Real time location (RTLS) with Wi-ue to its speed of operation, RFID technology can be used to create solutions known as Real-Time Location Systems (RTLS). RTLS are typically based upon the use of battery-powered RFID tags; they use multiple readers to 'triangulate' the position of a tag.

Where it gets interesting is at the convergence of RTLS with other wireless technologies, such as the 802.11 wireless network (Wi-Fi), or with a Global Positioning System (GPS).

Wi-Fi ID

Over the past few years, laptops have become ever-present at work, coffeehouses and even on forklifts where they can communicate with back-end systems in real time. Wireless networks enable this connectivity, and 802.11 networks are a standard in office buildings, manufacturing plants, distribution centers, and commercial environments.

As the 802.11 standards have evolved, the quantity of data that can be carried has vastly increased, and mass adoption has resulted in inexpensive equipment, which allows more access points and results in improved quality.

A plethora of devices exist today that use 802.11 wireless networks, including PDAs, laptops, wireless industrial equipment, and barcode scanners. Using Wi-Fi ID means any one of these devices can be located without modification. For everything else you want to track, there are 802.11 Wi-Fi tags. Like other active RFID tags, these devices can be attached to just about anything you want to track. They are powered either by rechargeable or disposable batteries. Some of these tags have motion sensors and others can interface with external devices.

Asset tracking, including people locating, is an application of Wi-FI ID systems. In industrial locations you could track forklifts and other materials handling equipment, as well as machinery and parts inventory; yard management for tractor-trailers and shipping containers. In healthcare: portable patient monitoring equipment as well as patients and caregivers. Hospitals are rapidly adopting Wi-Fi ID because many already have sophisticated 802.11 networks in place for data and voice. For security applications: visitor / personnel tracking and asset monitoring. There are Wi-Fi tags that are triggered when an internal motion sensor is activated. Alerts may be generated based on location, motion, or if a non-authorized 802.11 device enters an area.

How does Wi-Fi ID work?

WiFi ID is technically an active RFID system that uses the 802.11 standard of air communication in the 2.45GHz frequency spectrum. There are two methods used to determine location: Radio Signal Strength Information (RSSI) and Time Difference Of Arrival (TDOA). RSSI is best suited for tight indoor environments as signal strength degrades the further it travels. TDOA measures the time of arrival of the tag's signal and is better for outdoor or large open indoor environments. The big difference is RSSI can be performed using the same off-the-shelf wireless access points you can buy at the store and existing 802.11 network cards. TDOA requires a specific receiver and Wi-FI ID tags that communicate additional timing information.

Software determines the location of the device based on data being collected from multiple access points. The term most often used to describe this process is "triangulation". The software determines the X, Y, Z coordinates, heading, and even speed. The average accuracy is up to 1 meter (3.5 feet). The quality of location data varies depending on how often the tag transmits.

Things to be aware of

Network Traffic and Network Null spots

Battery life

Like all active tags, the life of the battery depends on the transmission interval. For a real time tracking where the tag is always moving, tag battery life will be shorter than applications where the tags only transmit when they are moved, such as a security system in a museum. One company publishes their tag life as follows: transmit every 5 minutes, up to 4 years; transmit every 30 seconds, over 2 years; transmit every 5 seconds, just over 4 months. There are some tags that can draw power from the item it is tagging, so the tag battery never has to be recharged.

Keep in mind, if you want to track laptops or other wireless devices using the built-in network card, they must be powered on, and the network card must be active. Otherwise, a Wi-Fi ID tag must be affixed to the device.

Tag Costs

The price of the average Wi-Fi ID tag is between $50 and $65 each depending on tag features. One announcement recently listed new tags as low as $40. This price points makes them best suited for high value assets, such as shipping containers, hospital equipment, people, etc.

What about active RFID systems that don't use 802.11?

Other active RFID systems use different communication protocols and frequencies. There are too many different types of active RFID systems for me to include a detailed comparison in this column. Generally speaking, active RFID systems utilizing lower frequencies than Wi-FI ID have further read range (especially in damp environments). There are also active RFID systems that encrypt their communication for security purposes. Where Wi-FI ID really shines is if you already have an existing 802.11 network infrastructure. Instead of adding a new infrastructure of active RFID readers, you can start with the access points you already have in place. This could save tens of thousands of dollars in equipment cost and installation.