conversions between LTE devices and 2G and 3G systems and relays this traffic to and from the Packet Data Network Gateway and earlier-generation gateways. Packet Data Network Gateway (PGW) This interfaces with the public data network (the Internet and carrier private data networks). It additionally allocates IP addresses to user devices and is responsible for policy enforce- ment. It can classify packets as requiring Quality of Service (QoS). It also generates usage records that it sends to the carrier's billing system, indicat- ing levels of customer voice and data usage. The PGW has links to the roam- ing database for functions related to roaming and billing roaming traffic. Mobile and Wi-Fi Networks The PGW and SGW transmit 2G and 3G traffic as well as LTE traffic to the Inter- net and other data networks. 2G and 3G traffic is handed off to these LTE gateways by earlier-generation controllers and signaling gateways. Databases in the LTE Core Gateways in core networks connect to a variety of databases that support roaming, bill- ing functions, voice messaging, and text messaging services. These types of databases also support 2G and 3G mobile networks. The following is a listing of key databases in core 4G networks. The Home Location Register (HLR) contains information services and bill- ing information on subscribers in 2G and 3G networks. The HLR also keeps track of the status and location of subscribers within its area. Visitor infor- mation is located in the Visitor Location Register (VLR) and keeps tempo- rary records of roaming subscribers. IP databases in 4G networks are Home Subscriber Services (HSSs). These databases keep location information on roamers. Databases that store telephone identities and authentication codes for digital phones are located in the LTE core. Messaging center databases and processors handle Short Message Service (SMS) messages, which are short text messages. More advanced systems store multimedia messages including voicemail, facsimile, and e-mail. They also support the ability