 to the media gateway controller, while the accounting function collects call accounting information for billing purposes. The accounting function can also have a broader role of authentication, authori- zation and accounting in remote access networks. Signaling Gateway The signaling gateway provides the gateway for signaling between a VoIP network and the PSTN, whether SS7/TDM or ATM based. It encapsulates and transports PSTN signaling pro- tocols over IP. Applications Server The applications server is the application execution entity. Its primary role is to provide the service logic and execution for one or more applications and/or services. The applications server provides enhanced service logic for applications such as voice mail, a conference bridge or a pre- paid calling card service. Other applications include delivering services and information to an end user. It can access data net- works such as the Internet to gain access to information systems to provide services to the user. 148 - The Telecom Handbook CHAPTER For example in a voice mail application, the applications server instructs a media server to play the ring that the caller hears, prompts the caller to enter the digits (touchtone) and re- port the collected digits back to the applications server. The applications server verifies the passcode and if the caller is a user, it determines whether there are messages. A Feature Server is a limited Applications Server. It uses a proprietary API (applications programming interface) while the Applications Server uses a standard protocol, SIP. Media Server A media server offers its resources to applications servers, feature servers and media gateway controllers. Its primary role is to operate as a server that handles requests from the applications server or the media gateway controller for performing media pro- cessing on packetized media streams. It supports multiple concurrent capabilities including: Digit detection and collection Streaming of tones and announcements Algorithmic tone generation Recordin