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Comprehensive Guide to IEC 15055-01:2026 – Inter-Exchange Signalling for Call Completion in Private Integrated Services Networks
Understanding the Protocol Architecture, Technical Specifications, and Implementation Considerations for ISO/IEC 15055-01
1. Scope and Purpose
IEC 15055‑01:2026, adopted in Canada as CAN/CSA‑ISO/IEC 15055‑01 defines the inter‑exchange signalling protocol for call completion supplementary services within Private Integrated Services Networks (PISN). The standard specifies messages, procedures, and protocol elements that enable a user to request that a network complete a call attempt when the called terminal becomes available (e.g., after being busy or after no reply). It supports both Call Completion to Busy Subscriber (CCBS) and Call Completion on No Reply (CCNR).
The standard sits within the ISO/IEC 15055 series and is fully aligned with ITU‑T Recommendations Q.733 and Q.733.1 for private networks. It is intended for use with PISN interconnection (Pinx) entities and assumes the underlying circuit‑mode or packet‑mode bearer services defined by ISO/IEC 11574 and ISO/IEC 15056.
Key benefit: IEC 15055‑01 enhances end‑user productivity by automating call retry procedures across private network boundaries without requiring repeated manual dialing or user intervention.
2. Technical Requirements
2.1 Protocol Architecture
The protocol defined in IEC 15055‑01 conforms to the OSI Reference Model layers 3, 4, 5, and 7. The signalling messages are exchanged over the Q reference point between PISN nodes using the Q‑Sig signalling system. The protocol relies on the generic functional procedures and message transport capability defined in ISO/IEC 11572 (Q‑Sig layer 2) and ISO/IEC 11582 (Q‑Sig layer 3).
Key protocol elements include:
- Invocation: A user requests call completion activation via a FACILITY message containing the appropriate supplementary service invocation identifier.
- Notification: When the called user becomes available, the originating network sends a SETUP message that includes call completion indicator information.
- Recall and release: Timers and state machines ensure that call completion requests are terminated after a configurable period or after a certain number of unsuccessful attempts.
2.2 Message Types and Information Elements
The standard defines a set of supplementary service control messages and associated information elements. The principal messages are shown in Table 1.
| Message | Direction | Purpose |
|---|---|---|
| FACILITY | Bidirectional | Invoke, activate, deactivate, or interrogate a call completion service |
| SETUP | Originating → Terminating | Establish a new call after the called party becomes available |
| RELEASE | Both | Release a call or call completion association (e.g., when the request is cancelled) |
| INFORMATION | Both | Carry supplementary service status and call state information |
| NOTIFY | Bidirectional | Indicate call completion events (e.g., recall initiated) |
Each message carries mandatory and optional information elements. The mandatory elements include protocol discriminator, call reference, message type, and a supplementary service indicator. Optional elements carry service‑specific parameters such as call completion indicator, time‑to‑live, and call identity.
Implementation note: To guarantee interoperability, implementers must carefully encode the call completion indicator field as a bit‑mask that distinguishes between CCBS, CCNR, and combined requests. Non‑support of the full mask may cause failed interworking between peer nodes from different vendors.
3. Implementation Highlights
Deploying IEC 15055‑01 in a PISN environment requires careful consideration of the existing network architecture and the interworking with public networks.
3.1 Interworking with Public Networks
When a PISN is connected to the public switched telephone network (PSTN) or an ISDN, the Q‑Sig call completion protocol must be mapped to the network‑specific signalling, such as DSS1 or SS7. IEC 15055‑01 describes a set of interworking functions that preserve the semantics of call completion requests across the boundary. Network operators should ensure that the public network side supports corresponding services (e.g., CCBS as defined in ETSI EN 300 356).
3.2 Timer and Retry Configuration
The standard defines a number of configurable timers (T100–T105) that control the activation, monitoring, and re‑attempt behaviour. Table 2 lists the most critical timers.
| Timer | Default (s) | Purpose |
|---|---|---|
| T100 – Activation timeout | 30 | Maximum waiting time for a response to an activation request |
| T101 – Recall delay | 60 | Delay before the first call re‑attempt after an available indication |
| T102 – Repetition interval | 180 | Interval between successive call attempts while the called user remains available |
| T103 – Overall service duration | 3600 | Maximum time a call completion request remains active |
Administrators should adjust timer values to match network load and user expectations. Short T101 values may cause premature recall attempts, while long T102 values may delay the connection.
