The PhoneBoy Blog


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A Brief Primer in VoIP over DIalup

Someone previously asked via a comment on a previous blog entry how to do VoIP over Dialup.

hey. PhoneBoy, can you just explain to me in detail how “VoIP over Dialup” works? what is an ATA device (could you give an example) & what is a router. Another thing do i have to subscribe to a VoIP service provider or is it that your “trick” doesn’t require that. Please reply as soon as possible. P.S. – expalin in simple terms so i can understand you. I am not very advanced in all this

VoIP over Dialup depends on how you use VoIP. If via a softphone, which presumably exists on your PC which dials up to the Internet via a modem, then it’s pretty straightforward to use dialup over VoIP. If your internet telephony service provider gives you a piece of hardware to make calls with (e.g. an ATA device), then it becomes a little tricky.

The first thing you will need to do, if you haven’t already done it, is get an ethernet card. Ethernet cards for PCs are dirt-cheap these days and come in a number of varieties. Make sure it has an RJ-45 port on it. Most modern Ethernet cards have this kind of port, some earlier models do not.

The second thing you will need to get is a way to hook up your Internet telephony device to the Ethernet port on your PC. There are two ways to do this:

  • Get a hub or switch and two Ethernet cables. This is the preferred method and will allow you to connect other devices later.
  • Get an Ethernet crossover cable. This allows you to hook two devices directly to one another over Ethernet. It’s cheaper, though it might be a little harder to find in some of your office supply stores. However, for a travel situation (e.g. with a laptop), a crossover cable might be more desirable.

After this, you will need to do is set up Internet Connection Sharing, something that is available in Microsoft Windows 98 and beyond. annoyances.org has a page that tells you How to set up Internet Connection Sharing on Windows 98 and beyond. You can do this under Linux or MacOS as well, but I don’t have any nifty resource pages to point you at.

Once you have set up Internet Connection Sharing and plugged your Internet telephony device into your computer, your device should be able to make and receive calls.

    The next issue you will have involves configuration with your Internet telephony service provider, and that’s changing the codec. A codec is a way of encoding and decoding something for storage, transmission, and playback over a computer. In this instance, a codec refers to how your voice is encoded and decoded to be sent and received over an IP network. 

    The problem is that the vast majority of codecs used for VoIP do not play well under the constraints of a dialup connection. G.711, which is the most widely deployed, requires about 2 to 3 times the amount of bandwidth available in a typical dialup connection. Most other codecs use a ltitle less, but it is still too much for a typical dialup connection.

    When using a dialup connection, the conventional wisdom is to use the G.723 codec. At the moment, this seems to be your best bet. Some people have reported success with G.729, but as I understand it, you’ve gotta have a pretty good dialup connection to use G.729 successfully. G.726-16 might also be appropriate for a dialup connection, but I’ve never heard any success stories with that codec.

    In order to change the codec, you will have to contact your Internet telephony service provider. They may or may not allow you to change this. The vast majority of providers will not provide support for their service over dialup, so keep that in mind as well.

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