Fax over IP

Best Practices for Faxing over IP (FoIP)

Faxing over IP networks presents challenges, which led to the introduction of the T.38 relay standard by the International Telecommunication Union (ITU) to facilitate real-time fax transmission over IP networks (FoIP). While T.38 doesn't handle call setup directly, it complements protocols like SIP or H.323 for negotiating fax sessions. It incorporates "fax-aware buffer management techniques" to mitigate missing or delayed packets, simulating a PSTN connection for fax machines.

Although some networks might handle fax transmission without T.38, enabling it is advisable. T.38 can reduce network bandwidth by a factor of 10 and correct packet loss, delay, and jitter.

Best practices for ATAs and voice gateways supporting faxing include:

1. Enabling T.38
2. Preferring uncompressed codecs like G.711 over compressed ones like G.729 to avoid signal distortion.
3. Considering G.711 passthrough alongside T.38 for added reliability.
4. Adjusting the network jitter buffer to its maximum size.
5. Disabling certain features like Voice Activity Detection, Echo cancellation, and Call waiting which can interfere with fax transmissions.

Fax machine settings can also be optimized:

1. Lowering transmission speeds (baud rates) to 9600 or 4800 bps if errors occur.
2. Disabling ECM (Error Correction Mode) as it might cause issues over VoIP.

However, FoIP solutions involve adapting technology beyond its original design, and reliability can vary due to vendor incompatibilities, network performance, or device limitations. In such cases, alternative solutions like using a dedicated analog PSTN line or a cloud-based internet fax service may be necessary.

In conclusion, with proper configurations and adherence to best practices, fax machines can be integrated into IP networks reliably, ensuring availability and functionality.