How much for a phone call?

ITworld.com |  Networking

I'll admit up front that the title of this piece, while not inaccurate, is a bit
misleading. This isn't about how much a phone call costs, although that could be an
interesting column, too. (Remember when a pay phone call cost a nickel?)

No, this is about how much bandwidth a phone call -- specifically, a voice-over-IP
(VoIP) phone call -- requires.

More than a million VoIP ports have shipped to date. And while the majority of those
have so far gone to service providers and carriers, it looks like enterprise networkers
are now poised to start catching up. Invariably, the first question that comes up in
planning for VoIP is: "Well, how much bandwidth does a VoIP phone call require, anyhow?"

Not to be flip, but the answer truly is that it depends.

After having conducted 1.2 bazillion tests with a dozen different IP PBXs, more than
two dozen different VoIP gateways, and hundreds of different vocoder settings, href="http://www.mier.com">Mier Communications graciously offers the following tips
and rules of thumb:

  • There can be a 20-fold difference in how much bandwidth a VoIP phone call
    requires -- from as little as 9 Kbps, to as much as 180 Kbps, in each
    direction.

  • The bandwidth required per VoIP call is fairly constant, which makes VoIP
    traffic load engineering a little easier. You don't have to worry about bandwidth
    spikes or surges, at least not from the VoIP traffic. (If VoIP is sharing the same link
    with regular data, though, you'll still see spikes and surges, though.)

  • The encoding rate, which is a measure of how many bits per second the
    vocoding algorithm generates in converting an analog voice signal to digital, is
    not the network bandwidth that a VoIP conversation requires. Don't assume,
    when a VoIP vendor tells you, "G.729 encodes at 8 Kbps, G.711 encodes at 64
    Kbps," and so forth, that those are the amounts of network bandwidth that each
    VoIP call needs.

  • In an environment where voice quality is paramount, and where bandwidth is
    plentiful, a typical VoIP conversation needs about 84 Kbps in each direction. (This is
    based on straightforward G.711 encoding with 20 milliseconds of voice samples per
    packet, WAN transmission via PPP encapsulation, and no other special options.)

  • In an environment where voice quality is important, but bandwidth efficiency is
    just as important, figure on about 27 Kbps, in each direction for a typical VoIP
    conversation. (This is based on G.729 encoding with 20 milliseconds of voice samples
    per packet, WAN transmission via PPP, and no other special options.)

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