Switching to VoIP

This is the Title of the Book, eMatter Edition

Copyright © 2007 O’Reilly & Associates, Inc. All rights reserved.

332

|

Chapter 14: Traditional Apps on the Converged Network

There are a few ways to tackle this problem. The first and most obvious is to avoid

lossy codecs and use only non-lossy codecs like G.711 and G.722. It won’t distort

the analog modem signals as long as jitter is under control. But avoiding lossy codecs

may not be possible, especially on bandwidth-starved WANs.

Fortunately, the ITU has two recommendations to aid in migration of fax to VoIP.

Both are discussed in this chapter.

There is no real answer to the matter of modems, however. The ITU’s V.150.1 stan-

dard describes how to relay modem signals over a converged network, but no open

reference implementations exist yet. One could argue that it’s a bit arcane to use a

modem in a converged network, since any data sent by the modem could be sent

hundreds of times faster using Ethernet. In other words, if you have an Ethernet con-

nection, use that instead of the modem.

Thanks to the insistence of the U.S. Department of Defense, Version 4.1

of CallManager, Cisco’s venerable softPBX, does indeed support

modem pass-through using V.150.1.

T.30, T.37, and T.38

The ITU’s T.30 recommendation describes how fax devices should work. Just about

all legacy fax machines (and fax software) are T.30-compliant.

The T.38 protocol is the ITU’s recommendation ...