become very successful in data centers and transport networks, such as those described in Jain et al. (2013). In 3GPP, the idea was first discussed in the context of 4G EPC, where S-GW and P-GW core network nodes were separated into S-GW-C/P-GW-C and S-GW-U/P-GW-U (3GPP TS 23.214) control- and user-plane nodes, respectively. In 4G, the concept remained confined to the core network. In 5G, the core network (5GC) was designed according to the principle of control- and user-plane separation from the beginning. This is also the case for NG-RAN, which since Release-15 supports both deployment options of co-located and separated control- and user-plane NG-RAN network nodes. The fact that both options have been specified is a bit unusual, as normally there is no need to do SO - it is sufficient to specify the separated option only and the implementation is free to "collapse" two network nodes in one, as this has no standards impact. 10 This reflects the fact that in the initial 5G study, control- and user-plane separation of RAN has attracted broad interest but also faced concerns from some companies (the concerns have been mostly not with the technology as such, but with standardizing an open multi-vendor interoperable network interface to support it, which has commercial implications). During the study of control/user-plane separation in NG-RAN (3GPP TR 38.806), three deployment options have been identified, as illustrated in Figure 4.4.1. These have the following benefits for each deployment option: 1. Allow to take maximum advantage of cloud technologies. 2. Ensure low latency for critical control-plane procedures. 9 E1 is one of the few examples of control-plane only network interfaces; all other NG-RAN interfaces are with both control- and user-plane protocols. 10 In EPC, for example, there are implementation that contain both S-GW and P-GW in a single network node, in which case the interface between them is internal. Some implementations go as far as containing the whole EPC in a single network node. Even though