Presence Service

Description

Presence service is a 3GPP-defined capability that supports the management of presence information between watchers and presentities. It is specified in TS 22.127 and TS 22.141. The service enables applications and services to make use of presence information by providing a standardized set of procedures for publishing, subscribing to, and notifying changes in presence information. The presence service acts as an intermediary between presentities (entities that provide presence information) and watchers (entities that consume presence information), ensuring that information is shared according to authorization rules.

The architecture of the presence service includes several functional entities: the presence server, the presence client (in the UE), and the presence XML document management server (XDMS). The presence server is the core entity that stores and manages presence information. It receives publications from presentities, handles subscriptions from watchers, and sends notifications when presence information changes. The presence client in the UE allows the user to publish their own presence information and to subscribe to others' presence information. The XDMS manages authorization policies and presence rules, such as who can see what information.

The presence service relies on the Session Initiation Protocol (SIP) for signaling. The SIP PUBLISH method is used by a presentity to upload presence information to the presence server. The SIP SUBSCRIBE method is used by a watcher to request presence information, and the SIP NOTIFY method is used by the presence server to deliver presence information to the watcher. The presence information itself is encoded in the Presence Information Data Format (PIDF), an XML-based format. 3GPP has extended PIDF to include mobile-specific attributes, such as network availability, roaming status, and device capabilities.

Authorization is a critical component of the presence service. Before a watcher can receive presence information, the presentity must grant permission. This is managed through presence authorization rules, which are stored in the XDMS. The rules specify which watchers are allowed to see which attributes of the presence information. The presence server enforces these rules when processing subscriptions and notifications. Additionally, the presence service supports privacy features such as anonymous subscriptions and blocking.

The presence service is used by a variety of applications, including instant messaging, push-to-talk over cellular (PoC), VoIP, and social networking. It enables these applications to adapt their behavior based on the user's availability and context. For example, a PoC session can be initiated only if the target user is available. The presence service also supports interworking with other presence systems, such as those based on the IETF SIMPLE framework. In 3GPP, the presence service is part of the IMS and is accessible via the Ut interface for configuration and the ISC interface for application servers.

Purpose & Motivation

The presence service was created to provide a standardized, interoperable way for applications to access and utilize presence information in mobile networks. Before its introduction, presence capabilities were fragmented and proprietary, making it difficult for applications to work across different networks and devices. The presence service solves this by defining a common architecture and set of protocols based on IETF standards (SIP, PIDF) and 3GPP specifications. This allows applications to query and subscribe to presence information regardless of the underlying network technology.

The service was introduced in 3GPP Release 99 as part of the early work on presence and instant messaging. It was motivated by the growing popularity of Internet-based presence systems and the need to bring similar capabilities to mobile users. The initial definition focused on the basic capability to manage presence information between watchers and presentities. Over time, the service has evolved to include richer features such as authorization, privacy, and interworking with other presence systems.

The presence service also addresses the need for efficient use of network resources. By providing presence information, it can help applications avoid unnecessary communication attempts, such as calling a user who is already busy. This reduces signaling load and improves user experience. Additionally, the presence service enables new revenue-generating services, such as presence-enabled advertising and context-aware content delivery. Its standardized nature ensures that these services can be deployed across different operators and vendors.

In summary, the presence service exists to enable applications to make use of presence information in a standardized, secure, and efficient manner. It provides the necessary infrastructure for publishing, subscribing, and notifying presence changes, while respecting user privacy and authorization. Its evolution reflects the increasing importance of context-aware services in mobile networks.