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In the early days of point-to-point voice-over-packet systems, this was actually a very important question. In these early implementations, especially for voice over frame relay, the packet technology was often used as a replacement technology for trunk lines between PBXes. The primary business case was, of course, to use a low-bit-rate voice technology instead of 64K bit/sec digital telephony circuits and/or analog circuits. The voice sounded great, but, as you may or may not remember, modem traffic does not sound like human speech. In fact, modems often use modulation techniques for encoding data that are not perceptible to the human ear.

Why would anyone ever use a modem? Why not just use the data network for everything? After all, taking a digital bit stream, putting it into an analog carrier, and then reconverting that analog stream to a differently encoded digital bit stream does seem counterintuitive.

Nevertheless, as VoIP permeates corporate networks, there are increasingly assumptions that the entire VoIP infrastructure is automatically a replacement for the traditional voice network. And it usually is - but not always. Traveling visitors to an office may indeed have modem access as their only way to gain e-mail access to their company. And the problem isn’t limited to modems.

Two other technologies, fax and TTY, also use modem-like transmission technologies. Admittedly, a great deal of the faxing of yesteryear has been replaced by e-mailing Word, PowerPoint, and Excel documents. But there are still instances where “real” faxing is needed. For instance, in some cases a “real” signature is needed.

TTY (or TDD) devices also are designed for analog transmission. These devices often are still in place simply because nobody has gotten around to replacing them. But in some cases, they are necessary for complying with regulations like the Americans with Disabilities Act.

The bad news, if any, is that you must be careful in your VoIP implementation to make sure that you’re appropriately accounting for the legacy devices - some of which cannot be replaced on a wholesale swap-out. The good news, though, is that the vendor community is actively addressing this issue.

Two quick notes. First, we’d like to hear how you are handling (or have handled) this issue in your implementation. We’ll pass along your real-world hints and war stories. Secondly, if you would like to dig deeply into the technical issues surrounding this discussion, see the link referenced below.