, EN-DC is also known as the 5G "Non-Standalone" (NSA) option and it was also the first 5G solution specified in the first version (known as the "early drop") of 3GPP Release-15. The other MR-DC options, NGEN-DC, NE-DC, and NR-DC, were introduced in the last version (i.e. the "late drop") of 3GPP Release-15.5 NGEN-DC shares the same benefit as EN-DC of allowing the reuse of an existing LTE radio infrastructure to provide basic service, which can be enhanced using the dual con- nectivity functionality when NR coverage is available. On top of this, NGEN-DC allows connection to the 5GC and the related services and enhancements, for example, including the support of the 5G QoS framework. 5 3GPP Release-15 is different compared with other releases not only because this is the first release introducing 5G, but also because it effectively contains three "sub-releases," also referred to as "drops": EN-DC/NSA in the "first drop," "standalone"/full: 5G released at the originally planned scheduled date, and NGEN-DC, NE-DC and NR-DC in the "late drop." 4 NG-RAN Architecture NE-DC also allows the reuse of an existing LTE radio infrastructure. The difference in this case is that the anchor is the NR access network and the LTE access is used to increase the performance. For instance, this architectural option is useful when the NR network operates in a lower band than the LTE one; then providing an even better coverage than the existing LTE network. While EN-DC is arguably the most important MR-DC option, at least in the first phase of 5G deployment, the benefits of NGEN-DC and NE-DC are somewhat less evident. While these options do have certain benefits, as described in this chapter, it is not clear whether these benefits justify the cost of their deployments. At the time of writing this book, the majority of 5G deployment announcements and plans are centered around EN-DC and "Standalone" 5G. The benefit of NR-DC is different: it is not related to the reuse of legacy core and radio access networks, as it actually applies to a 