Asterisk performance vs DSL bandwidth

I’m still (occasionally) working on my pro-bono Asterisk project for a non-profit here, as I have time.

(It’s going really slow because I need approvals just about every step of the way, which take a while to obtain, but I digress…)

The original plan was to install an Asterisk PBX on Fedora Core 5 in their facility, connect it up to their DSL, and provision the place with half a dozen IP phones placed here and there. Officers of the organization would be able to install XLite 3.0 on their home computers and connect to the PBX via their own broadband Internet connections. The idea was that outside callers could call in and reach any officer in his/her home just the same as in the office. Another objective is to support conference calling.

Then last week I found out that they can not get a static IP on their DSL, and worse yet, their DSL is 768K down and 128K up and they can’t get it raised to a reasonable speed.

So, I’m pulling back and re-thinking things.

I can give them space on our PBX here, so if for example they want to initiate a conference call, the PBX here can act as the hub, so that their DSL only needs to support as few as one call.

But, that’s only if one person in the office wants to participate. Two participants mean two simultaneous calls, etc.

Worse yet, that arrangement would mean that each IP phone in their office would route back here, so that if someone wants to call someone else in the next office, the call would route to here and then back to there, taking up two calls’ worth of bandwidth on their DSL, and forcing a call between two points 20 feet apart to have to pass via a 100-mile round trip.


So, here are my questions…

  1. Can I do some judicious selection of codecs to allow three or four calls to coexist simultaneously and reasonably happily on a 768K/128K DSL, or is that just asking too much?

  2. Can I provision them with a PC running Asterisk which would then connect to our PBX via IAX protocol, so that all their internal, intra-facility calls would just go over their own LAN and not get rerouted through here?

If I did (2), then how well would the single point-to-point IAX connection via their DSL support multiple simultaneous independent calls?

If, for example, they wanted to conduct a conference call involving four people in the office (each on his/her own phone) and 8 more people at home, then the four people in the office would crowd into the same 768K/128K DSL IAX pipe and connect to here, and our PBX would connect the calls individually to each home participant (we have plenty of bandwidth to do that). Any reasonable chance of success there?


Depends on how you set it up.

You definately want each office to have its own * server. That way if the DSL goes down at least they can still call each other (and out cuz presumably they have TDM trunks?).
If they only have 128k DSL that is going to create headaches. First you will need good qos controls on the DSL, and you will also need to use G.729 (which means buying licenses).

The way I would do it (this will be hard)- set up the conference room so that there is the same exten conference room on both servers. However, work it so that when the conference is established, a second call goes out that establishes it on the other server. This will probably require an AGI which creates a .call file. When somebody dials the exten for a conference room, you first check to see if that conference exists. If it doesn’t, you create it, and then generate a .call file which connects to it on the other box (where it is created dynamically).

The result is-
(many people in office) --sip calls-- office PBX --ONE iax call-- your PBX --sip calls-- (many people at home).
The ONE iax call bridges the two conference rooms.

The other option of course is to have them dial in via whatever lines you already have and connect that way, which will probably work better.

That said you should strongly encourage them to upgrade their DSL. This may be an uphill fight as that may require even more approvals but it will make your life much easier…

…the telcos and cable companies give you two speeds. One up and one down and the one up is always the slowest one…that way if you want to do something important they can charge extra for it…

Cynical viewpoint, perhaps. But this is why they will charge $15 per month for an extra residential phone line…ah, but if you tell them you’re a business they will charge you $65 per month for the same line.