Wi-Fi ID: How it Works

A plethora of devices exist today that use 802.11 wireless networks, including laptops, PDAs, wireless industrial equipment, barcode scanners, and vehicle-mounted terminals. They connect to a network via a wireless access point (WAP) or wireless-enabled broadband modem. Wi-Fi ID allows these devices to be located, and tracked, without modification using multiple WAPs and software. For everything else you want to track, there are 802.11 Wi-Fi tags. Like other active RFID tags in Real Time Location Systems (RTLS), these devices can be attached to just about anything you want to track.

How does Wi-Fi ID work?

Wi-Fi ID is technically an active RFID system that uses the 802.11 standard of air communication in the 2.45GHz frequency spectrum. There are a few methods used to determine location: Radio Signal Strength Information (RSSI) and Time Difference Of Arrival (TDOA). RSSI is suited for both tight indoor environments as signal strength degrades the further it travels as well as outdoors when the proper network planning has been done. TDOA measures the time of arrival of the tag's signal from multiple readers at the same time and is better for outdoor or large open indoor environments where multiple readers can get clear line of sight the tags at the same time. The big difference is RSSI can be performed using the same off-the-shelf wireless access points you can buy at the store. TDOA requires a vendor specific receiver and Wi-FI ID tags that communicate specific timing information.

In the RSSI model, software determines the location of the device based on data being collected from multiple access points. There are two ways of measuring RSSI:
1) signal strength from the tag perspective or
2) signal strength from the access point perspective.

The first option tends to be more accurate because signals transmitted from the access point tend to be stronger and more consistent than those being sent from the tag. This also helps to reduce the impact of multi-path or reflected signals.

The term most often used to describe this process of transforming the raw data into location data is "triangulation". "Tri" referring to three because if the signal strength from at least three access points is known, then the intersection of the three circles around the access point can yield the location of the tag. The problem with this method is when a signal is blocked or reflected - position accuracy is seriously degraded or becomes impossible to determine.

The Ekahau system uses a statistical probability model that differs from pure triangulation in that it also factors in the environment (such as walls, doorways, etc.) when calculating the location of an object. The Ekahau software can determine the X, Y, Z coordinates, heading, and even speed. The average accuracy is up to 1-3 meters (approximately 3-10 feet). The quality of location data varies depending on how often the tag transmits, the number of WAPs as well as the environment. To simply know if a tag is in a given facility or roughly what area of a large facility a single WAP and a single transmission from the tag are enough.

For the extreme expert:
Ekahau's probabilistic positioning model applies Bayesian statistics using given observations, initial observations (made during the site calibration), and prior observations (where was it last observed). In reality, this approach provides far more accurate results than triangulation alone.

Of all the companies in this space, Ekahau is the only company that works with any standard 802.11 Wireless Access Points simply by installing software (even an inexpensive $30-$50 model from your local electronics retailer). Ekahau is the first company to announce support for the 802.11n protocol which can produce data rates of up to 300 Mbps - approximately six times higher than current 802.11a and g technology, and about 30 times higher than 802.11b. Other companies require hardware devices, commonly called a "location appliance", that provides the functionality of the software described above. The problem with the appliance approach is that it requires a company to replace all of their existing access points with new devices that may cost from $600 to a couple thousand dollars a piece. Not only is this expensive to install, but expensive to maintain. It is also a proprietary approach to a specific vendor solution and really only works in a tightly controlled and managed closed loop environment.

Beyond the instant supply chain solution described in the article, Introducing Instant Real Time Supply Chain, here's an overview of how an Ekahau RSSI-based system is used for a real-time location of assets. To start with, four or more 802.11 wireless access points are required. What's cool is that you can go to the local electronics retailer and buy off-the-shelf equipment with prices starting around $30 per access point. It's recommended that you set up at least four wireless access points in an asymmetrical fashion, with some APs at the corners and some in the middle. The application software then connected the network to each access point is connected to. You simply download a graphic image of the floor plan for the space you are going to configure into the Ekahau system (for a multifoor or campus you can load in multiple maps). If you are just interested in what facility a certain item is located the Ekahau RTLS can provide this level of location tabular format without the need to survey the facility or import the facility map. Using a laptop connected via an 802.11 network card, you walk around and measure the RSSI at various intervals through-out the area. Depending on the size of the area, this could take some time. When considering the alternatives of wiring a proprietary system with power and Ethernet, this is like a “walk in the park”. As you walk around the laptop is taking signals strengths measurements of the WAPs in the area. The system leverages the measurement data, number of WAPs and the environment itself (which causes the signal to vary) to achieve location accuracy. As new wireless network devices connect to the network, an icon graphically depicting the device location is displayed. The icon follows the item in real time. See image below.

Things to be aware of

Network Traffic and Network Null spots

Wi-Fi ID solutions involve two-way communication between the Wi-Fi device and access point. The majority of the time, the device is only sending a header message that consists of the MAC address and around 60 bytes of data. Since the tags are not all broadcasting at the same time, and are distributed across access points, network traffic is not a problem, even in production environments with 5,000 or more tags on the floor. The amount of data transmitted by 5000 tags equates to once sending a 2-page word document across the network.

Depending on how many access points are in your facility, there may be null spots, an area without adequate signal, in the area covered by the 802.11 network. This problem may be solved by adding more access points to fill in gaps. Adding the WAP will not only cover the hole for the Wi-Fi ID system but will benefit other users of the network such as laptop computers, RFID scanners, VoWIFI phones, etc.

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 wheelchair in a hospital. Ekahau publishes their tag life as being able to go up to five years without needing to change batteries. This is dependent how frequently the tag blinks and if it is set to blink on motion or simply at a timed interval. A timed interval of say 20 minutes will allow the batteries to last to the five year period. The blink method and rate can be dynamically varied on the tag through commands sent through the network allowing the end-user full control of the tags and it’s batteries.

Ekahau also has the T-301B, a credit card sized tag that is rechargeable (see picture below) :

Tag Costs

The average Wi-Fi ID tag costs around $35-$50. These price points make them best suited for higher value assets, such as shipping containers, hospital equipment, people, etc. The re-use of the tag obviously drives the cost per use down significantly to where if it’s being used to track inventory, the use cost is pennies per tracked item.

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

Other active RFID systems use different communication protocols and frequencies. There are many different types of active RFID systems. 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 that has established security procedures and you have and IT organization that is familiar with its operation and management. 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 hundreds of thousands of dollars in equipment cost and installation.

If you're considering an Asset Tracking system and you already have an 802.11 wireless network, you should include these solutions in your evaluation.

Using RTLS, there are five ways in which assets and people can be located over thousands of miles or down to a few inches:

• Two-way associated mode – the “norm” is a algorithm that enables a remote “smart” tag to have a conversation with the server so that relevant information about the tracked items such as x-y-z coordinates, battery life, and so forth can be communicated. Software updates and new instructions can be communicated without physical contact with the tag.
• Beaconing mode – in certain cases, it may be optimal for a tag just to blast its location to a listening server. This would happen at pre-set intervals.
• Presence detection – Perhaps all is needed is a simple “I’m here” as a tag checks in and out of geographically dispersed Wi-Fi networks. This enables RTLS coverage between cities and even countries as needed.
• Virtual zones – Creating chokepoints and portal coverage through software, this mode can detect when unauthorized entrance or exit is happening. No hardware needed.
• Audio/visual location – Since a tag has a MAC address, it can be addressed and identified, even in a crowded storage closet full of other identical units.