iNODE T.38 Modem Setup using IOS VoIP Gateway

16 January, 2009

iNODE T.38 Modem Setup using IOS VoIP Gateway


iNODE v1.6.0 has support for T.38 fax calls origination and termination on its Fax Server. This feature is implemented by a special virtual modem which is recognized as Legacy Fax modem by Fax Server. Up to 256 T.38 modems can be handled by a single Fax Server with enough hardware resources (CPU, RAM). There is also a T.38 VoIP Dial plan configuration section for outgoing T.38 calls. We've done extensive interoperation tests successfully between iNODE Fax Server and Cisco IOS VoIP Gateways. It is known that it also working with T.38/T.30 Gateways of other vendors such as Patton.

In the current document we'll give step by step instructions for setting up a T.38 modem to work with a Cisco H.323 IOS Gateway for incoming and outgoing calls.


  • A Cisco IOS Gateway capable for H.323 T.38 calls.
  • An iNODE v1.6.0 or newer Fax Server

Step by step instructions

Step1. Create a T.38 Legacy Modem on the Fax Server via iNODE Web Management Interface

1a. Go to ':: Configuration :: Fax Service :: Legacy Fax-modems'

1b. Click on 'Add new modem' button

1c. Enter the Fax modem name on the field Name

1d. Enter the Fax machine ID on the Fax identifier field.

1e. Select T.38 modem on the Modem type radio button group.

Note: If you have not provided with such options, you need to enable T.38 Fax over IP Service on the page ':: Configuration :: Fax Service :: T.38 Fax over IP Settings'. You can find more details for this page on Step 2 below.

1f. Leave the port value to default value (it is automatically assigned as a virtual serial port of the T.38 Modem)

1g. Click on the button next and the new Modem will be automatically assigned and detected. The Legacy Modem form is now presented for extra modem parameters.

1h. Enter the exact phone number that this modem will hear to answer calls to.

Note: You should configure a proper dial-peer on your IOS Gateway to direct T.38 calls for this number to the current Fax Server. Such instructions are given below.

1i. Leave the rings before answer to a value greater or equal of 2. This is required for proper Caller ID recognition by the modem on incoming calls.

1j. Leave all fields with default values and enter a dial out prefix number for outgoing calls on PBX prefix field.

The T.38 Modem has been successfully created and it is active and ready to place calls. Some additional steps are required to complete configuration.

Step 2.Configure T.38 Dialplan on iNODE Fax Server

2a. Go to ':: Configuration :: Fax Service :: T.38 Fax over IP Settings'

2b. The default port of iNODE Fax Server to hear for T.38 calls is 1733. If you need to change it to the default, enter the value of 1720 to 'H.323 Listen port' field.

2c. If you have implemented an H.323 gatekeeper on your network you can use it by checking the H.323 gatekeeper field and entering its IP address. If have no H.323 gatekeeper leave it unchecked.

2d. Select the default carrier codec for T.38 calls to be G.723.1 or G.711. For better Fax quality leave the default codec to G.711.

2e. Choose the G.711 codec mode of operation from alaw to ulaw according to your requirements. For USA you should use ulaw, for Europe you should use alaw which is the default value.

2f. Enter the desired debug trace level for T.38 fax modems. The higher the value the more detailed trace log will be produced for debugging purposes. You should leave the default value.

2g. If you have a WAN with multiple gateways and you need to direct outgoing calls to the proper IOS Gateway according to the dialed phone number, you need to add static dialplan rules by clicking on 'Add Route' button. Enter any destination number(s), by using an asterisk as a wildcard character for multiple numbers matching, together with the IP address of the IOS Gateway that the call should be directed to.

2h. If you have a single IOS Gateway for all outgoing T.38 Calls enter the IP Address of this gateway and the port number on the 'Route all other to' field.

2i. If the T.38 Fax over IP Service Status is Disabled click on the Enable button on this page.

2j. Click on the 'Update settings' button and the T.38 Configuration on the iNODE Fax Server is complete.

