mber 2004 with the publication of 3GPP Release 6 (R6) standards. R6 adds support for SIP forking and multiway conferencing and the group management capabilities neces- sary for instant messaging and presence services. R6 also allows for interoperability between the IMS variant of SIP and the IETF SIP standard (RFC 3261). Finally, R6 adds interworking with WLANs. 3GPP Release 7 (R7), working together with TISPAN (Telecoms and Internet converged Services & Protocols for Advanced Networks) R1, adds support for fixed networks. TISPAN is a standardization body of ETSI that specializes in fixed network and Internet convergence. The TISPAN architecture is based on the con- cept of cooperating subsystems sharing common components. IMS products began to be introduced in 2005, and IMS-based consumer and entertainment services are expected in 2006 or 2007. One of the first applications to use IMS is likely to be Push-to-Talk over Cellular. (Push-to-talk is discussed in Chapter 16.) The Next-Generation Network Architecture Network architectures are in transition. In today's environment, time division and statistical multiplexers gather customer traffic for additional circuit-based aggrega- tion through a stable hierarchy of edge (i.e., local), tandem, and core switching offices in the carrier networks. Overlay networks, such as X.25, Frame Relay, ATM, and the Internet, have been put in place and have created the need to internetwork services, thereby eroding traditional network borders. As additional access in transport options-including cable, DSL, fiber, and wireless-began to be intro- duced, they joined traditional modems and brought their own high-density access aggregation devices into the picture. Meanwhile, in the core, SDH/SONET trans- port has been layered over DWDM, adding capacity and producing a variety of vendor-specific switching, routing, and management options. Figure 10.4 puts today's networks into a visual context. Residential customers on POTS connect through their first point of access, the Class 5 (i.e.,