- MBS: the MB-SMF and MB-UPF, with MBS extensions to other 5G Core functions and NG-RAN, and new reference points.
- Delivery: 5GC Shared delivery (one copy to each NG-RAN node) for broadcast and multicast; 5GC Individual delivery (one copy per UE) for multicast only.
MBS in the 5G System and NG-RAN
Two functions are added to the 5G Core: the MB-SMF, which manages MBS sessions and allocates TMGIs, and the MB-UPF, which handles the multicast and broadcast data. Existing functions are extended, among them the AMF, SMF, UPF, PCF, NEF, NRF and UDM, and NG-RAN and the UE. NG-RAN is the decision point for switching between PTP and PTM. In Release 19, MBS for roaming is not supported (TS 23.247 V19.3.0 clause 4.1); in Release 20, a roaming UE can receive the MBS of the VPLMN but not of the HPLMN (TS 23.247 V20.0.0 clause 4.1). Added in Release 19, a broadcast MBS session can also be transmitted over NR non-terrestrial networks (NTN) (TS 23.247 V20.0.0 clause 5.3.2.1 to 5.3.2.15, 4.1; TS 23.247 V18.8.0 clause 4.1 (comparison)).
MBS communication services
TS 23.247 defines two new logical communication services (TS 23.247 V20.0.0 clause 3.1):
- Multicast communication service: A 5GS communication service in which the same service and the same specific content data are provided simultaneously to a dedicated set of UEs i.e. not all UEs in the coverage of the MBS service area are authorised to receive the data (TS 23.247 V20.0.0 clause 3.1).
- Broadcast communication service: A 5GS communication service in which the same service and the same specific content data are provided simultaneously to all UEs in a geographical area i.e. all UEs in the broadcast coverage area are authorised to receive the data (TS 23.247 V20.0.0 clause 3.1). The content provider and network may not be aware whether authorised UEs are actually receiving the data being delivered (TS 23.247 V20.0.0 clause 3.1, note 1).
MBS sessions
TS 23.247 defines MBS sessions as follows (TS 23.247 V20.0.0 clause 3.1):
- MBS session: A multicast or broadcast session (TS 23.247 V20.0.0 clause 3.1).
- Broadcast MBS session: An MBS session to deliver a broadcast communication service. A broadcast MBS session is characterised by the content to send and the geographical area over which to distribute it (TS 23.247 V20.0.0 clause 3.1).
- Multicast MBS session: An MBS session to deliver a multicast communication service. A multicast MBS session is characterised by the content to send, the list of UEs that may receive the service and optionally by a geographical area over which to distribute it (TS 23.247 V20.0.0 clause 3.1).
Broadcast and multicast MBS sessions are time-bound (TS 23.247 V20.0.0 clause 4.2.1; TS 23.247 V20.0.0 clause 4.2.2). Their life-cycles are specified in clauses 4.2 and 4.3 of TS 23.247 (TS 23.247 V20.0.0 clause 4.2; TS 23.247 V20.0.0 clause 4.3).
The geographical areas for which the multicast and broadcast MBS sessions are available is determined by the coverage area of the sites/sectors from which the broadcast service is transmitted (TS 23.247 V20.0.0 clause 3.1, note 2). Refer to TS 23.247 clause 3.1 and TS 38.300 for more detail.
