Teraquant 'No Fluff' Blog

There are still many businesses, enterprises and government organizations which use Fax for their official activities. So, it makes sense for these organizations using VoIP to move their fax traffic onto the IP network as well. They can then converge their fax and voice communications into one network. This will enable lower operating costs, simplified infrastructure deployment, and eliminate costly analog lines.

Fax Over IP (FoIP) sends/receives fax through the IP network rather than through traditional phone lines and utilizes SIP, i.e., the same protocol used for setting up VoIP calls. The popular options for transmitting fax in a VoIP network are using either T.38 fax relay or using G.711 audio codecs as a fall back in case T.38 is not supported by some network elements along the call path. T.38 devices use standard call setup protocols to negotiate the T.38 call, e.g., H.323, SIP and MGCP.

In legacy systems fax transmission use T.30 fax protocol. However, in VoIP networks, T.38 protocol, an ITU-T recommendation for fax data to be carried over IP networks is used. T.38 media is preferred for fax, as opposed to normal voice codecs. Voice codecs are good for sending voice-based audio but are unreliable when it comes to transmitting fax.

T.38 sessions are described using the “image/t38” within the SDP part of the SIP message [ click here to learn more about Session Description Protocol (SDP) https://teraquant.com/understand-media-sip-session-description-protocol/ ]. T.38 fax/IP is detailed in RFC 3362. The SDP negotiation for SIP signaling for T.38 is nearly identical to a VoIP call with the exception of the listed Fax image media with media type formerly known as MIME. Data is transmitted directly using T.38 without being converted into audio streams. This saves the amount of bandwidth needed. Since audio streams are transmitted using RTP over UDP, there is a possibility of significant loss of packets (based on underlying network, traffic etc.), which might still be OK for audio (as you would prefer audio being sent immediately without delay rather than send all audio packets with significant delay) but not for images being sent for Fax. T.38 where reassembly of the image occurs after a significant part of the transmission. So, T.38 also supports data and control redundancy to mitigate the effects of packet loss.

The originating or terminating side could have analog fax machines which are connected to the VoIP network using adapters/gateway for SIP signaling and to convert the T.30 protocol to T.38. The modes of negotiating T.38 using offer/answer model within the network can be categorized into the following:

The T.38 recommendation defines the use of both TCP and UDP to transport T.38 packets. However, implementations mostly use UDP, due to TCP’s requirement for acknowledgement of packets and resulting re-transmission during packet loss, which introduces delays. More particularly, Fax transmissions use UDPTL (layer utilizing UDP) which makes use of redundancy and Forward Error Correction techniques for error correction as per ITU-T T.38 recommendation. UDPTL is preferred instead of RTP as UDPTL packet only requires 2 bytes per header, while RTP requires 12 bytes for its basic header.

A few issues that you can run into while faxing are listed below:

T.38 is the preferred way to send faxes over VoIP networks, however there are a few caveats to be kept in mind and appropriate configurations needed in the network to ensure best results from your faxing.