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In this interoperation and roaming scenario, the enhanced network elements for the LTE/EPS network are deployed both in the HPLMN and the VPLMN. There are two ways to transfer user data between the UE and the Internet in case of roaming between two different PLMNs with the enhanced network elements. To enable such roaming scenarios, new logical interfaces are used to interconnect among the network elements as described below:

 Routing of User Data by the HPLMN

In this interoperation arrangement, Figure 6.11, the roaming user data is routed from the VPLMN’s S‐GW back to the HPLMN packet data network (PGW) gateway. This means that a roaming user has access to the HPLMN’s services and resources from the VPLMN.

Figure 6.11 LTE/EPS Roaming: routing of user data by the HPLMN for roaming user.

To route the roaming user traffic by the HPLMN, the VPLMN S‐GW and the HPLMN PGW are interconnected through the S8 logical interface as shown in Figure 6.11. S8 interface uses the GTP to tunnel user data from the VPLMN S‐GW to HPLMN PDN. Apart from this, the HSS of the HPLMN and the MME of the VPLMN are interconnected through the S6a interface through which the UE’s current location and also the subscriber’s information are exchanged between them.

In Figure 6.11, the thin dashed line represents the boundary that separates the HPLMN and the VPLMN. The arrow represents that the UE has left its HPLMN and registered with the VPLMN.

 Routing of User Data by the VPLMN

In this interoperation arrangement, Figure 6.12, the roaming user data is routed by the VPLMN PGW gateway to the external PGW.

Unlike the previous scenario, Figure 6.11, no user data is routed back from the VPLMN to the HPLMN. Because of this, it is also known as the local breakout, refer to TS 23.401 [39], roaming scenario. Also, the Policy and Charging Rules Function (PCRF) deployed at the HPLMN and VPLMN requires to exchange of subscriber and policy control‐related information between them over the S9 interface. The PCRF is the centralized policy and charging‐related control network element that is part of the LTE/EPC; refer to Figure 1(b) TS 23.002 [29]. PCRF works based on the subscription information of a user and supports both the roaming and nonroaming users. PCRF meets the charging requirement of different categories of subscribers, for example, volume‐based, time‐based charging. PCRF may deny the information flow of a subscriber based on its subscription profile.

For a roaming user whose traffic is routed by the VPLMN, the UE has two associations with both the home PCRF (HPCRF) and visited PCRF (VPCRF). HPCRF and VPCRF exchange policy and charging‐related information for a particular subscriber and its profile. PCRF has a couple of logical interfaces toward other servers/nodes to perform various functions and requirements. Typical functions include the charging, policy enforcement, QoS control, usages monitoring, user traffic congestion detection, and mitigation at the radio access network level. For more information on PCRF, its reference architecture, and logical interfaces, refer to TS 23.203 [33].

Figure 6.12 LTE/EPS Roaming: routing of user data by the VPLMN for roaming user.

Figure 6.13 Roaming through interoperation of legacy network elements.

Source: © 2015. 3GPP ™ TSs and TRs are the property of ARIB, ATIS, CCSA, ETSI, TSDSI, TTA and TTC who jointly own the copyright in them. © 2015, 3GPP.