Radio frequency Identification (RFID) as the name implies uses radio frequency to exchange data between two entities for identification purpose. It is a wireless technology to collect information without any human interventions.
An RFID system is basically an integrated combination of various components which work together for detection and identification of objects or persons. These are the components which are primarily responsible for working of any RFID system whether basic or complex. Although there can always be additional components associated with RFID systems like sensors etc. but the following are amongst the key components of these systems:
- A tag (sometimes called a transponder), which is composed of a semiconductor chip, an antenna, and sometimes a battery.
- An interrogator (sometimes called a reader or a read/write device), which is composed of an antenna, an RF electronics module, and a control electronics module.
- A controller (sometimes called a host), which most often takes the form of a PC or a workstation running database and control (often called middleware) software.
The tag and the reader communicate information between one another via radio waves. When a tagged object enters the read zone of a reader, the reader signals the tag to transmit its stored data. Tags can hold many kinds of information about the objects they are attached to, including serial numbers, time stamps, configuration instructions, and much more. Once the reader has received the tag’s data, that information is relayed back to the controller via a standard network interface, such as an Ethernet LAN or even the internet. The controller can then use that information for a variety of purposes. For instance, the controller could use the data to simply inventory the object in a database, or it could use the information to redirect the object on a conveyor belt system.
1. RFID Tags
The most important classification of RFID tags is based on their power supply requirements. RFID tags are generally classified based on their modes of power supplies, and thus they can be defined in the following three major types:
- Active tags
- Semiactive tags
- Passive tags.
Active RFID tags have an on-board power supply in the form of a battery. An active tag uses battery power to amplify the signal and then transmit data back to the reader. Therefore, active tags do not need to use the RF carrier signal’s energy to energize the data processing section and hence have a longer reading range. Active tags can generally be differentiated by their digital section. The digital section provides the ID code as well as embedded security protocols and encryption techniques. The data processing and protocol execution are controlled by the processor, which, in some cases, has additional coprocessors to perform the encryption and data processing instructions. Active tags have the ability to process and store more data than passive tags due to the on-board power supply and are less sensitive to the strength of the reader’s interrogation signal.
When communicating with the reader, the tag is the first entity to be engaged in data transmission. Because the presence of the reader is not necessary for data transmission from the tag, an active tag can maintain a continuous data transmission without the presence of a reader. This type of communication between the reader and the tag is known as transponder driven. Although the active tag has an on-board power supply, additional techniques for extending the battery life with low-power consumption have been implemented in the form of sleep modes. Active tags that do not detect the interrogation zone of a reader hibernate by going into a sleep mode, and thus they do not waste power. The most significant advantage of active RFID transponders is that they are reprogrammable, and therefore, can be used on a variety of items repetitively until the battery power is exhausted.
The difference between the active tag and the semiactive tag is that a semiactive tag has the provision of the on-board power supply for minor signal processing tasks but this power is not utilized for amplification of received and transmitted signals. Thus a semiactive tag consumes much less power from the on-board battery and has a longer life compared to an active tag. However, due to this budgeted power allocation that is only for the signal processing unit, semiactive tags have less reading range compared to an active tag. Therefore, the semiactive tag is an in-between approach compared to a fully active tag and a battery less fully passive tag.
When communicating with the reader, the tag must first acknowledge the interrogation signal of the reader in order to reply; this communication protocol is known as interrogator driven. Some semiactive tags can still perform complex tasks such as data processing and encryption and can achieve reading range almost as good as active tags. These advantages are able to be exploited with the advent of very-low-powered highly efficient microprocessors available in today’s market.
Passive Tags do not possess an on-board power supply and therefore rely only on the power emitted from the reader for both data processing and transmission. Passive tags may or may not contain an IC, memory block, or application specific IC (ASIC). This means that some passive tags perform data processing, but others do not. These tags are usually in the form of Electronic Article Surveillance (EAS) tags commonly found in retail shops for security purposes or Surface Acoustic Wave (SAW) tags. Most passive tags have low power consumption and low cost due to the nature of their design. Because they rely solely on the reader’s emitted energy to cull its operating energy, all passive tags must have an RF front end, an analog circuit, and depending on their data processing techniques, a digital circuit.
