the same type. This part of the ISO/IEC ISP 12067 applies only to communications using PSDN subnetworks with permanent access on which virtual calls are used.