Fax For Asterisk FAQ

  1. What fax machines are compatible with Fax For Asterisk?
    As far as we know, every standards-compliant fax machine should work with Fax For Asterisk. In our lab and field testing, we have not yet found any standards-compliant fax machine that fails to be able to send or receive a fax when communicating with Fax For Asterisk.
  2. What is T.38 and how does it work with Fax For Asterisk?
    T.38 is a specification for transmitting faxes across IP networks. Asterisk 1.4 and later provide the capability to allow one T.38 enabled endpoint to setup a call to another T.38 enabled endpoint for the purposes of fax transmission. Asterisk 1.4 is not, though, capable of terminating those T.38 calls into Asterisk; they can only be passed through. Asterisk 1.6 and later provide the capability to terminate a T.38 call into Asterisk. Fax For Asterisk may be used to establish a fax session using T.38 between Asterisk and a T.38-enabled endpoint or VoIP service provider.

    Asterisk and Fax For Asterisk do not support T.38 relay, meaning that a fax session from a T.38 enabled endpoint cannot be directed in real-time to a fax machine on the PSTN. In order to accomplish this, it is necessary to use Fax For Asterisk to first receive one leg of the fax into a TIFF file, and then upon its completion, establish a second call and transmit the TIFF file to the remote fax machine.
  3. Does Fax For Asterisk guarantee reliable VoIP faxing using G.711 instead of T.38?
    No. VoIP faxing is made reliable by T.38. VoIP faxing in the absence of T.38 is unreliable and depends on the quality of your network. If you attempt VoIP faxing using regular G.711 instead of T.38, then network issues such as jitter or packet loss can / will cause your faxes to fail. Fax For Asterisk can be used for VoIP faxing using G.711, instead of T.38, but its reliability cannot be guaranteed.
  4. What if I have fax issues when using TDM/PSTN fax?
    Fax For Asterisk customers, please perform the following steps and contact Digium Technical Support.
    1. At the Asterisk CLI, type "fax set debug on"
    2. At the Asterisk CLI, type "core set verbose 6". Verbosity can be 6 or higher.
    3. At a Linux command prompt, type "dahdi_monitor -r -t "
    4. Redirect a manager session with "call" class permissions to a file.
    5. Reproduce the issue.
    6. Submit the Asterisk CLI output, dahdi_monitor recordings, and manager session output to Digium.
    Technical support is not provided for Free Fax For Asterisk users.
  5. What if I have fax issues when using T.38 fax?
    Fax For Asterisk customers, please perform the following steps and contact Digium Technical Support.
    1. At the Asterisk CLI, type "fax set debug on"
    2. At the Asterisk CLI, type "core set verbose 6". Verbosity can be 6 or higher.
    3. Redirect a manager session with "call" class permissions to a file.
    4. Reproduce the issue.
    5. Submit the Asterisk CLI output and manager session output to Digium.
    Technical support is not provided for Free Fax For Asterisk users.
  6. What versions of Asterisk are compatible with Fax For Asterisk?
    Fax For Asterisk is supported by Asterisk versions 12 and earlier. Asterisk 13 does not support Fax For Asterisk. Please use spandsp for Asterisk 13 and later versions.
  7. Does Fax For Asterisk provide the same capabilities when used with Asterisk 1.4 vs. Asterisk 1.6 and later?
    No. When used with 1.4 Asterisk, Fax For Asterisk provides only TDM fax capabilities — faxing to and from DAHDI-compatible (Digium) telephony boards. When used with Asterisk 1.6 and later, Fax For Asterisk provides both TDM fax capabilities and T.38 fax capabilities — faxing to and from T.38 compatible SIP endpoints and VoIP service providers.
  8. What defines a channel of Fax?
    A "channel" as it relates to Fax For Asterisk is an in-use instance of the fax modems provided by the res_fax_digium resource module. Every use of a modem requires one channel license. This means that every individual fax that you send or receive will claim and use one fax channel license. If you are sending and receiving a fax at the same time, then you are engaging two instances of the fax modems and would require two channel licenses. If you are only sending or only receiving one fax, then you would only require one channel license.
  9. What happens to a new fax call if I run out of fax licenses?
    In the event that all of your available fax channel licenses are in use, perhaps two calls are in progress and two licenses are available, and another call, the third, requests access to the fax modems, then the third call will fail. Asterisk does not queue up calls pending the release of a fax license from an active session because remote fax machines do not sit idle waiting to begin transmitting a fax, they respond better if the call is hung up — most fax machines include automated redial capabilities. Best practice then is to always maintain one more channel of Fax For Asterisk licenses then you think that you will need.
  10. How can I get my channel of Free Fax For Asterisk?
    Channels of Free Fax for Asterisk are available for acquisition from the Digium online store. Use your existing digium.com account, or sign up for a new one, and browse to the "Free Fax For Asterisk" product. Put the item in your cart (limit one channel and one line item per order) and check out. Because Free Fax For Asterisk is provided at no charge to users of Asterisk, no purchase method (credit card, Paypal, etc.) is required. Once the order enters Digium's processing servers, you will receive an e-mail in your Inbox with your Free Fax For Asterisk registration key and instructions on how to register it.
  11. Can I use more than one channel of Free Fax For Asterisk with a single instance of Asterisk?
    No. Only a single channel of Free Fax For Asterisk may be run with an instance of Asterisk. If you need more than one channel of fax capability with an instance of Asterisk, you will need to purchase channels of the paid Fax For Asterisk product.
  12. How can I get additional channels of Fax, beyond the first, working with Asterisk?
    Additional channels of fax capability, beyond the first, require purchase of the Fax For Asterisk product. Channels of Fax For Asterisk are available from your Digium reseller. Fax For Asterisk is also available from the Digium online store under the "Fax For Asterisk" product. Unlike Free Fax For Asterisk, Fax for Asterisk is a paid product and cannot be delivered for free, so a method of payment is required.
