Fax Adaptor / Interworking Function

Description

The Fax Adaptor at the Interworking Function (FA/IWF) is a critical network element defined in 3GPP specifications for enabling Group 3 fax services over mobile packet-switched networks, such as those based on GPRS, EDGE, or later UMTS/HSPA. Its primary role is to act as a gateway or adaptor, mediating between the standard analog fax signaling and modulation used by traditional fax terminals (or circuit-switched networks) and the packet-based protocols used within the 3GPP core network. The IWF is typically a functional entity that could be integrated within a Gateway GPRS Support Node (GGSN) or a dedicated Media Gateway.

Architecturally, the FA/IWF sits at the boundary between the packet-switched domain (e.g., the IP network) and an external network, which could be the PSTN/ISDN or another circuit-switched network. When a fax call is established from a mobile station (MS) towards a legacy fax machine, the data passes through the FA/IWF. It performs real-time conversion of the fax modem signals. This involves demodulating the analog fax signal from the circuit side, extracting the fax image data, packetizing this data using protocols like TCP/IP or UDP/IP, and transmitting it over the packet network to the mobile station's own Fax Adaptor (FA/MT). The reverse process occurs for faxes sent from the mobile station.

The operation relies on the T.38 fax relay protocol or similar mechanisms for real-time fax over IP. The FA/IWF implements the T.38 protocol stack, handling functions like spoofing to compensate for packet network jitter and delay, which are detrimental to traditional fax handshaking. It manages the call control signaling, often interfacing with SIP or H.323 in the IP domain and ISUP/PRI on the circuit side. Key components include the modem interworking function, the packetization/de-packetization engine, and jitter buffer management. Its role is to ensure transparent fax service, making the packet network appear as a reliable circuit-switched connection to both the fax terminal and the mobile user, thereby supporting a vital legacy business service in modern networks.

Purpose & Motivation

The FA/IWF was created to solve the problem of supporting legacy Group 3 fax services over emerging packet-switched mobile networks. As 3GPP networks evolved from circuit-switched voice (CSD) to more efficient packet-switched data (GPRS, UMTS), a mechanism was needed to carry real-time fax traffic, which is sensitive to delay and jitter, over these best-effort IP networks. Without an adaptor, fax communication would fail or be of poor quality.

The historical context is the transition phase in the early 2000s (around 3GPP Release 4) where mobile operators needed to maintain support for business-critical fax services while migrating their core networks to all-IP architectures. Previous approaches relied on Circuit Switched Data (CSD) for fax, which dedicated a full traffic channel for the duration of the call, an inefficient use of spectrum and network resources compared to packet-switched transmission.

The FA/IWF, alongside the FA/MT, addressed this by defining a standardized interworking solution. It enabled the reuse of existing fax machines and services, protecting operator investments and ensuring business continuity. Its creation was motivated by the commercial necessity to offer a complete suite of telephony services, including fax, over the new high-speed packet data networks, thereby making these networks viable replacements for all traditional circuit-switched services.

Key Features

• Interworks between analog/PCM fax signaling and packet-based (e.g., T.38) protocols
• Typically integrated within the network's IWF or Media Gateway node
• Supports real-time fax demodulation and remodulation for transport over IP
• Implements T.38 fax relay with jitter compensation and spoofing techniques
• Handles call control interworking (e.g., ISUP to SIP) for fax session establishment
• Enables transparent fax service for mobile users connecting to PSTN fax terminals

Evolution Across Releases

Defining Specifications