Fax services via VoIP networks.

Companies that switching over from traditional ISDN/analogue lines to VoIP, often would like to move their fax traffic also to the new VoIP network. However, VoIP networks are, like the name implies optimized for voice traffic and not for Fax.


Below is described what is done to make Fax via VoIP networks possible and why it is not so reliable compared when using the traditional ISDN/analogue lines as a transport media. For Gigaset to be able to provide any support if Fax via your provider is reliable, Interop testing with this provider is the minimum requirement. See the Interop section on this wiki.

The reliability of Fax via VoIP also depends on:

  • Provider
  • Protocol used
  • Network quality (Delay/Jitter/Packet drops)
  • Fax device
  • ...

Often providers offer online FAX services to be able to offer a MORE RELIABLE fax service.

The original fax protocol (T30) via the traditional networks includes mechanisms to handle background noise and spikes of interference on the telephone line. For example, poor signal quality can be accommodated by lowering the transmission speed, and spikes of noise can be handled by retrying any operations that are lost during the spike.

But T30 it is not designed for IP network problems like packet loss, which can result in large gaps in fax data and cannot be eliminated by a lower transmission speed. In fact, lower speeds may actually make the performance worse by generating additional network traffic that will be exposed to packet loss. And retries may not always recover missed data when packet loss is frequent, because the retries can suffer packet loss as well. Even the retry requests themselves may be lost.

When upgrading from traditional analog fax to a Fax over IP (FoIP) solution, it is necessary to provide a sufficient level of network performance to support reliable operation. Fax transport over an IP network can be provided by T.38 fax relay and G.711 pass through, each one having different network requirements for reliable operation.


T.38 fax relay is an ITU-T recommendation that allows for fax data to be carried over IP networks. Data is transmitted directly in T.38 without being converted to an audio stream, and results in a significant reduction in the bandwidth needed. T.38 also supports data and controls redundancy to mitigate the effects of packet loss.


G.711 is an ITU-T recommendation for Pulse Code Modulation (PCM) of voice frequencies. It uses an uncompressed format and requires high bandwidth, typically about 64 kbps. Using G.711 as the transport method for FoIP is an extension of traditional PSTN audio-based faxing. The digital fax data is converted to a PCM audio stream and then sent as G.711 Real-time Transport Protocol (RTP) packets.

G.711 has not been optimized for fax transport over IP networks, and does not typically support packet redundancy. Having been developed for voice, G.711 allows the transmission of missing audio because any gaps would be filled in by a human listener. But when used to transmit modem data, any loss of packets is significant, because the receiver has no way to recreate the missing data.

Some packet networks may not be fax-aware, and may optimize the G.711 stream for voice with the use of silence suppression, echo cancellation, or transcoding to a higher compression codec. Such optimizations can cause a loss of data and prevent FoIP from operating. This may force the use of a dedicated G.711 fax trunk to provide reliable fax performance. However, G.711 is an inherently simpler approach to fax than T.38, so interoperability issues between different vendors’ products is less common with G.711 than may be encountered with T.38. The cost for a G.711 approach may also be lower than T.38 because it can leverage voice data infrastructure.



Packet loss:

T38: 3% Packet loss can reduce the reliability back to 82%

G.711: 3% Packet loss can reduce the reliability back to less then 20%


More pages also decreases the reliability.


T38: 100 mseconds packet loss, can reduce the reliability back to 96%

G.711: 100 mseconds packet loss, will reduce the reliability back to 0%


T38: More then 3 seconds Round trip delay means no Fax is possible.

G.711: More then 2.5 seconds Round trip delay means no Fax is possible.


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