Access network: The user can connect to the IP-core of NGN in various ways, most of which use the standard Internet Protocol (IP). User terminals such as mobile phones, personal digital assistants (PDAs) and computers can register directly on NGN-core, even when they are roaming in another network or country. The only requirement is that they can use IP and Session Initiation Protocol (SIP). Fixed access (e.g., digital subscriber line (DSL), cable modems, Ethernet), mobile access (e.g. W-CDMA, CDMA2000, GSM, GPRS) and wireless access (e.g.WLAN, WiMAX) are all supported. Other phone systems like plain old telephone service and non-compatible VoIP systems, are supported through gateways. With the deployment of the NGN, users may subscribe to many simultaneous access-providers providing telephony, internet or entertainment services. This may provide end-users with virtually unlimited options to choose between service providers for these services in NGN environment.