Initiation Protocol is a protocol for the establishment, modification and termination of conferencing and telephone sessions over IP-based net- works. SIP uses text based messages much like HTTP (Hyper Text Transfer Protocol used for moving docu- ments around the Internet.) - Compliments of Newton's Telecom Dictionary circuit to packet formats or from analog/ISDN circuit to packet. It is always controlled by a media gateway controller. The media gateway interfaces the IP network with an ac- cess endpoint or network trunk, or a collection of endpoints and/or trunks. As such, it serves at the gateway between the packet and the external networks such as the PSTN, cellular network, etc. For example, the media gateway can provide the gateway between an IP and circuit network (e.g. IP to PSTN), or between two packet networks (e.g. IP to 3G mobile networks or ATM) Its primary role is to transform media from one transmission format to another, most often between cir- cuits and packets, between ATM packets and IP packets, or between analog/ISDN circuits and packets. The media gateway always has a master/slave relationship with the media gateway controller It may perform media processing functions such as transcoding, packetization, echo cancellation, jitter buffer management, packet loss compensation, etc. It may perform media insertion functions such as call progress tone generation, DTMF (touchtone) generation, comfort noise generation, etc. The Telecom Handbook 65 147 CONVERGENCE OF VOICE & DATA It may perform signaling and media event detections such as DTMF (touchtone) detection, on/off hook detection, voice activity detection, etc. Many media gateways are controlled with media gateway control protocol (see box describing MGCP). Note: An earlier meaning of "softswitch" referred to the me- dia gateway and controller. Now softswitch has come to refer to the interaction all of these servers mentioned here and their col- lective operation. Call Routing and Accounting The routing function provides call routing information