Presence Information

Description

Presence information is a fundamental concept in 3GPP presence services, defined in specifications such as TS 22.127 (Service requirements for the Open Service Access) and TS 22.141 (Presence service). It consists of a set of attributes that characterize a presentity, which is an entity (typically a user) whose presence information is being tracked. These attributes can include the presentity's current activity (e.g., busy, available, in a meeting), environment (e.g., at home, in the office), communication means (e.g., voice, video, instant messaging), and contact addresses (e.g., phone numbers, SIP URIs). Presence information is dynamic and can be updated by the presentity or collected from various devices associated with the presentity.

The architecture for presence information involves several key entities: the presentity, the watcher, and the presence server. The presentity provides presence information, which is stored and managed by the presence server. Watchers are entities that request and receive presence information about a presentity, subject to authorization. The presence server aggregates presence information from multiple sources, such as the presentity's devices and network elements, and distributes it to authorized watchers. Only the system and the presentity have direct access to the raw presence information; watchers receive filtered or processed views based on privacy rules.

Presence information is typically modeled using the Presence Information Data Format (PIDF) defined by the IETF, which is adopted by 3GPP. PIDF is an XML-based format that represents presence information as a set of tuples, each containing a status and optional communication address. 3GPP has extended PIDF to include additional attributes relevant to mobile networks, such as network availability, roaming status, and device capabilities. The presence server uses these attributes to provide rich presence information to applications.

The management of presence information involves several procedures: publication, subscription, notification, and authorization. A presentity publishes its presence information to the presence server using a PUBLISH request (e.g., SIP PUBLISH). Watchers subscribe to presence information using a SUBSCRIBE request, and the presence server sends NOTIFY messages when the presence information changes. Authorization policies determine which watchers can access which attributes. The presence server may also compose presence information from multiple sources, such as the presentity's calendar, location, and device status.

Presence information is used by a wide range of applications, including instant messaging, push-to-talk, VoIP, and social networking. It enables services to adapt their behavior based on the user's availability and context. For example, a call can be routed to voicemail if the user is busy, or a message can be sent as an SMS if the user is not online. In 3GPP networks, presence information is also used for network optimization, such as deciding whether to page a UE or to use a different access technology. The presence service architecture is defined in TS 23.141 and TS 24.141, and it relies on the IP Multimedia Subsystem (IMS) for session control and the Ut interface for configuration.

Purpose & Motivation

Presence information exists to provide applications and services with knowledge about the current state and context of users, enabling more intelligent and personalized communication. Before presence services, applications had no standardized way to determine whether a user was available, what devices they were using, or how they preferred to be contacted. This led to inefficient communication attempts, such as calling a user who was already on another call or sending a message to an inactive device. Presence information solves this by providing a structured, extensible set of attributes that describe the user's situation.

The concept was introduced in 3GPP Release 99 as part of the early work on presence and instant messaging. It was motivated by the success of Internet-based presence systems like ICQ and AIM, and the desire to bring similar capabilities to mobile networks. The initial definition focused on a minimal set of attributes (status, identity), but it quickly evolved to include richer information such as communication means and contact addresses. This evolution reflects the growing complexity of mobile services and the need to support multiple devices and access technologies.

Presence information also addresses privacy and authorization concerns. Users need to control who can see their presence information and what attributes are visible. The 3GPP presence architecture includes mechanisms for authorization, such as presence authorization rules and watcher authorization. This ensures that presence information is only shared with trusted parties. Additionally, presence information can be aggregated from multiple sources, providing a more complete picture of the user's state. This is essential in multi-device scenarios where a user may be active on one device but idle on another.

Overall, presence information is a key enabler for context-aware services in 3GPP networks. It allows applications to make informed decisions about how to reach a user, improving user experience and network efficiency. Its standardized format and procedures ensure interoperability across different vendors and networks.