Downlink data split over the NR air interface in the NSA Option 3x architecture is implemented at the medium access control (MAC) layer. The MAC layer is responsible for the management of data transmission over the air interface, including segmentation and reassembly of data packets, scheduling of transmission resources, and error correction. The other layers (RLC, PDCP, and physical) are not involved in the implementation of downlink data split.
https://ec.europa.eu/research/participants/documents/downloadPublic?documentIds=080166e5a964aa85&appId=PPGMS Preliminary results for multi-service support in link solution adaptation
https://ec.europa.eu/research/participants/documents/downloadPublic?documentIds=080166e5a964aa85&appId=PPGMS
https://ieeexplore.ieee.org/iel7/6287639/8948470/08998153.pdf
3GPP NR Sidelink Transmissions Toward 5G V2X
https://ieeexplore.ieee.org/iel7/6287639/8948470/08998153.pdf
https://www.3gpp.org/ftp/Inbox/Marcoms/ICT_6_1-2.pdf
journal of ict standardization - 3gpp
https://www.3gpp.org/ftp/Inbox/Marcoms/ICT_6_1-2.pdf
In the Non-Stand-Alone (NSA) Option 3x architecture, the control plane functions are handled by the 4G LTE network (EPC), while the user plane functions are handled by the 5G NR network. Data split is a technique that allows to split the user plane data between the 4G LTE and 5G NR networks.
The MAC (Medium Access Control) layer is responsible for controlling the access to the shared wireless medium, and it is where the downlink data split is implemented. In this architecture, the MAC layer in the 5G NR network receives the downlink data from the 4G LTE network and sends it to the physical layer for transmission.