- RAN delivery methods: PTM sends a single copy over the radio interface to multiple UEs; PTP sends separate copies to individual UEs.
- Delivery modes (labels used on this page): delivery mode 1 (multicast), delivery mode 2 (broadcast) and the unicast delivery mode.
- RRC states: broadcast can be received in all RRC states; multicast in RRC_CONNECTED, and in RRC_INACTIVE with PTM delivery.
RAN traffic delivery methods (TS 38.300, TS 38.331)
On receiving an MBS packet from the 5GC via the shared MBS traffic delivery method, the gNodeB can choose between two different RAN delivery methods, as described below (TS 23.247 V20.0.0 clause 4.1; TS 38.300 V19.4.0 clause 16.10.5.4).
Point-to-Multipoint (PTM) RAN delivery method
The PTM RAN delivery method is used only for MBS packets arriving at the gNodeB via the 5GC shared MBS traffic delivery method (TS 23.247 V20.0.0 clause 4.1). gNodeBs then deliver a single copy of the MBS data packets over the radio interface to multiple UEs (TS 38.300 V19.4.0 clause 16.10.5.4). The radio interface may use either delivery mode 1 (multicast) or delivery mode 2 (broadcast). The PTM RAN delivery method is always used to deliver the MBS packets of a Broadcast MBS Session (TS 38.300 V19.4.0 clause 16.10.6.5A; TS 23.247 V20.0.0 clause 4.1). The PTM RAN delivery method may be used to deliver the MBS packets of a Multicast MBS Session (TS 38.300 V19.4.0 clause 16.10.5.4). In some cases, however, the gNodeB may opt to use the PTP delivery method (see below) (TS 38.300 V19.4.0 clause 16.10.5.4; TS 23.247 V20.0.0 clause 4.1).
Point-to-Point (PTP) RAN delivery method
The PTP RAN delivery method may be used for MBS packets arriving at the gNodeB via either the 5GC shared MBS traffic delivery method or the 5GC individual MBS traffic delivery method (TS 23.247 V20.0.0 clause 4.1). gNodeBs then deliver separate copies of the MBS data packets over the radio interface to individual UEs (TS 38.300 V19.4.0 clause 16.10.5.4). The PTP RAN delivery method is always used for conventional PDU Sessions carrying packets from a unicast session (TS 23.247 V20.0.0 clause 4.1, note 2). The PTP RAN delivery method is always used in combination with the 5GC individual MBS traffic delivery method (TS 23.247 V20.0.0 clause 4.1). The PTP RAN delivery method may be used in combination with the 5GC shared MBS traffic delivery method. If a UE is configured with both PTM and PTP transmissions, the gNodeB dynamically decides whether to deliver multicast data by the PTM leg and/or the PTP leg for a given UE, based on information such as MBS Session QoS requirements, number of joined UEs, UE individual feedback on reception quality, and other criteria; the same QoS requirements apply regardless of the decision (TS 38.300 V19.4.0 clause 16.10.5.4). The PTP RAN delivery method is not used for broadcast MBS sessions (TS 38.300 V19.4.0 clause 16.10.6.5A).
RAN delivery modes
This page describes how a gNodeB delivers MBS data at Layer 2 with three labels: delivery mode 1 (multicast), delivery mode 2 (broadcast) and the unicast delivery mode. None of the three is a 3GPP term. TS 38.300 uses "delivery mode" for the 5GC Shared and the 5GC Individual MBS traffic delivery (TS 38.300 V19.4.0 clause 16.10.5.1), and TS 23.247 uses "unicast delivery" for delivery over a PDU Session, which is not the same as the 5GC Individual MBS traffic delivery method (TS 23.247 V20.0.0 clause 4.1, note 2). The unicast delivery mode here means the Layer 2 handling of the point-to-point RAN delivery method (TS 38.300 V19.4.0 clause 16.10.3, 16.10.5.4).
