Conversion Facility

Description

The Conversion Facility (CF) is a network element specified in 3GPP standards that provides protocol and format conversion between Teletext (Ttx) and Telex (Tx) services. Teletext is a television-based information retrieval service, while Telex is an older teleprinter communication system. The CF acts as a gateway that translates between the different signaling protocols, character encoding schemes, and transmission formats used by these legacy text services, allowing them to interoperate within 3GPP mobile networks.

Architecturally, the CF typically resides in the service layer of the network, interfacing with both the core network elements and external service platforms. It contains protocol stacks for both Ttx and Tx protocols, along with conversion logic that handles the mapping between different character sets (such as ITA2 for Telex and various national variants for Teletext), timing characteristics, and service-specific features. The CF maintains session state information for active conversions and ensures proper synchronization between the two services despite their different operational characteristics.

Key components of the CF include the protocol adaptation layer, which handles the physical and data link layer conversions; the message conversion engine, which translates message content and format; and the service logic that manages the overall interworking process. The CF also includes interfaces to network management systems for configuration, monitoring, and fault management purposes. It operates transparently to end users while ensuring that the essential characteristics of both services are preserved during conversion.

The CF plays a crucial role in maintaining service continuity for specialized applications that rely on these legacy text services, particularly in sectors like maritime communications, aviation, and certain government services. By providing this conversion capability, 3GPP networks can support a wider range of communication services without requiring users to replace existing equipment or change established operational procedures. The CF represents an important example of how mobile networks can evolve while maintaining backward compatibility with established communication technologies.

Purpose & Motivation

The Conversion Facility was created to address the need for interoperability between legacy text communication services and emerging 3GPP mobile networks. Teletext and Telex services were widely deployed in various industries and government applications before the widespread adoption of mobile data services. These services had established user bases, specialized equipment, and operational procedures that couldn't be easily replaced. The CF enabled these existing services to continue operating while allowing users to benefit from the mobility and coverage advantages of 3GPP networks.

Prior to the CF's introduction, Teletext and Telex services operated on separate, dedicated networks with incompatible protocols and formats. Users requiring both services needed separate equipment and connections for each. The limitations of previous approaches included duplicated infrastructure, higher operational costs, and the inability to leverage mobile network capabilities for these text services. The CF solved these problems by providing a standardized conversion mechanism that allowed both services to be delivered over a common mobile network infrastructure.

Historical context shows that the CF was particularly important during the transition from second-generation to third-generation mobile networks, when operators sought to maximize service offerings while minimizing infrastructure costs. By enabling Ttx/Tx interworking, mobile operators could attract users from specialized industries that relied on these text services, creating new revenue streams while providing valuable backward compatibility. The CF's development reflected 3GPP's commitment to supporting diverse communication needs as mobile networks evolved beyond simple voice services.

Key Features

• Protocol conversion between Ttx and Tx signaling protocols
• Character encoding translation between different text formats
• Session management for maintaining service continuity during conversion
• Transparent operation that preserves service characteristics
• Standardized interfaces to 3GPP network elements
• Support for legacy service features and operational requirements

Evolution Across Releases

Defining Specifications