[cisco-voip] T.38 Fax Issue

[Antonio Soares]

It was working before so I assume the other end's configuration was changed.
This is a SIP trunk and we have this configuration for the T.38 fax:

voice service voip
 fax protocol t38 ls-redundancy 0 hs-redundancy 0 fallback pass-through
g711alaw

We are now trying the following:

voice service voip
 fax protocol t38 ls-redundancy 0 hs-redundancy 0 fallback cisco

This should match their PT=96 request, right ?


Thanks.

Regards,

Antonio Soares, CCIE #18473 (R&S/SP)
amsoares at netcabo.pt


-----Original Message-----
From: Peter Slow [mailto:peter.slow at gmail.com] 
Sent: segunda-feira, 17 de Janeiro de 2011 11:50
To: Antonio Soares
Cc: cisco-voip at puck.nether.net
Subject: Re: [cisco-voip] T.38 Fax Issue

Usually t.38 Fax relay operates by switching to t.38 based on an MGCP
MDCX, a new h245 TCS/OLC etc, or a SIP re-invite. this is referred to
as protocol based switchover, and is generally how people's gateways
are configured for t.38 when it is being used. it would appear that
the other GW you are talking to is set up for NSE based switchover,
which AFAIK is a cisco proprietary method. You may be configured for
protocol based switchover still, while it sounds like perhaps the
other side went and changed something and is now trying to do NSE
based switchover...

Check out this link:
http://www.cisco.com/en/US/docs/ios/12_3/vvf_c/cisco_ios_fax_services_over_i
p_application_guide/t38.html#wp1156699

It is tough to help you out beyond what I've said without knowing what
protocol (h.323, MGCP..) you are using for call control. More insight
there, plus perhaps some asn1 or mgcp packet debugs would help, but
try setting your side to use nse based switchover if it isnt set that
way already.

-Peter



On Mon, Jan 17, 2011 at 5:48 AM, Antonio Soares <amsoares at netcabo.pt> wrote:
> Hello group,
>
> A few days ago, fax stopped working on one gateway. The "debug voip rtp
> session nse" gives me this:
>
>
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
> ++++++++++++++
>          s=VoIP d=DSP payload 0x60 ssrc 0x509B391F sequence 0x18E
timestamp
> 0xF8C0
>  <<<Rcv> Pt:96     Evt:136     Pkt:02 80 50
>          s=VoIP d=DSP payload 0x60 ssrc 0x509B391F sequence 0x18F
timestamp
> 0xF910
>  <<<Rcv> Pt:96     Evt:136     Pkt:01 40 50
>          s=VoIP d=DSP payload 0x60 ssrc 0x509B391F sequence 0x190
timestamp
> 0xF960
>  <<<Rcv> Pt:96     Evt:136     Pkt:01 40 50
>          s=VoIP d=DSP payload 0x60 ssrc 0x509B391F sequence 0x191
timestamp
> 0xF9B0
>  <<<Rcv> Pt:96     Evt:136     Pkt:01 40 50
>          s=VoIP d=DSP payload 0x60 ssrc 0x509B391F sequence 0x192
timestamp
> 0xFA00
> (...)
>
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
> ++++++++++++++
>
> Most likely, the problem is on the other end, a gateway that I do not
> control.
>
> I believe for this to work I need to receive PT=100 and EVT=200:
>
> Payload Type 100: Designates an NSE (Named Signaling Event packet). NSE
> packets can signal a variety of different messages so it is helpful to
know
> the NSE Event ID.
>
> Event 200: Triggered by the detection of a V.21 fax preamble when T.38 is
> configured.  This message initiates the switchover of the call from voice
to
> T.38. This  message is sent by the terminating fax gateway to notify the
> originating fax  gateway of the switch to T.38.
>
> And I'm receiving PT=96 and EVT=136. PT=96 means " Sent by a voice gateway
> to signal a switchover to Cisco fax relay". I wasn't able to find the
> meaning of EVT=136.
>
> With this information, can I assume that the problem is on the other end ?
> Can I do anything on my side to force the desired PT/EVT combination ?
>
>
> Thanks.
>
> Regards,
>
> Antonio Soares, CCIE #18473 (R&S/SP)
> amsoares at netcabo.pt
>
>
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