ateway). Gateways are carrier-class products that reside in the service provider network. They provide PBX-like telephony service to multi- ple business and telecommuting customers as well as basic telephony services for consumers. Another requirement is the softswitch (also called a call server or call agent) for call-processing functions and administrative software. End-user services are delivered by IP phones and other devices. Finally, IP-based voice enterprise sys- tems include IP PBXs, open-source PBXs, and service provider solutions. Figure 9.2 shows an example of the IPT network architecture, and the following sections describe the components. Media Gateways Gateways provide seamless interoperability between circuit- switching and packet-switching network domains. They interconnect with the Sig- naling System 7 (SS7) network and handle IP services. They support a variety of tele- phony signaling protocols, such as H.323, Megaco (H.248) and MGCP, and SIP. They support Class 4 (i.e., toll) switches and Class 5 switches (as in local exchange ser- vices). They operate in the classic public network environment, where call control is Chapter 9 IP Services Billing System Management System Softswitch SS7 Network Media Gateway Broadband Packet Network: Trunking IP, Frame Relay, ATM Trunking Gateway MPLS, etc. Gateway Media Gateway Trunking SIP Phone Gateway Media Gateway IP Phone Figure 9.2 IPT network architecture separate from media flow. They support a variety of traffic, data, voice, fax, multi- media, and SO on over a data backbone. Softswitches In enhanced applications such as network conferencing, network interactive voice response, fax serving, network voicemail, and directory services, a softswitch controls the voice or data traffic path by signaling between the media gateways that transport the traffic. The gateway provides the connection between an IP or another packet-based network (such as ATM or MPLS) and the traditional circuit-switched network, acting as a multiprotocol cross-connect. The softswitc