School of Specs 23.501v20.2.0

3GPP 23.501 v20.2.0 — the document's own text

5.6.3 Roaming

Taught in 12. The connection to a data network (The 5G system architecture, in depth), 18. Being served by somebody else's network (The 5G system architecture, in depth), 5. The connection, and what is promised on it (The 5G system architecture, overview), 7. Roaming, other accesses, the edge and the outside (The 5G system architecture, overview).

In the case of roaming the 5GC supports following possible deployments scenarios for a PDU Session:

  • "Local Break Out" (LBO) where the SMF and all UPF(s) involved by the PDU Session are under control of the VPLMN.
  • "Home Routed" (HR) where the PDU Session is supported by a SMF function under control of the HPLMN, by a SMF function under control of the VPLMN, by at least one UPF under control of the HPLMN and by at least one UPF under control of the VPLMN. In this case the SMF in HPLMN selects the UPF(s) in the HPLMN and the SMF in VPLMN selects the UPF(s) in the VPLMN. This is further described in clause 6.3. Home Routed with Session Breakout in VPLMN (HR-SBO) is described in clause 6.7 of TS 23.548 [130].

NOTE 1: The use of an UPF in the VPLMN e.g. enables VPLMN charging, VPLMN LI and minimizes the impact on the HPLMN of the UE mobility within the VPLMN (e.g. for scenarios where SSC mode 1 applies).

Different simultaneous PDU Sessions of an UE may use different modes: Home Routed and LBO. The HPLMN can control via subscription data per DNN and per S-NSSAI whether a PDU Session is to be set-up in HR or in LBO mode.

In the case of PDU Sessions per Home Routed deployment:

  • NAS SM terminates in the SMF in VPLMN.
  • The SMF in VPLMN forwards to the SMF in the HPLMN SM related information.
  • The SMF in the HPLMN receives the SUPI of the UE from the SMF in the VPLMN during the PDU Session Establishment procedure.
  • The SMF in HPLMN is responsible to check the UE request with regard to the user subscription and to possibly reject the UE request in the case of mismatch. The SMF in HPLMN obtains subscription data directly from the UDM.
  • The SMF in HPLMN may send QoS requirements associated with a PDU Session to the SMF in VPLMN. This may happen during the PDU Session Establishment procedure and after the PDU Session is established. The interface between SMF in HPLMN and SMF in VPLMN is also able to carry (N9) User Plane forwarding information exchanged between SMF in HPLMN and SMF in VPLMN. The SMF in the VPLMN may check QoS requests from the SMF in HPLMN with respect to roaming agreements.

In home routed roaming case, the AMF selects an SMF in the VPLMN and a SMF in the HPLMN as described in clause 6.3.2 and in clause 4.3.2.2.3.3 of TS 23.502 [3] and provides the identifier of the selected SMF in the HPLMN to the selected SMF in the VPLMN.

In roaming with LBO, the AMF selects a SMF in the VPLMN as described in clause 6.3.2 and in clause 4.3.2.2.3.2 of TS 23.502 [3]. In this case, when handling a PDU Session Establishment Request message, the SMF in the VPLMN may reject the N11 message (related with the PDU Session Establishment Request message) with a proper N11 cause. This triggers the AMF to select both a new SMF in the VPLMN and a SMF in the HPLMN in order to handle the PDU Session using home routed roaming.