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Fax Over IP (FoIP) Basics, Design & Best Practices

New Member
04-26-2011 12:00 AM PST thru 04-26-2011 12:00 AM PST

Here’s a condensed summary of our Facebook forum held in April 2011. We were honored to have David Hanes as our host who is both a Cisco expert and a Cisco press author.

A lot of our audience may be wondering what FoIP is and more importantly if Fax is still relevant in these days. What do you think?

FoIP or Fax over IP is similar to VoIP (Voice over IP). The difference is that a fax image is being transported over IP instead of voice samples. However, because fax images are digital data they need to be handled differently as any loss can degrade or cause the fax to fail. That is why fax specific transport protocols like T.38 are usually necessary.

People have been predicting the demise of fax for a long time but it is still the simplest, universal method for getting a document from one location to another. People are also very familiar with fax and have confidence in its reliability.

Can you give us a high level overview of how FoIP works?

With FoIP, gateways or ATA devices digitize the fax information using one of two methods - pass-through or relay. Pass-through is pretty much just like voice and uses a G.711 codec while relay demodulates the fax signal and packages the fax image into packets. T.38 is an example of the most popular fax relay protocol.

Do we need any dedicated devices for FoIP? Something like a VoIP router?Do you need a new IOS for FoIP to work?

Voice gateways and ATA devices usually have the ability to handle both voice and fax calls simultaneously.

You do not need a special IOS for FoIP. Any IOS that supports VoIP also supports FoIP. The only difference is that later versions of IOS may support newer features, like the ability to handle high speed or Super G3 fax transmission over T.38, which was introduced in 15.1(1)T.

For FoIP to work, do fax machines need a NIC port with PoE instead of a conventional RJ-11 port?

No, you can just plug the analog fax machine directly into an FXS port on the voice gateway. From the fax machine's perspective it does not know if it is connected to the PSTN or the voice gateway. It behaves the same.

Can you explain briefly why T.38 is superior to other transports like T.37, pass-through, and Cisco fax relay?

T.38 is superior because it is a standard that has been adopted by all of the major vendors. This helps with product interoperability. It also consumes the least amount of bandwidth of any fax transport protocol and offers multiple layers of redundancy if needed to make sure that your FoIP traffic is transported reliably. T.38 is also real-time unlike T.37 which uses SMTP or email to transport the fax information. T.37 can have problems with acknowledgments and verifying if the fax is successful or needs to be resent.

Most of our users answered this as false in our poll. "T.37 Store-and-Forward fax uses email to transport fax transmissions." What is the right answer?

The correct answer is that T.37 does in fact use email or the SMTP protocol to transport faxes across an IP network. Cisco voice gateways implementing T.37 act as a mail client and send an email to the mail server with the fax pages as image attachments.

Is FoIP a technology that is addressed in any of the certifications? What aspects of FoIP are relevant for Cisco certifications and labs?

Yes, I am not familiar with all of the Cisco certifications but I do know that FoIP is an area that is covered by the Voice CCIE certification. T.38 is the most important aspect of FoIP today and is what you should be the most familiar and comfortable with. You will need to know a little about the other transport mechanisms as well (Cisco fax relay, T.37, passthrough, etc.) but T.38 are where I would focus the most. You should know how to integrate T.38 with voice gateways and CUCM as these are the common UC devices applicable to FoIP.

What SIP Trunk carriers have you found to do a good job recently of supporting / implementing T.38?

Well, unfortunately, I am not in a position to point out specific carriers. What I can tell you is that our technical support staff has seen a lot of T.38 interoperability problems with just about every carrier. I highly recommend that you do extensive testing before running T.38 over a SIP trunk provider.

I've found a lot of T.38 over SIP trunks interoperability problems with just about every carrier, and often find the need to fall back on G.711 Pass Through for interoperability. Is this common?

I am not surprised. T.38 over SIP trunks has not been a smooth transition and there is a lot of work on this being done through groups like the SIP Forum. Hopefully things will improve but it has been painful for quite a few organizations.

Have you been finding implementations with V.34 in 15.1(1)T to be working well and stable? Are there any design issues there to be concerned with?

I wish I could say that V.34 over T.38 has gone smoothly but there have been quite a few interop problems there as well, especially when interoperating with fax servers. You should be ok running this between Cisco gear but there w...ere problems recently with falling back to G3 speeds from V.34 in certain scenarios. Most of the issues have been resolved so definitely run the latest IOS and if you are not in a rush and can wait another release or two, then that would be my advice.

What's a good place to start for someone who is interested in knowing more about FoIP?

There are a lot of FoIP resources on One good introductory resource that many folks have found helpful is an article I posted for Cisco's TS Newlsetter a while back. It gives a good overview of fax, FoIP, and the various transport mechanisms -

Do you have any tips or parting advice before we close?

I would like for everyone to remember to not treat their fax traffic as voice traffic. Fax is different than voice and has different demands in regards to being transported across IP. If at all possible, Cisco recommends using T.38 to transport your fax traffic. Thanks again for being part of this event!

To view the complete event visit!/CiscoSupportCommunity/posts/10150229459761412?referring_site=facebook&channel=CiscoSupportCommunity_notetab