Comtel-Networks and DIDX Prepare Fax DID Case Study

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Comtel-Networks and DIDX Prepare Fax DID Case Study Comtel-Networks and DIDX Prepare Fax DID Case StudyComtel-Networks and DIDX Prepare Fax DID Case Study

10 September 2009

DIDX and Comtel Networks will showcase sending and receiving fax over Internet via DID and open source software and PBX at ITEXPO West DIDX booth 125. The case study, which took place from Jul. 30, 2009 through Aug. 12, 2009 in order to reach this goal, uses T.38 protocol and DID from DIDX wholesale phone number marketplace. The process is complete and has resulted in new business on DIDX among veteran and new suppliers and buyers of DID/DDI phone numbers from over 60 countries. Comtel Networks hosted and wholesale consulting inquiries have increased phenomenally. The following steps of the case study will show the progress from start to finish.

* DIDx APIs at Vconexion showing T.38 Support and Channel Capacity

On July 30, 2009 at 12:44 AM, Comtel Networks consulted the DIDX fax API at, in particular the section on “How do I search the DIDs using API by T38 capability ?” to get started. Fax testing was done using FreePBX via Hylafax but the TIF was not found by the PDF converter in the dial-plan, which after converting is the script in charge of sending the file by e-mail.

*Sample fax received from a PSTN land-line into Asterisk.

After one hour of debugging and a discussion of the problem on Trixbox forum at, no solution had been found yet.

Finally, after several hours, Comtel Networks and DIDX discovered the culprit was a bug and missing file argument. FreePBX is supposed to work with rxfax / txfax, which uses a different port to the dsp engine than iaxmodem and hylafax. Both systems use spandsp as the core for dsp though. RxFAX uses Asterisk's calls' unique Ids to identify tif files, while Hylafax must use its own external dispatcher to check its own spool directory.

AGX Asterisk Add-ons:

A second trial begins on a different port and it worked. The perl script within FreePBX's dialplan,, gets the file correctly from Asterisk through FreePBX's dialplan. The conclusion was that after testing also with Hylafax's FaxDispatcher, a different script to send e-mails, the best is to use the one that matches naturally with FreePBX, AGX Asterisk Add-ons, TxFAX and RxFAX, that work with NV Fax Detection & Background, to receive & send faxes over VoIP. Once we got the software running, a total of 8 out of 10 faxes were sent successfully through a Comcast land line and received by FreePBX. As of today, reception rates have increased after tunning the system.

At this point, it was time to test the other way around. We looked for a carrier capable of terminating fax VoIP calls to PSTN and other VoIP carriers.

“Our original plan was to get a PSTN line at our ITEXPO booth to show sending and receiving faxes with DIDX DID but then thought it would be better to do everything over IP. Max Glucksmann, key programmer in this experiment, thought of using a PAP2 directly registered with one of the PBX we were working on, but finally figured out how to build a web to fax gateway, which by now has been thoroughly tested and will be working at our booth 125 at ITEXPO West,” states Suzanne Bowen of DIDX.

After almost 6 hours of testing and research, Comtel-Networks and DIDX had fax termination via VoIP working using spandsp and the app TxFAX; the fax was sent from a PBX in a data center into Max's PC that had FreePBX installed. We used Deltathree termination and DIDX origination. No need for PSTN line at the ITEXPO West DIDX exhibit. Even so, more testing would be done.

*FreePBX's Inbound Route Setup – NVFax is selected as Fax Detection Mechanism.

Below, DIDx's dashboard, where a DID shows the supported features. In this case study, “T.38 = yes” is a must.

During tests, we also examined Voipnow 2.0 by 4PSA. “The Voipnow system was impressive during testing because of its nice AJAX GUI and because it installs almost 'Linux CLI debug and hassle free' out of the box,” states Max Glucksmann of Comtel-Networks. “The main difficulty is that has several user levels for resellers, clients and end-users, so the routing and billing schedule can be a headache.”

Not being open sourced also makes it difficult to personalize and understand better, according to Max's remarks.

*Voipnow's Fax Center – Neat AJAX Web GUI

Having tested different platforms, Comtel-Networks decided to build a hybrid trying to gather the best features of all platforms studied. In the following images you can see the result and a have deep insight into how Comtel-Networks Web/Fax Gateway works, on the front-end and behind the scenes too. To illustrate it, we are showing an Asterisk server sending a fax using TxFAX and a separate receiving server using RxFAX.

*Comtel Networks Web to Fax Gateway

Comtel-Networks finally decided to go with the AGX's add-ons mainly because the setup is much simpler and easier than Hylafax's: there's no need to create iaxmdem soft-fax devices running in the background; besides that, compilation and installation is very straight forward also with AGX.

*Vconexion using Comtel Networks' Fax Gateway – Application TxFAX is called through a PHP AGI script.

In order to achieve the resulting Fax Gateway, we used similar logic to FreePBX's dialplan, which allows the user to configure a recipient e-mail per DID and also lets the user select if the call should be answered to check for incoming faxes or if it should be forwarded to a destination number or other URL, in case the user has its own fax end-point. To generate fax calls, first the fax file is uploaded; then, the Web GUI places a call file in Asterisk's spool directory. This file triggers in one end, a PHP AGI that calls the destination fax number, and once connected, another API is called to connect the channel to TxFAX, providing the uploaded file path in the process.

* FreePBX Receiving a Fax using TxFAX – Dialplan can be seen in Asterisk's CLI.

In conclusion, our goals were to be able to showcase the use of DIDX for T.38 DID for fax solutions at ITEXPO and to pass on that success to others to copy and perfect. We wished to deliver an honest case study of the successes and failures throughout the project. We used the DIDX API to show DID that support T.38 with the number of channels that are available.

Upon further study of the project at this point, Comtel-Networks will enhance the Web-to-Fax GW by adding examination of TxFAX's return code to provide delivery retry in case the connection fails, An E-mail-to-Fax Gateway, which at this point is still missing, will be part of future development at Comtel-Networks. Planned implementation of e-mail to fax is based on Inter7's API to qmail and Asterisk.

* See diagrams at the original case study and press release at

About Vconexion
A provider of PBX appliances, phones and other related communication products. Vconexion also offers services of DID (Direct Inward Dialing) or phone numbers, phone service for business customers, and wholesale termination.

About Comtel-Networks
The one stop for telephone needs, VoIP solutions, VoIP startups, and DIDX integration. Comtel-Networks will not only assist you with your current platform, but will provide you with the tools to build the products and packages you're looking for.

About DIDX
DIDX is a service of Super Technologies, Inc., a Florida-based company since 1999. It serves as a global marketplace of local telephone numbers and VoIP transport to IP communications service providers. DIDX is a wholesale VOIP Phone Number Trading Platform with over 12,500 telecom companies as its members, trading over 60 countries' numbers.

Super Technologies Inc.

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