MBMS is a point-to-multipoint service in which data is transmitted from a single source entity to multiple recipients. Transmitting the same data to multiple recipients allows network resources to be shared.
The MBMS bearer service offers two modes:
Broadcast Mode is supported for EPS and GPRS, and Multicast Mode is supported for GPRS. MBMS for EPS supports E-UTRAN and UTRAN. MBMS for GPRS supports UTRAN and GERAN.
MBMS architecture enables the efficient usage of radio-network and core-network resources, with an emphasis on radio interface efficiency.
MBMS is realised by the addition of a number of new capabilities to existing functional entities of the 3GPP architecture and by addition of a number of new functional entities.
The existing PS Domain functional entities (GGSN, SGSN, MME, E-UTRAN, UTRAN, GERAN and UE) are enhanced to provide the MBMS Bearer Service. In the EPS a functional entity MBMS GW exists at the edge between the CN and the BM-SC. In the bearer plane, this service provides delivery of IP Multicast datagrams from the Gi and SGi-mb reference points to UEs with a specified Quality of Service. In the control plane, this service provides mechanisms for:
managing the MBMS bearer service activation status of UEs (in the case of multicast mode);
outsourcing authorisation decisions to the MBMS User Service (i.e. to the BM-SC) (in the case of multicast mode);
providing control of session initiation/modification/termination by the MBMS User Service and managing bearer resources for the distribution of MBMS data (in the case or multicast and broadcast modes).
A particular instance of the MBMS Bearer Service is identified by an IP Multicast Address and an APN Network Identifier. A TMGI also can be used to identify one MBMS Bearer Service inside one PLMN.
For GPRS the boundary of the MBMS Bearer Service is the Gmb and Gi reference points as shown in Figure 1a below. The former provides access to the control plane functions and the latter the bearer plane. For EPS the boundary is the SGmb and the SGi-mb as shown in Figure 1b below.
A functional entity, the Broadcast Multicast Service Centre (BM-SC) provides a set of functions for MBMS User Services. BM-SC functions for different MBMS User Services may be supported from the same or different physical network elements.
Signalling between GGSN and BM-SC is exchanged at Gmb reference point. This represents the network side boundary of the MBMS Bearer Service from a control plane perspective. This includes user specific Gmb signalling and MBMS bearer service specific signalling.
MBMS bearer service specific Gmb signalling:
The GGSN establishes the MBMS bearer context and registers at BM-SC.
The GGSN or the BM-SC releases the MBMS bearer context and de-registers the GGSN from the BM-SC.
The BM-SC indicates session start, update and stop to the GGSN including session attributes like QoS and MBMS service area.
User specific Gmb signalling:
The BM-SC authorises the user specific MBMS multicast service activation (join) at the GGSN.
The GGSN reports to the BM-SC the successful user specific MBMS multicast activation (join) to allow the BM-SC to synchronise the BM-SC MBMS UE context with the MBMS UE contexts in the SGSN and GGSN.
The GGSN reports to the BM-SC when a user specific MBMS multicast service is released or deactivated (e.g. at implicit detach). The GGSN makes this report in order to synchronise the BM-SC MBMS UE context with the MBMS UE contexts in the SGSN and GGSN.
The BM-SC initiates the deactivation of a user specific MBMS bearer service when the MBMS user service is terminated.
BM-SC functions for different MBMS bearer services may be provided by different physical network elements. Further, MBMS bearer service specific and user specific signalling for the same MBMS bearer service may also be provided by different physical network elements. To allow this distribution of BM-SC functions, the Gmb protocol must support the use of proxies to correctly route the different signalling interactions in a manner which is transparent to the GGSN.
Mz is the roaming variant of the Gmb reference point with the same functionality as described under Gmb, i.e. with MBMS bearer and User specific signalling.
MBMS bearer and User specific Mz signalling is used between a BM-SC in the visited PLMN and a BM-SC in the home PLMN when MBMS services from the home PLMN are offered by the visited PLMN.
User specific signalling is used between a BM-SC in the visited PLMN and a BM-SC in the home PLMN when the visited PLMN offers MBMS user services to roaming users. This user specific Mz signalling provides home PLMN authorisation for MBMS user services that are provided by the visited PLMN. This mechanism supports only MBMS user service classes that are offered by the visited and by the home PLMN.
Mz may use proxying capabilities as described for Gmb, e.g. to proxy signalling between BM-SCs. An APN may be included in the signalling between BM-SCs, which is used to select an appropriate GGSN to access the MBMS service aiming for an optimized routing, resource saving, or operator policy.