Functions in the 5G System
Functions and their MBS roles (TS 23.247 clause 5.3.2)
| Function | MBS role, as TS 23.247 lists it |
|---|---|
| MB-SMF | MBS session management, including QoS control; configures the MB-UPF; allocates and de-allocates TMGIs; for broadcast, controls shared delivery through the AMF; for multicast, provides MBS Session Context information to the SMF and controls the MB-UPF for individual delivery |
| MB-UPF | Processes incoming downlink multicast and broadcast packets; enforces QoS; delivers data to NG-RAN nodes for 5GC Shared delivery, and multicast data to the UPF for 5GC Individual delivery |
| SMF | Discovers the MB-SMF for a Multicast MBS session; authorises join requests if needed; interacts with RAN and UPF for individual delivery. The SMF and MB-SMF may be co-located |
| UPF | Receives multicast data from the MB-UPF and delivers it to UEs through PDU Sessions, for individual delivery |
| AMF | Signalling with NG-RAN and the MB-SMF for MBS Session management; selects the NG-RAN nodes for broadcast distribution and for multicast activation notification of UEs in CM-IDLE |
| NG-RAN | Delivers MBS data over radio with PTM or PTP; controls switching between PTM and PTP per UE; supports multicast service continuity at Xn and N2 handover; group paging; may move multicast UEs to RRC_INACTIVE |
| UE | Receives multicast with PTM or PTP in RRC_CONNECTED, and if supported with PTM in RRC_INACTIVE; receives broadcast with PTM; joins and leaves Multicast MBS sessions |
| AF | Requests multicast or broadcast service with service information, including QoS requirements |
| NEF | Interface to AFs for MBS service provisioning, session and QoS management; selects the MB-SMF |
| MBSF | Service level functionality and interworking with LTE MBMS; selects the MB-SMF; controls the MBSTF if used |
| MBSTF | Media anchor; sourcing of IP multicast; packet transport functions such as framing and FEC; delivery of input files as objects or object flows |
| PCF | QoS handling and policy information for the MBS Session, when dynamic PCC is needed |
| UDM, UDR | MBS subscription data for authorising UEs for multicast MBS sessions |
| NRF | The MB-SMF and MBSF NF types; MB-SMF discovery |
Reference points and interfaces
New reference points (TS 23.247 clause 5.3.1)
| Reference point | Between |
|---|---|
| N3mb | RAN and MB-UPF (TS 23.247 V20.0.0 clause 5.3.1) |
| N4mb | MB-SMF and MB-UPF (protocols in TS 29.244) |
| N6mb | MB-UPF and AF/AS (TS 23.247 V20.0.0 clause 5.3.1) |
| N7mb | MB-SMF and PCF (TS 23.247 V20.0.0 clause 5.3.1) |
| N11mb | AMF and MB-SMF (TS 23.247 V20.0.0 clause 5.3.1) |
| N16mb | SMF and MB-SMF (TS 23.247 V20.0.0 clause 5.3.1) |
| N19mb | UPF and MB-UPF (TS 23.247 V20.0.0 clause 5.3.1) |
| N29mb | MB-SMF and NEF (TS 23.247 V20.0.0 clause 5.3.1) |
| Nmb1 | MB-SMF and MBSF (TS 23.247 V20.0.0 clause 5.3.1) |
| Nmb2 | MBSF and MBSTF (TS 23.247 V20.0.0 clause 5.3.1) |
| Nmb5 | MBSF and NEF (TS 23.247 V20.0.0 clause 5.3.1) |
| Nmb8 | MBSTF and AF/AS (TS 23.247 V20.0.0 clause 5.3.1) |
| Nmb9 | MB-UPF and MBSTF (TS 23.247 V20.0.0 clause 5.3.1) |
| Nmb10 | MBSF and AF/AS (TS 23.247 V20.0.0 clause 5.3.1) |
| Nmb12 | MBSF and PCF (TS 23.247 V20.0.0 clause 5.3.1) |
| Nmb13 | MB-SMF and AF/AS (TS 23.247 V20.0.0 clause 5.3.1) |
| Nmb14 | NEF and MBSTF (TS 23.247 V20.0.0 clause 5.3.1) |
| N101 | MB-SMF and CHF (TS 23.247 V20.0.0 clause 5.3.1). Added in Release 18 with the charging work item 5MBS_CH (TR 21.918 clause 12.1); it is in TS 23.247 V18.8.0 and not in V17.12.0 |
In functionality, Nmb13, N29mb and Nmb1 are identical, Nmb5 and Nmb10 are identical, and Nmb9 and N6mb are identical. The existing reference points N1, N2, N4, N5, N10, N11, N30, N33 and N40 are enhanced for MBS. Between NG-RAN and the MB-SMF, the AMF relays the N2 SM information transparently (TS 23.247 V20.0.0 clause 5.3.1; TS 23.247 V20.0.0 clause 5.1, NOTE 5; TS 23.247 V20.0.0 clause 8.1.2).
