services will continue to have a fruitful life for some years to come, but they will have to increasingly live with IP services, SO the obvious solution is a hybrid archi- tecture that enables a low-risk evolution while allowing for new service offerings. There are now new genres of equipment and protocols to ensure that packet- based telephony switching gateways can in fact support key legacy services and signaling features. For example, a next-generation gateway switch supports the tra- ditional Class 4, or toll switch, services and the Class 5, or local exchange switch, services. It is designed to support a wide variety of traffic-data, voice, fax, multi- media, and other emerging media types-over a data backbone. The next-generation gateway switch provides seamless interoperability between the circuits that net- work the PSTN and packet-switching networks, such as IP backbones, ATM net- works, Frame Relay networks, and emerging MPLS networks. We can use these gateway switches to connect with the SS7 network and to handle the IP services that are SO popular today. These gateway switches can support a variety of tele- phony signaling protocols (e.g., H.323, Session Initiation Protocol [SIP], and Media Gateway Control Protocol [MGCP]) for communicating with the underly- ing SS7 architecture. (Chapter 9, "IP Services," discusses all the call control signal- ing standards.) As shown in Figure 4.16, there are different forms of these next-generation gateway switches for different purposes. In the bottom right of Figure 4.16 is an ILEC and its resident circuit-switched network, which is a series of Class 5 offices at the perimeter. These Class 5 offices are connected to the SS7 network or the STPs via A links. These Class 5 offices then connect into a CLEC's packet-switched network, and their first point of interface is a gateway switch. Among other things, this gateway switch is responsible for digitizing and packetizing the voice to pre- pare it for transport over the packet-switched network. The CLEC's packet- sw