Step 3.Configure Cisco IOS Gateway for T.38 to direct incoming and outgoing calls to iNODE Fax Server

For each IOS Gateway you should configure the dial-peers needed to carry outgoing and incoming T.38 calls. Wherever possible you can summarize these dial-peers. In this how-to we will show a single dial-peer for incoming and outgoing calls.

3a. Open a text editor (e.g. Notepad) and paste the following IOS configuration commands text:

dial-peer voice 6963 voip
  destination-pattern ^953963$
  session target ipv4:x.y.z.n:1733
  incoming called-number ...
  codec g711alaw
  fax rate 14400
  fax protocol t38 ls-redundancy 0 hs-redundancy 0 fallback cisco
  no vad

3b. Change the dial-peer number (6963) with a proper number which should be valid for your configuration.

3c. Change the destination number from 953963 to the number(s) that the gateway should direct to iNODE Fax Server. You can enter the desired pattern that should be matched for your incoming calls but more details for entering IOS H.323 dial-peer patterns is out of the scope of this document.

3d. Enter the IP of iNODE Fax Server by substituting the x.y.z.n pattern with it.

3e. if you leave the default value for listening port on the step 2b above leave the suffix ':1733' as it is in the line starting with 'session target'. If you enter the default H.323 listening port (1720) then remove this suffix so that this line should be ending with the IP Address.

3f. Enter the numbers pattern, that this dial peer should match for outgoing T.38 calls (calls from iNODE Fax Server to the PSTN via the gateway), on the incoming called-number line.

3g. Select all the text and Copy it to the clipboard.

3h. Enter in configuration mode on the CLI console on your IOS gateway and paste the above config to it. If your terminal doesn't support paste function then enter each line in sequence as it is after changes to the CLI.

3i. Enter any translation profile rules needed for incoming PSTN calls to match your desired destination pattern. For example if your IOS Gateway connected via ISDN and you have DID enabled by your operator check the Called numbers that you received on incoming calls during a call setup. If the destination number is different than the destination pattern that you defined in step 3c above you may need to enter a translation profile rule.

Your configuration steps are now complete for both iNODE Fax Server and IOS Gateway. Further IOS Configuration instruction is out of the scope of this document. Consult with your network support partner for more specific configuration.

Note: You should make clear that the traffic from IOS Gateway(s) to iNODE Fax Server are not blocked by any kind of firewall along the traffic path. The control connection between them (tcp/1720 or tcp/1733) as well as the RTP media traffic (udp/rtp) should be flowing free.

FAQ - Troubleshooting T.38 Modem calls

Q. I have configured my T.38 Fax according with your instructions above but I have not seen any T.38 call arriving on iNODE Fax Server by monitoring ':: Monitoring :: Fax Service :: T38 Modem Calls Realtime Log'.

A. Check your IOS gateway configuration for proper call redirection. Use 'debug isdn q931' to see if the call is received and the destination number that is coming during call setup. Check with your translation profile if you need to translate this number to the one you have configured on the T.38 dial-peer. For a translation profile example check below.

Q. I have seen the call coming to the iNODE Fax Server in ':: Monitoring :: Fax Service :: T38 Modem Calls Realtime Log' but the modem is not answering.

A. Check the T.38 Legacy Modem configuration and see if it is configured to only send (Allow receive is unchecked). Then check if the fax number is the same as the destination-pattern number configured in the T.38 dial-peer. The Fax number field should be exactly the same as the called number. This T.38 Modem will answer only on incoming calls to this number.

Q. I've sent a fax via T.38 modem but the document cannot get delivered.

A. Check ':: Monitoring :: Fax Service :: T38 Modem Calls Realtime Log' and see if the call have been placed. If placed check on your gateway if the call is coming in. If no, check for any intermediate firewall.

Translation profile example

In the following example the calls to the number 2310953963 are coming to the Voice gateway from the operator. Operator are sending only the last 2 digits during call setup because the line have DID feature. So the number 63 are coming in. The following translation profile will be applied to the called number so the number is translated to 953963. The gateway dialplan will match this number to the dial-peer given in the Step 3 above.

voice translation-rule 1
  rule 1 /^63/ /953963/
voice translation-profile TP01
  translate called 1
voice-port 0/0/0
  translation-profile incoming TP01

iNODE Support Team