Internet Facsimile Transfer

Description

Internet Facsimile Transfer (IFT) is a standardized service within the 3GPP IP Multimedia Subsystem (IMS) framework, specified primarily in TS 26.114. It provides the capability to send and receive fax documents in an all-IP environment, bridging legacy facsimile technology with modern packet-switched networks. The service supports two primary modes of operation: voice-band data (VBD) transport and T.38 packet-based transport. In VBD mode, the fax modem signals are treated as audio, encoded using a codec like G.711, and transmitted within an IMS voice session. In T.38 mode, the fax modem signals are demodulated and packetized into a specific protocol (T.38) for more robust transmission over IP networks that may have packet loss, delay, or jitter.

Architecturally, IFT involves several key network functions. The User Equipment (UE) must support fax modem functionality and the appropriate IMS client capabilities. The IMS core handles session establishment using SIP (Session Initiation Protocol) and SDP (Session Description Protocol) to negotiate the fax mode and parameters. Media resources, such as a Media Resource Function Processor (MRFP), may be involved in transcoding or protocol interworking, especially in scenarios involving a transition between circuit-switched and packet-switched domains (e.g., fax to/from a PSTN line). The service defines specific payload formats, session negotiation procedures, and fallback mechanisms to ensure interoperability.

How IFT works begins with session initiation. A UE indicates its capability and desire to conduct a fax session within the SIP INVITE. The network authorizes the service and routes the session. During the media phase, the fax modems perform their standard handshake and training procedures, but the audio or data packets are carried over the IMS bearer. For T.38, the UE or a network gateway translates the fax HDLC frames into T.38 UDPTL or TCP packets. IFT includes procedures for handling network events like codec changes, session updates, and error conditions specific to fax transmission, ensuring reliable delivery of the fax document end-to-end.

Purpose & Motivation

IFT was created to address the declining but persistent need for fax services in a world transitioning to all-IP telecommunications networks. Traditional fax over circuit-switched networks (PSTN/ISDN) was incompatible with the packet-based nature of 3G/4G/5G mobile cores and IMS. The purpose of IFT is to provide a standardized, carrier-grade method to support fax over these new networks, ensuring business continuity for sectors like healthcare, legal, and logistics that still rely on fax for document transmission due to legal acceptance or legacy system integration.

The motivation stemmed from the introduction of IMS as the service delivery platform for multimedia communications. Without a standard like IFT, operators would face proprietary solutions, interoperability issues, and an inability to offer a reliable fax service to their mobile and fixed-line customers on next-generation networks. IFT solves the problem of transporting real-time fax modem signals over an IP network susceptible to packet impairment. By standardizing both VBD and T.38 approaches, it offers flexibility and robustness, supporting everything from simple audio-pass-through to error-corrected packetized fax, thereby future-proofing a legacy service.