vic3-2FXS/DID fax problem

[mingl]

I have a network in Beijing China below,

(1) E1 Pri interface to PSTN

(2) Vic3-2FXS/DID to fax machine

all these modules installed on Cisco 2900, when I call my fax machine from PSTN, the fax machine answered, but fax can't start.

I want to know is there any special consideration about the fax configuration to be worked.

Thanks for your advice.

Robert

2011/2/18

[slakhanp]

Hi Robert,

Yes there are special considerations to be taken while setting up fax connection as they are more prone to losses.

Try the below suggestions:

• channel used should be for 64 kbps and no compression algorithm should be used
• try lowering the baud rate
• remove VAD and Echo Cancellation

HTH!

Regards

Saurabh

[mingl]

Very thankful for you useful information, I'm not too clear about your answer;

• channel used should be for 64 kbps and no compression algorithm should be used;

    Do you mean every B-channel in PRI should be 64k, yes, in china, B-channel in China is 64K, I also noticed that the codec is g722-64, below is output:

   show voice call summary

  .................

   0/0/0:15.31     g722-64    y  S_CONNECT             S_TSP_CONNECT

• try lowering the baud rate

   Whick command should I used on router 2911? "fax rate" command, I think it isn't;

• remove VAD and Echo Cancellation

   I remember "no VAD" command can be only issued on voip dial-peer, How can I turn it off on POT dial-peer?

   Yes, I can use 'no echo-cannel enable' on voice-port configuration, but on FXS voice-port or on E1 signaling channel?

Thanks your further information

Robert Li

2011/2/18

[slakhanp · Accepted solution]

• yes, what i meant by 64 kbps channel was to use a codec using 64 kbps (e.g. G.711)
• yes the command used for selecting the baud rate is 'fax rate'
  
• use the 'no echo-cannel enable' command on the port connecting the TDM interface E1 line connecting to the fax machine
• also check which fax protocol you are using
  

[mingl]

Thanks

Robert Li

2011/2/18

[mingl]

One more quesiton:

    Please notice this scenario is a PRI---> FAX call, not a fax relay tranversing IP using VOIP Dial-peer, Are you sure fax rate and fax protocol is really related to this question?

Thanks

Robert Li

2011/2/18

[slakhanp]

Hi Robert, as per my understanding the router will use either of the three  methods mentioned below to handle fax:

- Fax relay

- Fax pass through

- Store and Forward

I would like others to comment too, if they feel otherwise.

[mingl]

Thanks anyway.

Robert Li

2011/2/18

[Bryan Deaver]

Hey Robert.

Since you have PRI and the fxs port on the same card, in theory there is not a whole lot you need to do with regards to the fax or port configuration on the router if this is H323 or SIP.  On our initial voip gateways even on port to port calls we would invoke fax relay, but on the ISR line with the internal tdm backplane, the dsp should be dropped from the call with minimal impairment of the connection through the gateway.

Now, this is true as long as you don't have "no local-bypass' configured under the voice-card.  Default is that this is not configured but it you do, you should change it to local-bypass.  "show voice dsp active" will show if the dsp is still in the loop on the call as well.

Definitely check the E1 controller ("show controller e1") for slips and errors.  This will kill a fax if present.

Looking back at your previous post though, I would not expect you to be seeing G722 on the output of the show voice call summary when the call connects between the two ports.

Are you using MGCP?

When the call is active between the two ports, what is the output of "show call active voice brief" and "show call active fax brief"?

There are a few good resources for troubleshooting fax issues.  An excellent source if the CiscoPress publication "Fax, Modem and Text for IP Telephony".  If you would like some of the scaled down versions, I have posted a couple of links that would be useful.

Bryan

https://supportforums.cisco.com/community/netpro/collaboration-voice-video/ip-telephony/blog/2010/10/26/fax-configuration-and-troubleshooting-project

http://www.cisco.com/en/US/docs/ios/12_3/vvf_c/voice_troubleshooting/old/vts_fax.html