How much for a phone call? –

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, 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.)

  • If you value bandwidth efficiency even more dearly than voice quality, you can take the previous situation and employ what is called silence suppression (also known as voice activity detection, or VAD). In most cases, this will cut the bandwidth requirement nearly in half. So figure that, with G.729A encoding, G.729B silence suppression, 20 milliseconds per packet, and WAN transport via PPP, you can get by allocating only about 14 Kbps per VoIP conversation in each direction.

There's one other trick that may be available to you for maximizing your bandwidth. That is what's called RTP compression, which you can enable on your backbone Cisco routers. Recent versions of Cisco's IOS router software support this feature on routers, including the 7000 series. RTP compression is unquestionably effective; it can cut your per-VoIP-call bandwidth down to a mere 8 to 9 Kbps (given the same G.729 encoding described in the scenario above). But there are two provisos to keep in mind with RTP compression:

  • It is processor intensive; so, while it can reduce VoIP bandwidth, it also eats up processing capacity on your router. If your backbone Cisco routers are running hot now -- frequently exceeding 50 percent CPU capacity, say -- adding to the burden by enabling RTP compression may not be a good idea.

  • RTP compression doesn't work with many vendors' VoIP traffic. In fact, the only vendor's VoIP traffic that we can say for certain will be compressed via Cisco routers' RTP compression is Cisco's. For all other VoIP gateway vendors, you'd better do some testing first.

So how much is a VoIP phone call? Regrettably, it can vary significantly on a case-by-case basis.

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