Bandwidth T.38 faxing support guide


Molly Katolas


T.38 protocol is one of several communication protocols used for sending and receiving faxes over an IP network instead of the public telephone network.

How does it work?

Our suggestions for attempting and maximizing the quality of T.38 faxing over VoIP on the Bandwidth’s network are provided below.

Note: Due to the variables involved with faxing over the public internet, and especially transcoding between carriers, there's no SLA or guarantee on specific performance parameters within faxing at this time.

  1. Set up a VoIP call with G711.
  2. A customer ReINVITE for T.38 must be offered and accepted.
  3. A handshake between faxing clients must occur and be successful (the handshake is when the faxing clients communicate and agree upon the terms of the transmission). If the ReINVITE between faxing clients is unsuccessful a 488 SIP message (Not Acceptable Here) will be sent and transmission should fall back to G711.


Recommended guidelines for VoIP faxing

Faxing over IP has known limitations based industry-­wide upon differing technologies across carriers. There are many VoIP companies, including Bandwidth, who use T.38 for sending faxes over the internet. However, because calls typically traverse multiple carriers before terminating to their destination, T.38 faxes become vulnerable unless all carriers in the call flow support T.38 fax protocols.

To help mitigate this issue, we suggest customers on fiber or high bandwidth Ethernet circuits send fax traffic as uncompressed g711u. For customers on cable/DSL/T1/low bandwidth/low-reliability circuits, fax traffic should be sent as G711 and then ReINVITE to T.38, but be prepared to fall back to G711 if a 488 SIP message is received. All codecs with compression won't work. Additionally, G711 still has issues with ECM (Error Correction Mode, which isn't supported today), and will be limited to 9600 or 14.4k.

With VoIP faxing there can be packet loss or jitter, which to a fax machine might seem like a hang-up and can keep some pages from fully transmitting. For best results, we recommend making sure your MaxDatagram is set to 176. Another recommendation is to set the fax machine at the lowest baud rate and turn off its error correction mode. If the end user is performing high volume faxing, a unified solution such as eFax is highly recommended.

Finally, we're fully vested in providing you with the quality services your business needs. If you're a company that's heavily dependent on regularly faxing large, multi-­page documents, it's recommended that you maintain an analog line and a traditional fax machine for reliable and consistent performance.

T.38 support

While Bandwidth is fully committed to bringing you the best and most reliable service possible, we can't guarantee that every fax transmission will complete 100% of the time, especially in instances where the diagnosed issue is occurring outside of our network. We'll work with you and help advise you on how to best mitigate fax reliability issues so that you and your end users have the best, most consistent faxing experience possible.

Note: Troubleshooting into the depths of the RTP/UDPTL data isn't supported at this time.

Questions? Please open a ticket with your Bandwidth Support Team or hit us up at (855) 864-7776!

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