PSTN Dialler

Typically, many of the engineering product designs are done with future expansion into consideration. Especially for those developed for industrial applications, for which the lifetime may be more than a few decades, it is essential to provide options to integrate new features that might be required throughout the product lifetime. Embien always considers such requirements and designs the product with provisions for such capability expansion. Also, we have developed many add-on cards that will work with existing products and increase the available functionalities.

Numerous examples can be given for such kinds of device designs such as protocol converters for solar inverters, new industrial interface cards for variable speed drives, FPGA based video format converters, etc. This case study covers a design of PSTN diallers done for the expansion of fire alarm panel functionalities. Done for a major OEM, this product offered the features of transmitting the events generated over the PSTN network to pre-configured receivers.

Building Management System

Building management systems are required for managing buildings and large facilities. It is essential to monitor events such as fire, smoke, track water and sewage flow, air conditioning status etc. Of various sub-systems in BMS, alarm systems are primarily implemented to warn the occupants of hazardous events to safely evacuate themselves. This might be critical to lives as it is possible that the building can house thousands of occupants at a time. There is also a legal conformance angle to this, imposed by regulatory bodies of different countries for the safety of these lives. The alarm panel manufactured by our customers can connect to thousands of detectors and monitor them. In case of any alarms or events, it can immediately push it to the cloud via Ethernet/WiFi network, from where further action can be taken.

Our customer wanted to add a feature to their device such that the events can also be communicated over the Public Switched Telephony Network (PSTN) to receivers. Embien analyzed the present design and developed the product meeting the customer expectations.

PSTN Dialler

Though the IP is becoming the primary channel of communication, still the PSTN network-based communication is being used either as primary or as a secondary channel. There are dedicated receivers that can be placed connected to the PSTN in, say corporate head offices. When the remote site detects an event, it will dial into this receiver and transmit information such as the alarm codes. The receiver can either simply log them in case of non-critical events or take corrective actions in case of other cases, such as dispatching to emergency services.

Analyzing the existing design, Embien found that the SPI as the choice of interface between the fire panels and the dialler. The design is done with ARM-based MCU with a DAA (Direct Access Arrangement) chip. The dialler is designed with two phone line interfaces that can be selected using a relay arrangement. The advantage is that the dialler can choose one line over another based on line-busy or fault status. Few GPIO’s are used as interrupt source/receiver for triggering notifications between dialler and panel. The panel, powered by Linux, houses all the configurations such as the number to dial based on the event code.

PSTN Dailler

On the firmware front, RTOS was used as the underlying layer and designed in a modular approach grouped based on its logical functions. Different modules perform respective functionalities such as once for Analog front end operation, one for line supervision, and another for running higher-level protocols on top of the PSTN call. Finally, the control module manages the overall operation of the device and monitors inter-module communication. A limited amount of event buffering is also supported.

ContactID and SiA Protocols

As mentioned above, there are different classes of receiver each supporting different high-level protocols. The two most popular protocols used for such PSTN signaling are Contact ID and SiA protocols. Of these, Contact ID is the most popular protocol that uses the DTMF signals to send a string of numbers representing the account, area, and zone of the particular area along with the event code. The process of the Contact ID protocol-based communication is depicted in the following diagram.

Process of Contact-ID protocol

  • Dialing - Sending the dial tone to central station
  • Initiation - Central station gives the 100mSec of 1400Hz ,100mSec silence and 100 mSec of 2300Hz pure-tone.
  • Transmitter sends the Reporting event message with 50 mSec long.
  • After successful reception 800 mSec of 1400Hz Kiss-off signal is received from central station.

If the alarm system does not receive the kiss-off tone in time, the system retransmits the message.

The SIA protocol uses FSK format for data transmission and uses pulse dialing instead of tone dialing. The Frequency Shift Keying (FSK) modulation scheme is used to convert the digital data into two different kinds of frequency. Initially a 2225Hz SiA handshake is received from the receiver side. Then the account block and event blocks are sent via pulse dialing followed up finally by zero event block. After successful transmission, the dialler waits for acknowledgement.

The firmware supported both the Contact-ID and SiA protocols that can be selected by the panel based on the events and target receiver. Automatic line supervision and switching, internal event generation, and other conformance related requirements are met in the design. Necessary modifications are also done in the panel side firmware including driver development, web page-based configuration etc.

Thus, Embien could successfully design and develop the add-on provision for the customer helping them expand upon their product feature set without affecting their present design.

About Embien

Since inception, Embien has been helping its customers with industrial product developments and made a mark in it. Whether it be a partial firmware development, or complete system development, our engineers give their best and help customers achieve more than they wanted for. Our expertise includes Cyber-security enabled designs, HMI development on Linux, WinCE and Android, protocol stack development such as CANOpen, DeviceNet, Ethernet/IP, Modbus, Data Acquisition system development, Protocol translators, Building management systems, etc. Feel free to Get in touchwith us to make your next product to bridge older technology with newer ones.