The Evolution of Wireless Security Systems
From the 1980s through early 1990s, security systems for homes and commercial buildings were wired. They required sensors for each window or door that then were wired back to a control panel. Once wireless technology became more readily available, there grew a demand for wireless security systems as well. The manufacturers of these security systems began searching for viable alternatives to the traditional wired technology.
Early years – 433 MHz
In the 1990s, people started to experiment with using similar technology for security sensors as was used for garage door openers. Section 15.231 of the FCC rules allows alarm systems to transmit at a frequency of 433 MHz.
The 433 MHz system included a central receiver and wireless sensors that were transmit only. The sensors included pyrolytic, or heat sensing, standard door and window sensors as well as other specialized sensors. The receiver was plugged into AC power and was always on and “listening” 24/7. The sensors would wake and transmit a check-in message and go back to sleep and send actual alert messages when needed. Because there was no bidirectional communication, the receiver could not report any information back to the sensors or provide firmware updates.
Essentially, this system uses the same type of sensors as wired systems. The difference is in how the message is relayed from the sensors back to the receiver.
First evolution – 2.4 GHz
One issue with 433MHz sensors is that specific band is a US allocation. As security companies want to deploy the same product in other countries, they looked for a band that was available elsewhere and the 2.4GHz band met that need.
The primary issue with the 2.4 GHz is poor propagation through walls and objects in general, as well as general distance propagation. There are also significant issues more recently with wifi saturation causing interference and further lowering the useful range. All too often, the sensors’ messages back to the security base station experience interference from other wireless traffic.
2.4 GHz is still common. The main solution to the range issues is to put multiple receivers in the building. Those receivers can forward messages back to a central control panel via another wireless or wired link.
Many security system distributors stuck with the 433 MHz system because it was fairly cheap, worked reasonably well, and you knew what you were getting into with regard to sensor issues.
Continuing sensor issues
A significant problem with the sensors used in security systems is that there is no bi-directional communication. You can’t update firmware wirelessly or “ask” the sensors anything. The sensors are waking up periodically, reporting any changes, and then going back to sleep. There is no way to tell the sensor that the message was received.
Additionally, there is an expectation that, if the sensor is triggered, it will immediately wake up – and will continue to do so as long as it reasonably can. The sensors, however, have a short battery life. The sensors can typically send battery status information when they check in, but it’s very common for users to never get around to changing the batteries. This could potentially keep a sensor vulnerable for the hour or so until it is scheduled to report back in. It is also easy to jam the sensor’s communication frequency. Once a signal is jammed, an intruder could easily open a window or door and then disable the battery.
Another issue is the inability to coordinate sensor transmissions. Because of this, two sensors may try to “talk” to the receiver at the same time. As a result, multiple radio transmissions from multiple sensors may collide, causing a delay in spotting any potential breach issues. A well designed sensor addresses this with randomization of the transmissions, but it can still result in a delay before an alert can get through. This is particularly the case with an event that affects multiple sensors at the same time, such as a large set of windows all getting broken at the same time.
Current evolution – 900 MHz
Recent years have seen development of LoRa, Z-Wave, and custom protocols, which use a 900 MHz transmit frequency. With FCC 15.249, which covers low power transmissions in the ISM bands, higher transmit power is now available. This provides many benefits over the traditional 433 MHz system. Some systems can even use 15.247 rules which allow for much higher transmit power.
With a 900 MHz LoRa system, you can potentially have sensors with +20 DBM (equal to 100-250 milliwatts) of transmit power. The sensors work at much longer range, transmit well through walls, and have a more viable battery life. These protocols are already bidirectional, so these sensors can receive as well as transmit. This allows for firmware updates as well as “listen before talk” and acknowledge or non-acknowledge (ACK/NAK) functionality that helps to ensure messages are repeated until they get through. And even though this system uses a sensor receiver that uses more power, sensor messages get through in one transmission vs multiple ones, which makes it more power efficient because the whole sensor can sleep once the message is received.
It is important to note that the 900 MHz band is only available in North America. If you need a solution for more worldwide markets, customers may need to use the 2.4 GHz band even though it will be less effective indoors than an equivalent 900 MHz system.
This brings us to the current point of security system evolution, as well as the current challenge. With the 900 MHz systems, which are basically high-power radios with multiple receivers, you can’t just use the usual sensors from the usual suppliers. The 433 MHz sensors have the advantage of being inexpensive, which is a good thing since so many are needed to cover an entire structure. Instead, the 900 MHz systems require something more customized, which can be a daunting task for any individual security system manufacturer. Instead, security system companies often look to outside firms to supply the sensors in their systems. This can avoid having to create an entire line of sensors but it also limits the company’s control over the sensor IP and supply chain. A common solution to this is to create a single sensor product for use in a wide variety of applications and then de-populate the parts of the device that may not be needed.
If you want a high-quality, long-range, secure 900 MHz security system solution, turn to the experts at F3. We understand the firmware requirements and security details involved, how to get maximum battery life out of the sensors, and how to obtain optimal system performance for both bandwidth and range through proper antenna selection and implementation.