‘Mission possible’: A low-cost alarm system for public safety 800MHz communications
Placing remote site alarms at a location that is continuously monitored can be accomplished without spending a great deal of money, adding bandwidth or increasing space requirements in the dispatch center.
The assignment is the needs assessment, preliminary design, and project management of a large public safety communications project. The system is a state-of-the-art 800MHz trunked simulcast network consisting of four sites that are interconnected with digital microwave. Additionally, a four-site, two-channel, conventional mobile data system shares the same locations as the trunked voice system.
The challenges The first hurdle that I cleared was getting remote site alarms to a location that was continuously monitored without spending a great deal of money. After checking with the popular alarm system suppliers, I found they were all relatively expensive and required a certain amount of bandwidth to operate. These suppliers offered simple analog systems that would activate lights and sound audible alarms. There were also sophisticated software driven arrangements that would report to a master computer, typically a PC. The computer would then generate an impressive multicolor display that a somewhat knowledgeable person would have to translate into action of some sort. Fine and dandy, but the obvious problems were: * The alarm system would be an additional cost that was not in the budget. * The alarm system would take up additional bandwidth that was not originally allocated for that purpose. * Some of the alarm systems were not truly user-friendly. * The display would be located on an already overcrowded dispatch su-pervisor’s work location (CAD display, SIMS II display, CRT trunked radio dispatch display, and telephone panel).
Because the microwave was an all-digital system, I decided to try using the existing signaling capabilities of the network. The trunked simulcast radio system was supported by special channel modems (DSMs), and standard digital PCM channel banks supported the non-simulcast audio functions of the network.
Close examination revealed that the DSMs were fully used, in that even the “E” and “M” leads were used. The “E” and “M” leads provided a method of remotely switching a function from one end of the microwave link to the other. For example, a change of state of the “M” lead at one end of the link would cause a corresponding change of state of the “E” lead at the other end of the link. The good news was that the standard PCM channel banks did not use the “E” and “M” switching leads, and they were available for other duties.
Now, the remote site alarms that were of interest to me were: * open door – intrusion * high temperature * smoke detector * ac power failure * generator run * waveguide pressure * tower lights * battery charger failure * low temperature
The solution Each of these alarms could be configured to cause a form “C” contact closure upon detection of any of the above conditions. This was good! I could use the contact closure to cause activation of the desired “M” lead at the remote site. This was the beginning of the solution as I could now use existing “E” and “M” signaling to send remote site alarms without using up any additional channel bandwidth.
With this information now available at the microwave “hub,” the next step was to display it at a location where a human could benefit from it. The reverse process was applied at the near end. For example, the door alarm contact closure at the remote end causes the “E” lead at the hub (near end) to change states (another switch closure). This was exactly what I was looking for. But now, what could we do with the contact closure? We could use it to light a light, sound a buzzer or maybe do something a little more informative that the lay person could use immediately.
When the new simulcast trunked system was installed, the existing console electronics bank was upgraded from conventional operation to trunked operation. This process left a lot of auxiliary input/output (AUX I/O) cards that were no longer in use. These cards would take a switch closure input and cause something to happen on the pre-programmed dispatch consoles. Now things were starting to get interesting! I could take a contact closure at a remote site and ultimately cause something to happen on a continuously monitored dispatch console.
The trunked system installation also included display-type consoles (expensive PCs). By having a Motorola field engineer program the consoles properly, we could now have the remote switch closure (door alarm, for example) cause the console to beep and display “door open” at “site name.” We even had the choice of a “momentary” or “latched” alarm. If we programmed the “door open” alarm to be a “latched” alarm, it would cause the console to stay in alarm even after the door was closed. In this case, the console operator would have to “clear” the door alarm manually. This is desirable because a momentary door open alarm might be missed by a busy dispatch supervisor, allowing unauthorized personnel to quickly enter the site without detection. I also made the “ac power” alarm a “latched” alarm so that I could display and verify momentary power outages and fluctuations. All other alarms were designed to clear automatically when the alarm condition was corrected. To avoid additional work load on the busy dispatchers, only the dispatch supervisor consoles were equipped with the remote site alarm functions.
Successful operation This system has been in place for more than two years, and it works well. The remote site alarm system design is simple, and it addresses the original four challenges: * The cost was minimal. It required some technician time to wire in the alarm sensors and to make the connections from the channel banks to the central electronics bank that operates the consoles. It also required some of the Motorola field engineer’s time to program the central electronics bank so that the appropriate consoles would display the desired messages. * No additional bandwidth was required, such as using microwave channels specifically for this purpose. The “E” and “M” signaling was unused, so it was virtually “free” bandwidth. * The display is simple and user-friendly. We have supplied a chart to each dispatch supervisor that prioritizes each alarm and recommends the course of action to be taken. * The display took no additional space at the already overcrowded dispatch supervisor’s work location. It was integrated into the existing dispatch screen.
As an added bonus, a failed microwave link is immediately noted, as all alarms start flashing at once. Of course, there are other indications as well, such as, no radio coverage in the area around the affected microwave site. (No shortage of people to report this problem!)
The only down side is that the microwave radio alarms are not available in this user-friendly and efficient format. Those alarms are “networked” so that they appear on the display of all microwave terminals (or repeaters) at each site. We still have to go to the microwave radio (typically at the “hub”) to check for alarms. I will let you know when we figure out an inexpensive solution to that challenge.
Tedrick is president of CCI Telecom, El Paso TX, a privately held corporation, which provides telecommunications engineering and project management services. He is a member of the Radio Club of America.