Operators design 5G networks for the deployment of evolved Mobile BroadBand (eMBB) services. These services will most likely be deployed in the mainstream 5G frequency range of band n78 (3,4GHz – 3,8GHz). According to the expectations for evolved Mobile BroadBand, the existing MMEs can support the 5G enhancements defined by 3GPP. Based on the non-stand-alone network model 3.x, the NR is only handling the S1 user plane directly towards the serving gateway (S-GW) while the 4G eNodeB is kept as the signaling point towards the MME.
Regarding redundancy, there are no new aspects compared to the existing implementation of LTE. LTE and 5G access will be commonly using the MME. As a result, the same pool of MME addresses is used. There will be no need to configure new pools on the RAN side since a dedicated core network (DECOR) is not required for 5G NR early deployment. GW redundancy relies on the same mechanisms as today. In the Domain Name System (DNS), multiple GWs can be configured and sent to the MME. The MME will then choose the GW based on round-robin or apply the same selection mechanism as today.
GW Selection via DNS
For GW selection, the 3GPP standard has introduced a new suffix “+nc-nr,” which stands for “Network Capabilities – New Radio.” This is introduced to the app-protocol name in the DNS /NAPTR (Name Authority Pointer) records. It enables the MME to indicate towards the DNS that an Access Point Name (APN) shall be resolved for 5G access. In the DNS server, the IP addresses of the 5G Overlay GWs shall be configured for this nc-nr type. Therefore the MME will receive from DNS the IP Addresses and their capabilities.
Moreover, the MME uses the service parameters received from the DNS server. When a Dual Connectivity with New Radio (DCNR) capable UE attempts to register in the SGSN, and all DCNR validations are successful for dynamic gateway selection, the SGSN uses the DNS server’s service parameters (in NAPTR response) over other service parameters to select the NR capable gateway. SGSN can also use the same mechanism as the MME. In this way, the 5G terminals are anchored in 5G Overlay GWs, even when attached in 2G or 3G coverage.
A specific vendor MME supports an option to apply a standard preconfigured 5G profile, that is 5G support with 4G subscription. This enables Operators to introduce 5G services without having 5G subscriber profiles in the Home Subscriber Server (HSS). If an operator has problems upgrading an HSS or IT provision in time for an early start, this option can bridge the time until the HSS is upgraded.
GW aspects
The introduction of 5G does not change the service or policy design for GWs. For MBB use cases, exactly the same services will apply as for 4G access. Therefore, the GW configuration for APNs enabled for 5G will be equal to the existing GW configurations. The final throughput /AMBR values, which shall be negotiated, will be received via the Gx interface /PCRF. It is assumed that a 5G subscription will be provisioned with 4,2 Gbps, and the Policy Charging Rules Function (PCRF) will assign an Aggregate Maximum Bit Rate (AMBR) value smaller than 4.2 Gbps. With these values, it is guaranteed that the signaling towards the PCRF/Gx and Online Charging System OCS/Gy will not be impacted and/or changed compared to 4G access.
The introduction of 5G NSA does not have a major impact on the CDR (Charging Data Records) structure and attributes. Generally, ePC is only aware that the terminal is 5G capable, and RAB modification procedure is used to re-anchor the payload GTP tunnel from eNodeB to 5G NR. This implies that all traffic seen in the bearer for GW charging is either sent by 5G or 4G. So existing charging mechanisms for offline and online charging are applicable but are not 4G or 5G agnostic. The complete payload for the default bearer is counted and reported via CDR and Online Charging (Gy-Interface).
For offline Charging/CDRs, one option is that a new container for 5G counting is not activated, if the IT does not understand new CDR. S-GW-CDRs, and P-GW CDRs will count the overall 4G, and 5G traffic in volume count. CDRs will have an unchanged CDR structure with one exception, the QoS parameter. The QoS information field might be impacted if more than 4 Gbps traffic is used. New parameters are extended to signal higher speeds.
Another option is that a new container for 5G counting is activated if IT understands new 5G specific CDRs. S-GW and P-GW will produce separate CDR containers to count 5G traffic only. This information relies on the periodic update delivered by the RAN usage report. In this way, fixed frequency or accuracy is not guaranteed. This information is recommended to be used only for data warehouses and statistics. It is not recommended to use this for applying data tariffs as the accuracy and timing do not fulfill accuracy requirements. Online charging is so far only applicable for the combined 4G and 5G data stream. Therefore secondary RAT reporting cannot be used to steer the Gy-Interface. Consequently, it is not possible to support a fair use policy for 5G only.
Roaming aspects
For the initial phase, it is requested to suppress any 5G support for inbound roamers. As happened with VoLTE, it is assumed that there will be 5G roaming agreements after some time. Therefore the network shall allow 5G access based on the MNC/NCC level. There are no special functions or features for the initial phase, which allow such a sophisticated suppression. The MME shall suppress any inbound roamer to use 5G services, even though the subscription would allow this. This generic suppression is supported by specific vendors.
In case that the MME does not have a generic switch for roamers, a Diameter Routing Agent DRA-based solution is proposed to be implemented to have a generic suppression for all inbound roamers.
In this case, the MME retrieves the subscriber data via the S6a interface towards the HSS. DRA routes all diameter messages towards the home HSS. DRA will manipulate the received subscriber profile sent from the HSS by overwriting the Parameter “Access Restriction” to the value ‘5G not allowed’, since NR as secondary RAT in E-UTRAN is not allowed. With this change, the MME will not allow any 5G service and signal it down to the RAN. Then, the terminal will only be serviced by 4G instead of 5G. For outbound roamers, the visited network will interrogate the HSS to get the subscriber information. DRA shall look into Diameter messages and shall overwrite Access Restriction to “5G not allowed” for any Diameter requests from “foreign” MMEs.
Design for Lawful Interception (LI)
There are no new functional requirements concerning the Lawful Interception interface for 5G introduction. X1, X2, and X3 interfaces are applicable as they are used today. In case a subscriber is intercepted, the GW will report signaling and user plane via the well-known X1, X2, and X3 interfaces. Performance requirements towards the GW will be increased due to the new throughput classes defined for 5G. For the initial deployment, a maximum of 4.2 Gbps bearer will be used and subscribed. The existing assumption is that the terminal will support up to 2Gbps. Therefore, the GW has to be capable of delivering up to 2Gbps towards the X3 interface. For some vendors, the committed throughput capabilities are up to 1,6 Gbps. Discussion is ongoing, when and how this can be increased. Other vendors have not raised any limitations for the X3 interface.
