4aa4f.shtml) contains examples of what to look for within the out- put of these commands. Troubleshooting CHAPTER 30 Voice over IP (VoIP) has been the biggest thing to happen to network since IP itself. Maybe that's a stretch, but there's no denying the way VoIP has become a big deal for networking folks. So let's see what's involved, how it all works, and how to build a small phone system using a router, a switch, and some IP phones. I'll be using Cisco gear for this chapter, just like all the others. Sure, there are other solutions out there, but chances are, if you're reading this book, you'll be at a Cisco shop or studying for a Cisco exam, SO Cisco is what I'm using. If you're hell-bent on not using Cisco (and there are many execs who feel this way, especially when it comes to telephony), I recommend you check out the open source Asterisk project. Using SIP phones (read on to learn about SIP) and a Linux server, you can build a powerful phone system for little money. There's even an O'Reilly book to help you out: Asterisk: The Future of Telephony, Second Edition, by Jim Van Meggelen et al. (http://oreilly.com/ catalog/97805960096251). VoIP is a huge topic, and there are entire books devoted to its use and configuration. As always, my goal here is to get you started, show you how it works, and provide a real-world example. This chapter will not explain everything there is to know about VoIP. It will show you how to make a small office run on Cisco VoIP using Call Manager Express and Cisco 79xx phones. How VolP Works From a network standpoint, VoIP has two main functions: call control and the voice call itself. We'll cover call control in a minute, but first let's talk about the voice data stream. Telephony is a means whereby an analog representation of a caller's voice is sent over wires to another phone, where it is heard. In the early days, this was all done with simple electricity. As telephony advanced, the analog signal was converted to digital for mul- tiplexing on trunks such as T1 lines. See Chapter 2