Voip Blog
How Does A Phone Work?
Thursday, May 08, 2008
Here's an interesting video from the 1960s that explains how a telephone works:
The video goes a bit longer, but for the purposes of this discussion, it covers the basics. Sound is modulated in via the mouthpiece and turned into electrical impulses, which are then transmitted out the wire to the central office, where it is then sent to the other party, who hears it faithfully reproduced on the other end through the earpiece.
That's the way it works with a traditional telephone. In the voice over IP world, it's even more complicated than that. Sure, there's something, be it a telephone handset or a headset on your microphone that modulates your voice into electrical impulses. While VoIP does go out a wire, the wire isn't analog like a traditional telephone line, it's digital, or at least the part it uses is.
Specifically, the voice must be translated into something that can be sent and received on an IP network. The voice has to be turned into ones and zeros--relatively quickly, I might add--to get to the other end quick enough that the conversation seems natural. Too much delay in that process and the call becomes difficult to have.
Sampling your voice is fairly straightforward. Transmitting it efficiently while maintaining the quality of your voice is a completely different matter. This is done through codecs, which compress your voice into data and decompress it at the other end.
G.711 is the codec most often used to create "toll-quality" voice. If you want support dial-up modems, faxes, or other non-voice uses of a voice over IP line, then G.711 is what you must use. Assuming there is no packet loss and the jitter remains relatively constant, G.711 provides the closest thing to a call over a conventional landline.
The problem with G.711 is the amount of bandwidth you need. It encodes 8-bit samples at 8kHz , which gives you 64 kilobits per second of transmission. When you add the extra overhead of communicating over IP, it takes roughly 87 kilobits per second. (Incidently, that's more bandwidth than a traditional telephone line requires).
There are other codecs as well, which we will get into next time.
