VOIP Over DSL Lines

There are three possible approaches to hooking up the two locations via VoIP:

[1] Use three single-line ATAs (such as Linksys 3102) on either location and hook them up to three unused phone lines on your PBX. The cost for this equipment will be about USD 300. Any user on the PBX may now dial one of the special lines and be connected to the respective "other" side that connects to the remote PBX.

The advantage of this solution is that it is simple and straightforward to implement. Linksys ATAs support G.729, so you may be able to fit even two VoIP conversations into that DSL line. However, 512 kbit/s is not much (I assume this is downstream, so upstream is probably no more than 128 kbit/s).

Here, I put together a list of codecs and their bandwidth requirements. The page is in German, but you will find figures and names, anyway: http://wiki.ip-phone-forum.de/voip:codecs:start

Consider getting a second DSL line just for VoIP. This way, your router will not be burdened with QoS/traffic shaping. Your bandwidth is so small, the second line would be useful, anyway. The cost should nevertheless be lower in total than having three additional outside lines from your telecom carrier.

Disadvantages: people using these connections will have to "know" which lines are free and try maybe each one of the three in turn until they find a free one.

[2] Use at either location a VoIP gateway connected to the PBX as some more "outside lines". You may need three of them or some ISDN links (two channels each). The VoIP gateway may be one of the Patton/Inalp products, Lancom or Intertex routers, or an Asterisk of some form. This gateway needs the proper number of incoming lines from your PBX (assuming it really can support three more outside lines) and an Internet connection.

Advantages: you can upgrade to more outside lines any time, e.g., by moving to an E1 link between the VoIP gateway and the PBX, or you may partly use VoIP instead of your normal fixed-line termination. Some limited number of outside lines (fixed-line) could be kept for backup. The remote location would appear to be just another set of extensions on your local PBX. You could even call through the other site, if necessary, to make "local" calls at lower rates even from the respective other location.

Disadvantages: the setup is a bit more complex and depends on the ability of your existing PBX to support three more outside lines.

[3] Setup an Asterisk server on each location as in the previous example, but place it between the existing outside lines and the PBX. Users who make plenty of calls will get an IP phone and connect directly to the Asterisk. This way, a soft migration from the old PBX is possible.

Advantages: easier migration, less complex setup. You may implement least-cost routing in the Asterisk and add VoIP providers as necessary, without having to change any prefixes people dial to get outside lines.

Disadvantages: this requires a bit more thought, but that's not really a disadvantage :-)

--gandalf.