Last Call for Landlines
The Mockingbird Telephone Exchange emulates the switching hardware at a landline telephone exchange to simulate subscriber and trunk lines.
Once a fixture in nearly every home, landline telephones are rapidly disappearing. And with them, so is the supporting infrastructure — market research firm Gartner estimates that only about five percent of the once great landline infrastructure will still be in operation by 2030. Of course this has been decades in the making, with landline service availability continually shrinking over the years as cell phones replaced them in the market. Now this fading technology is finally about to take its last gasp.
Enthusiasts do not let old technologies die quite so easily, however. Whether due to nostalgia for a time past, or the opportunity to learn from a simpler, easier to understand technology, hardware hackers love to breathe new life into old tech. Steve Rodgers is numbered among this group of committed electronics enthusiasts, and recently decided to build a landline telephone exchange, because who doesn’t need their own landline infrastructure these days?
In particular, Rodgers built what he calls the Mockingbird Telephone Exchange. It is designed to mimic the Western Electric Number Five Crossbar Switching System, which has been in use in the public switched telephone network since the 1940s. This is not just old hardware that has been dusted off and repaired — Mockingbird is a modern day implementation that emulates the original hardware. As configured, the system supports eight subscriber lines and three trunk lines.
By way of background, a telephone switching system is the hardware that makes a physical connection between the caller and the receiver. A telephone exchange is equipped with these switches to service calls in a local area. Trunk lines are special connections that link local exchanges and enable long-distance communication between them.
To reproduce the functions of an exchange, Rodgers designed and built a set of four custom dual line cards, with each card supporting two subscriber lines. Additionally, a set of three trunk cards were designed to facilitate communication between the line cards, as if they were geographically separated. Both the line and trunk cards are equipped with STM32F103C8T6 microcontrollers. The exchange is controlled by the STM32F767ZI Nucleo development board, which was programmed with the STM32CubeMX development environment in Eclipse. This development board communicates with the line and trunk cards designed by Rodgers.
The Mockingbird Telephone Exchange also has a custom tone plant and DTMF decoder card. The tone plant generates familiar sounds like the dial tone and busy signal. The DTMF decoder is used to detect and decode the tones generated by pressing the keys on a phone’s keypad.
Source code has been released on GitHub, and Rodgers notes that more information is coming on the custom boards that were designed for the project. So, if you want to experiment with landline phone technologies for fun or education, be sure to check out the project write-up. Also take a look at the video which shows calls being placed through the Mockingbird Telephone Exchange.
