Internet Facsimile Protocol

Description

The Internet Facsimile Protocol (IFP) is a 3GPP-specified protocol that facilitates the transport of traditional Group 3 fax (G3F) services over packet-switched networks, specifically within the IP Multimedia Subsystem (IMS) framework. It acts as a gateway protocol, translating between the legacy analog/digital fax world and the all-IP world. IFP does not define a new fax compression or signaling method; instead, it encapsulates the existing ITU-T T.30 procedural signaling and T.4/T.6 image data protocols within Real-time Transport Protocol (RTP) packets for transmission over an IP network.

Architecturally, IFP is implemented in functional entities called Media Gateways (MGWs) or, more specifically, in Interworking Functions (IWFs) that are part of the Media Resource Function (MRF) or stand-alone gateways. When a fax call is placed from a legacy Public Switched Telephone Network (PSTN) or Integrated Services Digital Network (ISDN) fax machine to an IMS user (or vice versa), the call is routed through a Media Gateway Control Function (MGCF) and a Media Gateway (MGW). The MGW contains the IFP adaptation function. This function terminates the T.30 signaling from the circuit-switched side, interprets the fax handshake (e.g., CNG, CED, DIS/DCS), and converts the T.4/T.6 image data into a packet stream.

The protocol works in two main phases: the T.30 procedure phase and the image data phase. During the T.30 procedure phase, key signals like Called Station Identification (CED), Digital Identification Signal (DIS), and Digital Command Signal (DCS) are not transmitted as audio tones over the voice path. Instead, they are extracted by the gateway, represented as IFP packets (specifically, T.30-indicator packets), and sent over the IP bearer. This 'demodulation' prevents these signals from being distorted by voice codecs. During the image data phase, the scanned image data, modulated using V.27ter, V.29, or V.17, is demodulated by the gateway. The raw compressed image data (e.g., Modified Huffman, Modified READ) is then packetized into RTP payloads according to the IFP format (T.4/T.6 data packets) and sent over IP. The receiving gateway performs the reverse process, modulating the data back for the destination fax machine. This end-to-end process ensures the fax transmission is reliable and efficient over IP networks.

Purpose & Motivation

IFP was developed to solve a critical interworking problem during the transition from circuit-switched telephony networks (PSTN/ISDN) to packet-switched, all-IP networks like IMS. Traditional fax (G3F) is highly sensitive to latency, jitter, and packet loss, and it relies on precise in-band audio-frequency signaling (T.30) and modulated image data. Standard voice codecs (like AMR, G.711) used in VoLTE or VoIP severely distort these modem signals, making fax transmission over vanilla VoIP impossible. Early 'fax over IP' attempts using G.711 pass-through (no compression) were bandwidth-inefficient and still prone to failures due to network impairments.

The creation of IFP was motivated by the need to support a ubiquitous and legally significant business service—fax—within the new 3GPP IMS architecture. It addressed the limitations of previous approaches by moving the complex modem signal processing out of the audio path and into the network gateways. By demodulating signals at the edge of the IP network and transporting them as structured data packets, IFP makes fax transmission robust against codec distortion and general IP network impairments. It allows operators to retire legacy circuit-switched networks while maintaining backward compatibility for millions of fax machines, fulfilling regulatory and commercial requirements for continued fax service support in an all-IP era.