Description
Transmission side facsimile (T-FAX) is a 3GPP term that specifies the network-side components and procedures responsible for transmitting facsimile data within a cellular network. It is part of the broader 3GPP facsimile service specifications, which ensure backward compatibility with the ubiquitous Group 3 (G3) fax standard used on the Public Switched Telephone Network (PSTN). In a circuit-switched context (e.g., GSM, UMTS), T-FAX functionality is typically implemented within the Mobile Switching Centre (MSC). The MSC acts as the intermediary between a mobile station (the receiving or terminating side) and the PSTN or another fax terminal.
The operation involves several key stages. First, during call setup, the MSC identifies the call as a facsimile session based on bearer capability information. It then negotiates the fax parameters (like data rate and compression) with both the mobile station and the remote fax terminal. During the transmission phase, the T-FAX entity in the network is responsible for adapting the fax modem signals. It converts the audio-band modem signals from the mobile station (which are carried over the air interface and the core network) into a format suitable for transmission over the PSTN, which typically uses a 64 kbps PCM (Pulse-Code Modulation) channel in the TDM (Time-Division Multiplexing) network. This involves demodulation, error correction, and potentially rate adaptation.
With the evolution to IP-based networks like the IP Multimedia Subsystem (IMS), the T-FAX concept was adapted. In IMS, the Media Gateway Control Function (MGCF) and Media Gateway (MGW) often embody the T-FAX functionality for interworking with the PSTN. Furthermore, 3GPP defined "fax over IP" procedures where the T-FAX role involves handling the Real-time Transport Protocol (RTP) streams that carry the fax modem signals as audio packets (using codecs like G.711). The network must ensure these packets are delivered with minimal jitter and packet loss to maintain fax session integrity. The T-FAX procedures are detailed in 3GPP TS 23.146, which covers the technical realizations and interworking scenarios for fax services across different network generations.
Purpose & Motivation
T-FAX technology was standardized to support a critical legacy service—facsimile—over emerging digital cellular networks. When GSM was developed, fax machines were a dominant business communication tool. A key requirement for cellular network adoption by enterprises was the ability to send and receive faxes from mobile devices or via fixed-mobile integration. The purpose of defining T-FAX (and its counterpart, R-FAX for the receiving side) was to create a standardized, reliable method for transporting fax modem signals over the radio and core network, which are inherently different from the analog PSTN.
It solved the problem of signal degradation and incompatibility. The digital encoding, compression, and potential for errors on the radio link could easily disrupt the sensitive analog modem handshaking and data transmission of a fax session. T-FAX procedures include specific error correction modes (like ECM) and precise rate adaptation algorithms to mitigate these issues. This ensured that a fax sent from a mobile phone had the same reliability and quality as one sent from a traditional landline, which was essential for business and legal document transmission.
As networks evolved to 3G, 4G, and 5G, the continued support for T-FAX, even as an optional feature, was driven by the need for long-term service continuity. Certain verticals, such as healthcare, legal, and maritime, continued to rely on fax for its simplicity, security perception, and legal standing. Standardizing T-FAX in IMS allowed operators to maintain this service while transitioning their core networks to all-IP architectures, ensuring they could still interconnect with the vast installed base of PSTN fax machines worldwide.
Key Features
- Support for ITU-T Group 3 facsimile protocols over 3GPP networks
- Interworking function between cellular bearer services (e.g., UDI, 3.1 kHz audio) and PSTN fax transmission
- Implementation of fax modem signal adaptation and rate adaptation
- Support for Error Correction Mode (ECM) to ensure data integrity over error-prone links
- Procedures for both circuit-switched (CS) and IP Multimedia Subsystem (IMS) network architectures
- Call control and bearer negotiation specific to facsimile services
Evolution Across Releases
Initial comprehensive specification for facsimile services in 3GPP, defining T-FAX and R-FAX roles. Covered circuit-switched fax interworking for GSM and UMTS, including bearer service negotiation, modem signal handling, and support for transparent and non-transparent data services over the air interface.
Defining Specifications
| Specification | Title |
|---|---|
| TS 23.146 | 3GPP TS 23.146 |