Unicast delivery mode
gNodeBs always use the unicast delivery mode for the point-to-point RAN delivery method. This includes the delivery of (TS 38.300 V19.4.0 clause 16.10.5.4):
- Unicast packets arriving at reference point N3 from a PDU Session.
- MBS packets from a Multicast MBS Session arriving at reference point N3 in a PDU Session corresponding to 5GC individual MBS traffic delivery (TS 23.247 V20.0.0 clause 4.1).
- MBS packets from a Multicast MBS Session arriving at reference point N3mb using the 5GC shared MBS traffic delivery method when the gNodeB has chosen to use the point-to-point RAN delivery method (TS 23.247 V20.0.0 clause 4.1).
The unicast delivery mode cannot be used for broadcast communication services (TS 38.300 V19.4.0 clause 16.10.6.5A). Payloads delivered in the unicast delivery mode are intended for specific UEs; they are not intended to be decoded by more than one UE. The NG-RAN uses a UE-specific PDCCH with a CRC scrambled with a UE-specific RNTI (e.g., C-RNTI) to schedule a UE-specific PDSCH which is scrambled with the same UE-specific RNTI (TS 38.300 V19.4.0 clause 16.10.5.4). Unicast can only be received by UEs in RRC_Connected state (TS 38.300 V19.4.0 clause 16.10.1).
Delivery mode 1 (Multicast)
A gNodeB uses delivery mode 1 for a Multicast MBS Session when it has chosen to use the PTM RAN delivery method (TS 38.300 V19.4.0 clause 16.10.5.4). Delivery mode 1 supports HARQ feedback, retransmissions, and basic mobility with service continuity (TS 38.300 V19.4.0 clause 16.10.1; TS 38.300 V19.4.0 clause 16.10.5.3.2). Delivery mode 1 payloads are intended for multiple UEs served by a particular gNodeB. The gNodeB uses a group-common PDCCH with the CRC scrambled by a group-common RNTI to schedule a group-common PDSCH scrambled with the same group-common RNTI (TS 38.300 V19.4.0 clause 16.10.5.4). To receive the multicast service the UE first performs the MBS Session Join procedure, and the gNodeB may then send an RRCReconfiguration message with the relevant MBS configuration (TS 38.300 V19.4.0 clause 16.10.5.2). Delivery mode 1 can only be received by compatible UEs in RRC_Connected state, or in RRC_Inactive state with PTM delivery (TS 38.300 V19.4.0 clause 16.10.1; TS 38.300 V19.4.0 clause 16.10.5.2). Delivery mode 1 is not used for a broadcast MBS session (TS 38.300 V19.4.0 clause 16.10.6.5A).
TS 38.300 V19.4.0 clause 16.10.5.7 defines two HARQ-ACK reporting modes for MBS: in the first the UE sends ACK or NACK; in the second the UE does not transmit a PUCCH that would include only ACK values, i.e. NACK only. HARQ-ACK feedback for multicast can be enabled or disabled per G-RNTI or G-CS-RNTI, and is not supported in RRC_INACTIVE.
Delivery mode 2 (Broadcast)
gNodeBs always use this delivery mode for a Broadcast MBS Session (which always uses the point-to-multipoint RAN delivery method) (TS 38.300 V19.4.0 clause 16.10.6.5A). Delivery mode 2 does not support HARQ feedback (TS 38.300 V19.4.0 clause 16.10.6.6); retransmissions are not triggered by feedback, and the UE combines the received data of a transport block with the data in the soft buffer when the transmission is a retransmission (TS 38.321 V19.4.0 clause 5.3.2.2). For service continuity as a UE moves from one cell to another, the UE can read the MCCH/MTCH from neighbour cells (TS 38.300 V19.4.0 clause 16.10.6.5.1); lossless handover is specified for multicast (TS 38.300 V19.4.0 clause 16.10.5.3.2). Delivery Mode 2 payloads are intended for any UE served by a particular gNodeB. The gNodeB uses a group-common PDCCH with the CRC scrambled by a group-common RNTI to schedule a group-common PDSCH scrambled with the same group-common RNTI (TS 38.300 V19.4.0 clause 16.10.6.5A). The g-RNTI of a broadcast session is carried in MBS-SessionInfo in the MCCH (TS 38.331 V19.4.0 clause 6.3.6). Delivery mode 2 can be received by UEs in all RRC states (RRC_Connected, RRC_Idle and RRC_Inactive) (TS 38.300 V19.4.0 clause 16.10.1; TS 38.300 V19.4.0 clause 16.10.6.2).
