Discover workarounds for fax over IP challenges

If you've tried to send a fax using a VoIP network, you're aware of the potential problems. Deb Shinder describes the fax over IP, or FoIP, process and how you can minimize glitches.

One of the biggest challenges for consumers switching from a public switched telephone network (PSTN) to Voice over IP (VoIP) phone services is getting their fax machines or fax software to work reliably. Many businesses depend heavily on fax communications, so this can be a potential obstacle to replacing all of your landlines with VoIP lines. Let's take a look at the reasons fax transmissions over VoIP present a problem, and examine some of the solutions and workarounds.

The problem with FoIP

Fax over an IP network (FoIP) presents a special problem because of the nature of the fax protocols, which rely on very precise timing mechanisms: Fax transmissions are more sensitive to the latency and delays that are inherent in a packet-based network than are voice transmissions.

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There is no "jitter," or unevenness of the speed of transmission, in the PSTN network. On the other hand, one of the big problems with sending fax transmissions over the Internet is that there is no Quality of Service control.

The fax transmission protocol, also called the T.30 protocol, defines the five separate phases of a fax transmission, which proceed consecutively in a timed sequence. The phases are:

  • Phase A/Call establishment: The calling terminal dials the number and answers the call. This also includes the exchange of fax tones.
  • Phase B/Pre-message procedure: The calling and called terminals exchange information regarding capabilities and identification such as phone numbers and organization names.
  • Phase C/In-message procedure and message transmission: The in-message procedure sends control signals for error detection and correction and synchronization. The message transmission includes the coding scheme, modulation, and demodulation. This phase is sometimes further broken into two subparts, but these two activities occur at the same time.
  • Phase D/Post-message procedure: The calling terminal sends multi-page, end-of-message, and end-of-facsimile signaling, and the called terminal's confirmation messages confirm receipt of the calling terminal's messages.
  • Phase E/Call release: The calling terminal sends a disconnection signal to release the call.
  • It's the nature of IP networks for some packets to occasionally drop or be lost. If this happens with a voice transmission, you may hear a quick "cut out" when the packet loss is small, but you'll still probably be able to understand what's being said. However, if it happens during a fax transmission, it can create an error in the fax transmission.

    Working around the problems

    Does this mean you must retain PSTN lines for your fax communications? Not necessarily. There are workarounds, although none is yet perfect.

    Some VoIP providers have better results with fax communications than others. Some don't support fax over VoIP at all, whereas others take special measures to support fax operations. Some VoIP providers offer a dedicated fax line as part of their business plans.

    Unlike the T.30 protocol, the T.38 protocol and g.711 codecs support faxing over VoIP. If you need to be able to send faxes over your VoIP line, you'll want to go with a provider that supports one of these protocols.

    T.38 works like regular PSTN faxing but instead works over the IP network. A T.38 fax machine can plug directly into the Ethernet network with an RJ-45 connector, or it can use software running on a computer. T.38 addresses many of the problems inherent in faxing over IP. Using T.38, lost packets don't cause the modems to lose synchronization, and jitter buffering is less often necessary.

    Some other methods used by the providers to help increase the quality of fax transmission include:

  • Slowing down the speed at which the packets are transmitted.
  • Using special codecs (encoding and decoding software).
  • You may need to configure settings, such as reducing the fax baud rate to 9600 and setting the codec to 711u on your VoIP devices and/or fax equipment.

    In addition to real-time faxing over VoIP, some providers use a "store and forward" method for transmitting faxes. In this method, a FoIP gateway decodes and stores the faxes. Via SMTP, the fax transmission forwards to a FoIP gateway at the receiving end. A T.37 specification defines the store and forward delivery of faxes.

    Web-based fax services

    One solution to the fax dilemma is to use VoIP for your voice calls and an online fax service to send and receive faxes. Companies such as MyFax and eFax allow you to send and receive faxes over the Internet for a monthly fee. Plans are available for personal, small business, and corporate use. You're assigned a fax number (local or, in some cases, toll-free) that others can use to send you faxes. You can send faxes via the Web or by e-mail and receive your faxes in your e-mail inbox.

    Alternatives to fax

    Many companies are moving away from faxes for transmitting documents, since there are many ways to accomplish the same thing. For example:

  • You can send and receive documents as e-mail attachments. If signed documents are required, digital signatures are binding in many jurisdictions.
  • You can transfer files via most instant messaging programs.
  • You can make documents available on an FTP site or through other file sharing services.
  • One concern regarding sending documents by these methods has to do with security. Fax transmissions over PSTN lines use a point-to-point connection over relatively private phone lines, whereas e-mail and other computerized transfer methods (and fax over VoIP, for that matter) use the public Internet. However, you can address this by encrypting the files before sending or making them available over the Internet.

    Another concern that some fax users have is the ability to be sure from whom an e-mail document originates (fax machines are required by law to include the sending phone number on the fax). You can solve this problem by using digital signatures to verify the identity of the sender of the e-mail document.


    Faxing over a VoIP line presents some special challenges—challenges now addressed by new protocols and technologies. The need for fax capabilities should no longer be a deterrent to those considering giving up their landlines for the cost and convenience benefits of VoIP.