Fax Passthrough

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Aug 16th, 2009
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Hi, We have recently installed a Cisco 2811 at our customer site where we are providing SIP services. This GW connects to the customer's Nortel PBX via an E1 and also to PSTN via an E1, it also connects to the SIP provider via IP. We want to route all the calls including fax to the SIP provider.

Though the VOIP calls are working fine but the incoming and outgoing fax don't work, below is the config for fax passthrough that we want to use. I have also attached the debug output of the fax call but I am struggling to interpret it.

voice service voip

fax protocol pass-through g711ulaw

Outgoing Dial-peer

dial-peer voice 107 voip

description "Voip out to SIP Provider"

translation-profile outgoing SIP-Outbound

destination-pattern [4,6,8,9].......

voice-class codec 1

session protocol sipv2

session target dns:sip.xxxx.com

dtmf-relay rtp-nte

no vad

Incoming Dial-peer

dial-peer voice 109 pots

description "Voice in from PSTN"

incoming called-number .


port 0/1/0:15

Thanks for your help.

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Nicholas Matthews Mon, 08/17/2009 - 07:38
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It looks like you're switching over to passthrough.

Couple of other things to check:

make sure network-clock-select 1 E1 0/0/0 is on the router connected to the PSTN/PBX. Slips with 'show controller t1/e1' will kill a fax.

Check for packet loss with 'show call active voice brief' in the lost:0/0/0 field correlating with the IP address and originating and terminating caller ID for the call.



hadisharifi Mon, 08/17/2009 - 17:37
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Thanks for your help. I have the following configuration: I have also changed the fax passthrough to Modem passthrough since the terminating GW is also Cisco.

voice service voip

modem passthrough nse codec g711ulaw redundancy maximum-sessions 10

network-clock-participate wic 0

network-clock-participate wic 1

network-clock-select 1 E1 0/1/0 "This E1 connects to the PSTN"

E1 0/0/0 connects to the PBX

Clocking for E1 0/0/0 is configured as internal.

E1 0/0/0 is up.

Applique type is Channelized E1 - balanced

Description: "Connection to Nortel PABX"

No alarms detected.

alarm-trigger is not set

Version info Firmware: 20071129, FPGA: 20, spm_count = 0

Framing is CRC4, Line Code is HDB3, Clock Source is Internal.

CRC Threshold is 320. Reported from firmware is 320.

Data in current interval (355 seconds elapsed):

2 Line Code Violations, 4 Path Code Violations

0 Slip Secs, 0 Fr Loss Secs, 2 Line Err Secs, 0 Degraded Mins

2 Errored Secs, 0 Bursty Err Secs, 0 Severely Err Secs, 0 Unavail Secs

Total Data (last 24 hours)

148 Line Code Violations, 250 Path Code Violations,

0 Slip Secs, 0 Fr Loss Secs, 148 Line Err Secs, 0 Degraded Mins,

148 Errored Secs, 0 Bursty Err Secs, 0 Severely Err Secs, 0 Unavail Secs

Nicholas Matthews Mon, 08/17/2009 - 20:24
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It looks like a clocking issue with the PBX. Maybe it is also set to internal/master, but the slips and LCV/PCV are going to mess up your faxes.


hadisharifi Mon, 08/17/2009 - 22:23
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Hi, The PBX is set to user or to get it's clocking from the Cisco GW as per the Nortel engineer. I was going to change the fax protocol to T38 but the SIP provider doesn't support T38. So I am really not sure what to do.

Nicholas Matthews Tue, 08/18/2009 - 05:54
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If you have slips on your T1 your fax protocol won't matter. You may have hardware or wiring problems between the routers.


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