Service-based interfaces and their services (TS 23.247 clause 9)
| Interface | Service | Operations | Consumers |
|---|---|---|---|
| Nmbsmf (MB-SMF) | Nmbsmf_TMGI | Allocate, Deallocate | NEF, MBSF, AF (TS 23.247 V20.0.0 clause 9.1.1) |
| Nmbsmf_MBSSession | Create, Update, Delete, StatusSubscribe, StatusUnsubscribe, StatusNotify | MBSF, NEF, AF (TS 23.247 V20.0.0 clause 9.1.1) | |
| ContextStatusSubscribe, ContextStatusUnsubscribe, ContextStatusNotify | SMF (TS 23.247 V20.0.0 clause 9.1.1) | ||
| ContextUpdate | AMF, SMF (TS 23.247 V20.0.0 clause 9.1.1) | ||
| Npcf (PCF) | Npcf_MBSPolicyControl | Create, Update, Delete | MB-SMF (TS 23.247 V20.0.0 clause 9.2.1) |
| Npcf_MBSPolicyAuthorization | Create, Update, Delete | AF, NEF, MBSF (TS 23.247 V20.0.0 clause 9.2.1) | |
| Namf (AMF) | Namf_MBSBroadcast | ContextCreate, ContextUpdate, ContextRelease, ContextStatusNotify | MB-SMF (TS 23.247 V20.0.0 clause 9.3.1) |
| Namf_MBSCommunication | N2MessageTransfer | MB-SMF (TS 23.247 V20.0.0 clause 9.3.1) | |
| Nnef (NEF) | Nnef_MBSTMGI | Allocate, Deallocate, ExpiryNotify | AF (TS 23.247 V20.0.0 clause 9.4.1) |
| Nnef_MBSSession | Create, Update, Delete, StatusNotify, StatusSubscribe, StatusUnsubscribe | AF (TS 23.247 V20.0.0 clause 9.4.1) | |
| Nmbsf (MBSF) | defined in TS 26.502 |
NGAP procedures between NG-RAN and the AMF (TS 38.413 clauses 8.17 and 8.18)
| Procedure | Purpose |
|---|---|
| Broadcast Session Setup | Set up MBS session resources in the NG-RAN node for a broadcast MBS session (TS 38.413 V19.4.0 clause 8.17.1) |
| Broadcast Session Modification | Update the resources or the area of a broadcast MBS session (TS 38.413 V19.4.0 clause 8.17.2) |
| Broadcast Session Release | Release the resources of a broadcast MBS session (TS 38.413 V19.4.0 clause 8.17.3) |
| Broadcast Session Release Required | Lets the NG-RAN node trigger the AMF to release a broadcast MBS session (TS 38.413 V19.4.0 clause 8.17.4) |
| Broadcast Session Transport | Assign NG-U resources for a broadcast MBS session (TS 38.413 V19.4.0 clause 8.17.5) |
| Distribution Setup | Assign NG-U resources for a multicast MBS session (TS 38.413 V19.4.0 clause 8.18.1) |
| Distribution Release | Release the NG-U resources of a multicast MBS session, or of one area session of it (TS 38.413 V19.4.0 clause 8.18.2) |
| Multicast Session Activation | Activate the resources of a multicast MBS session in an NG-RAN node (TS 38.413 V19.4.0 clause 8.18.3) |
| Multicast Session Deactivation | Deactivate them (TS 38.413 V19.4.0 clause 8.18.4) |
| Multicast Session Update | Update the resources or the area of a multicast MBS session (TS 38.413 V19.4.0 clause 8.18.5) |
All of them use non-UE-associated signalling (TS 38.413 V19.4.0 clause 8.17.1.1; TS 38.413 V19.4.0 clause 8.17.2.1; TS 38.413 V19.4.0 clause 8.17.3.1; TS 38.413 V19.4.0 clause 8.17.4.1; TS 38.413 V19.4.0 clause 8.17.5.1; TS 38.413 V19.4.0 clause 8.18.1.1 to 8.18.5.1).
Delivery methods
Between the 5G Core and NG-RAN there are two delivery methods. TS 23.247 does not call them unicast, multicast or broadcast:
- 5GC Shared MBS traffic delivery: the 5G Core receives a single copy of the MBS data packets and delivers a single copy to an NG-RAN node, which delivers it to one or more UEs. It is used for broadcast and multicast MBS sessions, and is required in all MBS deployments (TS 23.247 V20.0.0 clause 4.1; TS 23.247 V20.0.0 clause 3.1).
