Interchange Circuit 107

Description

CT107, or Interchange Circuit 107, is a specific control signal defined within the ITU-T V.24 recommendation for the interchange circuits between data terminal equipment (DTE) and data circuit-terminating equipment (DCE). In the context of 3GPP specifications, particularly TS 23.045 and TS 43.045, CT107 is referenced to ensure compatibility with legacy modem interfaces and circuit-switched data services. It operates as part of a physical interface, typically using a 25-pin connector (like the EIA-232 standard), where it is assigned to a specific pin to carry the Data Set Ready (DSR) signal. This signal is a control line that indicates the readiness of the DCE (often a modem or network termination) to establish a communication link.

The primary function of CT107 is to provide a handshaking mechanism between DTE and DCE. When the DCE is powered on, properly configured, and connected to the communication line, it asserts the DSR signal on CT107 by setting it to an 'ON' (or positive voltage) state. This informs the DTE that the DCE is operational and ready to accept commands or data. If the DSR signal is 'OFF' (negative voltage), the DTE should not attempt to transmit data, as the DCE is not ready, which could be due to issues like power loss, faulty connection, or being in a test mode. This simple yet critical signal prevents data loss and ensures synchronized communication initiation.

Architecturally, CT107 is one of several interchange circuits defined in V.24, each serving distinct control functions such as Request to Send (RTS on CT105), Clear to Send (CTS on CT106), and Data Terminal Ready (DTR on CT108.2). In a typical data call setup, the DTE first asserts DTR to indicate its readiness, and the DCE responds with DSR on CT107 once it is ready. This exchange is part of a larger sequence that may involve additional circuits for carrier detection and data transfer. Within 3GPP systems, especially in early releases, CT107's role was crucial for supporting circuit-switched data services over GSM networks, where mobile stations or terminal adapters interfaced with modems or network equipment using these standardized control signals.

CT107's operation is based on voltage levels, typically +3 to +15 volts for the 'ON' state and -3 to -15 volts for the 'OFF' state, as per EIA-232 specifications. It is a unidirectional signal from DCE to DTE, meaning it only conveys status from the network side to the terminal. In modern 3GPP networks, while packet-switched data has largely replaced circuit-switched services, references to CT107 in specifications like TS 43.045 (for GSM) ensure that legacy equipment and interfaces remain supported, particularly for fallback scenarios or in specific industrial applications. Its inclusion underscores 3GPP's commitment to backward compatibility and interoperability across generations of telecommunications technology.

Purpose & Motivation

CT107 exists to standardize the Data Set Ready (DSR) control signal for reliable data communication between data terminal equipment (DTE) and data circuit-terminating equipment (DCE), as defined by ITU-T V.24. It solves the problem of unsynchronized data transmission by providing a clear, hardware-based indication of when the DCE (e.g., a modem) is ready to operate, preventing data corruption or loss that could occur if the DTE attempted to send data before the DCE was prepared. This handshaking mechanism was essential in early telecommunications systems, including GSM, to ensure robust circuit-switched data services where precise timing and equipment coordination were critical for successful connections.

The historical context for CT107 dates back to the era of analog modems and leased-line communications, where ITU-T V.24 (first published in the 1960s) established a widely adopted standard for interface circuits. Before such standardization, proprietary interfaces led to interoperability issues between different manufacturers' equipment, hindering the growth of data networks. CT107, as part of V.24, addressed these limitations by providing a universal signal for DSR, enabling seamless integration of DTEs and DCEs from various vendors. In 3GPP, its adoption in releases like Rel-4 ensured that mobile networks could interface with legacy data equipment, supporting services like fax and dial-up internet over GSM.

Motivated by the need for backward compatibility and reliable control signaling, CT107's inclusion in 3GPP specifications allowed for the gradual evolution from circuit-switched to packet-switched data without abruptly discontinuing support for existing infrastructure. It addressed the limitations of ad-hoc control methods by offering a deterministic, voltage-based signal that reduced errors in data call setup. While its relevance has diminished with the rise of all-IP networks, CT107 remains a foundational element in telecommunications history, exemplifying the importance of standardized interfaces for interoperability and network stability.

Key Features

• Standardized Data Set Ready (DSR) signal as per ITU-T V.24
• Unidirectional control circuit from DCE to DTE
• Voltage-based signaling with ON/OFF states for readiness indication
• Part of physical interface handshaking for data communication
• Ensures synchronization between terminal and network equipment
• Supports backward compatibility in 3GPP for legacy circuit-switched services

Evolution Across Releases

Defining Specifications