To establish acceptable location accuracy, additional wireless access points — or even an auxiliary technology such as infrared (IR) transmitters or RFID — are added to offset the limitations of a Wi-Fi-only infrastructure. While a dual-technology solution can better ensure accurate location of a duress event, the added infrastructure usually nullifies the financial advantages of using an existing Wi-Fi network.
Another concern with 2.4 GHz is that in many cases, the signal will not penetrate through and around building materials and structures to provide complete coverage over campus-sized distances without a significant build-out of the network. The inability of 2.4 GHz to cope with obstacles leads to null spots in coverage and brief outages — which are not necessarily a problem for most of the functions performed by RTLS, since it is easy to report an asset missing due to loss of signal. However, it is much harder to locate something or someone no longer “in range.”
What it is: ZigBee was created to provide an economical, standards-based wireless networking solution that supports low data rates and low power consumption. ZigBee systems use a mesh network to send small data packets through a series of nodes, where each node of the network repeats the messages from its neighbor until the message reaches the head-end. As this is a relatively new and still evolving technology, only a few ZigBee or ZigBee-based RTLS mobile duress applications have hit the market.
The advantages: A ZigBee RTLS system can be installed easily and quickly, and, because of the interoperability of the IEEE 802.15 open standard and the nature of the ZigBee mesh network, the same system that provides mobile panic buttons can also perform other functions, including temperature monitoring and building automation.
ZigBee systems are scalable, and, because the devices are low-power mesh network nodes, it is usually far cheaper to build out a complete system using ZigBee than it is using Wi-Fi. Zigbee, like Wi-Fi, can be ideal for asset and patient tracking, temperature monitoring and building automation.
The drawbacks: Like Wi-Fi, most ZigBee-based RTLS systems operate on the 2.4 GHz frequency, presenting the same concerns about signal propagation, and the low-power nature of the ZigBee further restricts the 2.4 GHz frequency band. ZigBee offsets this by deploying a staggering number of devices — on large campuses, this can mean tens of thousands of nodes, with every wireless access point necessitating a dedicated wall outlet. Even with the low cost of each individual node, this can come with a sizable price tag.
These large networks also have a serious impact on system latency, which is the lag time between a wireless message’s initial transmission and its final receipt. A ZigBee mesh network consists entirely of transceivers, each of which is responsible for retransmitting each of the other transceiver’s messages. This means that every time a message is transmitted, each transceiver in the network must wake up, listen for the transmitted message, and resend it, which can have a significant impact on latency. Moreover, though Wi-Fi and ZigBee operate using different standards, they both operate on the same frequency band, and increases in Wi-Fi traffic will increase ZigBee system latency even further.
Both latency and interference immunity must be seriously considered for panic button applications, where response time is evaluated in seconds and coverage is extremely important.
Proprietary Frequency Hopping Spread Spectrum Repeater-Based 900MHz
What it is: While Wi-Fi and ZigBee are the most popular, there also exist a number of proprietary wireless technologies designed specifically for life safety applications, including mobile duress. In these cases, the wireless technology has been carefully balanced to ensure the best possible mix for life safety.
A frequency-hopping spread-spectrum technology will send redundant messages across multiple channels to avoid electronic interference and the effects of physical obstacles. A repeater, or a series of repeaters, is the backbone that ensures multiple transmission paths to the receiver — similar to the mesh architecture of ZigBee, but with much greater distance between nodes.