Description
The Message phase (MSG) is a defined procedural state within the ITU-T T.30 protocol for Group 3 facsimile communication. A T.30 session is divided into distinct phases: Call Establishment, Pre-Message Procedure, Message Transmission, Post-Message Procedure, and Call Release. The MSG phase encompasses the actual transmission of the facsimile image data. During this phase, the scanned document image is encoded using Modified Huffman (MH), Modified READ (MR), or Modified Modified READ (MMR) compression schemes and transmitted as a series of digital frames over the established bearer connection.
In the context of 3GPP networks, supporting the MSG phase involves the interworking between the 3GPP core network and the facsimile terminal adapters or gateways. For circuit-switched (CS) fax services, this occurs over a traditional voice channel (e.g., in GSM or UMTS), where the modem tones carrying the T.30 signaling and the MSG phase image data are treated as audio. The network must ensure transparent transmission of these tones without disruptive processing like voice activity detection or compression that could corrupt the data. In packet-switched implementations, such as Fax over IP (FoIP) using the T.38 protocol, the MSG phase data is packetized and transmitted over the IP network with specific mechanisms for redundancy and error correction to cope with packet loss.
The technical implementation involves several network functions. The Mobile Switching Center (MSC) or IP Multimedia Subsystem (IMS) must recognize a fax call based on the called number or early in-band tones (the Called Station Identification - CSI). Once identified, the network may disable voice codecs and enable a transparent 64 kbps unrestricted digital bearer (in CS) or invoke the T.38 protocol stack (in PS). During the MSG phase, the network's role is primarily to provide a reliable bit-pipe. Any transcoding, echo cancellation, or jitter buffers must be configured to preserve the integrity of the high-frequency modem signals that constitute the image data. The successful completion of the MSG phase is confirmed by the T.30 Post-Message Procedure, which includes the confirmation (MCF) or retrain (RTP) signals.
Purpose & Motivation
MSG, as defined by ITU-T T.30, is the fundamental operational phase for facsimile transmission. Its support within 3GPP standards was necessary to ensure backward compatibility and continued service for fax machines, which were, and in some sectors still are, a critical business tool. Early mobile networks (GSM) needed to support fax as a basic telephony service alongside voice. The primary problem was adapting a network designed primarily for compressed voice to carry the high-frequency, analog-modem signals of a fax session without degradation.
The motivation for its inclusion and continued reference in 3GPP specs stems from the need for interworking and service transparency. As networks evolved from circuit-switched to packet-switched architectures, the challenge shifted from preserving analog tones to reliably transporting real-time fax data over IP networks prone to delay, jitter, and packet loss. The T.38 standard, referenced in later 3GPP releases, was developed specifically to address this, defining how the T.30 protocol phases, especially the critical MSG phase, are encapsulated and protected in IP packets. Supporting the MSG phase allows mobile operators to offer fixed-mobile convergence for fax services and support specialized devices like medical or legal fax machines that rely on this legacy technology.
Key Features
- Defined procedural state within the ITU-T T.30 facsimile protocol
- Phase where the actual scanned document image data is transmitted
- Utilizes image compression standards like MH, MR, and MMR
- Requires network transparency or specific adaptation (T.38) for signal integrity
- Supported over both 3GPP circuit-switched bearers and packet-switched IP networks
- Involves interworking with facsimile gateways or terminal adapters in the network
Evolution Across Releases
Formally referenced the T.30 MSG phase in 3GPP specifications for 3G circuit-switched facsimile services. Support was defined for transparent transmission of fax signals over UMTS bearers, relying on the network's ability to provide an uncompressed digital path for the duration of the fax call, including the Message phase.
Enhanced support for Fax over IP (FoIP) in the context of IMS and packet-switched networks. Specifications began to incorporate references to the ITU-T T.38 protocol, which defines how the T.30 protocol (including the MSG phase) is adapted for real-time transport over IP networks, including handling packet loss and jitter.
Defining Specifications
| Specification | Title |
|---|---|
| TS 23.045 | 3GPP TS 23.045 |
| TS 26.969 | 3GPP TS 26.969 |
| TS 43.045 | 3GPP TR 43.045 |