(PLMN Application E-UTRAN Application (Stationary) LTE-Uu E-UTRAN LTE-Uu LTE-Uu LTE-Uu (Vehicle) (Vehicle) Application Application V2X Application PLMN A PLMN B Nonroaming V2X network architecture Roaming V2X network architecture Figure 7.7 Roaming/non-roaming network architecture for LTE-based V2X communication 2 V2X Application server S1-MME MB2-C E-UTRAN MBMS GW BM-SC LTE-Uu SGi-mb Figure 7.8 Reference architecture for eMBMS for LTE-Uu-based V2X communication via MB2 2]. updates for the area, or suggested speed. These two operation modes may be used by a UE independently for transmission and reception; for example, a UE can use eMBMS for recep- tion without using LTE-Uu for transmission. A UE may also receive V2X messages via LTE-Uu unicast downlink. V2X application servers (e.g., for traffic management services) in different domains can communicate with each other to exchange V2X messages. The interface between V2X application servers and the methods of the exchange of messages between V2X application servers is out of scope of 3GPP specifications. ProSe discovery feature can be used by a V2X-enabled UE and it is up to UE implementation. The RSU is not an architectural entity, but an implementation option. The high-level roaming and non-roaming reference architec- tures for PC5 and LTE-Uu-based V2X communications are shown in Fig. 7.7. Fig. 7.8 shows the high-level reference architecture with eMBMS for LTE-based V2X com- munication. The V2X application server may apply either MB2 or xMB reference points 804 Chapter 7 when managing eMBMS service-related information via a broadcast multicast service center (BM-SC), where MB2 reference point provides the functionality related to group communi- cation and xMB reference point provides an interface for any content and supports security framework between content provider and BM-SC. In the V2X reference architecture, the V2X control function is a logical function that is used for network-related functionalities required for V2X. It is assumed that there is only one l