XO Communications' Blog

To FoIP or Not to FoIP, That is the Question

[ 1 ] July 26, 2012 | By

So you thought the good ol’ facsimile, aka the fax, was dead?  Not really.

Although a lot of companies are now using “e-signature” for document signing, fax is still used by many businesses because you can easily (and cheaply) capture a signature for legal documents. When a business decides to move over to Voice over IP (VoIP) from their traditional telephony system, how a fax will work over IP network can become a major concern. This blog post discusses the most common Fax over IP (FoIP) methods and their pros and cons.

FoIP is a technology that enables the interworking of a fax machine with an IP network. Using FoIP, a fax is transmitted via the IP network rather than the traditional method of sending faxes via a PSTN line.

There are two most common methods for FoIP:

  1. Pass-through (G.711u) method
  2. Relay (T.38) method

Pass-through method
With this method, fax data is sampled and encoded with G.711 codec and encapsulated in Real Time Protocol (RTP) to transport over the IP network. This method is similar to the way normal voice gets transfer over IP network.


  •  Obvious benefit of this method is its simplicity. It’s like a voice call. This inherent simpler approach to fax leads to lower interop issues between different vendors.


  • Fax data is converted to Pulse Code Modulation (PCM) audio stream and then sent as G.711 RTP packets. This requires higher bandwidth compared to the T.38 relay method.
  • As this technique treats voice calls and faxes same way, there’s no built in mechanism to address the impairments of the IP network.For example, this method is very vulnerable to packet loss as there is no redundancy mechanism, a small amount of packet loss can cause bad fax quality output.

Relay (T.38) method
This method is specifically designed to handle real-time faxing over an IP network. It’s designed to overcome impairments when a fax is being sent on the IP network.


  • Higher fax success rate as it addresses most of the issues faxes can face over IP. It uses ECM (error correction mode) to ensure error free transmission of pages and redundant data packets to overcome packet loss.
  • Uses less bandwidth than G.711 Pass-through.
  • For higher interoperability, T.38 fax relay can fall back to G.711 Pass-through if initial T.38 negotiation fails.


  • Effectiveness of T.38 depends on its implementation by the vendor. Interoperability issues are common between various vendor T.38 implementations. Testing is required before deployment.

In summary, some customers like G.711 Pass-through for its simplicity and some prefer T.38 relay as it’s a de facto standard for fax transport over IP network. SIP service providers work with various IP-PBX and fax vendors to ensure interoperability and certify the fax works so that a customer can have a smooth transition when they move from a traditional telephony system to VoIP.

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Tags: fax over IP, fax server, FoIP, Pass-through method, Relay (T.38) method

Category : Communications, VoIP

About Himanshu Kapadia: I’m a Data Network Engineer for XO Communications. My role includes performing interop tests with XO SIP services. I have over 8 years of experience in Voice over IP (VoIP) field. I hold a Master’s degree in Electrical engineering with Telecom specialization from Southern Methodist University (SMU) at Dallas. I also hold a bachelor's degree in Electronics engineering from South Gujarat University, India. My hobbies include astronomy and photography. View author profile.

Comments (1)

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  1. Chris A. says:

    This is a great article and breaks it down so it’s easy to understand for the uninitiated. Most end users don’t realize how Fax over SIP works and this is going to be a boon for me. Thanks

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