Multimedia Broadcast Multicast Service Selected Services

Description

MBMS Selected Services refers to a specific subset of MBMS activated services in broadcast mode that a User Equipment (UE) has indicated to the UTRAN (UMTS Terrestrial Radio Access Network) as being selected for reception. This concept is defined in 3GPP specifications 25.331 and 25.346 and is crucial for managing UE interest in broadcast/multicast services. When upper layers in the UE (such as the application or middleware) determine that a particular MBMS service should be received, the UE uses RRC (Radio Resource Control) procedures to inform the network of this selection. The network can then use this information to optimize resource allocation and paging.

The mechanism operates within the context of MBMS broadcast mode, where services are transmitted to all interested UEs in a cell without a dedicated connection. However, not all UEs in a cell may be interested in all available services. By signaling the selected services, the UE helps the network understand which services are actually being consumed, allowing for more efficient use of radio resources. For example, if no UEs have selected a particular service, the network may decide to stop transmitting it in that cell, thereby saving resources.

The RRC procedures used for this purpose include the MBMS Modification Request and the MBMS Service Selection procedure. When a UE selects a service, it sends an RRC message to the UTRAN indicating the service identity. The UTRAN acknowledges this and may update the UE's context. The UE must also inform the network when it deselects a service, so that the network can maintain an accurate view of service demand. This information is particularly important for the counting mechanism, where the network may decide to switch between point-to-multipoint and point-to-point transmission based on the number of interested UEs.

In terms of architecture, the MBMS Selected Services concept involves the UE, the UTRAN (specifically the RNC), and the core network (SGSN and GGSN). The RNC maintains a list of selected services per UE and uses this for radio resource management. The SGSN may also use this information for session management and charging. The signaling is carried over the RRC connection, which must be established if the UE is in idle mode. This implies that the UE may need to transition to connected mode to signal its selections, which has implications for battery life and signaling load.

The concept has been part of MBMS since Release 6 and has remained largely unchanged in definition, although its implementation has evolved with the introduction of new radio technologies. In later releases, similar concepts were adapted for LTE and 5G, where the signaling is handled by the eNB/gNB and the MME/AMF. The fundamental principle remains: the UE informs the network about which broadcast services it is receiving, enabling efficient resource management.

Purpose & Motivation

MBMS Selected Services exists to provide the network with knowledge of which MBMS broadcast services are actually being consumed by UEs in a cell. Without this information, the network would have to assume that all UEs are interested in all services, leading to inefficient use of radio resources. By having UEs explicitly signal their selections, the network can optimize transmission parameters, such as whether to use point-to-multipoint or point-to-point transmission, and can decide to stop transmitting services that no one is receiving.

Historically, this mechanism was introduced in Release 6 as part of the initial MBMS specifications. It addressed the need for a counting mechanism to determine the optimal transmission mode. In broadcast mode, services are transmitted to all UEs, but if only a few UEs are interested, it may be more efficient to use point-to-point connections. The selected services signaling provides the necessary input for this decision. It also helps in mobility scenarios, where a UE moving to a new cell needs to inform the new RNC about its selected services to ensure continuity.

The purpose remains relevant in modern networks, where multicast/broadcast services are used for applications like live streaming, public safety, and IoT firmware updates. The signaling of selected services allows the network to tailor its resource allocation to actual demand, improving overall network efficiency and user experience.