Unquestionably, the best way to learn IP Telephony is to get the equipment and start using it. Put it together in a lab and play with it. Use a protocol analyzer and watch the traffic as you setup calls and enter digits, then use features like conferencing and hold. Watch the traffic fluctuate as you speak.
The catch of course, is that much of this equipment is tens of thousands of dollars. Even the software, like SIP and H.323 proxy servers, is often $40,000 or more! So how can you learn?
These developers have written some fabulous software that runs on any win32 platform as well as Linux and other UNIX. It may not be as pretty as some of the expensive commercial versions, but it works, and it's open. That means that not only is it free, but you can look at the source code and even modify it if you wish.
What's almost as impressive as the price tag is how difficult it was to set up. I downloaded and unzipped three files (the executable and two .dlls) onto two laptops running Windows 2000, in approximately 3 minutes. Next, I double-clicked "openphone.exe" on both computers. Finally, I clicked "Call > Make call" and typed an IP address. A second later, I was hearing my voice out of the other PC. Elapsed time, about 5 minutes! Of course, you may have to tweak the codecs some before your sound quality is acceptable, but that's what learning is all about.
After you master these simple steps, there is plenty more to learn. This program also supports H.323's H.261 video standard, so if you have some video equipment, you can have a videophone too. Then it's time to move on to a little more complicated subject: Multipoint Control Units (MCU), PSTN gateways, and gatekeepers.
So go download. Tweak some jitter buffers. Play with silence suppression. Set up a VoIP answering machine. Learn!
Thomas Alexander Lancaster IV is a consultant and author with over ten years experience in the networking industry, focused on Internet infrastructure.