Introduction

RFID (Radio Rrequency IDentification) is a technology that incorporates the use of electromagnetic or electrostatic coupling in the radio frequency (RF) portion of the electromagnetic spectrum to uniquely identify an object, animal, or person. RFID is coming into increasing use in industry as an alternative to the bar code. The advantage of RFID is that it does not require direct contact or line-of-sight scanning. An RFID system consists of three components: an antenna and transceiver (often combined into one reader) and a transponder (the tag). The antenna uses radio frequency waves to transmit a signal that activates the transponder. When activated, the tag transmits data back to the antenna. The data is used to notify a programmable logic controller that an action should occur. The action could be as simple as raising an access gate or as complicated as interfacing with a database to carry out a monetary transaction. Low-frequency RFID systems (30 KHz to 500 KHz) have short transmission ranges (generally less than six feet). High-frequency RFID systems (850 MHz to 950 MHz and 2.4 GHz to 2.5 GHz) offer longer transmission ranges (more than 90 feet).

A typical RFID system is shown in Figure 18. It consists of a tag, a reader, and some sort of data processing equipment, such as a computer. The reader sends a request for identification information to the tag. The tag responds with the respective information, which the reader then forwards to the data processing device. The tag and reader communicate with one another over an RF channel.

RFID systems can be classified according to the frequency band in which they operate – either high, intermediate, or low. RFID systems can further be broken down according to some characteristics of the tags being used, namely, whether the tags are active or passive. And, taking the classification process yet another step further, tags, regardless of whether they are active or passive, can be either read-only, write-once-read-many (WORM), or read-write.

RF Tag Types

There are two types of tags (or transponders): active, and passive.
Active tags have an on-board power source, such as a battery. The advantages of active tags are that they reduce the power requirements of the reader, can transmit their information over relatively far distances, and offer better noise immunity and higher data rates when used at high frequencies. The disadvantages are that they have a limited operating life, can only be used in certain environments, and are bigger and more expensive than passive devices since they require battery.
Passive RFID tags have no internal power supply. The minute electrical current induced in the antenna by the incoming radio frequency signal provides just enough power for the tag to transmit a response. Due to limited power and cost, the response of a passive RFID tag is brief — typically just an ID number (GUID). They are smaller, lighter, and less expensive than active tags. Furthermore, passive tags have a virtually unlimited lifetime. Some disadvantages of passive tags are that they have a shorter read range than active tags, and require a much higher-powered reader. Lack of an onboard power supply means that the device can be quite small: commercially available products exist that can be embedded under the skin. As of 2005, the smallest such devices commercially available measured 0.4 mm × 0.4 mm, which is thinner than a sheet of paper; such devices are practically invisible. Passive tags have practical read distances ranging from about 10 mm up to about 6 metres.
Both active and passive tags can be either read-only, write-once-read-many (WORM), or read-write. Of these, read-write tags are typically the most expensive, while read-only tags are generally the least expensive.