[cisco-voip] T.38 Fax Issue

[Peter Slow]

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_ip_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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