- 5GC Individual MBS traffic delivery: the 5G Core delivers separate copies to individual UEs, each through a PDU Session associated with the Multicast MBS session. It is used only for multicast, and is required for mobility where NG-RAN nodes do not all support MBS (TS 23.247 V20.0.0 clause 4.1; TS 23.247 V20.0.0 clause 3.1).
Over the radio, NG-RAN has two delivery methods: (TS 23.247 V20.0.0 clause 4.1)
- Point-to-Multipoint (PTM): a single copy over the radio interface to multiple UEs (TS 23.247 V20.0.0 clause 4.1).
- Point-to-Point (PTP): separate copies over the radio interface to individual UEs (TS 23.247 V20.0.0 clause 4.1).
NG-RAN may combine PTP and PTM. For a broadcast service only 5GC Shared delivery with PTM applies. For a multicast service, an NG-RAN node that supports MBS is served with 5GC Shared delivery; the same multicast session may be delivered with Shared delivery to some UEs and Individual delivery to others (TS 23.247 V20.0.0 clause 4.1).
| 5GC Shared | 5GC Individual | |
|---|---|---|
| MBS sessions | Broadcast and multicast | Multicast only (TS 23.247 V20.0.0 clause 4.1) |
| From the MB-UPF to | NG-RAN nodes, at N3mb | UPFs, at N19mb; the UPF forwards the data in the N3 tunnel of each associated PDU Session (TS 23.247 V20.0.0 clause 6.7; TS 23.247 V20.0.0 clause 7.2.1.3 step 18) |
| Transport | A common GTP-U tunnel per MBS session with IP multicast, otherwise a GTP-U tunnel per MBS session per NG-RAN node | A common GTP-U tunnel per MBS session with IP multicast, otherwise one per MBS session per UPF (TS 23.247 V20.0.0 clause 6.7) |
| Shared by | All UEs that joined the multicast session through that NG-RAN node (one tunnel per session, MBS service area and node) | All associated PDU Sessions served by that UPF (TS 23.247 V20.0.0 clause 7.2.1.3 step 14; TS 23.247 V20.0.0 clause 6.7) |
| Radio | PTM or PTP | Unicast radio bearers of the associated PDU Session (TS 23.247 V20.0.0 clause 4.1; TS 23.247 V20.0.0 clause 3.1) |
The MB-UPF is the MBS Session Anchor. When the MBSTF is involved, the MBSTF is the media anchor. The MB-UPF adds a sequence number to the MBS data of each MBS QoS Flow (TS 23.247 V20.0.0 clause 6.7; TS 23.247 V20.0.0 clause 7.2.3.5).
NG-RAN
The overall NG-RAN architecture applies to NR MBS. The Layer 2 stack for MBS uses the same sublayers, with a reduced set of functions: SDAP maps MBS QoS flows to MBS Radio Bearers (MRBs), and PDCP keeps transfer, sequence numbers, header compression, reordering and duplicate discarding (TS 38.300 V19.4.0 clause 16.10.2; TS 38.300 V19.4.0 clause 16.10.3).
