Binary Coded Signalling

Description

Binary Coded Signalling (BCS) is a critical protocol phase within the ITU-T T.30 standard for facsimile communication, specifically designed for digital signalling. In the context of 3GPP networks, BCS is utilized to support circuit-switched fax services, such as Group 3 fax, over mobile systems. The BCS phase occurs after the initial handshake and before the actual image data transmission (the message phase). It employs a high-level data link control (HDLC) framing structure to exchange binary-encoded control messages between the transmitting and receiving fax terminals. These messages negotiate transmission parameters, confirm capabilities, and manage the call procedure, ensuring both terminals agree on modulation schemes, data rates, and error correction modes before proceeding.

Architecturally, BCS operates within the user plane of the circuit-switched domain in 3GPP networks. It is implemented in the terminal equipment (e.g., a fax machine or fax modem) and supported by network elements like the Mobile Switching Centre (MSC) for call routing and bearer management. The BCS messages are transported over a transparent or non-transparent data bearer service, typically using an asynchronous circuit-switched data call. Key components include the HDLC flag sequences for frame delimitation, address and control fields for link management, and a frame check sequence (FCS) for error detection. The information field contains the actual T.30 binary commands and responses, such as Digital Identification Signal (DIS), Digital Command Signal (DCS), and Confirmation to Receive (CFR).

In operation, BCS enables a structured negotiation: the called terminal sends a DIS message indicating its capabilities (e.g., supported data rates, resolution). The calling terminal responds with a DCS message to select the parameters, followed by a training sequence to optimize the physical link. Upon successful training, a CFR message confirms readiness, transitioning to the message phase for fax page transmission. BCS also handles mid-call control, like post-page commands for multi-page documents. Its role in 3GPP is to ensure interoperability between mobile fax services and traditional PSTN/ISDN fax systems, maintaining reliability through standardized error handling and retransmission procedures for corrupted frames.

The integration of BCS into 3GPP specifications, such as TS 23.046 and TS 43.045, defines adaptations for radio access and core network interactions. This includes mappings to radio interface protocols and interworking with PSTN for end-to-end fax calls. BCS is essential for legacy fax support in 2G (GSM), 3G (UMTS), and early 4G (LTE) networks where circuit-switched services are prevalent, though its relevance has diminished with the decline of fax and transition to all-IP networks.

Purpose & Motivation

BCS was created to address the limitations of earlier analog signalling methods in facsimile transmission, such as the T.30 pre-message procedures that used tone-based signals (e.g., CNG, CED). These analog methods were prone to errors in noisy environments and offered limited flexibility for negotiating complex capabilities. BCS introduced a digital, binary-encoded signalling phase to enhance reliability, speed, and functionality. By using HDLC framing and error detection, BCS reduces the risk of misinterpretation and enables more sophisticated parameter negotiation, supporting higher data rates and advanced features like error correction mode (ECM). This was crucial for improving fax quality and efficiency over both PSTN and mobile networks.

In 3GPP, the purpose of incorporating BCS, starting from Release 99, was to ensure seamless support for circuit-switched fax services in mobile telecommunications. As GSM and UMTS networks evolved, there was a need to maintain compatibility with existing fax infrastructure while leveraging digital mobile technology. BCS allows mobile terminals to act as fax endpoints, negotiating capabilities with remote fax machines over the network. It solves problems like interoperability between mobile and fixed fax devices, managing call setup in a radio environment with potential latency and errors, and providing a standardized method for fax over circuit-switched data bearers. This was motivated by the widespread use of fax for business and legal documents, requiring reliable mobile fax solutions before the advent of widespread internet-based alternatives.

Historically, BCS addressed the transition from analog to digital fax, enabling features like superfine resolution and faster modulations. In 3GPP, it supported services like fax mail and store-and-forward fax, as defined in specifications. However, with the phasing out of circuit-switched domains in favor of VoLTE and IP Multimedia Subsystem (IMS), the relevance of BS has decreased, though it remains part of legacy support in specifications up to recent releases.