Fax Adaptor / Mobile Terminal

Description

The Fax Adaptor at the Mobile Terminal (FA/MT) is the client-side functional component that enables a 3GPP mobile device to participate in Group 3 fax sessions over a packet-switched network connection. It is the counterpart to the network-based FA/IWF (Fax Adaptor at the Interworking Function). The FA/MT is implemented in software and/or hardware within the User Equipment (UE), such as a smartphone with fax software, a dedicated fax modem dongle, or a data card in a laptop.

Architecturally, the FA/MT resides above the device's packet data protocol stack (e.g., PPP over the 3GPP data bearer) and interfaces with the user's fax application or virtual fax modem driver. When the user initiates a fax send operation, the application generates standard fax modem signals. The FA/MT intercepts these signals, digitizes and packetizes the fax image data and control signals according to the T.38 fax relay protocol or a similar scheme. It then transmits these IP packets over the established packet data protocol (PDP) context to the network's FA/IWF. For incoming faxes, the process is reversed: the FA/MT receives T.38 packets from the FA/IWF, reconstructs the fax modem signals, and presents them to the fax application as if they came from a direct analog connection.

Key to its operation is the management of the real-time fax session over a potentially jittery IP connection. The FA/MT includes a jitter buffer and implements spoofing techniques to trick the local fax application into believing it has a stable, low-latency circuit connection. It handles the T.38 protocol procedures, including error correction mode (ECM) if supported, and interacts with the device's telephony layer for call control (initiating a data call for fax). Its role is to provide a complete fax modem emulation locally, abstracting the complexity of the packet network and interworking from the user's fax software, thereby enabling seamless mobile fax functionality without requiring a traditional circuit-switched voice call.

Purpose & Motivation

The FA/MT was developed to empower the mobile user equipment to act as a fax terminal in a packet-switched network environment. Prior to its standardization, sending a fax from a mobile device required a Circuit Switched Data (CSD) call, which tied up a voice channel and offered low data rates (e.g., 9.6 kbps). This was inefficient and often required specific hardware support. The FA/MT, introduced alongside the FA/IWF, solved this by leveraging higher-speed packet data connections (GPRS, EDGE, UMTS) for fax transmission.

The creation was driven by the need for mobile data services to fully replace legacy circuit-switched services, including fax, which remained important for business documents, legal contracts, and healthcare. It addressed the limitation of requiring a dedicated CSD-capable modem in the terminal and the poor user experience of slow fax transmission speeds.

By implementing the fax adaptor logic in the terminal, it allowed for more flexible and software-upgradable fax solutions. Users could potentially use a single mobile device for voice, internet, and fax over a shared packet data subscription. The FA/MT and FA/IWF together facilitated the migration path for fax from a circuit-switched telephony service to an application running over mobile broadband, aligning with the industry's move towards all-IP networks.