Roaming Support

Feature Description

AMF supports the following roaming types:

  • Inter-operator roaming

  • Roaming on N9 and S8 interface

  • Steering of roaming, roaming restrictions and operator Policy Support

N9 and S8 Roaming

Feature Description

Table 3. Feature History

Feature Name

Release Information

Description

Local Break Out (LBO) Support for N9 Roaming

 2024.01.1

AMF supports local breakout for N9 roaming irrespective of prsence of NRF configuration. This LBO support ensures a seamless network connectivity for roaming subscribers.

AMF checks if the subscriber is a roamer or homer, during registration. For a roamer subscriber, AMF selects one of the following roaming procedures:

  • Home-routed roaming: This procedure enables the subscribers to access the visited network through the home PDN gateway (H-PGW) and obtain services provided by their home networks.

  • Local Break Out (LBO) roaming: This procedure enables the subscribers to obtain visitor network provided services. LBO routes the traffic to the visited network, without routing through the home network before sending to or from the end destination.

AMF discovers the following NFs using NRF services, as per the selection of homer or roamer:

  • AUSF

  • PCF

  • UDM

  • SMF

Prerequisites

NRF configuration must be available.

How it Works

This section describes how this feature works.

Call Flows

This section describes the key call flows for this feature.

N9 Home Routed Roaming Call Flow

This section describes the N9 Home Routed Roaming call flow.

Figure 1. N9 Home Routed Roaming Call Flow
Table 4. N9 Home Routed Roaming Call Flow Description

Step

Description

1

The UE sends the Create PDN Request to AMF.
2, 3a, 3b

AMF performs the SMF selection and sends Nsmf_PDUSession_CreateSMContext_Request to vSMF and receives response.

4

vUPF performs the UPF selection.

5a, 5b

vSMF sends the N4 Session Establishment Request to vUPF and receives a response from it.

6

vSMF sends Nsmf_PDUSession_Create_Request to the hSMF.

7

Registration or subscription data retrieval is performed.

8

PDU session Authentication or Authorization procedure is performed.

9-13

See 3GPP TS 23502, version 15.4.0, section 4.3.2.2.2.

14

vSMF sends Namf_communication_N1N2MessageTransfer to SMF.

15, 16

AMF sends N2 PDU Session Request to RAN and receives a response.

17, 18

AMF sends Nsmf_PDUSession_updateSMContext_Request to vSMF and receives a response.

Note

For Local Breakout, the AMF uses the local SMF configuration in the absence of an NRF configuration.
S8 Home Routing Call Flow

This section describes the S8 Home Routing call flow.

For the preparation phase, see 3GPP TS 23502, Release 15.4.0, section 4.11.1.2.2.2-1—EPS to 5GS (4G to 5G) handover using N26 interface.

The home-routed roaming scenarios support the following functionalities:

  • H-PLMN supports the following nodes:

    • PGW-C and SMF

    • UPF and PGW-U

  • AMF selects a default vSMF per PDU session, and invokes the Nsmf_PDUSession_CreateSMContext service operation with the following:

    • UE PDN connection contexts

    • AMF ID

    • SMF

    • PGW-C address

    • S-NSSAI

  • The S-NSSAI is configured for interworking and is associated with default vSMF.

  • The default vSMF puts S-NSSAI in the N2 SM information container.

  • AMF selects PGW-C and SMF through NRF from S8 FQDN (obtained from the MME).

Standards Compliance

This feature complies with the following standards specifications:

  • 3GPP TS 23.501 "System Architecture for the 5G System"

  • 3GPP TS 23.502 "Procedures for the 5G System"

  • 3GPP TS 24.501 "Non-Access-Stratum (NAS) protocol for 5G System"

  • 3GPP TS 29.503 "Unified Data Management Services"

  • 3GPP TS 29.513 "Policy and Charging Control signalling flows and QoS parameter mapping"

  • 3GPP TS 29.518 "Access and Mobility Management Services"

Feature Configuration

Configuring this feature involves the following steps:

  • Configure the LBO—This configuration provides the commands to configure LBO roaming. For more information, refer to Configuring the LBO.

  • Configure the MNC digits in SUPI—This configuration provides the commands to configure the number of MNC digits in SUPI. For more information, refer to Configuring the MNC bits in SUPI.

    Configure the MNC bits in SUPI to discover the exact peer NF in roaming.

  • Configure the Globally Unique AMF ID (GUAMI) for AMF selection—This configuration provides the commands to configure the GUAMI. For more information, refer to Configuring the GUAMI for AMF Selection.

Configuring the LBO

To configure Local Break Out, use the following configuration:

config 
    amf-global 
        dnn-policy policy_name 
            lbo-roaming-allowed { true | false } 
            end

NOTES:

  • dnn-policy policy_name —Specify the DNN policy name.

  • lbo-roaming-allowed { true | false } —Specify LBO roaming allowed or not. Configuring lbo-roaming-allowed has minimum priority. The priorities are as follows:

    1. When UE includes a DNN name in the PDU Establishment Request, the DNN name is validated with the UDM subscription data. The LBO flag in the UDM subscription data has maximum priority.

    2. When UE doesn't include DNN name in PDU Establishment Request, AMF checks for the default DNN in UDM subscription. If the default DNN is available in the UDM subscription, the LBO flag is considered from the UDM subscription data.

    3. When UE doesn't include DNN name and UDM doesn't provide any default DNN, AMF checks for the configured default DNN. The corresponding DNN policy is checked for the configured default DNN and the LBO is configured using Configuring the LBO.

    When lbo-roaming-allowed configured as true, or UDM sends this flag, LBO roaming is considered. Otherwise Home-routed roaming is considered.


Note

AMF provides a lbo-roaming-allowed CLI within the DNN Policy. This CLI allows overriding the LBO flag sent in the UDM subscription. Specifically, when the LBO flag is false or missing, you can configure the override CLI to true.

Configuration Example

The following is an example configuration.

config
   amf-global
      dnn-policy dn1
         lbo-roaming-allowed true
         end

Configuring the MNC bits in SUPI

To configure the MNC bits in SUPI, use the following configuration:

config 
    amf-global 
        call-control-policy policy_name 
            supi 
                mnc number_of_mnc_bits 
                end

NOTES:

  • call-control-policy policy_name —Specify the call control policy name.

  • mnc number_of_mnc_bits —Specify the number of MNC bits. Must be either 2 or 3.

Configuration Example

The following is an example configuration.

config
   amf-global
      call-control-policy local
         supi
            mnc 2
            end
Configuration Verification

To verify the configuration:

show running-config amf-global call-control-policy local
amf-global
call-control-policy local
supi mnc 2

Configuring the GUAMI for AMF Selection

To configure the GUAMI for AMF selection, use the following configuration:

config 
    profile network-element amf amf_name 
        query-params guami 
        end

NOTES:

  • profile network-element amf amf_name —Specify AMF name. Must be a string.

  • query-params guami —Specify query parameters as GUAMI.

Configuration Verification

To verify the configuration:

show running-config profile network-element amf amf1 query-params
profile network-element amf amf1
query-params guami

Configuring the 5GC Inter-PLMN Roaming

To configure this feature, use the following configuration:

config 
    amf-global 
        call-control-policy policy_name 
        local-cause-code-map registration-restriction cause-code-5gmm plmn-not-found 
        end

NOTES:

  • call-control-policy policy_name —Specify the call control policy name.

  • local-cause-code-map registration-restriction cause-code-5gmm plmn-not-found —When the subscriber is a roamer and has registration restrictions, the AMF rejects the subscriber with the plmn-not-found cause setting.