Common Presence and Instant Messaging

Description

Common Presence and Instant Messaging (CPIM) is a comprehensive 3GPP service framework that provides standardized mechanisms for presence and instant messaging within the IP Multimedia Subsystem (IMS). It is not a single protocol but a set of specifications that define how presence information (such as user availability, willingness to communicate, and terminal capabilities) is managed, published, and subscribed to, and how instant messages are routed and delivered in a secure, interoperable manner. The architecture leverages IMS core elements like the Call Session Control Function (CSCF) for session control and routing, and the Home Subscriber Server (HSS) for user data. For presence, it typically employs a presence server that collects, aggregates, and distributes presence information from various presence sources (like user terminals or network elements) to authorized watchers (subscribers). For instant messaging, it defines two primary modes: session-based messaging, which establishes a SIP session for a message exchange, and page-mode messaging, which uses standalone SIP MESSAGE requests for immediate delivery without a session.

The technical operation of CPIM is deeply integrated with SIP (Session Initiation Protocol) and its extensions, particularly SIP for Instant Messaging and Presence Leveraging Extensions (SIMPLE). Presence involves a publication mechanism where a Presence User Agent (PUA) or a Presence Network Agent (PNA) sends PUBLISH requests containing Presence Information Data Format (PIDF) documents to the presence server. Watchers then subscribe to this information using SUBSCRIBE requests, receiving NOTIFY messages with updated presence data. The presence server acts as a central aggregator, applying authorization policies based on rules often stored in an XML Document Management Server (XDMS). For instant messaging, page-mode messages are sent as SIP MESSAGE requests routed through the IMS core (P-CSCF, I-CSCF, S-CSCF) to the recipient's S-CSCF for final delivery. Session-mode messaging involves an initial SIP INVITE to establish a messaging session, often using the Message Session Relay Protocol (MSRP) within the established session for the actual message transfer, providing benefits like delivery reports and support for larger messages or file transfers.

Key components in a CPIM-enabled network include the Presence Server (PS), which is the central entity for presence operations; the Resource List Server (RLS), which can handle subscriptions to lists of presentities (entities being watched) to reduce signaling; and various XDMS servers for managing presence authorization and group lists. The IMS core (CSCFs, HSS) provides the essential registration, routing, and subscriber management. CPIM's role is to provide a standardized, scalable, and secure service layer on top of IMS, ensuring that presence and IM services work consistently across multi-vendor networks and different service providers. It forms the foundation for enriched communication services, enabling features like "see what I see" and integration into broader service scenarios defined in later 3GPP releases.

Purpose & Motivation

CPIM was created to address the lack of standardized, interoperable presence and instant messaging services in mobile networks. Prior to its introduction, proprietary or internet-based IM and presence services (like those from individual vendors or over-the-top providers) were common, but they lacked integration with core network capabilities, guaranteed quality of service, robust security, and the ability for operators to monetize and control the service. This fragmentation also hindered the user experience, as services from different operators or on different devices often could not interoperate. The motivation for CPIM was to leverage the emerging IMS architecture—a standardized IP-based service delivery platform—to define a carrier-grade, billable, and reliable framework for these real-time communication services.

Historically, the growth of internet messaging and the desire for richer, more contextual communication (knowing if a contact is available before calling or messaging) drove the need for a standardized approach within the telecom ecosystem. 3GPP initiated work on CPIM in Release 6 to provide operators with the tools to offer competitive, value-added services beyond basic voice and SMS. It solved the problem of service silos by defining a common protocol suite based on IETF standards (like SIP/SIMPLE) but adapted and profiled for the controlled environment of a mobile operator's network. This allowed for integration with subscriber management, authentication, charging systems, and network policies, addressing limitations of best-effort internet services such as lack of guaranteed delivery, weak security, and no seamless mobility support.

Furthermore, CPIM provided the essential building blocks for more advanced communication services envisioned for future releases, such as Rich Communication Services (RCS). By establishing a standardized presence and IM core, it enabled a service ecosystem where features could be reliably composed and deployed, fostering innovation while maintaining interoperability and network control for operators.