ISO/IEC 10177:1993

Overview

ISO/IEC 10177:1993 defines how an intermediate system or interworking unit (IWU) uses ISO/IEC 8208 (X.25 Packet Layer Protocol) to provide the OSI connection-mode Network internal layer service (NILS). The standard expresses a protocol mapping between NILS (as defined in ISO/IEC 10028) and X.25 Virtual Call (VC) and Permanent Virtual Circuit (PVC) services, and includes a PICS (Protocol Implementation Conformance Statement) proforma to support conformance testing per ISO/IEC 9646.

Keywords: ISO/IEC 10177, X.25, ISO/IEC 8208, Network internal layer service, NILS, interworking unit, VC, PVC, PICS, SNDCP.

Key topics and requirements

• Scope and purpose: Specifies the method for an IWU to support the OSI connection-mode Network service using X.25 at a subnetwork point of attachment. Applies where X.25 is used (public/private packet-switched networks, LANs, switched access via ISDN/CSDN/PSTN, DTE/DXE mode).
• Protocol mapping: Formal mapping of NILS primitives and parameters to/from X.25 packets and procedures (establishment, data transfer, release, errors). The mapping is defined so NILS primitives are direct abstractions of packet fields.
• NILS features covered: NC establishment, QOS negotiation, normal data transfer, flow control, expedited data, receipt confirmation, NC reset, NC release, connection control, error diagnostics.
• PVC support and SNDCP: Defines Subnetwork Dependent Convergence Protocol (SNDCP) procedures enabling a PVC to support one Network connection at a time and to be reused for different connections over time.
• Conformance and PICS: Provides a normative PICS proforma and modified PICS requirements for ISO/IEC 8208 in line with ISO/IEC 9646-1/2. Static conformance requirements list mandatory and optional NILS features.
• Compatibility notes: Covers compatibility with post-1984 X.25 and specifies limits regarding compatibility with 1980 X.25 for VC vs PVC.

Practical applications and users

Who uses ISO/IEC 10177:

• Network equipment vendors implementing IWU/intermediate systems that bridge OSI network-layer services over X.25.
• Telecom and network architects integrating legacy X.25 infrastructures with OSI-based internetworks.
• Protocol implementers developing packet layer entities (DTE/DXE interfaces) and SNDCP procedures for PVCs.
• Conformance testers and certification bodies using the PICS proforma to validate implementations.
• System integrators and operators managing packet-switched public/private networks, LAN-X.25 interworking, or switched access (ISDN/PSTN).

Practical benefits:

• Enables predictable interworking between OSI network services and X.25 subnetworks.
• Provides a standardized basis for testing and certifying IWU behavior.
• Aids migration and interoperability in environments where X.25 remains in use.

Related standards

• ISO/IEC 8208 - X.25 Packet Layer Protocol
• ISO/IEC 10028 - Relaying functions of a Network layer intermediate system (NILS)
• ISO 7498 / ISO/IEC 8348 - OSI Reference Model / Network service definition
• ISO 8648 - Internal organization of the Network Layer
• ISO/IEC 9646-1 / -2 - Conformance testing methodology and PICS guidance

This standard is essential when designing, implementing, or certifying intermediate systems that must map OSI connection-mode network behavior onto X.25 virtual circuits.