). For location changes additional filters can be defined to further narrow down the events that will be reported (e.g. report some or all tracking area changes only). In conclusion, the modularized service approach fosters reuse since it makes it simple to access information like UE location for other standardized or proprietary 5GC network functions. In a similar manner, the functionalities of most 5GC network functions have been spec- ified as services (see clause 5.2 in 3GPP TS 23.502 for an overview of all network function services defined in 5GC). It is worth mentioning that interfaces between the 5GC control plane and the UPF as well as the interfaces between 5GC and NG-RAN are still following the traditional point-to-point model, that is, are not based on network function services. Another design decision that can be expected to simplify reuse, foster network pro- grammability, and generally lower the entry barrier for software add-ons on top of the standardized 5GC network functions is the shift from telecom-specific protocols like Diameter (Fajardo et al. 2012) and the GPRS Tunneling Protocol (GTP) (3GPP TS 29.274) to web protocols such as HTTP/2 (Belshe et al. 2015) and JavaScript Object Notation (Bray 2017) as the basis for the 5GC control plane (further protocol details can be found in 3GPP TS 29.500). 3.2.2.2 Overview of 5GC Control-Plane Functions As illustrated in the previous section, 5GC is based on a new system architecture. Never- theless, many concepts and system functionalities are similar to those in EPC. This section presents a high-level overview of the 5GC control plane and the roles of the network functions it consists of (Figure 3.2.2) while also pointing out the key differences to their EPC counterparts. 3 5G System Overview The 5GC equivalent of EPC's MME is the AMF. AMF's key responsibilities include: Registration management: before a UE can use network services, the UE needs to perform an initial registration with the AMF. As part of this step, UE and AMF (supported by other functi