Background to NSA
In terms of initial deployments for 5G networks, the general consensus in the mobile telecoms industry was that standards-compliant 5G networks were expected to be deployed by around 2020. However, for many in the industry 2020 wasn’t early enough; vendors and service providers alike wanted to see large scale trials and deployments to begin as early as 2019. Consequently, the 3GPP agreed to deliver an intermediate milestone to the complete 5G technical specifications.
This milestone was termed “Non Standalone” 5G New Radio and was based on utilization of the existing LTE radio and core network for mobility management, session management and macro coverage, but with the addition of a 5G radio carrier to boost user data capacity.
As such, in December 2017 the 3GPP delivered the first drop of the Release 15 specifications related to 5G, specifically focused on Non Standalone operation. Although this fulfilled the industry backed desire to see an early version of the 5G 3GPP specifications, it was not designed to fulfil the overall requirements of IMT-2020. For example, Non Standalone 5G only really addresses the eMBB (Enhanced Mobile Broadband) aspect of IMT-2020, leaving URLLC (Ultra Reliable and Low Latency Communication) and MIoT (Massive Internet of Things) to be addressed in the full 5G Standalone specifications. Although this approach received some criticism, the introduction of Non Standalone 5G has allowed vendors and service providers to embark on their 5G projects in earnest.
Non Standalone Access typically centres around the operation of EN-DC (E-UTRA New Radio Dual Connectivity) in the network, whereby a master RAN node and a secondary RAN node work together to increase the overall data rate offered to the device.
In EN-DC configuration, the master node takes the form of a Master eNB and the secondary node takes the form of an en-gNB. With the latter, this RAN node supports the 5G radio link to the device, but has X2 connectivity with the Master eNB. From the perspective of the core network, everything remains centred around the EPC (Evolved Packet Core). The Master eNB will control all aspects of EN-DC operation, including when to add, change or remove an en-gNB and also determining how a data bearer should be configured eg only handled by the master or secondary, split between the master and secondary, etc.
The diagram shows how a “Master Cell Group Split Bearer” is configured in terms of the user data plane.
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