RTLS advances RFID to the next level

Industrial Embedded Systems — September 19, 2008

6One of RFID's cousins is Real-Time Locating Systems (RTLS), and although the two technologies are related, they have different capabilities. Judson explores the benefits of using RTLS to track asset movement over long distances.

For simplicity's sake, RTLS is often classified as an active RFID technology. Sure, the basic definition fits. Both systems use battery-powered tags to track asset movement. But this designation minimizes the benefits of RTLS systems, especially those that operate over Wi-Fi networks.

Wi-Fi-based RTLS offers numerous advantages over traditional active RFID systems. The primary difference is that Wi-Fi-based RTLS provides accurate automatic tracking using any existing 802.11 infrastructure that businesses deploy for voice and data communication.

RTLS can be deployed in unique ways over Wi-Fi networks, even those located in geographically dispersed areas, enabling location tracking applications on a campus, across the country, and even around the world.

Two-way communication: Wi-Fi roaming

The classic RTLS application is automatically tracking an object or person on the move, typically indoors. Examples where RTLS tracking is commonly used include an intravenous pump being pushed around a hospital, a forklift moving through a warehouse, or a giant scoop going back and forth along mile-deep mine shafts.

A Wi-Fi tag is attached to every asset in these applications. The Wi-Fi tags are smart two-way communication devices much like laptops or PDAs that associate with a Wi-Fi network. Once activated on a tracked asset, the tag reports its location to a central location engine using the received signal strength indication of access points within the Wi-Fi network. The system then estimates the x, y, and z coordinates of tagged assets within a Wi-Fi environment. Figure 1 illustrates this associated roaming method of tracking across a Wi-Fi network.

Figure 1

Because these tags are capable of two-way communication, they can send data to the engine and receive software updates and new instructions, as well as change the timing and frequency of when to blink.

Despite the two-way communication capability, packet traffic from tags never exceeds more than about a few bytes on a multimegabit network. So the traffic generated by the tags is basically unnoticed, yet the tags can function as essential components capable of supporting intelligent programming and updates.

One-way communication: beaconing mode

If a system can handle intelligent two-way communication, why would anyone want a simplistic, one-way beaconing mode tag? In some instances, this type of application makes sense.

While two-way communication is essential for sophisticated location applications, certain uses require that the tag simply broadcast the location of the item it is tracking. This one-way beaconing method of location (Figure 2) occurs when the tag periodically sends its signal, which is collected by the Wi-Fi access points and delivered to the location server via the network controller. The tag has no knowledge of its host and cannot learn or be managed by the network.

Figure 2

Because RF capture takes place at the access point and not the tag, the captured data does not represent an accurate view of the RF environment, allowing accuracy to be compromised. Also, the speed of providing the location information is limited by the network controller's ability to collect data from the access points and feed it to the location engine. Thus, locating mobile objects (and especially people) in real time is not very practical using this method.

One-way beaconing is used in places where pinpoint location accuracy is not paramount or where intelligence is not needed in the tag. Because beaconing doesn't require authentication with the host Wi-Fi network, it consumes less power and allows the tag battery to last longer.

The downside of the beaconing mode is that one-way communication and access point capture are not included in the IEEE 802.11 standard. As a result, this method requires proprietary integration to each Wi-Fi network vendor's infrastructure, and each vendor must have its own version of this setup.

Some systems can support both methods, combining the best elements of each to provide a solution that has proprietary components but can still adhere to global standards so that the RTLS investment is well protected into the future.

Creating virtual portals

Establishing choke points or detection zones has traditionally required hardware installation to assure that all tagged items entering or leaving a designated area can be recorded.

Today's modern RTLS systems come with powerful software tools that create virtual zones or logical areas without requiring users to run cables or hang readers. RTLS software enables users to draw boundaries of the designated areas on a digital map and label each area for its specific restrictions. The system can then detect if any of the RTLS tags or clients are entering or leaving these zones and determine if those tagged objects or persons are authorized to be in that area.

The software enables an RFID type of location detection commonly referred to as a virtual portal, as shown in Figure 3. For example, by defining zones on either side of a door, the location server can identify any tagged asset or person that entered or exited the door. By defining two of these areas, one inside and one outside the door, the software can detect the direction in which the object is moving and ascertain if it is leaving or entering the area. Business rules can then sound alarms, close doors, alert security, and begin other event-driven actions.

Figure 3

Used in combination with highly accurate location algorithms, this method eliminates the need to use a physical RF portal or choke point, which would otherwise necessitate hardware installation, increase system cost, and require continued maintenance. What a virtual portal can do in a few minutes would take several days and thousands of dollars using more traditional methods.

Software-based portals also eliminate the 125 KHz electromagnetic interference issue, which is endemic in certain devices that can potentially create risks to hospital patients.

Presence detection

Another innovative function facilitated by a Wi-Fi infrastructure is a tag's ability to communicate with a series of networks as it moves from facility to facility, regardless of geographic distance to the central server.

Many believe RTLS tagging is limited to covering a single Wi-Fi network, but this is not the case. A relatively simple application that has recently emerged enables supply chain partners to find out if items have arrived and at which facility they are located.

The Wi-Fi RTLS system achieves this open presence supply chain application by providing location roaming across separate Wi-Fi networks. Using existing Wi-Fi networks in geographically dispersed facilities, a user can track the whereabouts of a tagged item on a truck traveling across a large supply chain covering thousands of miles and even spanning business partners foreign locations. Determining the tagged item's exact location within a facility is not expected in this scenario, just whether or not it is on-site.

The onboard tag signs on to the local Wi-Fi network when it reaches each destination much like a laptop connects to a Wi-Fi network at an office or home. Of course, the tag must have permission to log into the network, but it accomplishes this easily through normal authentication protocols.

From location to location, the tag talks to the home server/centralized engine in very small snippets where data is collected and reported. This capability validates and time-stamps the item's movement along a dispersed supply chain, enabling the system to report the presence of objects from point to point along the route, as illustrated in Figure 4.

Figure 4

As long as the tag has the correct access credentials, it can roam to another network and "phone home" from a remote shipping or receiving dock. This capability does not require users to integrate or install proprietary hardware or software at each site, but rather leverages standard off-the-shelf Wi-Fi access points at any given location, regardless of the equipment's brand or generation.

Simplified but still accurate

By attaching a Wi-Fi tag such as Ekahau's T301-A or T301-B pictured in Figure 5, users can easily locate a person or mobile object traveling over long distances. Wi-Fi-based RTLS thus provides better accuracy than active RFID without the complications of requiring cabling, readers, exciters, and other infrastructure. More importantly, Wi-Fi-based RTLS has a broad range of uses, offering open presence applications for location tracking across the supply chain, a capability impossible to achieve using a typical active RFID system. The wide deployment of Wi-Fi networks enables this versatility at a fraction of the cost of conventional systems.

Figure 5

Judson Vaughn is marketing manager for Ekahau, Inc., an RTLS provider based in Reston, Virginia. Judson started his career as a journalist and has spent most of his work life in telecom marketing with companies including Alcatel, MCI, and Sprint. He also was principal in his own marketing agency for a decade.