-plane function (SGW-C) and PGW control-plane function (PGW-C) control 3 5G System Overview S2b-C S2a-C GxGy Gz Gw Gn/Gp-C S5/8-C Serving Gateway-C Gateway-C Operator's IP S11-U Serving Services Gateway-U Gateway-U (e.g. IMS, PSS etc.) S5/8-U Gn/Gp-U S2a-U S2b-U Figure 3.2.3 Separation of user plane and control plane in EPS. (Source: reproduced with permission from © 3GPP). the user-plane forwarding within their respective user-plane functions by providing rules to classify incoming packets, add/remove header information, and forward packets to next hop user-plane nodes. While following a similar approach, 5GC offers a more flexible method for control and user-plane separation since CUPS was not superimposed onto an existing architecture as in EPC. Instead, separation of control and user plane was a design target from the beginning during the 5GS study phase. This resulted in the following key characteristics: Only a single UPF has been specified, which can be configured by the SMF to take differ- ent roles (e.g. the role of an intermediate user-plane entity like the SGW-U in EPS, which forwards packets between two tunnels, or the role of a session anchor like the PGW-U in EPS, which adds/removes tunnel headers, classifies packets for charging, and QoS rule enforcement, etc.). The number of UPFs that can be chained for a PDU session is not restricted by specifica- tions, i.e. 5GC supports PDU sessions using only a single or multiple UPFs. This flexibility enables the SMF to chain UPFs for different scenarios, for example, to enable access to both local and central data networks using a single PDU session (see Figure 3.2.4). Section 3.2.5 provides more details and examples of how the UPF can be configured for such scenarios. 3.2.4 Common Access-Agnostic Core Network One of the goals during the design phase of the 5G system architecture was to define a common core network that could be used for 3GPP accesses (e.g. E-UTRA and NR) but also for non-3GPP access technologies (e.g. WLAN, digital subscriber line [DSL], or