Overview
ISO/IEC 10732:1993 specifies how the X.25 Packet Layer Protocol (PLP, ISO/IEC 8208) is used to provide the OSI Connection-mode Network Service (CONS) over the telephone network. It covers both leased-circuit and switched-circuit telephone links and defines the mapping of CONS primitives and parameters onto X.25 elements and procedures required when a switched telephone connection is present.
Two primary configurations are addressed:
- DTE-to-DTE over a telephone network (leased or switched)
- DTE-to-PSDN (DTE-to-DCE) via a telephone network (leased or switched)
The standard complements CCITT Rec. X.223 / ISO 8878 by adding provisions specific to switched telephone network operation.
Key Topics and Requirements
- Mapping CONS to X.25 PLP: Specifies how CONS primitives/parameters are translated to and from X.25 packet-layer elements (as referenced in X.223 / ISO 8878).
- Control of underlying connections:
- No extra procedures for leased circuits.
- For switched connections, a successful N-CONNECT request triggers phone-call setup (V-series modems, V.25 or V.25 bis signaling). Failure is reported as an N-DISCONNECT indication.
- Procedures for unexpected disconnection and recovery are defined; mapping to OSI N-DISCONNECT indications applies.
- Data link layer constraints:
- ISO 7776 (LAPB-compatible) single-link basic mode (modulo 8) is mandatory; extended mode (modulo 128) may be implemented.
- Addressing: Calling end = A, called end = B on switched links unless pre-agreed.
- Recommendations: T1 timer adjustable to 5 s for long-delay links (e.g., satellite); support for 1031-octet frames and possibly large windows.
- Packet layer considerations:
- Telephone-network addressing is used to establish switched connections.
- For DTE-to-DTE over telephone links the X.25 Address Field is empty; Called/Calling NSAPs are carried in address extensions. Any Address Field contents received should be ignored.
- Identification exchange: When needed (billing/security), identification methods follow X.32 procedures; governed a priori for DTE-to-DTE or by PSDN for DTE-to-DCE.
Applications and Who Uses It
- Network and telecom engineers implementing X.25 over PSTN/PSDN.
- Equipment vendors building modems, DTEs, DCEs, and packet-switching interfaces that must interoperate with OSI CONS.
- System integrators deploying legacy X.25-based applications across telephone networks.
- Standards and compliance teams verifying conformance with international X.25/OSI mappings.
Keywords: ISO/IEC 10732:1993, X.25, OSI CONS, telephone network, switched connection, leased circuit, DTE, DCE, PSDN, ISO/IEC 8208, ISO 7776, V.25
Related Standards
- CCITT Rec. X.223 / ISO 8878 (mapping CONS to X.25 PLP)
- ISO/IEC 8208 (X.25 Packet Layer Protocol)
- ISO 7776 (LAPB-compatible data link procedures)
- CCITT Rec. X.32 (DTE access procedures and identification)
- CCITT Rec. X.213 / ISO/IEC 8348 (OSI Network Service definition)
- V.25 / V.25 bis (telephone signaling for switched calls)