 of the network provider or network platform that you are connected to. This intelligent programmable network requires some form of communication between the network elements. In the public switched telephone network (PSTN), this communication occurs through the use of high-speed common-channel signaling systems that allow real-time communications between the network elements. In essence, it's like a private subnetwork. No voice, data, or image traffic is carried on these channels-only the signaling information that dictates who is calling, what rights they have, what features and services they want to use, and SO on. Because there are many manufacturers and providers of network platforms, it is important Introduction that the programmable platforms use open application programming interfaces. In today's converged network infrastructures, the IP Multimedia Subsystem (IMS) creates a telephony-oriented signaling network that overlays an underlying IP net- work, replacing traditional telco SS7 signaling and acting as a control plane for both wireless and wireline networks. (See Chapter 4, "The PSTN," and Chapter 10, "Next-Generation Networks.") The new public network requires a new broadband infrastructure that has very high capacities and offers multichannel service (i.e., one physical medium can carry multiple conversations). The two dominant media types in the broadband arena are high-speed fiber (run as close as possible to the customer) and broadband wireless (over the last few meters to the customer, if needed). (See Chapters 11 and 15.) It is important that the new public network be a low-latency network. Humans cannot suffer much delay-only on the order of 650 milliseconds-in receiving information before it becomes unintelligible. To give you some perspective on this, on a satellite call, the delay between the time you say "Hi" to the time you hear the response is annoying, but it lasts only 500 milliseconds. Current infrastructures, such as the Internet, may impart as much as 1,000 or 2,000 milliseconds of 