4 General Concept

23.2473GPPArchitectural enhancements for 5G multicast-broadcast servicesRelease 18TS

4.1 Overview of multicast and broadcast communication

Multicast and Broadcast Service (MBS) is a point-to-multipoint service in which data is transmitted from a single source entity to multiple recipients, either to all users in a Broadcast service area, or to users in a multicast group as defined in TS 22.146 [2]. The corresponding types of MBS session are:

– Broadcast MBS session

– Multicast MBS session.

The MBS architecture defined in clause 5 follows the 5G System architectural principles as defined in TS 23.501 [5], enabling distribution of the MBS data from the 5GS ingress to NG-RAN node(s) and then to the UE. The MBS architecture provides:

– Efficient usage of RAN and CN resources, with an emphasis on radio interface efficiency;

– Efficient transport for a variety of multicast and broadcast services.

Multicast/Broadcast Service for roaming is not supported in this release.

Interaction between Multicast/Broadcast Service and support of deployments topologies with specific SMF Service Areas is not specified in this Release.

The collection and reporting of MBS specific charging information are not specified in this Release.

The MBS also provides functionalities such as local MBS service and location dependent MBS service, authorization of multicast MBS and QoS differentiation. Refer to clause 6 for more details.

MBS traffic is delivered from a single data source (e.g. Application Service Provider) to multiple UEs. Depending on many factors, there are several delivery methods which may be used to deliver the MBS traffic in the 5GS.

NOTE 1: For clarity, delivery methods are not referred to as unicast/multicast/broadcast but as described below. The term "unicast delivery" refers to a mechanism by which application data and signalling between the UE and the application server are delivered using PDU Session within the 3GPP network and using individual UE and application server addresses (e.g. IP addresses) between the 3GPP network and the application server. It is not equivalent to 5GC Individual MBS traffic delivery method defined in this clause.

Between 5GC and NG-RAN, there are two possible delivery methods to transmit the MBS data:

– 5GC Individual MBS traffic delivery method: This method is only applied for multicast MBS sessions. 5GC receives a single copy of MBS data packets and delivers separate copies of those MBS data packets to individual UEs via per-UE PDU sessions, hence for each such UE one PDU session is required to be associated with a Multicast MBS session.

– 5GC Shared MBS traffic delivery method: This method is applied for both broadcast and multicast MBS sessions. 5GC receives a single copy of MBS data packets and delivers a single copy of those MBS packets to an NG-RAN node, which then delivers the packets to one or multiple UEs.

The 5GC Shared MBS traffic delivery method is required in all MBS deployments. The 5GC Individual MBS traffic delivery method is required to enable mobility when there is an NG-RAN deployment with non-homogeneous support of MBS.

For the Multicast MBS session, a single copy of MBS data packets received by the CN may be delivered via 5GC Individual MBS traffic delivery method for some UE(s) and via 5GC Shared MBS traffic delivery method for other UEs.

Between the NG-RAN and the UE, two delivery methods are available for the transmission of MBS data packets over radio interface:

– Point-to-Point (PTP) delivery method: NG-RAN delivers separate copies of MBS data packets over radio interface to individual UE(s).

– Point-to-Multipoint (PTM) delivery method: NG-RAN delivers a single copy of MBS data packets over radio interface to multiple UEs.

NG-RAN may use a combination of PTP/PTM to deliver an MBS data packets to UEs.

NOTE 2: The PTP and PTM delivery methods are defined in RAN WGs.

As depicted in the following figure, 5GC Shared MBS traffic delivery method (with PTP or PTM delivery) and 5GC Individual MBS traffic delivery method may be used at the same time for a multicast MBS session.

Figure 4.1‑1: Delivery methods

For MBS broadcast communication, only 5GC Shared MBS traffic delivery method with PTM delivery is applicable.

For MBS multicast communication, if the NG-RAN node supports MBS, the network shall use the 5GC Shared MBS traffic delivery method for MBS data transmission.

NOTE 3: The exception is when the UE moves between NG-RAN node not supporting MBS (with 5GC Individual MBS traffic delivery method) and NG-RAN node supporting MBS, there is temporary co-existence between 5GC Shared MBS traffic delivery method and 5GC Individual MBS traffic delivery method. Refer to clause 6.3 for details.