Security consideration: Call completion requests can inadvertently create loops if not properly bounded. The standard requires that each request carry a loop detection information element. Networks that fail to check this element risk signalling congestion and unintended call charges.
4. Compliance and Certification
Products claiming conformance to IEC 15055‑01 must undergo a series of static and dynamic tests. The conformance testing methodology is described in ISO/IEC 15055‑10 (TS). Key compliance areas include:
- Protocol behaviour: All state transitions and message sequences must exactly match the finite state machine specified in the standard. This is verified by a formal Abstract Test Suite (ATS).
- Encoding rules: All messages and information elements must be encoded using the ASN.1 PER (Packed Encoding Rules) scheme defined in the standard or alternatively the basic encoding rules (BER) for Q‑Sig baseline implementations.
- Interoperability testing: The standard encourages plug‑tests between different vendors’ equipment. A list of mandatory and optional capabilities must be declared in a PICS (Protocol Implementation Conformance Statement).
Non‑compliance risk: Failure to implement the mandatory call completion indicator exactly as specified can cause silent call failures. In multi‑vendor environments this may lead to revenue loss and customer dissatisfaction. Always verify that your product passes the ATS before market introduction.
For manufacturers, obtaining a certificate from an accredited testing laboratory (e.g., based on ISO/IEC 17025) is highly recommended. The certification body will review the PICS, execute the controlled tests, and issue a report valid for three years.
In Canada, the standard is published as CAN/CSA‑ISO/IEC 15055‑01 and compliance may be required for equipment operating within federal regulated telecommunications networks. Implementers should consult the latest edition (2026) and any national deviations published by the Canadian Radio‑television and Telecommunications Commission (CRTC).
Frequently Asked Questions
Q: What is the difference between CCBS and CCNR in IEC 15055‑01?
A: CCBS (Call Completion to Busy Subscriber) is activated when the called party is busy; the system monitors the busy state and completes the call as soon as the line becomes idle. CCNR (Call Completion on No Reply) applies when the called user does not answer within a predefined time; the network continues to ring and establishes the connection once the user answers.
A: CCBS (Call Completion to Busy Subscriber) is activated when the called party is busy; the system monitors the busy state and completes the call as soon as the line becomes idle. CCNR (Call Completion on No Reply) applies when the called user does not answer within a predefined time; the network continues to ring and establishes the connection once the user answers.
Q: Is IEC 15055‑01 backward‑compatible with earlier private network signalling protocols?
A: The standard was designed to interoperate with existing Q‑Sig implementations defined in ISO/IEC 11572 and ISO/IEC 11582. However, older nodes that do not support call completion supplementary services will ignore the relevant FACILITY messages. A gateway function may be required for full interworking.
A: The standard was designed to interoperate with existing Q‑Sig implementations defined in ISO/IEC 11572 and ISO/IEC 11582. However, older nodes that do not support call completion supplementary services will ignore the relevant FACILITY messages. A gateway function may be required for full interworking.
Q: What testing equipment is recommended for compliance verification?
A: Specialized protocol analyzers supporting Q‑Sig and the IEC 15055 family are available from vendors such as Spirent, Empirix, and Tektronix. Additionally, open‑source conformance test frameworks (e.g., Tetramino) can be adapted to run the ATS against the implementation under test.
A: Specialized protocol analyzers supporting Q‑Sig and the IEC 15055 family are available from vendors such as Spirent, Empirix, and Tektronix. Additionally, open‑source conformance test frameworks (e.g., Tetramino) can be adapted to run the ATS against the implementation under test.
Q: Does the 2026 edition introduce any breaking changes compared to the 2020 edition?
A: The 2026 edition primarily clarifies ambiguous state diagrams and adds a new optional information element for call control priority. Implementers should review the Annexes carefully; most changes are backward‑compatible, but the new priority element may affect call queuing behaviour if activated.
A: The 2026 edition primarily clarifies ambiguous state diagrams and adds a new optional information element for call control priority. Implementers should review the Annexes carefully; most changes are backward‑compatible, but the new priority element may affect call queuing behaviour if activated.