l vertical approach was not efficient and, based on this understanding, began to pursue a horizontally layered architecture. The main objectives of this horizontal approach were to support the universal delivery of enhanced services, regardless of the access technology used by the subscriber, while minimizing the costs for the service provider. The base IMS functionality was first defined in the 3GPP Release 5 (R5) stan- dards, with a focus on 3G mobile telecommunications systems. R5 was optimized for use by GSM UMTS wireless networks. 3GPP Release 6 (R6) standards add a specification for fixed-wireless and wireline networks, as well as interworking with WLANs. 3GPP Release 7 (R7), working together with Telecoms and Internet Con- verged Services & Protocols for Advanced Networks (TISPAN; www.etsi.org/tispan) R1, adds support for fixed networks. TISPAN is a standardization body of the Euro- pean Telecommunication Standards Institute (ETSI; www.etsi.org) that specializes in fixed network and Internet convergence. The TISPAN architecture is based on the concept of cooperating subsystems sharing common components. IMS is such an important development that even wireline carriers are looking toward IMS as the basis for achieving a single enhanced services architecture for delivering any service, using any media, to reach any customer, regardless of how The IP Multimedia Subsystem the person connects to the network. (Chapter 10, "Next-Generation Networks," discusses the details of IMS. This section focuses on the relevance of IMS to wire- less networks.) IMS is key to IP for 3G networks because its purpose is to create a telephony- oriented signaling network that overlays an underlying IP network. IMS is designed to support IP-based multimedia services, and as you know, multimedia is the main application driving the requirement for broadband infrastructures in both the wired and wireless realms. The 3GPP IMS and the 3GPP2 (www.3gpp2.org) version, Multimedia Domain (MMD), provide the technological basis for the provi- si