Recommended guidelines VoIP faxing


Chase Greiser


Faxing over IP has known limitations industry-­­wide based upon different 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 becomes vulnerable unless all carriers in the call flow support T.38 fax protocols.

To help mitigate this issue, we suggests customers on fiber or high bandwidth Ethernet circuits to 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 a 488 SIP message is received. All codecs with compression will not work. Additionally, G711 still has issues with ECM (Error Correction Mode, which is not supported today), hang-up 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, 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 volume faxing, a unified solution such as eFax is highly recommended.

Finally, Bandwidth is fully vested in providing you with the quality services your business needs. If your company is 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. 

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