For MBS multicast communication, the switching between 5GC Shared MBS traffic delivery method and 5GC Individual MBS traffic delivery method is supported. The UE mobility between RAN nodes both supporting MBS, and between a RAN node supporting MBS and a RAN node not supporting MBS is supported, for details see clause 6.3.

For MBS multicast communication, the switching between PTP and PTM delivery methods for 5GC Shared MBS traffic delivery shall be supported. NG-RAN is the decision point for switching between PTP and PTM delivery methods.

4.2 MB service provisioning

4.2.1 Multicast data provisioning

An example for the sequence of phases for multicast data provisioning is described in the figure below:

Figure 4.2.1-1: Phases of Multicast data provisioning example

The following phases are performed for a specific UE:

– UE Session Join: UE Session Join is the process by which a UE joins an MBS Session, i.e. the UE indicates to 5GC that such UE wants to receive Multicast data identified by a specific MBS Session ID.

– UE Session Leave: UE Session Leave is the process by which a UE leaves a MBS Session, i.e. the UE no longer wants to receive Multicast data identified by a specific MBS Session ID.

The following phases are performed for a specific service:

– MBS Session Creation: It is the phase that the information of Multicast MBS session is created as described in clause 4.3. This step is optional.

– Service announcement: Service announcement is used to distribute information toward UEs about the service required for service reception (e.g. IP multicast address(es)) and possibly other service related parameters (e.g. service start time). This step is optional.

– Session Establishment: It is the phase that Multicast MBS session is established as described in clause 4.3.

– No data receiving: It is the phase when no multicast data is received by 5GC. This step is optional.

– Data transfer: It is the phase when Multicast data are transferred to the UEs.

– Session Release: It is the phase that the resources for Multicast MBS session is released as described in clause 4.3.

– Session Deletion: It is the phase that Multicast MBS session is deleted as described in clause 4.3.

NOTE: After session establishment, Multicast MBS session state could be switched between Active and Inactive several times, triggered by AF or User Plane event, see clause 7.2.5. 5GC further updates Multicast MBS session state towards NG-RAN nodes after Session Establishment.

The phase of Multicast data provisioning is illustrated with the following example of timeline:

Figure 4.2.1-2: Multicast service timeline example

4.2.2 Broadcast data provisioning

An example for the phases of broadcast data provisioning is described in the figure below:

Figure 4.2.2-1: Phases of Broadcast data provisioning

The following phases are performed for a specific service:

– MBS Session Creation: MBS Session Creation is used by the AF to create the MBS Session towards 5GC. MBS session creation can occur in several steps (e.g. TMGI allocation, provisioning information about MBS session, request to activate the MBS session). The last step of the MBS session creation triggers resource establishment for transmitting the DL Broadcast data between 5GC and NG-RAN.

NOTE: For broadcast communication, after MBS Session Creation and Session Establishment, the established resources are not only between 5GC and NG-RAN, but also between the AF to 5GC.

– Service announcement: Service announcement is used to distribute information towards UEs about the service required for service reception (e.g. IP multicast address(es)) and possibly other service related parameters (e.g. service start time). This step can occur in parallel or after the MBS session configuration. However, TMGI allocation is required before. The information of the service announcement could be pre-configured at the UE side, see clause 7.3.1.

– Data transfer: It is the phase when broadcast data are transferred in the air interface.

– Session Release and Deletion: It is the point at which there will be no more need to transmit Broadcast data. At Session Release, the resources in 5GS are released and the broadcast MBS session is deleted.

The phase of Broadcast data provisioning is illustrated with the following example of timeline:

Figure 4.2.2-2: Broadcast service timeline

4.3 Multicast session state model

The following illustrate the states for the Multicast MBS session:

– Configured state: Information about the Multicast MBS session (e.g. QoS information) is available in 5GC NFs (e.g. MB-SMF) serving the Multicast MBS session, but no User Plane resources towards NG-RAN are reserved and no MBS data can be transmitted. Only resources at MB-SMF, NEF and MB-UPF are reserved and no multicast data are transmitted. A TMGI can be allocated for the Multicast MBS session. UEs may be allowed to join (subject to authorization check and configuration), but the first accepted UE join request will trigger the Multicast MBS session establishment towards the NG-RAN and the UE, see clause 7.2.1.

