Wireless networks and Radio Frequency Identification (RFID) have always been closely linked: One of Wi-Fi's first applications was transferring data from portable RFID readers. But the popularity of RFID has led some to envision whole networks of readers embedded throughout a building's infrastructure. Start-up Reva Systems has announced what it calls a Tag Acquisition Network (TAN), an array of RFID readers that work with passive, batteryless tags. Meanwhile, established networking vendors led by Cisco Systems are promoting Wi-Fi itself as a way to enable RFID-like applications.

Reva hasn't announced any products yet, but the basic TAN architecture it's promoting is identical to a switched Wi-Fi network: A central appliance controls and mitigates interference between many radios, which are analogous to access points. The difference is that whereas an access point can communicate with a client hundreds of feet away, passive RFID tag readers have a reliable range of only a few inches, so ubiquitous coverage is impossible. Even with the millions of radios needed to cover an entire room, the physical infrastructure holding them in place would prevent human occupation.

Instead, Reva is targeting smaller, more restricted areas. Its technology is currently being trialed by shops and warehouses, which embed readers into shelves and tags into the items sitting on them. When a customer picks up an item from a shelf, the system sends an XML message to the store's inventory database and can notify a staff member if the item is replaced incorrectly.

But RFID isn't just for stocktaking. Arrays of readers can also be placed around door and window frames to monitor the movement of objects or people into a room or building. Many factories and warehouses already use similar systems from vendors such as Symbol Technologies, but these are usually based on active, battery-powered RFID tags. The battery gives the signal a longer range, letting a single reader cover a doorway or a small room.

While most passive tags and readers now speak the same language, EPCglobal Gen2, the active variety is still proprietary. However, that may change, thanks to PanGo Networks and AeroScout, which make active tags that use Wi-Fi instead of transmitting on dedicated RFID frequencies. The advantage here is that RFID can use the same Wi-Fi network already in place for communications, provided that the access points include location-tracking capability. Most of the new enterprise-class Wi-Fi hardware do, though vendors vary in accuracy and in how the data is made available to external applications. There are also other trade-offs: Active systems increase the tags' cost from cents to dollars, and thickness from a millimeter to an inch.