  13. Can I move my Fax For Asterisk or Free Fax For Asterisk key to a different computer?
    When you run the register utility to enable Fax For Asterisk, your registration key is tied to the hardware on which you perform the registration. Your key was provided to you with three (3) registrations. The first is used when you first register your Fax Key. You have two more available registrations with your key, beyond the first, in the event that you need to move your key to a different system. Digium reserves the right to refuse requests to provide additional registration attempts — this applies more generally to Fax For Asterisk customers are Free Fax For Asterisk users can always register a second Free key and disable the first.
  14. Switchvox SMB 4.0 and later provide advanced fax capabilities. Does Fax For Asterisk provide these same functions?
    No. Fax For Asterisk provides the building blocks necessary to build your customized fax application. Inside of Switchvox, Digium has added the ability for desktop users to print their documents directly to Switchvox and then fax them, and for users to receive inbound faxes to a dedicated IMAP mailstore. Digium has used Fax For Asterisk as the building block to create these advanced applications, but they are not inherently part of Fax For Asterisk. Basic examples of how to use Fax For Asterisk from Asterisk's dialplan are provided in the Fax For Asterisk Administrator's guide.
  15. Do Free Fax for Asterisk keys work with Switchvox?
    No. Switchvox only works with Fax For Asterisk keys. Switchvox will ignore any attempts to apply a Free Fax for Asterisk key.
  16. Will users of running systems be able to use Fax For Asterisk without reinstalling?
    Yes. If they are currently running a new version of Asterisk 1.4 or later, then they can put the res_fax and res_fax_digium modules into the Asterisk modules directory, run the register utility, and load the fax modules from the Asterisk CLI.
  17. Does Fax For Asterisk provide analog modem termination / origination capability in addition to its fax modems?
    No. Fax For Asterisk does not provide the capability to talk to regular analog modems for the purposes of data communication, Remote Access Servers, etc. Fax For Asterisk provides fax modems for the purposes of originating or terminating fax sessions.
  18. I have significant timing slip between a PRI card and an analog card, will Fax For Asterisk help?
    In the event that you experience significant timing slips between a PRI card and an analog card such that a direct card-to-card fax is not possible, Fax For Asterisk can be used to build a store and forward application. On the first leg of the call, Fax For Asterisk can be used to receive the fax. After receiving the fax, Asterisk can be instructed to place a second call out to the analog card's FXS port and then send the fax to an attached fax machine.
  19. My VoIP service provider uses G.729. Can I use Fax For Asterisk?My VoIP service provider uses G.729. Can I use Fax For Asterisk?
    G.729 is a voice compression codec, it is not related to fax except that it is not possible to send a fax across VoIP when G.729 compression is used. The preferred method of faxing across VoIP when using Fax For Asterisk is T.38 (a standard developed to make internet faxing reliable). If T.38 is not possible, Fax For Asterisk can fall back to G.711 faxing, but the success rates will be lower than T.38 and connections with excessive jitter, packet loss or latency will not reliably pass fax calls. When faxing across VoIP, T.38 is always recommended.
  20. How can I convert a .pdf file into a .tiff file?
    You can convert PDF files into TIFF files by using the command-line Gostscript utility as such: # gs —q —dNOPAUSE —dBATCH —sDEVICE=tiffg4 —sPAPERSIZE=letter\ # —sOutputFile=myfile.tiff myfile.pdf
  21. How can I convert a .tiff file into a .pdf file?
    You can convert TIFF files into PDF files by using the command-line tiff2pdf utility as such: # tiff2pdf —o myfile.pdf myfile.tiff
  22. What kind of CPU will I need to run Fax For Asterisk?
    Fax For Asterisk can successfully transmit or receive two faxes safely, while still running other Asterisk applications like calls and transcoding, on a 1.2GHz Intel Celeron class CPU.
  23. Does Fax For Asterisk negotiate different speeds when talking to fax machines across the PSTN?
    Yes. Fax For Asterisk is capable of operating at 2400, 4800, 7200, 9600, 12000, or 14400 baud.
  24. Does Fax For Asterisk support T.30e, color faxes?
    No. Fax for Asterisk does not support faxing in color per ITU T.30e.
  25. When Digium releases new versions of Fax For Asterisk, how do I find out and how do I download them?
    By registering Fax for Asterisk, customers are automatically subscribed to an announce-only mailing list that provides product update announcements. Anyone (even non-customers) can subscribe to these mailing lists by visiting https://www.digium.com/en/users/manage_email_subscriptions.php. Digium provides the Fax For Asterisk software for download at http://downloads.digium.com/pub/telephony/fax
  26. How does Fax For Asterisk differ from spandsp?
    Fax For Asterisk is very similar to spandsp. From Asterisk's perspective, it entirely replaces the capabilities of spandsp — with a few exceptions noted in the Fax For Asterisk Administration manual. Remember that Asterisk versions 13 and later do not support Fax For Asterisk, thus spandsp is an alternate option in these cases.
  27. Does Fax for Asterisk support G3, Super G3, and G4?
    Fax for Asterisk does support G3 and G4 Fax image compression. Super G3 (V.34 modem) is not supported.
  28. Does Free Fax For Asterisk come with technical support?
    No. Free Fax For Asterisk does not come with technical support. It is provided under license, as-is. Users requiring a supported option should purchase Digium's commercial Fax For Asterisk product.
Documentation Support
  • Locate answers to your support questions.
  • View product faq, data sheets and literature, technical whitepapers, installation guides, sample configuration files and related data.
Bookshelf