Section: 16.10.3 Protocol Architecture
Content: Figure 16.10.3-1 and 16.10.3-2 depict the downlink Layer 2 architecture for multicast session and broadcast session respectively, where MBS protocol stack comprises the same layer 2 sublayers as described in clause 6 with the following differences: - SDAP sublayer provides only the following functionalities: - Mapping between an MBS QoS flow and an MRB; - Transfer of user plane data. - PDCP sublayer provides only the following functionalities: - Transfer of user plane data; - Maintenance of PDCP SNs; - Header compression and decompression using the ROHC protocol or EHC protocol; - Reordering and in-order delivery; - Duplicate discarding. - For a multicast session, gNB provides one or more of the following multicast MRB configuration(s) to the UE via dedicated RRC signalling: - Multicast MRB with DL only RLC-UM or bidirectional RLC-UM configuration for PTP transmission; - Multicast MRB with RLC-AM entity configuration for PTP transmission; - Multicast MRB with DL only RLC-UM entity for PTM transmission; - Multicast MRB with two RLC-UM entities, one DL only RLC-UM entity for PTP transmission and the other DL only RLC-UM entity for PTM transmission; - Multicast MRB with three RLC-UM entities, one DL RLC-UM entity and one UL RLC-UM entity for PTP transmission and the other DL only RLC-UM entity for PTM transmission; - Multicast MRB with two RLC entities, one RLC-AM entity for PTP transmission and the other DL only RLC-UM entity for PTM transmission. - For a multicast session, gNB may change the MRB type using RRC signalling. Figure 16.10.3-1: Downlink Layer 2 Architecture for Multicast Session - For broadcast session, gNB provides the following broadcast MRB configuration to the UE using broadcast RRC signalling: - Broadcast MRB with one DL only RLC-UM entity for PTM transmission. Figure 16.10.3-2: Downlink Layer 2 Architecture for Broadcast Session