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RFID tagsRFID: How it works

Posted on: 3rd, December 2015

In the second part of our introduction to RFID, we’re going to look at TTF and ITF protocol, the components of a RFID system and its many characteristics.

Who speaks first – the tag or the reader?

This question may appear insignificant but becomes important when several tags are simultaneously within the reader field or when the tags are not static and just pass through the field generated by the reader antenna.

As is often the case for RFID, the readers first task is to transmit power to the tag. In order to do this, the reader emits a fixed frequency signal (without modulation).
At this point, communication between the reader and tag has not officially begun.

Once the tag’s chip has received power, it can either immediately transfer information to the reader (TTF protocol – Tag Talk First) or respond to a query from the reader (interrogator) (ITF protocol – Interrogator Talk First).

The choice of protocol depends heavily on radio resource management issues and the management of the possible presence of multiple tags in the reader’s field (anti-collision protocol). In order to give us an idea of the implication of the choice of protocol on collision management, let’s imagine a classroom. The teacher is the interrogator and the students are the RFID tags.

  • > For TTF systems – we can imagine that at the start of the lesson, each student gives his or her name as they enter the classroom. Excepting a few latecomers, the students arrive on time for their lesson and each give their name more or less at the same time, and it is unlikely that the teacher (interrogator) will be able to understand each individual name and identify each of the students (tags). In order to solve this problem, the students can be asked to listen and ensure that no-one else is speaking before giving their name. This variant to TTF protocol is called TOTAL – Tag Only Talk After Listening.
  • > For ITF systems – the teacher (interrogator) asks the first question and asks the students to give their name. All the students in the classroom therefore respond to teacher’s request. As in the previous example, it can be difficult, or even impossible for the teacher to identify each student because they will all reply simultaneously to the teacher’s demand.
    This example shows that the two protocols are incompatible. Furthermore, the presence of a TTF tag in an ITF reader field can lead to disruption in ITF tag communication.

One of the advantages of the TTF protocol is the speed with which it is possible to identify a tag when it is alone in the reader field. It is also worth noting that a reader which isn’t communicating with tags only emits a non-modulating RF signal. The signal therefore only occupies a small part of the electromagnetic spectrum. This reduces the risk of interference with other communications. The main advantage of the ITF protocol is that the communication is triggered by the interrogator. All tag responses can therefore be easily superimposed for “bit-level” collision detection or easily sequenced in order to single out the tags.

RFID tags

The components of an RFID system

RFID Application: Dedicated software and a host computer system.

RFID Reader: These devices are radio-0frequency (RF) emitters which activate the tag by passing near the antenna in order to activate the tag and send a command. A simple command consists of asking the tag its unique identifier.  The frequencies used by RFID readers depend on the application and performance in terms of read range and/or environmental constraints. RFID systems are considered as non specific short range devices. Regulation issues are detailed in the part “RFID frequencies”

RFID tag: the RFID tag is made up of a microchip attached to an antenna. The tag picks up signals from and sends signals to a reader. The tag contains a unique serial number. Tags have lots of different form factors depending on the application. RFID tags can be active, passive or battery-assisted.

Passive tag characteristics

A passive tag is an RFID tag that does not contain a battery; the power is supplied by the reader. When radio waves from the reader are encountered by a passive RFID tag, the coiled antenna within the tag forms a magnetic field. The tag draws power from it, energizing the circuits in the tag. The tag then sends the information encoded in the tag’s memory.

The advantages of a passive tag are:

The tag functions without a battery; these tags have a useful life of twenty years or more.

The tag is typically much less expensive to manufacture

The tag is much smaller (some tags are the size of a grain of rice). These tags have almost unlimited applications.

RFID tag/reader connection

Connection between tag and reader can be either:

Magnetic – in the case of near field communication (few cm up to 1m). Reader uses LF (Low frequencies) or HF (High Frequencies).

Electromagnetic – in the case of far field communication (up to 6m in line of sight communication and European regulations). Interrogator uses UHF (Ultra High Frequencies) or SHF (Super High Frequencies)

The RFID frequency spectrum

RFID is considered as a non-specific short range device. It can use frequency bands without a license. Nevertheless, RFID has to be compliant with local regulations (ETSI, FCC etc.)

LF :    125 kHz -134,2 kHz : low frequencies,

HF :   13.56 MHz : high frequencies,

UHF : 860 MHz – 960 MHz : ultra-high frequencies,

SHF : 2.45 GHz : super high frequencies 

Radio frequency spectrum with the RFID frequency bands

Radio frequency spectrum with the RFID frequency bands

Deploying an RFID system necessitates multiple factors and many different components. Typically, installing a system requires basic hardware including RFID tag, readers as well as reader control and application software. When all of these components come together, an infinite number of creative applications are possible.

RFID system applications can help improve the quality of business operations and asset integrity whilst saving time and money in a variety of industries.

Want to know more?

If you’d like more information on our RFID enabled products, such as our intrinsically safe infrared windows and ultrasonic testers, then visit our KnowledgeHUB for a whole library of useful resources.

In the meantime, why not download our FREE eBook on ‘Ultrasonic Thickness Gauges in Potentially Hazardous Environments’.

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