Mobile Network, First Edition. André Perez. C ISTE Ltd 2016. Published by ISTE Ltd and John Wiley & Sons, Inc. VoLTE and ViLTE The mobility management entity (MME) located in the visited network is connected to the home subscriber server (HSS) located in the home network via the S6a interface. The S9 interface between the home policy charging and rules function (H-PCRF) and the V-PCRF (Visited PCRF) entity is optional. If the S9 interface is not deployed, the rules applying to mobile traffic in roaming are stored in the subscription profile repository (SPR) associated with the V- PCRF entity. During the mobile registration, the S9 interface carries the DIAMETER messages of the Gx interface exchanged between V-PCRF and PGW entities for the establishment of the default support (default bearer) assigned to the SIP flow. During the establishment of the session, S9 interface carries the DIAMETER messages of the Rx interface exchanged between the V-PCRF entity and the proxy call session control function (P-CSCF) in the IMS for the establishment of the dedicated bearer allocated to the RTP stream. 8.1.2. Roaming applied to the IMS network The functional architecture of roaming applied to the IMS network is described in Figure 8.2, when the RTP stream passes through the home network. The roaming interface is between the P-CSCF entity located in the visited network and the S-CSCF (Serving CSCF) entity located in the home network. The interconnection border control function (IBCF) can make the translation of IP addresses and port numbers, corresponding to the network address and port translation (NAPT). The IBCF entity can perform the translation of IP addresses, port numbers and conversion of IPv4 to IPv6, corresponding to the NAPT-PT (Protocol Translation) function. The IBCF entity performs the withdrawal of some headers of the SIP message based on the rules established by the operator, corresponding to the function topology hiding interconnect gateway (THIG). Roaming The transition gateway (TrGW) may perform filtering, 