IEC 13873-04 (CAN/CSA-ISO/IEC 13873-04): Private Integrated Services Network – Call Diversion Supplementary Service – Inter-exchange Signalling Protocol

Scope and Applicability

IEC 13873-04, also adopted as CAN/CSA-ISO/IEC 13873-04, specifies the signaling protocol for the Call Diversion supplementary service at the Q reference point (QSIG) of a Private Integrated Services Network (PISN). The standard is part of the PISN application layer protocol family and defines inter-exchange signaling procedures required for the diversion of incoming calls to alternative destinations.

Call Diversion allows the served user (the diverting party) to redirect incoming calls intended for a specific terminal to another terminal or number. The standard supports three diversion conditions: immediate (unconditional), busy, and no answer. It also covers the interaction of Call Diversion with other supplementary services and basic call control.

The target audience includes network equipment developers, test engineers, and compliance assessors involved in the design, integration, and certification of PISN components. The standard applies to both public and private network environments where QSIG signaling is deployed.

Technical Requirements and Specifications

Protocol Architecture

The Call Diversion protocol is based on the QSIG layer 3 signaling standard and uses the generic functional protocol defined in ISO/IEC 13872. It operates on top of the basic call control protocol (ISO/IEC 11572) and reuses the FACILITY message to convey diversion-specific information elements.

Diversion Types and Procedures

• Diversion Immediate (CFU): All incoming calls are redirected without any condition check.
• Diversion Busy (CFB): Calls are redirected only when the served user’s terminal is busy.
• Diversion No Answer (CFNR): Calls are redirected if the terminal does not answer within a predefined time.

The standard defines state machines for both the diverting user’s SSP (Signalling Pinch Point) and the diverting destination’s SSP. Message sequences include Diversion Request/Return Result pairs and Diversion Notification.

Information Elements

Each message carries mandatory and optional information elements, including:

• Diversion number (diverted-to address)
• Diversion reason (busy, no answer, immediate)
• Diversion status (active, inactive, pending)
• Notification indicator

Parameter coding follows the ASN.1 notation and encoding rules defined in other PISN standards.

Implementation Highlights

Interoperability Considerations

To ensure seamless operation across vendors, implementers must strictly follow the message sequencing and timer definitions (e.g., T_DIV for response timeout). The standard specifies three application protocol data unit (APDU) forms: network-side and user-side. It is critical to map these correctly for both originating and terminating exchanges.

Handling of diversion loops (e.g., A diverts to B, and B diverts both to A or another diverter) is addressed through hop count limits and error handling procedures. Implementations must include loop detection mechanisms and prevent infinite call routing.

Compatibility with Other Services

Call Diversion interacts with services such as Call Transfer, Conference, and Call Completion. IEC 13873-04 defines priority and conflict resolution rules. For example, when a call is diverted, the original called party may still receive a notification, but the diverting service takes precedence over Call Waiting.

Security and Privacy Aspects

The standard does not explicitly address security, but because diversion changes the intended recipient, unauthorized activation could be used for interception. Implementers should enforce authentication and authorization for diversion requests, especially in public or multi-tenant PISN environments. It is recommended to support the Restricted Diversion model where only authorized users can activate diversion.

Compliance and Certification Notes

Conformance Testing

Conformance to IEC 13873-04 is typically verified using a protocol analyzer against a suite of test cases defined in the associated Implementation Conformance Statement (ICS). The standard references a set of abstract test suites (ATS) that evaluate message format, sequence, and timer behavior. For CAN/CSA adoption, test laboratories accredited by the Standards Council of Canada may perform certification.

Relation to Other Standards

This standard forms part of a larger set of PISN inter-exchange signaling standards. A compliance matrix is often used to track dependencies:

• ISO/IEC 11572 – Basic call control
• ISO/IEC 13872 – Generic functional protocol
• ISO/IEC 11582 – Generic functional protocol for supplementary services

Adoption in Canada

The CAN/CSA version (CAN/CSA-ISO/IEC 13873-04) is identical to the international edition but includes a Canadian foreword and may have additional compliance notes regarding regulatory requirements for telecommunication equipment in Canada. Suppliers wishing to market in Canada should ensure that their product meets the CAN/CSA version and any applicable Industry Canada specifications.

Frequently Asked Questions

This article is based on the official text of IEC 13873-04 (2003 edition) and its Canadian adoption. All information is provided for technical reference. For regulatory or certification purposes, consult the latest version of the standard.