Communication DIVersion

Description

Communication DIVersion (CDIV) is a standardized set of supplementary services within the 3GPP IP Multimedia Subsystem (IMS) framework that provides dynamic control over the routing of incoming communication sessions. These sessions primarily include voice calls but can extend to other multimedia sessions like video calls and messaging, depending on the service configuration. The core principle involves intercepting an incoming session request (e.g., a SIP INVITE) at a network control point—typically the Serving-Call Session Control Function (S-CSCF) in the IMS core—and applying user-defined logic to determine whether to complete the call to the original destination or divert it elsewhere. This logic is executed by an Application Server (AS) hosting the CDIV service logic, which queries subscriber data and rules stored in the Home Subscriber Server (HSS) or a dedicated service profile repository.

The service operates based on a comprehensive set of diversion criteria and conditions defined in 3GPP specifications. Key diversion triggers include Busy (when the user's line is engaged), No Reply (when the user does not answer within a configurable time), Not Reachable (when the user's device is detached from the network or in a radio shadow area), and Unconditional (diverting all incoming sessions regardless of state). Additionally, more advanced conditions like diversion on user-not-logged-in or based on the calling party's identity (e.g., specific numbers or anonymous calls) can be configured. When a trigger condition is met, the CDIV AS instructs the S-CSCF to reroute the session signaling to a new destination address, which could be another telephone number, a voicemail server, or an instant messaging inbox. This rerouting is performed using standard SIP redirection responses (e.g., 302 Moved Temporarily) or by the AS acting as a back-to-back user agent (B2BUA) to establish a new leg to the diversion target.

Architecturally, CDIV is deeply integrated with the IMS service delivery platform. The initial Filter Criteria (iFC) in the user's service profile, downloaded from the HSS to the S-CSCF during registration, ensure that all relevant session initiation requests are forwarded to the designated CDIV Application Server for processing. The service logic on the AS evaluates the subscriber's active diversion settings, which the user can typically manage via Ut interface (XCAP) for self-provisioning or through operator-administered profiles. CDIV plays a critical role in the IMS service layer by decoupling the user's public identity (the dialed number) from the actual endpoint, enabling rich personal mobility and service personalization. Its implementation must also handle interactions with other supplementary services like Communication Waiting (CW) and Communication Hold (HOLD), following standardized precedence rules to avoid conflicts, and ensure proper charging records (CDRs) are generated for both the original and diverted call legs.

Purpose & Motivation

CDIV was created to provide a standardized, network-based call diversion capability for IP-based multimedia services, addressing the limitations of legacy circuit-switched diversion services and enabling seamless service continuity in the evolving IMS architecture. Prior to IMS, diversion features like Call Forwarding were implemented in circuit-switched mobile networks (GSM) and fixed networks, but these were often proprietary, limited in scope, and difficult to integrate with new IP-based applications. The transition to all-IP networks required a new, flexible mechanism that could work across diverse access networks (e.g., LTE, 5G, WLAN) and support multimedia sessions beyond simple voice calls. CDIV solves the problem of user accessibility and service personalization in a complex, multi-device environment by allowing users to define intelligent routing rules that reflect their availability and preferences.

The primary motivation was to replicate and enhance the popular call forwarding features from legacy systems within the IMS framework, ensuring backward compatibility for service providers migrating their networks while also enabling new, richer diversion scenarios. It addresses key user needs such as avoiding missed calls, screening unwanted communications, and creating a single point of contact that can reach the user on any device. From a network operator perspective, CDIV improves network resource utilization by efficiently redirecting sessions that would otherwise result in failed call attempts and unnecessary signaling load. By standardizing CDIV within 3GPP starting in Release 7, it ensured interoperability between different vendors' IMS equipment and allowed for the creation of a consistent user experience across various service providers and device types, forming a foundational service for commercial VoLTE and VoNR deployments.