In many cases 5G will come to smartphones, tablets and other devices in the form of 4G-5G dual-connectivity, also referred to as EN-DC (E-UTRAN New Radio – Dual Connectivity). While the mobile is in Idle state it will camp on a 4G LTE cell. A 5G NR cell is only added by the network once the device enters the RRC Connected state. Interesting conundrums that now present itself are for example if and how the mobile device can indicate to the user that it is ‘connected’ to 5G even while in Idle state and thus only really connected to a 4G cell!? Also, how does an LTE eNodeB know whether a 5G NR cell should be added when a mobile transfers to RRC connected state!? After all, network operators could decide that not all of their subscribers should have access to 5G. These problems do not exist today as the situation that a device talks to more than just one RAT (Radio Access Technology) is new. Some answers can now be found in the LTE core network specification 3GPP TS 23.401.
Here are some quotes from version 15.0.1 of the spec that was published mid-January 2018 that shed some light on these questions:
4.3.2a Support for Dual Connectivity
If the MME has an Access Restriction for NR for a UE (either signalled from the HSS, or, locally generated by VPLMN policy in the MME) then the MME shall signal this to the E-UTRAN as part of Handover Restriction List and to the UE in Attach and TAU Accept as defined in clauses 126.96.36.199, 188.8.131.52.3, 184.108.40.206.3 and D.3.6 respectively [NR-Restricted bit].
An eNB supporting Dual Connectivity with NR checks whether the UE is allowed to use NR. If the UE is not allowed to use NR, the eNB shall not establish Dual Connectivity with NR as a secondary RAT.
The MME uses “UE support for dual connectivity with NR” for SGW and PGW selection when the UE indicates support for NR and there is no Access Restriction for NR for the UE.
An E-UTRAN cell, based on operator configuration, broadcasts whether it is capable of supporting dual connectivity with locally available NR secondary cell(s).
220.127.116.11 E-UTRAN Initial Attach
If the UE is not allowed to use NR as Secondary RAT, the MME indicates that to the UE in the Attach Accept message.
5.7.1 HSS Data
Defines access restriction for NR as a secondary RAT
5.11.3 UE Core Network Capability
If the UE supports dual connectivity with NR (see clause 4.3.2a), then the UE shall indicate its support in a NAS indicator.
The HSS stores information if a subscriber is allowed to use 5G NR or not. The MME signals to the UE and the eNB during attach and tracking area updates if the subscriber is allowed to use the NR cell. And finally, the eNB broadcasts information if a 5G NR cell is co-located so UEs know even in Idle state that a 5G carrier can be added to 4G when it connects to the network. Together with the information of whether it is allowed to use 5G NR or not in the attach and tracking area update message, the UE has enough information to decide whether to show a 4G or 5G icon in the status bar.