Push the Panic Button

Aug. 2, 2012
A look at three wireless options for mobile duress system transmission

Workplace violence can strike anywhere, at any time. Roughly two million Americans are victims of workplace violence every year; in fact, according to a 2006 U.S. Department of Labor Statistics Survey, nearly five percent of the 7.1 million private industry business establishments in the United States had an incident of workplace violence within the previous 12 months. Worse, these represent only the reported incidents; some studies say only one in five is reported.

While it is impossible to entirely eliminate violence in the workplace, there are measures that can greatly reduce the risk of events occurring, or reduce the risk of events escalating to undesirable outcomes.

After surveying the range of security options available to the healthcare industry — one of those hardest hit by the epidemic of workplace violence — the Institute for Emergency Nursing Research, in a 2011 study, concluded that panic button/silent alarm systems lowered the odds of physical violence.

To be effective, a mobile panic button system must provide reliable operation in demanding environments, must provide alerts immediately and must provide accurate location information and coverage — not only in visible and high-traffic areas, but also in stairwells, parking garages, underground walkways and outparcels, since these are often locations where incidents occur.

The performance of a mobile panic button system, also called enterprise mobile duress, is dictated by its underlying technology. This article will compare three of the most popular mobile panic button wireless technologies: IEEE 802.11 WLAN (Wi-Fi); IEEE 802.15.4 ZigBee; and proprietary, repeater-based, 900MHz Frequency Hopping Spread Spectrum. Each has pros and cons that must be carefully weighed.

What it is: Wi-Fi is the term used for high-speed wireless local area networks (WLAN) based on IEEE 802.11 specifications. This well-established, file-transfer specified technology is capable of moving large amounts of voice and data over a moderate distance — usually measured in hundreds of feet — making it a fit for mobile computing applications. Airports, hotels, restaurants, and entire communities now offer public access to the Internet using Wi-Fi.

The advantages: The main attraction to using Wi-Fi for a mobile panic button is the appeal of leveraging an existing infrastructure and the apparent versatility of the generic Wi-Fi network.
One such versatility is when a Wi-Fi-based real-time location system (RTLS) for tracking assets, monitoring patient flow, and staff services is used in conjunction with panic buttons for duress.

Establishing location: In the RTLS application, location is established by measuring the signal strength of a transmitting tag as received by multiple Wi-Fi access points to estimate the distance the tag is from those access points. A simple trilateration calculation is performed to determine location. In many instances, just knowing which access point the tag is located closest to is sufficient.

The drawbacks: For RTLS systems that use Wi-Fi as a sort of “carrier signal,” the relatively short 2.4 GHz waveform is easily interrupted by moving assets — carts, beds, equipment, etc. — and especially human bodies. In a dynamic commercial environment, the result is unpredictable signal loss, making meaningful determination of location difficult. We have all felt this effect with our own laptops, and it can be resolved by simply repositioning, sometimes as little as a few feet.

When tracking assets or people for the purpose of workflow management, Wi-Fi RTLS is preferred. For the purposes of a mobile panic button application, however, where a staff member in danger cannot afford to be outside coverage,even for a brief time period, Wi-Fi or Wi-Fi-based RTLS has its challenges.

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.

The advantages: Because a life safety network does not need the high voice and data rate of Wi-Fi, most opt for a 900 MHz network designed to carry a moderate amount of data, while providing superior penetration and propagation abilities. The repeater-based 900 MHz technology is also ideal for indoor or outdoor location needs because of the repeater separation, system scalability and coverage zone.

Since the 900 MHz physical wavelength is longer than 2.4 GHz, the repeater infrastructure is significantly less than what would be required to convert a voice and data based 2.4 GHz network into a life safety equivalent.

Proprietary technologies are also, as a general rule, more secure than non-proprietary technologies. Because they are not accessible by the general public via off-the-shelf tools used to disable or interfere, either intentionally or not, these networks are much less susceptible to such interruptions and attacks.

The drawbacks: Like Wi-Fi and ZigBee, 900 MHz is a frequency range allocated by the Federal Communications Commission (FCC); however, most 900 MHz systems are proprietary, which could be considered a negative characteristic by some. Another potential drawback is that 900 MHz is a U.S.-only standard. Wi-Fi and ZigBee are based on frequencies that have been adopted internationally for public use.

Mark Jarman is president of Inovonics. He has served in the security and wireless industry for more than two decades. Email him at [email protected].

About the Author

Mark Jarman

Mark Jarman is President of Inovonics (www.inovonics.com). He has served in the security and wireless industry for more than two decades. He can be reached at [email protected].