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TDM vs T.38. Challenges With Fax In An IP Network

By June 18, 2012No Comments

People often ask me if T.38 fax is as reliable in an IP network compared to traditional analog or PRI faxing. My answer is “Yes and No”. This question seems simple enough yet there are so many variables that need to be examined and understood.

Decoding the Codec:

T38 vs G711; Which codec is better? Most of the time we suggest T.38 for fax but there are customer and carrier network environments that lack certain IP transport capabilities within the gateways they are using so G.711 ends up working better. I can’t tell you how important Error Correction is in fax. ECM is part of the fax communication process between two fax devices. ECM does all kinds of cool stuff like deciding how to compress the transmission and which lines in the fax transmission need to be resent to the receiving device to complete the fax, and so on.

Some media gateways and PBX manufactures such as Avaya do not support ECM at all. So you may be faced with the decision of doing the best with what you have and choosing T.38 with no ECM or G.711 with ECM. In an Avaya environment we always suggest a dedicated fax gateway such as the Dialogic or OpenText Fax Gateways. These gateways are designed especially for fax traffic and do an excellent job of converting analog or PRI into SIP.

FoIP and the Public Switched Telephone Network:

One of the most common issues we see here at Ingenium with fax in an IP enviornment is right after a company switches their telephone provider from analog or PRI to a VoIP carrier and completely overlooks the need for fax support; specifically T.38. Not all carriers are equal.

Just because your carrier can support voice over VoIP does not mean they can support Fax. Many local carriers have been moving their backbones to SIP wholesalers so they can get cheaper rates. When fax traffic hits these SIP backbones, we see all kind of issues like delay, lost packets and altered data.

Take for instance AT&T, who has one of the largest telephone networks in the United States. AT&T has so many local POPs, it would be financially impossible for them to replace all of the legacy TDM gateways and switches they have in the field to support T.38. So they do crazy things like pushing SIP services from behind MPLS connections that do not offer QOS or error logging for T.38/SIP invites. So, they can’t troubleshoot their own network when a customer complains about failed calls, disconnect errors and false positive fax transmissions.

In our experience, Verizon also has issues with providing T.38 over H.323 inside their network in some local markets. Verizon recently started upgrading their voice gateways in the south east to Avaya. But remember, Avaya does not support ECM.

Our favorite carrier for supporting T.38 is Global Crossing, who was recently acquired by Level 3. These guys have a superior IPv6 network from the ground up. I have been in the belly of this global network and have personally seen the core switching environment which, by immaculate design, converts all transports and signaling into SIP on the fly, and back to TDM or whatever transport the call route ends up being delivered to. They also monitor every single SIP session and can analyze all SIP conversations to scientifically and undisputedly peel back the twisted pairs and tell you exactly why the call failed or who’s network is not speaking the right codec. I have been totally impressed with their technical capabilities and support execution. We have a 99% fax success rate with Level 3. That’s better than most TDM networks.

Setting Customer Expectations:

Many of our clients have sucessfully virtualized their RightFax servers and are using SIP/T.38 for faxing. In order to be successful, you have to inspect what you expect. We suggest trying out this technology first before you make a buying commitment. Ingenium offers trial RightFax 10 software with the Dialogic T.38 codec option for validating your network and carrier compatibility with FoIP.

Our expert systems engineers can remotely install and configure the trial system in less than a day, in most cases. Our engineers will work with your IT department to help fine tune and validate your network for supporting fax over ip. For high volume users, we can also stress test the system to ensure network performance under a live fax load. By testing before the customer buys the software, everyone involved has an opportunity to see how fax over ip will work within their specific network.