The RF front end of the passive RFID tag consists of the antenna and the impedance matching circuit in order to minimize signal reflection between the antenna and transponder circuit. The analog part of the passive tag may comprise an LC tuning circuit and a rectifier. The rectifier supplies the required dc voltage to the digital circuit. The digital circuit of the RFID passive tag is completely optional and may have an IC, ASIC, or just a memory block of a few kilobits. Most passive tags has precisely designed microchips and/or ICs that contain digital logic sectors, which process data rapidly.
Passive RFID tags can be made using printing techniques. There have been tremendous efforts and interests in direct printing of RFID tags on plastic, fiber, and other low-cost laminates to compete with the ultra-low-cost optical barcodes. Also, all ink-jet-deposited processes capable of creating high quality passive devices for RFID applications have been envisaged and are being developed. Due to the absence of on-board power supplies, passive RFID tags have a much shorter reading range (up to 2m). They are more vulnerable to environmental effects and have poorer or no data processing abilities at all and hence can’t be easily reprogrammed. The advantages of passive RFID systems are low cost and low maintenance. Due to these salient features, passive tags are used in a wide range of applications such as medical, supply chain management, and wireless sensing.
There are different kinds of readers depending on the technical system used. However all readers can be reduced to two functional blocks: the RF-interface, consisting of transmitter and receiver, and the control unit.
The tasks of the RF interface are the emission of a RF field for energy supply for the tag, modulation of the transmitter signal for the transmission of data to the tag and the demodulation of data received from the tag. There are two different branches for data sent to the tag and for the data received, called transmitter branch and receiver branch. Full Duplex (FDX) systems can send and receive at the same time while Half Duplex (HDX) systems can only do one at a time. Sequential or pulsed systems provide the RF field in pulses to supply the tag with energy and to transmit data to the tag. Breaks in energy supply are used to transmit data from the tag to the reader.
Communications with the application as well as the execution of commands are taken care of by the control unit. The control unit also manages communications with the tag according to the master/slave principle and does the encoding and decoding of the signal. In more complex systems the control unit also executes an anti-collision algorithm, deciphers and enciphers transmitted data and manages an authentication procedure between tag and reader. To carry out these tasks efficiently, the control unit has a microprocessor in its core. For communication between the application and the reader in most cases an RS232 or an RS485 interface is used. High-end readers also support TCP/IP or USB communications. The interface between the control unit and the RF interface contains a binary representation of the state of the RF interface.
3. Backend Database
Readers may use tag contents as a look-up key into a back-end database. The back-end database may associate product information, tracking logs or key management information with a particular tag. Independent databases may be built by anyone with access to tag contents. This allows unrelated users along the supply chain to build their own applications. It is assumed that a secure connection exists between a back-end database and the tag reader. For protocol analysis, it may sometimes be useful to collapse the notion of reader and back-end database into a single entity. In other cases, the reader may be treated simply as an untrusted channel between tag and database.
In many ways, tags are only useful if corroborated with a database in some way. This is particularly true if tags do not contain explicit data, such as manufacturer and product codes. Tags could contain pointers, randomized IDs or encrypted data. While anyone could build a database from scratch using these values, it will often be more economical to subscribe to a database already containing tag associations.
RFID middleware is a system software that collects a large volume of raw data from heterogeneous RFID environments, filters them and summarizes into meaningful information, and delivers the information to application services and middleware platform software that standardizes common functions necessary for the development of RFID applications and provides them as components. The major basic functions of middleware are supporting the independency of the protocol of heterogeneous readers, managing data through real time collection, filtering and summarizing, and interoperating with legacy system through being integrated with application programs. Other functions include process modeling, real-time execution and controlling, and middleware should have a structure of high scalability and availability.