Overview
ISO/IEC 9574:1992 defines how to provide the OSI Connection‑mode Network Service (CONS) by packet‑mode terminal equipment connected to an Integrated Services Digital Network (ISDN). The standard specifies the mapping of CONS primitives and parameters to and from the protocol elements used by two packet‑mode terminal types: an X.25 DTE (TE2) and a packet‑mode ISDN terminal (TE1) implementing the X.25 Packet Layer Protocol (PLP, ISO/IEC 8208). ISO/IEC 9574:1992 aligns with CCITT/ITU recommendations (notably X.31 and X.223/X.25) and is published as an IS/CCITT joint text.
Key topics and requirements
- Mapping CONS ↔ X.25 PLP: Specifies how CONS primitives and parameters are carried over the X.25 packet layer (ISO/IEC 8208) to implement the OSI connection‑mode network service.
- Terminal types covered: Defines behavior for TE1 (packet‑mode ISDN terminals) and TE2 (X.25 DTEs) attached at S/T and R ISDN reference points.
- ISDN attachment cases: Covers use with ISDN packet handling (Virtual Circuit Bearer Service) and access to packet‑switched data networks (PSDN) via:
- D‑channel based packet service,
- B or H channel - semi‑permanent or demand (circuit‑switched) connections,
- Direct circuit‑switched B/H channel links between terminals.
- Use of virtual calls: CONS mapping uses X.25 Virtual Calls (VCs); the standard explicitly excludes X.25 Permanent Virtual Circuits (PVCs) extension (left for further study).
- Interworking with ISDN signaling: References Q.921/Q.931 and ISDN layer specifications where additional signalling or call control procedures are required.
- Scope limits: Does not address TE2s using terminal adaptors (TAs) in certain DTE/DTE circuit‑switched modes.
Practical applications
- Implementers of ISDN packet‑mode terminals (TE1) and legacy X.25 DTE (TE2) software/firmware use ISO/IEC 9574 to ensure correct delivery of OSI network services over ISDN infrastructures.
- Network equipment vendors (packet handlers, access units, terminal adaptors) apply the mapping rules to interwork CONS with ISDN bearer and signalling services.
- Systems integrators and telecommunications engineers use the standard when connecting OSI‑based applications across ISDN links or when accessing PSDNs via ISDN.
- Useful in migration/interop projects where X.25 CONS semantics must be preserved across ISDN access.
Related standards
- ISO/IEC 8208 (X.25 PLP), CCITT/ITU‑T X.25, X.223 / ISO 8878 (use of X.25 for CONS)
- CCITT/ITU‑T X.31 (Support of packet‑mode terminal equipment by ISDN)
- Q.921, Q.931 (ISDN data link and layer 3 signalling)
- ISO/IEC 8348 (NS service definition for OSI)
Keywords: ISO/IEC 9574, CONS, OSI Connection‑mode Network Service, ISDN, X.25, TE1, TE2, packet mode, X.31, PLP, virtual calls, ISDN signaling.