The 5GS System (5GS), just as the previous mobile telecom systems, consists of the Access Network (AN) and the Core Network (CN). R15 of the 3GPP standards defined the 5G New Radio (NR) and E-UTRA (commonly referred to as LTE) as the 3GPP Access Type, and Untrusted WiFi as the non-3GPP Access Type. In R16 the list of Access Types is was extended, with Trusted WiFi and Wireline accesses added as options within the non-3GPP Access Type.
The Trusted WiFi access added in the R16 3GPP standards, termed “Trusted Non-3GPP Access” is using the Trusted Non-3GPP Access Point (TNAP) for connectivity with the UE over the radio, and the Trusted Non-3GPP Gateway Function (TNGF) to terminate the N2 and N3 interfaces to the AMF and UPF, respectively, and the IPsec tunnel towards the UE.
For this scenario a UE needs to first perform a PLMN selection (over 3GPP access) and only afterwards will be allowed to select a TNGF within that PLMN. In most cases a UE utilizing the 3GPP and Trusted Non-3GPP accesses simultaneously will be serviced by one AMF and will have one common 5G-GUTI allocated, with the AMF terminating two different N1 connections and running two separate Registration Management (RM) and Connection Management (CM) state machines for the UE. The only exception is when a roaming UE uses different connection options (Local Breakout vs Home-Routed) for the 3GPP and for the non-3GPP accesses.
R16 standards add an option of Wireline access for the 5G UEs. The specifications mention two types of connections: from the Broadband Forum (BBF) and Data Over Cable Service Interface Specification (DOCSIS): In both cases, a Wireline Access Gateway Function (W-AGF) is used to terminate the N2 and N3 interfaces. The standards define a 5G-aware 5G Residential Gateway (5G-RG), or allow for a non-5G aware Fixed Network Residential Gateway (FN-RG) to be used. In the first case the 5G-RG acts as a 5G UE, terminating the N1 interface, while in the latter – the W-AGF has to take over the handling of the N1.
Roaming scenarios with the 5G-RG and the FN-RG are not specified in the current release of the standards.
With these new definitions the 5G is well on its way to offer wireless and wireline convergence, with a uniform handling of the UE access, security, mobility procedures as well as application-specific QoS, traffic steering and charging handling .
For more information on the 5G CN go to our in-depth 5G Core Network Architecture course.