Comparison of the delivery modes
| Dimension | Unicast delivery mode | Delivery mode 1 (Multicast) | Delivery mode 2 (Broadcast) |
|---|---|---|---|
| HARQ / retransmission support | HARQ feedback and retransmission can be applied to PTP transmission in RRC_CONNECTED (TS 38.300 V19.4.0 clause 16.10.1). | Supports HARQ feedback, retransmissions, and basic mobility with service continuity (TS 38.300 V19.4.0 clause 16.10.1; TS 38.300 V19.4.0 clause 16.10.5.3.2). | No HARQ feedback (TS 38.300 V19.4.0 clause 16.10.6.6); a retransmission is soft-combined by the UE (TS 38.321 V19.4.0 clause 5.3.2.2). For service continuity as a UE moves from one cell to another, the UE can read the MCCH/MTCH from neighbour cells (TS 38.300 V19.4.0 clause 16.10.6.5.1); lossless handover is specified for multicast (TS 38.300 V19.4.0 clause 16.10.5.3.2). |
| Payload scrambling | Payloads are intended for specific UEs and are not meant to be decoded by more than one UE. The NG-RAN uses a UE-specific PDCCH with a CRC scrambled with a UE-specific RNTI (e.g., C-RNTI) to schedule a UE-specific PDSCH which is scrambled with the same UE-specific RNTI (TS 38.300 V19.4.0 clause 16.10.5.4). | Payloads are intended for multiple UEs served by a particular gNodeB. A group-common PDCCH with the CRC scrambled by a group-common RNTI schedules a group-common PDSCH scrambled with the same RNTI (TS 38.300 V19.4.0 clause 16.10.5.4). | Payloads are intended for any UE served by a particular gNodeB. A group-common PDCCH with the CRC scrambled by a group-common RNTI schedules a group-common PDSCH scrambled with the same RNTI; the g-RNTI is carried in the MCCH (TS 38.300 V19.4.0 clause 16.10.6.5A; TS 38.331 V19.4.0 clause 6.3.6). |
| RRC states that can receive it | RRC_Connected only (TS 38.300 V19.4.0 clause 16.10.1). | RRC_CONNECTED, and RRC_INACTIVE with PTM delivery (Release 18: reception in RRC_INACTIVE via a dedicated multicast MCCH, SIB24, Multicast MCCH-RNTI) (TS 38.300 V19.4.0 clause 16.10.1; TS 38.300 V19.4.0 clause 16.10.5.2; TS 38.321 V19.4.0 clause 5.3.1). RRC_IDLE is not supported for delivery mode 1 (TS 38.300 V19.4.0 clause 16.10.5.2). | All RRC states: RRC_Connected, RRC_Idle and RRC_Inactive (TS 38.300 V19.4.0 clause 16.10.1). |
| Broadcast vs multicast applicability | Cannot be used for broadcast communication services (TS 38.300 V19.4.0 clause 16.10.6.5A). Covers: unicast packets arriving at reference point N3 from a PDU Session; MBS packets from a Multicast MBS Session arriving at reference point N3 in a PDU Session corresponding to 5GC individual MBS traffic delivery; and MBS packets from a Multicast MBS Session arriving at reference point N3mb using the 5GC shared MBS traffic delivery method, when the gNodeB has chosen to use the point-to-point RAN delivery method (TS 23.247 V20.0.0 clause 4.1). | Used for a Multicast MBS Session (via the PTM RAN delivery method). Not used for a Broadcast MBS Session (TS 38.300 V19.4.0 clause 16.10.5.4; TS 38.300 V19.4.0 clause 16.10.6.5A). | Always used for a Broadcast MBS Session (TS 38.300 V19.4.0 clause 16.10.6.5A). |
RRC messages and system information for MBS
TS 38.331 defines the system information blocks and RRC messages that carry the MBS configuration. This page lists which ones are used for MBS and what each carries; their ASN.1 is in TS 38.331.
