Evolutionary Rollout of 5G
Many of the benefits of 5G, such as low user plane latency, high data rate and support of high radio frequencies, can be achieved simply by transmitting data over the 5G New Radio. Furthermore, there are market pressures on network operators for an early roll-out of 5G networks in which those benefits are delivered to the user. To reflect these issues, the 5G New Radio is designed to support an evolutionary roll-out in which it inter-operates closely with LTE. The resulting architecture is closely based on the architecture for DC in LTE.
In the first phase, the mobile continues to communicate with the EPC and with an eNB, but it also communicates with a 5G gNB. The LTE base station takes on a role known as a MeNB (Master eNB), in which it delivers not only data packets to and from the user, but also signalling messages that control the UE’s operation. The 5G base station takes on a role known as a SgNB (Secondary gNB), in which it only delivers data. The RAN is still the E-UTRAN, because the base stations communicate in the same way that they do in LTE. The configuration is a NSA (Non Standalone) deployment of the 5G New Radio, as the gNB cannot act alone.
In the second phase, the EPC is replaced by the 5G core network. Furthermore, the E-UTRAN is replaced by the NG-RAN, because the base stations now communicate using the new protocols and procedures for 5G.
The third phase is a SA (Standalone) deployment of the 5G New Radio, in which a gNB can act alone. The 5G base station takes on the role of a MgNB (Master gNB), in which it delivers both data and signalling messages. It can be supported by an LTE base station, which takes on the role of a SeNB (Secondary eNB), and only delivers data. That role can also be filled by a SgNB, in a deployment that uses 5G alone. These three configurations can co-exist within a single operator’s network. For example, the E-UTRAN can coexist with the NG-RAN, and the EPC can co-exist with the 5GC. An individual UE is served either by the EPC or by the 5GC, but not by both at the same time. Generically, an MeNB or gNB is known as a MN (Master Node), and an SeNB or gNB is known as a SN (Secondary Node).