[Freeswitch-users] False Answer Supervision (FAS) - Question

[Vlasis Hatzistavrou (KTI)]

Hello Alfredo,

Please see comments inline:

On 10/4/10 6:51 μμ, Alfredo Quiroga-Villamil wrote:
> All:
>
> A while back I tried to solve a false answer supervision issue I was
> intermittently receiving from underlying carriers. Back then I tried
> to find a solution using asterisk but had other pending things and put
> this off until now.
>
> Does anyone have any recommendations on how to possibly handle or get
> around FAS using FS. If I am not mistaken what would be needed is to
> have something that upon receiving the first 200 message, it simply
> ignores it, never propagating it and waits for the next 200. I can
> control this now a little bit better since it's only happening when
> the calls are sent to a couple of GrandStreams (FXO).
>    
As you have already noticed, when you get FAS, you receive a 200 before 
the call is answered. But I have never seen a second 200 when the call 
is really answered by the remote end, because after you receive the 
first (and only) 200 message, the call is considered as being properly 
connected, with RTP going both ways and all. A FAS giving carrier will 
take care to not send you another 200 when the call is answered.

So, I don't believe there is a way to detect and remove FAS 
successfully, because to your equipment, it just looks like a properly 
answered call. What you can do is gather batches of CDRs and extract 
statistics about the time delay from the INVITE you send to the 200 you 
receive. If you measure it too short, then you PROBABLY have FAS, and 
you'd still need to place a test call yourself you verify it.

You will not be able of course to tell which exact calls had FAS, but 
you'll be able to have a clue about whether there is FAS present on the 
route or not. And this means that you'd still have to do a lot of work 
yourself, like watching stats, placing test calls etc.

Even if you were able to find a way to remove FAS on a route, that would 
mean that your termination carrier would still charge you for the time 
difference between the 200 that he sends you and the BYE. And you'd 
still pay for a longer duration than what you would charge to your 
customer if you were able to remove FAS.

This is why my opinion is to watch statistics, place test calls and in 
the event of a carrier giving FAS, remove it from your routing. Sooner 
or later, they will either stop giving you FAS or you will stop doing 
business with them.


Best regards & good luck,
Vlasis Hatzistavrou.