NOTE 1: The SMF is not involved in the Multicast MBS session while the Multicast MBS session is in created state.

NOTE 2: There may be several interim states in the created state, e.g. TMGI requested, or information about the Multicast MBS session provided, but these interim states will not be specified in this release.

– Active state: Multicast MBS session is established and MBS data can be transmitted to the UEs that have joined th Multicast MBS session. Radio resources for the Multicast MBS session are established. To receive multicast MBS session data, UEs that joined the Multicast MBS session shall be in CM-CONNECTED state for receiving data of the Multicast MBS session. UEs are allowed to join the Multicast MBS session (subject to authorization check). 5GC resources and radio resources for the Multicast MBS session are reserved for UEs that joined the Multicast MBS session.

– Inactive state: Multicast MBS session is established but no MBS data is transmitted to the UEs that have joined the Multicast MBS session. Radio resources for the Multicast MBS session are released, and the UEs that joined the Multicast MBS session may be in CM-CONNECTED or CM-IDLE state. UEs are allowed to join the Multicast MBS session (subject to authorization check).

The following procedures are defined which result in transition of the Multicast MBS session state:

Multicast Session Creation: The AF provides information about the Multicast MBS session and optionally requests the allocation of a TMGI, see clause 7.1.1.2 and 7.1.1.3. Alternatively, the information about the Multicast MBS session can be pre-configured in the network. The creation may indicate whether the Multicast MBS session may be established in active or inactive state and when a Multicast MBS session can become active. The AF may perform creation in several steps, e.g. to first request TMGI and then provide full information about the Multicast MBS session and allow it to be established, or to update the information whether the Multicast MBS session is to be in Active or Inactive state after establishment. Multicast MBS session state transitions from "Start (NULL)" to Configured state.

NOTE 3: A Multicast MBS session can also be created by the operator via OAM configuration or be established without prior creation.

– Multicast Session Establishment: When the join request of the first UE for the Multicast MBS session is accepted, the Multicast MBS session is established towards the NG-RAN node and the UE, see clause 7.2.1. Multicast session state transitions from "Start (NULL)" or Configured state to either Inactive or Active state.

– Multicast Session Activation: See clause 7.2.5.2, Triggered by the 5GC, the radio resources for the Multicast MBS session are established and Multicast MBS session data starts to be transmitted to the UE. UEs in CM-IDLE state and CM-CONNECTED with RRC Inactive state that joined the Multicast MBS session are notified. Activation can be triggered by AF request or data notification from the MB-UPF. Multicast session state transitions from Inactive state to Active state.

NOTE 4: The AF could not be aware, and the NEF will not be aware, whether a session is in created or established state. An AF may therefore update the session state to request the activation of a session prior to the establishment of the session, and this will determine that the session is subsequently established in Active state when the first UE joins, but will not trigger the Multicast Session Activation state transition.

– Multicast Session Deactivation: See clause 7.2.5.3. Triggered by the 5GC, the radio resources for the Multicast MBS session are released and Multicast MBS session data stops to be transmitted to the UE. Deactivation can be triggered by AF request or no reception of multicast data by the MB-UPF. Multicast session state transitions from Active to Inactive state.

– Multicast Session Release: Triggered by the last UE leaving the Multicast MBS session (see clause 7.2.2.2), or Multicast Session Deletion procedure (7.1.1.4 or 7.1.1.5), the resources for the Multicast MBS session are released in both 5GC nodes and RAN nodes, see clause 7.2.2. Multicast session state transitions from Active or Inactive state to Configured.

– Multicast Session Deletion: All information about the Multicast MBS session is removed from the 5GC, and the TMGI for the Multicast MBS session (if allocated during Multicast Session Configuration) is deallocated, see clause 7.1.1.4 or 7.1.1.5. The deletion may be triggered by an AF request. Multicast session state transitions from Configured, Active or Inactive state to "End (NULL)".

Figure 4.3-1: Multicast session states and state transitions

Figure 4.3-2: Multicast session states and state transitions in MB-SMF

Figure 4.3-3: Multicast session states and state transitions in NG-RAN

NOTE 5: Multicast session states and state transitions in NG-RAN is for illustration purpose, normative procedures are provided by RAN WGs.

Figure 4.3-4: Multicast session states and state transitions in SMF