Multimedia Broadcast Multicast Service User Service Discovery and Announcement

Description

MBMS user service discovery and announcement are complementary processes that enable UEs to find and access MBMS user services. User service discovery refers to the methods by which a UE obtains the list of available MBMS user services along with information about each service, such as the service identifier, session description, and scheduling. User service announcement refers to the methods by which the MBMS service provider makes this list and associated information available to UEs. Together, they form the entry point for users to access MBMS-based content and services.

Architecturally, the announcement function is typically implemented in the BM-SC (Broadcast Multicast Service Center), which is responsible for service provisioning and content delivery. The BM-SC generates service announcements, which are then distributed to UEs via various channels. These announcements can be delivered using different transport mechanisms, such as HTTP, FLUTE (File Delivery over Unidirectional Transport), or SMS. The choice of mechanism depends on the service requirements and network capabilities. For example, in LTE, announcements are often delivered via the Service Announcement Function (SAF) using FLUTE over MBMS bearers or via unicast HTTP.

The discovery process on the UE side involves receiving and processing these announcements. The UE may use a combination of push (network-initiated) and pull (UE-initiated) methods. In push mode, the network broadcasts announcements periodically, and the UE listens for them. In pull mode, the UE queries a discovery server (e.g., via HTTP) to obtain the service list. The UE then presents the available services to the user, who can select one to join. The service information includes details like the TMGI, session start time, media type, and codec information, which the UE needs to configure its receiver.

Key components include the Service Announcement Function (SAF), which is part of the BM-SC and handles the generation and distribution of announcements. The announcement itself is typically an XML document conforming to the MBMS User Service Description (USD) schema, which is defined in 3GPP TS 26.346. The USD contains metadata about the service, such as the service name, description, language, and the session description (often in SDP format). The UE parses the USD to extract the necessary information to join the service.

The discovery and announcement mechanisms have evolved over releases, with improvements in efficiency and flexibility. For instance, Release 17 introduced a minor change in the definition, capitalizing 'User Services' in the announcement part, but the core concept remained the same. The specifications 26.237, 26.346, 26.852, and 26.946 provide the detailed protocols and procedures for these functions. Overall, MBMS user service discovery and announcement are essential for making MBMS services accessible to users, ensuring that they can easily find and consume broadcast/multicast content.

Purpose & Motivation

MBMS user service discovery and announcement exist to solve the problem of how users can find and access broadcast/multicast services in a mobile network. Without these mechanisms, users would have no way of knowing what MBMS services are available or how to join them. The announcement function allows service providers to advertise their services to a wide audience, while the discovery function enables UEs to retrieve this information and present it to users. This is analogous to an electronic program guide (EPG) in traditional broadcast TV, but adapted for mobile networks.

Before these mechanisms were standardized, MBMS services would have required manual configuration or out-of-band information, which is impractical for mass-market services. The introduction of standardized discovery and announcement in Release 8 made MBMS services user-friendly and interoperable. It allowed operators to offer a variety of services, from mobile TV to emergency alerts, with a consistent user experience. The mechanisms also support dynamic updates, so if a service changes (e.g., session time shifts), the UE can be informed.

The need for efficient discovery is particularly important in mobile environments where users expect instant access to content. The 3GPP designed these mechanisms to be flexible, supporting both push and pull models, and to work over different transport channels. This ensures that even in challenging network conditions, users can discover services. The evolution of the definition over releases has been minimal, indicating that the original design was robust and has stood the test of time. The capitalization change in Release 17 is cosmetic, reflecting editorial consistency rather than a functional change.