Overview
ISO/IEC 8473-5:1997 (identical to ITU‑T X.625) specifies how the Connectionless-mode Network Protocol (CLNP, ISO/IEC 8473-1) can use ISDN circuit‑switched B‑channels to provide the underlying network service. It defines the Subnetwork Dependent Convergence Function (SNDCF) behaviour for ISDN B‑channels, the assumptions CLNP makes about the subnetwork, and includes a PICS proforma (Protocol Implementation Conformance Statement) for conformance reporting.
Keywords: ISO/IEC 8473-5, CLNP, ISDN B-channel, SNDCF, connectionless-mode network service, PICS proforma.
Key topics and requirements
- SNDCF model: Defines how SN-UNIT-DATA requests/indications from CLNP map to subnetwork-specific UNIT-DATA requests/indications or to connection setup when the subnetwork access protocol is connection‑mode.
- ISDN B-channel usage: Applies when ISDN 64 kbit/s bearer (B‑channel) is used to carry CLNP PDUs between End Systems (ES) or Intermediate Systems (IS).
- Subnetwork user data (SN‑Userdata):
- Transferred as an ordered multiple of octets, transparently between subnetwork points of attachment.
- Underlying service must support an SDU size of at least 512 octets.
- If subnetworks involved support sufficiently large SDUs, segmentation need not be used; implementations may choose the full protocol or the non‑segmenting subset.
- Connectionless vs connection‑mode access: The standard describes both connectionless subnetwork access protocols and connection‑mode alternatives (logical channels/links). If a connection cannot be established for a connection‑mode subnetwork, the SN‑UNIT‑DATA request is discarded.
- Conformance and testing: Annex A provides the PICS proforma to document which protocol capabilities and options an implementation supports, per CCITT X.290 / ISO/IEC 9646‑1 testing methodology.
Practical applications and users
- Network equipment vendors implementing CLNP over ISDN links.
- Developers of ISDN terminal adapters, routers, gateways or intermediate systems that must interoperate over B‑channels.
- Standards engineers and testers preparing PICS/PAS documents and validating conformance to CLNP over ISDN.
- Service providers and integrators maintaining legacy ISDN‑based data services or interworking with packet networks.
Related standards
- ISO/IEC 8473‑1 (CLNP protocol specification)
- ISO/IEC 8473‑3, ‑4 (other subnetwork provisions)
- ITU‑T Rec. X.625 (identical text)
- CCITT/ITU‑T X.200, X.213, X.290 and relevant ISDN recommendations (e.g., I.231, Q.921, DSS1 / Q.931)
This standard is essential for implementing and testing CLNP over ISDN B‑channels and for documenting conformance via the provided PICS proforma.