Overview
ISO/IEC 8878:1992 specifies how to use the X.25 Packet Layer Protocol (PLP) to provide the OSI Connection‑mode Network Service (CONS). The standard defines a mapping between the abstract CONS primitives/parameters and the concrete X.25 packet fields and procedures. The main body addresses X.25/PLP‑1984 using Virtual Calls (VCs) and the annexes provide alternative mappings and implementation guidance (including an SNDCP for X.25/PLP‑1980 and a PVC method).
Keywords: ISO/IEC 8878:1992, X.25, OSI Connection‑mode Network Service, CONS, X.25/PLP‑1984, virtual calls
Key topics and technical requirements
- Primitive-to-packet mapping: Detailed relationships between CONS primitives/parameters and X.25 PLP packets/fields for connection establishment, data transfer, release and reset phases.
- Procedure sets and implementation classes: Two sets of procedures are specified from which three classes are described - System‑1984, System‑1980, and Compatible implementations - to support interworking and migration.
- Data transfer services: Support for normal data transfer, receipt confirmation, expedited transfer and reset services, including packet/field mappings and procedures.
- Annexed protocols and guidance:
- Annex A: Subnetwork Dependent Convergence Protocol (SNDCP) for X.25/PLP‑1980.
- Annex B: Classification of systems and interworking rules.
- Annex C: Mapping for Permanent Virtual Circuits (PVCs).
- Annex D: Protocol Implementation Conformance Statement (PICS) proforma.
- Annexes E–I: additional considerations (NPAI addressing, priority negotiation, transit delay calculations, differences vs. X.223).
- Conformance and optional features: Clause 13 and annex D define conformance criteria and PICS requirements for claimants.
Keywords: primitive mapping, PICS, SNDCP, PVC, conformance, X.25 VC
Practical applications
- Enabling legacy and current X.25 subnetworks to support OSI‑model network services where CONS semantics are required.
- Providing a migration path from older X.25 implementations (1980) to X.25/PLP‑1984 features while preserving interoperability.
- Designing or testing network layer implementations (end systems and relay nodes) that must interoperate across mixed X.25 environments.
- Documenting conformance for product certification and interoperability testing.
Keywords: legacy networks, migration, interoperability, X.25 subnetwork, network layer
Who should use this standard
- Network protocol engineers and architects implementing OSI Network Layer over X.25.
- Vendor product teams building X.25 DTE/DCE or network‑layer relay software.
- Test labs and compliance teams preparing PICS and interoperability test plans.
- Organizations operating packet‑switched or integrated networks (public/private X.25, LAN gateways, ISDN interworking).
Related standards
- CCITT/ITU‑T Recommendation X.223 (related mapping)
- CCITT/ITU‑T X.25 (Packet Layer Protocol) and ISO/IEC 8208 (X.25 PLP for DTE)
- ISO/IEC 8348 / CCITT X.213 (Network Service definition)
Keywords: X.223, ISO/IEC 8208, ISO/IEC 8348, X.25 PLP