[Asterisk-Users] ZtDummy vs Hardware

[Rich Adamson]

> >
> > So, 215 - 40 - 110 = 65kbps
> >
> > You only have about 65kbps to spare, and all of this is based on ideal 
> > (theoritical) conditions.  I doubt that those 12 calls will sound 
> > okay, or even work at all...
> >
> > But, you can always try!
> 
> The thing is: how do I do that? What tools are there to test how many 
> channels can be safely fit through?

The only way to know for sure is to try it in a test environment. Place
6, 10, 12 calls to someone else's asterisk box, measure the bandwidth
consumed, and listen to the quality for one of the calls in each test.
Lots of free tools out there to help measure/display the actual bandwidth
consumed. And, you certainly don't need much of a system to test this, 
but you would need to commit to the dsl circuit obviously.

Doing the pencil calculations is fine for starters, but there are lots
of other parameters that can impact the pencil-best-case such as
propagation delays, ISP bandwidth throttling (for many different reasons),
incorrect half vs full duplex settings anywhere along the entire path,
dsl modem irregularities, etc. You might also find that iax trunking 
doesn't work the way you thought it would on paper, etc.

> Another thing is that I was thinking that it would be possible to save 
> bandwith by lengthening the frame length to 50 or even 90ms. I have no 
> idea how this works or how to do it though :(

That might be a consideration _after_ you've confirmed the tests noted
above, but you are really talking about some rather small incremental
bandwidth improvements. (Particularly if you use iax trunking, which I
think you mentioned in an earlier post.) Since changing the frame length 
is not something that a lot of people actually try, you're also likely 
to stumble across coding errors, etc, that have not yet been uncovered.