The concept is to provide combined services through the same access technology. This project represents a big endorsement for SIP. As a 3G network, PacketCable addresses real customer needs using SIP with some exten- sions. This type of project is a critical source of feedback and new ideas for the SIP community. New requirements lead to the implementation of new extensions. Cable modems are everywhere in much the same way phones are, and the PacketCable project will provide SIP multimedia services to a much larger number of users. Building Applications with the SIP Toolkit Architecture The PacketCable architecture defines a set of nodes and the protocols that will be used between them. SIP is one of the protocols used in this archi- tecture, augmented by a set of extensions. The most important nodes related to SIP operation in the PacketCable architecture are the Multime- dia Terminal Adaptor (MTA) and the Call Management Server (CMS). The MTA resides on user premises and has a user interface that resem- bles a traditional telephone (POTS). Between the CMS and an MTA, a pro- tocol called the Network Call Signalling (NCS) protocol comes into play. NCS is an extended variant of the master/slave IETF protocol Media Gate- way Control Protocol (MGCP) [RFC 2705]. The MTA acts as the slave, send- ing events and responding to commands issued by the CMS. For instance, the MTA informs the CMS when the user goes off hook or when he or she types some digits. The CMS can then order the MTA to alert the user or to open a media channel for voice. PacketCable architecture will probably evolve SO that the protocol used between an MTA and the CMS is SIP instead of NCS. The CMS is basically a SIP server. One CMS handles a domain and thus typically controls several MTAs, as shown in Figure 7-6. The pro- tocol used between CMSs is the Call Management Server Signalling (CMSS) protocol. This is basically SIP with some extensions such as the reliable delivery of provisional responses and Quality of Service (QoS) preconditions. Call Flo