The RFID system architecture consists of a reader and a tag (also known as a labelor chip). The reader queries the tag, obtains information, and then takes action based on that information. That action may display a number on a hand held device, or it may pass information on to a POS system, an inventory database, or relay it to a backend payment system thousands of miles away.
Let's look at some of the basic components of a typical RFID system.
RFID units are in a class of radio devices known as transponders. A transponder is a combination transmitter and receiver, which is designed to receive a specific radio signal and automatically transmit a reply.
In its simplest implementation, the transponder listens for a radio beacon, and sends a beacon of its own as a reply. More complicated systems may transmit a single letter or digit back to the source, or send multiple strings of letters and numbers. Finally, advanced systems may do a calculation or verification process and include encrypted radio transmissions to prevent eavesdroppers from obtaining the information being transmitted.
Transponders used in RFID are commonly called tags, chips, or labels, which are fairly interchangeable, although "chip" implies a smaller unit, and "tag" is used for larger devices. The designator label is mainly used for the labels that contain an RFID device. (The term "tag" is used for the purposes of this book.)
As a general rule, an RFID tag contains the following items:
• Encoding/decoding circuitry
• Power supply
• Communications control
Tags fall into two categories: active
(see Figure 24.4).
126.96.36.199 Passive vs. Active Tags
Passive RFID tags do not contain a battery or other power source; therefore, they must wait for a signal from a reader. The tag contains a resonant circuit capable of absorbing power from the reader's antenna. Obtaining power from the reader device is done using an electromagnetic property known as the Near Field. As the name implies, the device must be relatively near the reader in order to work. The Near Field briefly supplies enough power to the tag so that it can send a response.
In order for passive tags to work, the antenna and the tag must be in close proximity to the reader, because the tags do not have an internal power source, and derive their power to transmit from coupling to the Near Field of the antenna. The Near Field takes advantage of electromagnetic properties and generates a small, short-lived electrical pulse with the passive tag that can power a tag long enough for it to respond.
Figure 24.4: Passive and active tag processes
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The Near Field is a phenomenon that occurs in a radio transmission, where the magnetic portion of the electromagnetic field is strong enough to induce an electrical field in a coil. As the name implies, the Near Field occurs in an area near to the antenna. Just how big the Near Field is, depends on the wavelength of the radio signal being used.
r = λ/2π
where λ is the wavelength. For example, a common RFID frequency is 13.56 MHz and the wavelength of 13.56 MHz is approximately 22 meters. Therefore:
22/2π = 22/6.28 = 3.5 meters.
The Near Field for an RFID device operating at 13.56 MHz is 3.5 meters or 11.5 feet. Passive tags requiring the Near Field have to be within that area in order to operate correctly.
The alternative to a passive tag is an active tag. Active tags have their own power source, usually an internal battery. Since they contain a battery to power the radio circuitry, they can actively transmit and receive on their own, without having to be powered by the Near Field of the reader's antenna. Because they do not have to rely on being powered by the reader, they are not limited to operating within the Near Field. They can be interrogated and respond at further distances away from the reader, which means that active tags (at a minimum) are able to transmit and receive over longer distances.
Semi-passive tags have a battery to power the memory circuitry, but rely on the Near Field to power the radio circuits during the receiving and sending of data.