Bearer channels for user data = Signaling channels eNodeB MME = Mobility Management Entity PGW = Packet Data Network Gateway Radio access network SGW = Serving Gateway Figure 7-5 Fourth-generation (4G) Long-Term Evolution (LTE) architecture in the Radio Access Network (RAN) and the evolved packet core. The Streamlined Infrastructure of Long-Term Evolution Architecture The Three Elemental Functions of the LTE IP Core Traffic from hundreds of cell sites passes through a mobile carrier's core network. Signaling gateways for 2G and 3G networks are located in the core along with fourth- generation (4G) equipment. The LTE core network, also referred to as the evolved packet core, routes this traffic to data networks, the Internet, other mobile carriers, and other cell sites within the carrier's mobile network. Equipment and software located in the core also perform signaling and tracking calls for billing purposes. In addition, signaling links are carried separately from user data and IP voice. Sig- naling is used to keep track of usage, perform security functions (to keep the network free from hackers and malware), and enforce policies. Policies include rules on the number of bits included in various data plans. Signaling enables carriers to monitor this volume SO that a message can be sent to the user's device notifying him that he has used up his planned quota and will be charged extra for additional transmissions. Functions within the LTE evolved core network are divided into three functional elements. Two functions, the serving gateway and packet data network gateway, are generally located in the same router. The functional elements making up the LTE's IP core are as follows: Mobility Management Entity (MME) This performs the signaling func- tions in the IP core. It sends and receives signaling information needed to set up, bill, and address calls to the eNodeB and to the Serving Gateway. In addition, the MME contains the security protocols for authentication and authorization. Serving Gateway (SGW) The SGW forwar