Completion of Communications on Not Logged In

Description

Completion of Communications on Not Logged In (CCNL) is a service defined within the IP Multimedia Subsystem (IMS) framework to handle incoming communication requests for a user whose device is not currently registered with the IMS network. This state, known as 'not logged in,' occurs when a user's User Equipment (UE) is powered off, out of coverage, or has explicitly deregistered from IMS to conserve battery, yet the network still needs to deliver an incoming session, such as a Voice over LTE (VoLTE) call or a multimedia session. The CCNL service is invoked by the Serving-Call Session Control Function (S-CSCF) when it receives an initial SIP request (e.g., INVITE) for a user who is not registered. The S-CSCF, upon determining the user is not logged in, checks the user's service profile, which is stored in the Home Subscriber Server (HSS). If the CCNL service is subscribed and active, the S-CSCF triggers the service logic.

The core mechanism involves the S-CSCF interacting with an Application Server (AS) that hosts the CCNL service logic. This AS can be a Telephony Application Server (TAS) or a dedicated service platform. The AS, upon receiving the trigger from the S-CSCF, evaluates the request based on the user's service configuration and potentially subscriber preferences (like time-of-day rules or caller filtering). If the session is to be completed, the AS instructs the S-CSCF to perform a 'network-initiated' or 'third-party' registration on behalf of the user. This process temporarily registers the user's Public User Identity in the IMS, allowing the incoming SIP request to be routed correctly. The S-CSCF then resumes processing the original session establishment request as if the user were registered, forwarding it towards the UE, which must power on or re-attach to the network to accept the session.

Key architectural components involved include the S-CSCF, which acts as the central node for session control and service triggering; the HSS, which stores the user's service profile including CCNL subscription data; and the AS, which executes the service logic. The interface between the S-CSCF and the AS is typically the IMS Service Control (ISC) interface using SIP. CCNL plays a critical role in enhancing user experience by preventing missed communications during temporary network absences. It is particularly important for VoLTE and Rich Communication Services (RCS), where seamless call completion is expected. The service also interacts with other IMS enablers like Terminating Access Domain Selection (T-ADS) to choose the appropriate access network (e.g., CS fallback) if IMS registration fails, ensuring robust delivery.

Purpose & Motivation

CCNL was introduced to solve the problem of missed incoming communications in IMS-based networks, such as those delivering VoLTE and multimedia services. Prior to CCNL, if a user's device was not registered with the IMS (e.g., due to being switched off, in a deep sleep state, or out of coverage), incoming SIP session requests would be rejected by the network, leading to a 'user not registered' response and a missed call or message notification. This was a significant degradation compared to legacy circuit-switched (CS) networks, where the network could often page a device and complete a call even if it was in an idle state. The motivation for CCNL stemmed from the need to provide service parity and continuity as operators migrated voice and messaging services to all-IP IMS architectures.

The creation of CCNL was driven by the requirement to support power-efficient devices and user-controlled registration states without sacrificing reachability. Users might explicitly deregister from IMS to save battery, or devices might enter power-saving modes that involve periodic registration. CCNL addresses the limitations of the basic IMS registration model by allowing the network to temporarily re-establish registration context for the specific purpose of completing an incoming communication. This solves the business and technical problem of ensuring users remain reachable, which is a fundamental expectation for telephony services. It also enables new service scenarios, such as deferred session establishment, where the network can hold a session request until the user becomes available.