PSTN dialler Development with Contact ID and SIA protocols

CASE STUDY SNAPSHOT

Customer : A Fortune 100 Industrial conglomerate
Size : >1000 – 10000
Project vertical : Industrial
Challenge : Enable legacy PSTN connectivity for model fire alarm panel.
Solution : Design and develop a custom PSTN dialler module with Contact ID and SIA protocol.
Services & Products Availed :  Product engineering services, Industrial Protocol Stacks, Embedded Application Development
Tools and Technologies :

• Key Hardware: STM32L4R5
• Software Development: C, JNI
• Frameworks: Contact ID and SIA
• Frameworks: PSTN. SPI etc

INTRODUCTION

Embien collaborated with a Fortune 100 conglomerate, a global leader in fire and safety solutions, to enhance their state-of-the-art fire alarm panels. Their advanced, IP-based systems needed a reliable, add-on communication channel using the Public Switched Telephone Network (PSTN) to broaden market compatibility and ensure redundant alarm transmission.

CHALLENGE

The primary challenge was to seamlessly integrate a legacy communication method (PSTN) with a modern, Linux-based fire alarm panel without any modifications to the core hardware. The solution had to be a non-intrusive, add-on module. It required robust support for critical, industry-standard alarm protocols like Contact ID and SIA, which use distinct signaling methods (DTMF and FSK). Furthermore, the design demanded high reliability for life-safety applications, including automatic failover between two separate telephone lines to guarantee that critical event data was transmitted even during a line fault.

SOLUTION

Embien's Custom PSTN Dialler Module

Embien engineered a comprehensive solution involving a sophisticated hardware add-on card and modular firmware, creating a powerful PSTN Dialler that met every customer requirement. Our approach was rooted in a deep analysis of the existing system to ensure flawless integration and performance.

1. System Analysis and Interface Selection

Our initial analysis of the customer's existing fire panel revealed that the SPI (Serial Peripheral Interface) was the ideal choice for high-speed communication between the main panel and our new dialler module. This allowed for a clean, efficient data exchange without altering the panel's core architecture. We also designated several GPIOs to handle interrupt-based signaling, enabling the panel to instantly trigger the dialler upon detecting an event.

2. Hardware Design and Redundancy

The hardware was designed for reliability and performance.

• Processing Core: We selected a powerful ARM-based MCU to manage the complex protocol stacks, line supervision, and communication with the host panel.
• PSTN Interface: A specialized DAA (Direct Access Arrangement) chip was used to provide a safe, certified, and reliable interface to the PSTN lines.
• Dual-Line Failover: A key feature of our design was the inclusion of two phone line interfaces managed by a relay arrangement. The firmware constantly monitors the status of both lines. If the primary line is busy or develops a fault, the system automatically switches to the secondary line, ensuring that critical alarm transmissions are never missed. This redundancy is paramount in fire and safety systems where communication is non-negotiable.

3. Modular RTOS-Based Firmware

To handle the real-time demands of PSTN communication and protocol management, we built the firmware on a Real-Time Operating System (RTOS). This provided a stable and predictable foundation. The firmware was designed with a modular approach for maintainability and future expansion:

• Control Module: The central brain of the dialler, managing overall device operation and communication between all other modules.
• Analog Front-End (AFE) Module: Handled the low-level operations of the DAA chip, managing tones and signals on the phone line.
• Line Supervision Module: Continuously monitored the voltage and availability of the two PSTN lines, providing the data needed for the automatic failover logic.
• Protocol Module: This module contained the logic for the higher-level Contact ID and SIA protocols.
• Host Communication Module: Managed the SPI interface and interrupt handling for seamless data exchange with the fire alarm panel.

The firmware also supported a limited amount of event buffering, allowing it to store events temporarily if both phone lines were momentarily unavailable.

4. Deep Protocol Stack Implementation

A significant part of the project was implementing the two primary alarm communication protocols, Contact ID and SIA, which are the de-facto standards in the security industry.

• Contact ID Protocol: As the most widely used protocol, Contact ID utilizes DTMF (Dual-Tone Multi-Frequency) signals—the same tones your phone makes when you press a key—to transmit data. Our firmware meticulously implemented the entire handshake and data transmission process:
1. Dialing: The dialler goes off-hook and dials the pre-configured number for the central monitoring station.
2. Handshake: The receiver acknowledges the call by sending a specific sequence of tones (1400Hz and 2300Hz).
3. Data Transmission: The dialler transmits the event data as a string of DTMF digits. This string contains the account number, the event code (e.g., fire alarm, low battery), a partition number, and a zone number.
4. Kiss-off: Upon successful receipt of the message, the central station sends back a "kiss-off" tone (1400Hz) to acknowledge. If this tone is not received, our firmware automatically re-transmits the message.
• SIA (Security Industry Association) Protocol: This protocol uses FSK (Frequency Shift Keying) modulation to transmit digital data over the analog phone line. Our implementation managed its unique sequence, which includes pulse dialing and blocks of data for the account and event information, followed by a final acknowledgement check.

By supporting both protocols, the dialler ensured compatibility with a vast range of monitoring stations and receivers, maximizing the customer's market reach.

5. Seamless System Integration

The project was not just about building a dialler; it was about integrating it into a larger ecosystem. Embien's team also worked on the Linux-based fire alarm panel, developing the necessary drivers to manage the SPI communication. We extended the panel's web-based configuration interface, allowing system administrators to easily configure the dialler's settings, including phone numbers and protocol selection, through a familiar interface.

BENEFITS

• Seamless Feature Expansion: Enabled the addition of a critical communication feature without a costly redesign of the existing core product.
• Enhanced System Reliability: The dual-line automatic failover mechanism provides robust redundancy, ensuring alarm data is transmitted even during a line failure.
• Increased Market Compatibility: Support for both Contact ID and SIA protocols makes the fire alarm system compatible with the vast majority of monitoring centers worldwide.
• Future-Proof Design: The modular, RTOS-based firmware allows for easy updates and the addition of new features or protocols in the future.
• Accelerated Time-to-Market: Our expertise in embedded systems design and protocol development delivered a ready-to-deploy solution, significantly reducing the customer's development timeline.

CONCLUSION

Embien successfully designed, developed, and delivered a feature-rich PSTN Dialler add-on card, enabling our Fortune 100 client to expand their product's capabilities and reinforce its position as a leader in the fire and safety market. This project showcases our ability to execute complex legacy system integration and develop highly reliable industrial-grade electronics. Our deep expertise in both hardware and firmware ensures our clients receive a complete, robust, and market-ready solution. If you need to enhance your products with new communication interfaces or bridge older and newer technologies, contact Embien today.