MBS Radio Bearers (TS 38.300 clause 16.10.3)
| Session | UE state | MRB configurations |
|---|---|---|
| Multicast | RRC_CONNECTED, by dedicated RRC signalling | RLC-UM or RLC-AM for PTP; DL-only RLC-UM for PTM; or two or three RLC entities that combine a PTP leg (RLC-UM or RLC-AM) with a PTM leg (DL-only RLC-UM) (TS 38.300 V19.4.0 clause 16.10.3). The gNB may change the MRB type |
| Multicast | RRC_INACTIVE, by broadcast and/or dedicated RRC signalling | DL-only RLC-UM for PTM (TS 38.300 V19.4.0 clause 16.10.3) |
| Broadcast | RRC_IDLE, RRC_INACTIVE and RRC_CONNECTED, by broadcast RRC signalling | One DL-only RLC-UM entity for PTM (TS 38.300 V19.4.0 clause 16.10.3) |
| Item | Role (TS 38.300) |
|---|---|
| MTCH (MBS Traffic Channel) | PTM downlink channel for the data of a multicast or broadcast session (TS 38.300 V19.4.0 clause 16.10.4) |
| DTCH | PTP channel for the data of a multicast session (TS 38.300 V19.4.0 clause 16.10.4) |
| MCCH (MBS Control Channel) | PTM downlink channel for broadcast or multicast control information (TS 38.300 V19.4.0 clause 16.10.4). Broadcast MCCH and multicast MCCH are independent channels; the multicast MCCH is used only for reception in RRC_INACTIVE |
| G-RNTI | Identifies dynamically scheduled PTM transmissions of MTCH (TS 38.300 V19.4.0 clause 8.1) |
| G-CS-RNTI | Identifies PTM transmissions of MTCH scheduled with a configured grant (TS 38.300 V19.4.0 clause 8.1) |
| MCCH-RNTI | Identifies MCCH transmissions and change notifications for broadcast (TS 38.300 V19.4.0 clause 8.1) |
| Multicast MCCH-RNTI | Identifies MCCH transmissions and change notifications for multicast in RRC_INACTIVE (TS 38.300 V19.4.0 clause 8.1) |
| SIB20, SIB21 | MCCH configuration, and service continuity information, for broadcast (TS 38.300 V19.4.0 clause 7.3.1) |
| SIB24 | Information to acquire the multicast MCCH and MTCH configuration for RRC_INACTIVE (TS 38.300 V19.4.0 clause 7.3.1; TS 38.300 V19.4.0 clause 16.10.5.2). Added in Release 18 (TS 38.331 V18.11.0 clause 5.10 compared with V17.17.0, which has no SIB24) |
| SIB27 | The intended service areas (ISAs) of MBS broadcast services in an NTN cell (TS 38.300 V19.4.0 clause 7.3.1; TS 38.300 V19.4.0 clause 16.14.12; TS 38.300 V18.11.0 clause 7.3.1). Added in Release 19 |
In a PTP transmission, the gNB schedules a UE-specific PDSCH with a PDCCH whose CRC is scrambled by a UE-specific RNTI, such as the C-RNTI. In a PTM transmission, it schedules a group-common PDSCH with a group-common PDCCH whose CRC is scrambled by a group-common RNTI. When a UE has both a PTM and a PTP leg, the gNB decides dynamically which to use, based on information such as the session QoS requirements, the number of joined UEs and the UE feedback on reception quality. The same QoS requirements apply whichever it chooses (TS 38.300 V19.4.0 clause 16.10.5.4).
Multicast session states
| State | In TS 23.247 clause 4.3 |
|---|---|
| Configured | Information about the session (for example QoS) is held in the 5G Core, but no user plane resources towards NG-RAN are reserved and no data can be sent. A TMGI can be allocated |
| Active | The session is established and data can be sent to the UEs that joined it. Radio resources are established |
| Inactive | The session is established but no data is sent. Radio resources are released |
The first accepted join request establishes the session towards NG-RAN and the UE. For a broadcast MBS session, TS 23.247 clause 4.2.2 describes phases: MBS Session Creation and establishment by the AF, service announcement, data transfer, and Session Release, at which the 5G System releases the resources and deletes the session.
Sources for this page
- MBS communication services and sessions: TS 23.247 V20.0.0, clauses 3.1, 4.2 and 4.3.
- MBS, functions, reference points and services: TS 23.247 V20.0.0, clauses 5.1, 5.3.0, 5.3.1, 5.3.2, 8.1 and 9; TS 38.413 V19.4.0, clauses 8.17 and 8.18.
- Delivery methods: TS 23.247 V20.0.0, clauses 3.1, 4.1, 6.7 and 7.2.1.3.
- NG-RAN: TS 38.300 V19.4.0, clauses 3.1, 7.3.1, 8.1, 16.10.2 to 16.10.4 and 16.10.5.4; SIB27 compared with TS 38.300 V18.11.0, clause 7.3.1.
- Multicast session states: TS 23.247 V20.0.0, clauses 4.2.2, 4.3 and 7.2.1.1.
- Figures: TS 23.247 V20.0.0, figures 4.1-1, 4.3-1, 5.1-1, 5.1-2 and 6.7-1; TS 38.300 V19.4.0, figures 16.10.3-1 and 16.10.3-2.