cessed May 29, 2020). 3GPP Technical Specification 29.281 (2019). General Packet Radio System (GPRS) Tunnelling Protocol User Plane (GTPv1-U). Available at: www.3gpp.org (accessed May 29, 2020). 3GPP Technical Specification 38.401 (2019). NG-RAN; Architecture description. Available at: www.3gpp.org (accessed May 29, 2020). 3GPP Technical Specification 38.425 (2019). NG-RAN; NR user plane protocol. Available at: www.3gpp.org (accessed May 29, 2020). 3GPP Technical Specification 38.470 (2019). NG-RAN; F1 general aspects and principles. Available at: www.3gpp.org (accessed May 29, 2020). 3GPP Technical Specification 38.471 (2019). NG-RAN; F1 layer 1. Available at: www.3gpp.org (accessed May 29, 2020). 3GPP Technical Specification 38.472 (2019). NG-RAN; F1 signalling transport. Available at: www.3gpp.org (accessed May 29, 2020). 3GPP Technical Specification 38.473 (2019). NG-RAN; F1 Application Protocol (F1AP). Available at: www.3gpp.org (accessed May 29, 2020). 3GPP Technical Specification 38.474 (2019). NG-RAN; F1 data 'transport', www.3gpp.org (accessed May 29, 2020). Request for Comments 4960 (2007). The Internet Engineering Task Force (IETF), Network Working Group, Stream Control Transmission Protocol. Available at: www.ietf.org (accessed May 29, 2020). 4 Those specifications define the user-plane protocol used on many NG-RAN and 5GC interfaces and not everything is relevant to the F1 interface. 4 NG-RAN Architecture Multi-Radio Dual Connectivity Sergio Parolari ZTE Corporation, Italy Most of the NG-RAN architectures described elsewhere in this chapter (e.g. in the previous section) are concerned with various options for splitting the gNB functionality into multiple logical network nodes. Multi-radio dual connectivity (MR-DC) described in the present section is different as, in this case, gNB functionality is split across two fully functional base stations, which are otherwise capable of serving UEs by themselves, that is, also without the cooperation of another base station. From the UE point of view, MR-DC ref