Wireless Network

Location Tracking using Wi-Fi based RFID Tags


This article gives an introduction to the Wi-Fi based RFID Tags, how the location of assets can be identified by the combination of Enterprise Wi-Fi and RFID technologies, what can be accomplished by such a system, advantages and limitations of Wi-Fi based RFID Location tracking technology.

Tracking Active Wi-Fi Devices: Location Tracking is not a new technology. Certain Wireless LAN vendors have Location engine built in to their Wi-Fi solution or have a separate dedicated device to perform the function of a Location engine. This used to track active Wi-Fi devices in the organization (with an accuracy of 3 to 5 meters) and report it in the floor plans that were previously integrated with the Wireless Controller. Some Wi-Fi solutions can also indicate a live coverage of RF signal levels in the organization. Here, the Wireless controller decides the probable location of the active Wi-Fi device by measuring the time taken for the signals to reach the Wi-Fi device from at least three neighbouring Access Points, whose location is already known. This is used to locate Laptops, Voice Over Wireless LAN handsets and other active Wi-Fi based devices in the network. Of course, the laptops need to be ‘On’ for tracking them.

Tracking Non Wi-Fi devices through RFID and Wi-Fi Network: But Active Radio Frequency Identification Tags (RFID Tags) that comply with 802.11 WLAN standard are currently used with Real Time Location Systems (RTLS Engines) to monitor the location of Non Wi-FI based devices like crates, vehicles etc. The RFID Tags come with a in-built battery and they keep sending 802.11 WLAN based information periodically to the access points, which communicate the same to the Location Engine(Which is generally a server with RTLS software) over the Wi-Fi / IP Network to locate moving and stationary objects within a warehouse that have been tagged with the Wi-Fi RFID Tags, for example.

Since, these messages are being transmitted periodically to Access Points, the WLAN infrastructure needs to identify them to pass them on to the RTLS Engines. So, the RTLS vendors work with the WLAN vendors at the firmware level to identify and integrate their Wi-Fi network/ Access Points with Wi-Fi based RFID Tags.

These Wi-Fi based RFID Tags are battery operated devices which may be an inch long, and their battery life is expected to be around 4 years. The Tags are optimised not to send information to the Access Points when the objects to which they are attached to are stationary, to save battery power. Optional temperature/ heat sensors can be integrated to these Tags to make them communicate specific information to the RTLS Engine based on real time temperature/heat parameters. These tags can have an accuracy of 3-5 meters when determining the location and a large number of tags can communicate simultaneously with a single access point without choking the Wi-Fi network. These tags are generally capable of bi-directional data exchange.

There could also be certain choke points, like the entry and exit gate of the main door of a warehouse, which can be connected with readers such that when objects containing these tags pass through them, the tags automatically intimate the Wi-Fi infrastructure and hence the RTLS engine about the movement of goods outside the warehouse, which can inturn integrate with an alarm system to trigger an alarm.

Advantages and Dis-advantages: The advantages are obvious: Security automation, monitoring and location tracking for high cost devices that don’t have built in Wi-Fi sensors, reduce costs by using the existing 802.11 based WLAN infrastructure for communication instead of having an RFID reader network (with its own frequencies) which can be very costly to set up over a huge area. The disadvantages include the high cost of active RFID Tags and changing batteries after a certain number of years for all the tags.

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