y of the DNS server that handles the country where the mobile is registered; - level 2: these servers have authority over the country code and contain pointers to the level-3 servers. The DNS level-2 servers manage the <country code> e164enum.net domain. The response to the request relating to a phone number contains the identity of the operator's DNS server that handles the mobile; - level 3: these servers have authority over the codes assigned to operators and subscribers. The response to the request relating to a phone number contains the SIP URI of the mobile identity. Network Architecture 1.8. IPX network The internet protocol exchange (IPX) network provides the interconnection between mobile network operators A mobile network requires only a single connection to an IPX network to be able to interconnect with other networks of fixed and mobile operators. IPX network can offer several types of services: - transport service which is to route IP packets; - proxy service which is to route DIAMETER messages, to route SIP signaling messages and to switch RTP streams; - virtual roaming service that allows an operator to replace multiple bilateral roaming agreements by a single agreement with the virtual roaming operator; - ENUM database service by taking into account the level-1 DNS server that manages the e164enum.net domain. Signaling Protocols 2.1. NAS protocol The non-access stratum (NAS) protocol is the signaling exchanged between the user equipment (UE) and the mobility management entity (MME). The NAS protocol is transported by the radio resource control (RRC) protocol over the radio interface LTE-Uu and by the S1-AP (Application Part) protocol over the S1-MME interface. The NAS protocol comprises the following two protocols: - EPS mobility management (EMM): this takes care of controlling mobility and security; - EPS session management (ESM): this controls the bearer establishment. From the point of view of the MME entity, the mobile can be in one of two operational states: EMM-REGISTERED or EMM-DEREGISTERED