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Community Member

Fax is failing - Sip Server to Cisco AS5350


There is an IP box which is sending Fax to Cisco AS5350 via a Sip server.

The protocol used by IP box is t38 only.

The Fax configuration in Cisco is :

voice service voip

fax protocol t38 ls-redundancy 2 hs-redundancy 2 fallback pass-through g711ulaw


modem passthrough nse payload-type 110 codec g711ulaw


dial-peer voice 1001 voip

application remote_ip

incoming called-number 1234T

voice-class codec 1

session protocol sipv2

session target sip-server

dtmf-relay h245-signal h245-alphanumeric

fax-relay ecm disable

fax rate 9600

no vad


dial-peer voice 1002 pots

destination-pattern 1234T

progress_ind setup enable 3

progress_ind alert enable 8

progress_ind progress enable 1


port 3/6:D

What i see on cisco in the call history is this:

2CE4 : 2094254390ms.925931 +11130 +22340 pid:1001 Answer 31xxxxxxxx

dur 00:00:11 tx:1004/160640 rx:79/37920 41 (bearer capability not implemented (65))

IP rtt:0ms pl:14000/13720ms lost:0/5/0 delay:185/100/185ms t38

2CE4 : 2094254520ms.925970 +11000 +66400 pid:1003 Originate 12340031xxxxxxx

dur 00:00:55 tx:79/37920 rx:3214/514240 10 (normal call clearing (16))

Telephony 3/7:D (926370) [3/7.12] tx:17360/4000/0ms t38 noise:-61dBm acom:89dBm

Any idea why the fax is failing?

Thank you



Re: Fax is failing - Sip Server to Cisco AS5350

The POTS leg of the call is matching 1003, not 1002. What does that peer look like? Why are you locking the fax rate at 9600? What codec are you using for voice?

Are you sure your fax server is configured for T38 redundancy?

Community Member

Re: Fax is failing - Sip Server to Cisco AS5350

The pots dial peer 1003 is identical with 1002, but with a lower preference - going to another E1 port.

I locked the fax rate and I tried with the default fax rate... but was the same.

The codecs used for voice are in voice-class codec 1:

voice class codec 1

codec preference 1 g723r63

codec preference 2 g729r8

codec preference 3 g729br8

codec preference 4 g723r53

codec preference 7 g726r16

codec preference 8 g726r24

codec preference 9 g726r32

codec preference 10 g711alaw

codec preference 11 g711ulaw

codec preference 12 g723ar53

codec preference 13 g723ar63

codec preference 14 gsmfr

The fax server is using t38 only.

This is an update of the problem:

Call flow for T.38 FAX:

1) Voice channel is opened with remote UA and Cisco GW when the call is established (media stream with port - 16001).

2) After detecting the FAX tone, another logical channel will be opened (media stream with another port - 16003- normally this is udp + 2)

3) Once the fax is sent, logical channel will be closed and call will be disconnected on audio channel.

In trace received from sip box, T.38 is supposed to be established on 16003, but negotiation happened from Cisco side on 16001, after accepting 200 OK message from sip box.

It seems that Cisco is sending T.38 packets to the RTP port instead of T.38 port.

The IOS version is 12.3 on an AS5350.

Anyone have an idea about ?



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