| Name | Direction and channel | Carries |
|---|---|---|
| SIB20 | gNodeB to UE, system information | The information required to acquire the MCCH/MTCH configuration for MBS broadcast (TS 38.331 V19.4.0 clause 6.3.1, 5.9.1.1). |
| SIB21 | gNodeB to UE, system information | The mapping between the current and/or neighbouring carrier frequencies and MBS Frequency Selection Area Identities (FSAI) (TS 38.331 V19.4.0 clause 6.3.1, 5.9.1.1). |
| SIB24 | gNodeB to UE, system information | The information required to acquire the multicast MCCH/MTCH configuration for MBS multicast reception in RRC_INACTIVE (TS 38.331 V19.4.0 clause 6.3.1, 5.10.1.1). |
| SIB27 | gNodeB to UE, system information | The intended service area(s) of an MBS broadcast service in an NTN cell (TS 38.331 V19.4.0 clause 6.3.1, 5.9.1.1). |
| MBSBroadcastConfiguration | gNodeB to UE, MCCH | The control information for MBS broadcast services transmitted via broadcast MRB: the broadcast sessions provided in the cell and their scheduling information, and optionally a list of neighbour cells providing the same services (TS 38.331 V19.4.0 clause 6.2.2, 5.9.1.1). |
| MBSMulticastConfiguration | gNodeB to UE, multicast MCCH or RRCRelease | The control information for MBS multicast services transmitted via multicast MRBs to UEs in RRC_INACTIVE (TS 38.331 V19.4.0 clause 6.2.2, 5.10.1.1). |
| RRCReconfiguration | gNodeB to UE, dedicated | The relevant MBS configuration for a multicast session when the UE is in RRC_CONNECTED, including the MRBs to add or modify (TS 38.300 V19.4.0 clause 16.10.5.2; TS 38.331 V19.4.0 clause 5.3.5.6.1). |
| RRCRelease | gNodeB to UE, dedicated | The PTM configuration for multicast reception in RRC_INACTIVE (TS 38.300 V19.4.0 clause 16.10.5.2; TS 38.331 V19.4.0 clause 5.3.8.3, 5.10.1.1). |
| MBSInterestIndication | UE to gNodeB | That the UE is receiving, or is interested in receiving, or is no longer receiving or interested in receiving, MBS broadcast service(s) via a broadcast MRB (TS 38.331 V19.4.0 clause 6.2.2). |
Sources for this page
- RAN delivery methods: TS 23.247 V20.0.0, clause 4.1; TS 38.300 V19.4.0, clauses 16.10.5.4 and 16.10.6.5A.
- Delivery modes: TS 38.300 V19.4.0, clauses 16.10.1, 16.10.5.1 to 16.10.5.4, 16.10.5.7, 16.10.6.2, 16.10.6.5.1, 16.10.6.5A and 16.10.6.6; TS 38.321 V19.4.0, clauses 5.3.1 and 5.3.2.2; TS 38.331 V19.4.0, clause 6.3.6.
- RRC messages and system information: TS 38.300 V19.4.0, clause 16.10.5.2; TS 38.331 V19.4.0, clauses 5.3.5.6.1, 5.3.8.3, 5.9.1.1, 5.10.1.1, 6.2.2 and 6.3.1.
- Figures: TS 38.300 V19.4.0, figures 16.10.3-1 and 16.10.3-2.