The PhoneBoy Blog


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VoIP over Dialup

Recently, I was asked if you could do Voice over IP using a dialup connection. The answer is a qualified yes.

In general, it comes down to bandwidth, or a lack thereof on dialup. Assuming you have a 56k modem connection, which may really only give you about 33k depending on where you live, what kind of phone lines you’ve got, what modem you got, what phase of the moon it is, and so on. Most of the codecs used in Voice over IP assume a bit more bandwidth than this.

A codec is a way to compress your voice so it can be sent over an IP network. The various codecs use varying amounts of bandwidth. For a good overview, have a look at Grandstream‘s FAQ on Codecs. For those of you not wanting to read an Adobe Acrobat file, here’s the basic information you need:

  • G.711 — 107.2 kbps bit rate;
    • G.722 — 107.2 kbps bit rate;
      • G.726-32 (also referred as G.721) — 75.2 kbps bit rate;
        • G.728 — 188.8 kbps bit rate;
          • G.729 — 53.2 kbps bit rate;
            • G.723 — either 19.7 kbps (for 5.3 frame bit rate) or 20.8 (for 6.4kbps frame bit rate);
              • iLBC — either 36.8 Kbps (for 20ms frame length) or 27.7 Kbps (for 30ms frame length

              G.723 is the only reasonable choice in a dialup situation. Some people have reported success with G.729 over dialup as well. Basically, your VoIP provider must support G.723 for proper use over dialup.

              Now, the other trick is: making your VoIP call over a modem connection. There are two ways to do this: Use a soft phone from your PC that has a dialup connection, or use Internet Connection Sharing for your hardware device (e.g. Vonage ATA). The basic idea is:

              1. Put an Ethernet card in your PC if it doesn’t already have one.
                • Get a crossover cable between your ATA device and your PC or hook both to a hub/switch
                  • Enable Internet Connection Sharing in Windows. Instructions for this varies by Windows version
              What will happen is that your ATA, which is typically set up for obtaining an IP via DHCP, will use your computer as a sort of “router” to route the traffic over the Internet, which is provided by your dialup connection.

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