lane function function function function Control plane User plane Figure 1.13 Control-plane/user-plane protocol stack over Sxa, Sxb, Sxc, and combined Sxa/Sxb [8]. 36 Chapter 1 an individual PDN connection, TDF session, or this can be a standalone session and not tied to any PDN connection/TDF session, for example, for forwarding DHCP/RADIUS/ Diameter signaling between the PGW-C and PDN over SGi. Sx node-related procedures include Sx association setup/update/release procedures, monitoring peer PFCP, load and overload control procedures to balance loading across user-plane functions, and to reduce signaling toward user-plane function under overload conditions, Sx packet flow description (PFD) management procedure to provision PFDs for one or more application identifiers in the user-plane function. Sx session-related procedures include Sx session establishment/modification/deletion procedures; Sx session report procedure to report traffic usage or specific events. Data forwarding between the control-plane function and user-plane function is supported by GTP-U encapsulation on the user plane and PFCP on the control plane where the latter pro- tocol supports reliable delivery of messages. A set of new domain name system (DNS)²4 procedures are defined for user-plane function selection. The control-plane function selects a user-plane function based on DNS or local configuration, the capabilities of the user-plane function and the overload control information provided by the user-plane function. Figs 1.14 and 1.15 show two [example] deployment scenarios based on higher layer functional split between the central unit and the distributed unit (DU) of the base station reusing Rel- 12 dual-connectivity concepts. The control-plane/user-plane separation permits flexibility for different operational scenarios such as moving the PDCP to a central unit while retain- ing the radio resource management (RRM) in a master cell; moving the RRM to a more central location where it has oversight over multiple cells, allowing independent 