Figure 4.2.1-1 depicts the MBS network architecture defined in clause 5.1 of TS 23.247 using the reference point representation.

The functions and reference points involved in providing MBS User Services within the MBS System are highlighted in green. In particular:

• Reference point Nmb10 is used by the AF/AS to provision MBS User Services in the MBSF by invoking the Nmbsf service defined in clause 7.2.
• Reference point Nmb2 is used by the MBSF to configure and control MBS User Services distribution methods in the MBSTF by invoking the Nmbstf service defined in clause 7.3.
  When the MBS User Service Announcement Channel is in use (see clause 4.2.4), the MBSF may additionally push object manifests describing a set of User Service Announcement objects into the MBSTF at this reference point.
• Reference point Nmb8 is used by the MBSTF to ingest content from the AF/AS.

MBS User Services enable high-level applications to make use of the low-level features of the MBS System. An MBS User Service is provided by the MBSF and MBSTF working in combination to support configuration option 2 and configuration option 3 defined in Annex A of TS 23.247. In addition to the Network Functions defined in TS 23.247:

• The MBS AF provides unicast User Service Announcements to the MBSF Client in the user plane and to the MBSTF.
• The MBS AS provides unicast services such as Object Repair to the MBSTF and MBSTF Client.
• The MBSSF supports the User Plane security procedure according to clause W.4.1.3 of TS 33.501 and provides a user plane authentication service to the MBSF Client.

Together, these functions enable a complete service offering to an end-user, via a set of APIs that allows the MBS Client to activate or deactivate reception of MBS User Services. The MBS User Services architecture depicted in Figure 4.2.2-1 shows the MBS-related entities involved in providing MBS User Services delivery and control. These are described in the following clauses. The MBS Application Provider plays the role of the AF/AS.

The distribution methods defined for use by MBS User Services in clause 6 of the present document make use of MBS Sessions (see clause 4.1 of TS 23.247) to deliver MBS data packets to the MBS Client. The distribution methods may use either a multicast MBS Session or a broadcast MBS Session. A set of MBS distribution methods is provided by the MBSTF. These provide functionality such as security and key distribution, reliability control (by means of FEC techniques) and associated delivery procedures.

• Object Distribution Method: A distribution method that delivers discrete binary objects over an MBS Session. This may be used to support real-time distribution of media segments (as special objects) including Low-Latency CMAF segments.
  The use of MBS Sessions by the Object Distribution Method is specified in clause 6.1.
• Packet Distribution Method: A distribution method that supports streaming of packetized media data over an MBS Session where Service Data Units (SDUs) are conveyed to the UE as part of Protocol Data Units (PDUs) or IP flows. Examples of upper layer SDUs are generic IP/UDP datagrams.
  The use of MBS Sessions by the Packet Distribution Method is specified in clause 6.2.

The above distribution methods may use either a multicast MBS Session or a broadcast MBS Session to distribute content to an MBS Client and may also make use of the Object Repair feature, as defined in clause 4.2.5 below.

The MBS User Service Announcement provides service access information needed by the MBS Client to discover and activate the reception of one or more MBS User Services. Depending on the User Service advertisement mode(s) provisioned by the MBS Application Provider (AF/AS) at reference point Nmb10 (or N33+Nmb5) MBS User Service Announcements may be delivered:

1. Via a regular unicast PDU Session at reference point MBS-5.
2. Via a dedicated MBS Distribution Session provisioned and managed by the MBSF called the MBS User Service Announcement Channel at reference point MBS-4-MC. User Service Announcements are delivered using the Object Distribution Method.
3. By application-private means via reference point MBS-8.

The baseline information conveyed in User Service Announcements is defined in clause 4.5.7. The procedures for User Service advertisement are defined in clause 5.4.

Reporting by the MBS Client to the MBSF is for further study and may be realised by instantiating the data collection and reporting architecture specified in TS 26.531. This may include, for example, dynamically adjusting the Application Layer FEC redundancy level in the MBSTF according to the Packet Error Rate (PER) reported by the MBS Client.

When using the Object Distribution Method, Object Repair procedures may be provisioned by the MBS Application Provider. This feature allows an MBS Client to retrieve from the MBS AS (via reference point MBS-4-UC) missing portions of objects not received intact from the MBSTF at reference point MBS-4-MC. The following Object Repair procedures are defined in clause 5.6 of the present document:

• In-session Object Repair during the MBS Distribution Session, including the ability to repair objects in streaming distribution.
• Post-session Object Repair after the completion of the MBS Distribution Session, including a randomized back-off period for different MBS Clients in order to avoid overloading the MBS AS.

The functions of the MBS AS are defined in clause 4.3.4.

Aligned with the specification of time synchronization for MBMS specified in clause 4.6 of TS 26.346, the MBS Client is required to be time-synchronized with the 5G System (i.e. MBSF, MBSTF and/or the MBS AS) with a precision of at least ±1s to support certain MBS User Service functions. Similar to MBMS, two options are provided in order to achieve this functionality:

1. Usage of SIB9: As specified in TS 38.331, NR SIB9 conveys information in its timeInfo parameter related to a Coordinated Universal Time (UTC) wallclock to an accuracy of 10 ms. The MBS Client may obtain time information from SIB9 to synchronize the system clock of the UE to UTC wallclock time. If SIB9 is carried in the radio frequency carrying the MBS Distribution Session, the relevant MBS functions shall be time-synchronized to the same UTC wallclock as the NR SIB9 information to a tolerance of ±100 ms or better.
2. Time service: The MBS AS hosts a time service. The MBS Client may obtain time information from this service to synchronize its clock. The MBS Client should use the time service no more often than needed to maintain time synchronization accurate to at least ±1 s with the 5G System. Specific features may require tighter synchronization and tighter tolerances. If more than one time service endpoint is provided to the MBS Client, the MBS Client may choose any of these.

At least one of the two above options shall be provided by the network. If both options (i.e., SIB9 as well as the MBS AS time service in the MBS AS) are offered by the 5G System:

• The timing source in the MBS AS shall be time-synchronized to the same wallclock time as used for NR SIB9 information.
• The MBS Client should preferably use the information in NR SIB9.