Overview
ISO/IEC 22535:2009 specifies how to tunnel QSIG over SIP within a corporate telecommunication network (CN). It defines procedures for encapsulating QSIG messages inside SIP requests and responses so that calls and call‑independent signalling originating in a PISN (Private Integrated Services Network) can traverse an IP network using SIP and be delivered intact to another PISN. The standard applies to interworking units (gateways) that bridge QSIG‑based circuit‑switched domains (PINXs) and SIP‑based corporate IP networks. Tunnelling through public IP networks is outside the normative scope, though the mechanisms are applicable when combined with address translation, security and other functions.
Key topics and technical requirements
- Tunnelling model: Encapsulation of QSIG messages within SIP dialogs so QSIG functionality is preserved across an IP hop using SIP.
- Gateway roles: Definitions of ingress and egress gateways and their responsibilities when handling QSIG SETUP and connectionless QSIG messages.
- Procedures: Message handling procedures for:
- Encapsulation in SIP INVITE / responses
- Handling subsequent QSIG messages within an established SIP dialog
- Terminating the SIP dialog
- SDP/offer‑answer: Handling late availability of SDP parameters and a mandatory payload renegotiation with reversed offer/answer direction introduced for enhanced interoperability.
- Backwards compatibility: An indicator to detect and fall back to earlier signalling procedures when required.
- QSIG connectionless transport: Support for connectionless QSIG message types carried in SIP context.
- Security considerations: Guidance on securing the tunnelled QSIG/SIP exchange (see clause on security considerations).
- Conformance language: Uses RFC 2119 keywords (MUST / SHOULD / MAY) to indicate requirement levels.
Applications and who uses it
- Enterprise network architects and telecom engineers deploying mixed environments containing PISN/PINX and SIP‑based IP telephony.
- Gateway vendors implementing interworking functionality between QSIG and SIP to preserve supplementary services across IP links.
- Systems integrators connecting legacy PBX/QSIG islands across corporate IP networks.
- Network planners who require a standards‑based method to carry QSIG signalling over SIP within a private intranet while minimizing loss of QSIG capabilities.
Related standards
- ISO/IEC 11572, ISO/IEC 11582 (QSIG fundamentals and supplementary services)
- ISO/IEC 17343 (basic call interworking between QSIG and SIP)
- RFC 3261 (SIP), RFC 3264 (SDP offer/answer), RFC 3204 (MIME types for ISUP/QSIG), RFC 3840 (UA capabilities)
- RFC 2119 (requirement keyword usage)
ISO/IEC 22535:2009 is essential reading for those building interoperable gateways and corporate telecommunication solutions that require reliable tunnelling of QSIG over SIP.