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

[Alfredo Quiroga-Villamil]

Hello Vlasis. I'll set something up this afternoon and send it to you
to make sure I am interpreting this correctly. Essentially the issue I
am experiencing in this case seems to be due to the VOIP ==> PSTN
conversion being done by the GrandStream.

I wasn't really using wireshark; just looking at the SIP trace after
having enabled SIP debug messages. I'll setup a test today and capture
the output of it and send it your way. If I am not mistaken what I am
seeing is a 200 OK message being sent by the GrandStream immediately
after doing the bridge from the VOIP leg to the PSTN side. Once the
bridge is created and the call sent over the PSTN, if an answer is
received, the GrandStream will then propagate a second 200.

A second set of eyes wouldn't hurt before I try to implement a hack
for this using OpenSips. In this case I seem to have the ability as
Michael previously pointed out to intercept Signaling and manipulate
it if needed. The basic idea I have is since I will always receive a
200 OK, (assuming I was correctly looking that day at the SIP trace)
is to simply ignore the first one and propagate just the second one
received. Which could also be a 486 or whatever other message. At that
point I wouldn't care, I would just propagate whatever I get after the
first 200.

I really appreciate the help with this since this could be a tricky
issue at times. Simplified in this case since I will always receive
the same result.

Alfredo

2010/4/13 Vlasis Hatzistavrou (KTI) <vhatz at kinetix.gr>:
> Hello Alfredo,
>
>
> On 12/4/10 8:23 μμ, Alfredo Quiroga-Villamil wrote:
>> Thanks for the response Vlasis. We run statistical analysis for FAS in
>> our production routes.
>>
>> This request would be for a controlled environment where I am using
>> GrandStreams 488 to convert from Voip to PSTN. I was thinking that
>> perhaps FS has a way to allow me somehow access to the Signaling
>> portion of the protocol. If that was the case, I would be able to hack
>> my way around it and perhaps ignore the first 200 I receive and not
>> propagate it to the origination gateway. This is the setup I am
>> looking at:
>>
>>
>
> Are you absolutely certain that when your carrier (or test rig) gives
> FAS you receive two 200 messages instead of one? Have you verified it
> using Wireshark captures?
>
>> A =>  B =>  GrandStream Device =>  PSTN
>>
>>     SIP   SIP
>>
>> A/B - Both Gateways
>>
>> This is a test environment I am using, something I am just playing
>> with. So in every case I will receive a FAS from the GrandStream
>> device.
>>
>> Do you know if there is a way to launch an application from the
>> DialPlan and have access to intercept Signaling; something that would
>> allow me to see the messages coming in and make decisions based on
>> them like propagate or do not propagate that message. I know this is a
>> tough thing since I am literally asking for access to the SIP Stack.
>>
>>
>
> Perhaps there is a way to create an app to detect when a SIP 200 message
> arrives, but I still have reservations whether 2 such messages arrive...
> I'd like to help you in this, so, do you have a packet capture file to
> share where 2 200 SIP messaes are sent?
>
> Regards,
> Vlasis.
>
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