Tag Types

RFID tags can be categorized in many ways. They can be divided by communication methods, frequencies, classes, polarization, construction, or packaging. In this chapter, we will discuss all of these specifications and start with tag types according to the way they communicate.

If tags are able to transmit radio frequency (RF) signals to the environment without being near an interrogation zone or without needing power of some sort from an outside source, they are called active tags . If they are not able to transmit the signal by themselves, but need energy from the reader to do this, then they are called passive tags or semi-passive tags. I know you're thinking it can't be that simple or CompTIA wouldn't have created a whole section of the test just for tags-and you're right, so let's take a deeper look at tags and how they work.

Active Tags

As mentioned earlier, active tags can actively transmit an RF signal carrying data to the environment. They can broadcast this radio signal to the outside world because of how they are made. Active tags carry a transmitter and a battery, in addition to a chip and an antenna that are common to almost all tags. The battery is used to power the chip's circuitry as well as the transmitter. This enables the active tag to transmit its signal far into the environment (I have seen tags that could reach distances close to 1,000 meters) and to respond to a much lower-level interrogation signal than passive tags can handle. Because of the presence of a battery, active tags can also carry large amounts of memory and various sensors, and perform processing functions. An active tag has all the functionality of a minicomputer, without the keyboard, video, and mouse components.

Active tags can be programmed to beacon, which means that they broadcast their information to the environment, saying, "I am here, I am here" at regular intervals. Tags can beacon continuously or in preset time intervals. Setting the broadcast is determined by the application or "use case." Active tags can wait for an interrogation signal from the reader (reader-talks-first, or RTF) or send out the signal first (tag-talks-first, or TTF). They respond only after the reader contacts them.

Tip 

When deciding how often you will want your tags to beacon, keep in mind that beaconing discharges the tag's battery. Continuous beaconing not only uses more energy, but also may create unwanted RF noise in the environment.

Many techniques are used to program active tags. You can set them up to beacon until they are in the interrogation zone of a reader. After the communication link is established, the reader can issue commands to the tag and the tag will respond accordingly. Or, the reader can use the beacon signal only as a sign of the tag's presence and then will continue polling for other tags.

Active tags can operate on several frequencies; most often you will see them working at 433 MHz or 2.45 GHz. The most common active tags are the ones used by the U.S. Department of Defense (DoD) and made by Lockheed Martin's Savi Technology group. They operate at 433 MHz, and are on every container that the DoD sends outside the continental United States (OCONUS).

Active tags are significantly more expensive than passive and semi-passive tags because of how sophisticated their processing and capabilities are. The DoD has been using active RFID tags for almost two decades for container management, supply-chain management, equipment and asset tracking within a business process, and unit deployment efforts. Other commercial uses of active RFID technology include transportation companies and railroads using the tags in much the same way the DoD does for identifying goods within the transportation pipeline, asset tracking, and business process tracking, which is most commonly found within manufacturing processes of expensive industrial equipment. A newcomer to active RFID is real-time location system (RTLS), an exceptionally powerful tool that provides the ability to physically locate an item within 1 or 2 meters of its position. (RTLS providers usually guarantee accuracy for 7 to 15 feet.)

Note 

You can find more details about RTLS in Chapter 6, "Peripherals."

The following are examples of active tag applications:

• Hospitals track the inventory and location of medical equipment (medical carts, heart-monitoring machines, and so forth) by using RTLS.

• The military and industries such as transportation and retail use active tags for container tracking and location. This could be combined with monitoring of environmental conditions as well as route and container access and yard management.

• The automotive industry uses key fobs for remote car locking and unlocking, alarm activation, starting up the car, and so on.

The users benefit not only from the long read ranges, but also from the tag's memory, which can carry information such as tag ID, container number, content of the container, dates of departure and arrival, route, and so on, up to several megabytes. Active tags can also be used with a global positioning system (GPS) as well as other satellite systems or cellular communication networks. The tag is able to collect, for instance, data about its route or data from environmental sensors that track temperature, humidity, and other information. These data then can be communicated in real time to the back-end system, sent in batches, or downloaded at the end of the tag's assignment. Another critical component that makes active tags attractive is that they don't need to be hooked up to the Internet to get useful information. Even in the middle of a desert with a handheld reader, a soldier can determine the contents of a shipping container without opening it, because the data are entirely on the tag.

Passive Tags

Passive tags differ greatly from active tags. They do not carry a battery or a transmitter and are not able to transmit an RF signal by themselves to the environment. Passive tags use the signal received from the reader to power their chip circuitry and send a response back to the reader. To communicate with a reader, passive tags use either inductive coupling or passive backscatter techniques (both of which will be discussed further later in this chapter) depending on whether they are getting energy from a magnetic field or an electric field.

The construction of a passive tag is quite simple. A passive tag consists of a chip (integrated circuit, or IC) and an antenna, which are connected and placed on a substrate. These three components together are often called an RFID inlay. Usually, the RFID inlay is not used by itself but is integrated into some kind of packaging or media, most often labels but sometimes buttons, cards, or wristbands. Because a passive tag has a very thin profile, often as thin as a sheet of paper with a small bump for the chip, it also can be integrated into product packaging or in some cases inside the product itself.

Passive tags can operate on various frequencies, from 135 kHz to 2.4 GHz, and their characteristics differ by the frequency used. Generally, passive tags have a considerably shorter read range than active tags. Passive tags can communicate in distances up to 10 meters (30 feet) under ideal conditions, but usually their read ranges are shorter, and they vary by frequency and communication method used and the type of material the tags are attached to.

Because of their relatively simple construction and fast manufacturing, passive tags are the least expensive types of tags. (I have been referred to as the Great Tagnac recently because of my ability to predict price. For those of you who read RFID For Dummies, you'll see that I accurately predicted the price of tags dropping below 10 cents by 2006 and being close to 5 cents by the end of 2006. Chalk that up to even a blind squirrel finding a nut occasionally.) Passive tags are easily produced in large quantities and are suitable for use on high-volume, low-cost products. Passive tags are used for tracking pallets, cases, or items throughout military, retail, automotive, pharmaceutical, and other industry supply chains; for inventory management; and for access control, personnel identification, parking, subway toll collection, theft prevention, and many other applications. Just to give you an idea about the flexible uses of passive RFID tags, here are a few examples:

• Retail Passive tags are used for tracking pallets and cases, and sometimes items. Tags are used for theft prevention, inventory management, promotion management, product display, and availability, as well as a multitude of other applications.

• Pharmaceutical Industry Tags are used for item tracking to provide the drug pedigree (history of the drug and its progress throughout the whole supply chain from the manufacturer to the retailer or hospital) and prevent counterfeiting. For this application, tags are placed on vials with pills, as well as on cases.

• Garbage Collection Tags are placed on garbage bins. The tag ID is cross-referenced with a database carrying data about paid and unpaid accounts.

• Automotive Industry Passive tags are used in keys to provide key/vehicle/owner identification. When the key is positively identified, it can turn on the ignition. Tags are also used for inventory control and tracking of various parts to provide a history.

• Hospitals Babies are tagged to provide identification as well as theft prevention. Patients are tagged to provide identification, medical records, and a list of medicines provided.

• Access Control Badges and access cards are equipped with tags that can be read without being swiped through a reading device. This is used for access control to buildings, parking garages, secure areas, and the like.

• Libraries Tags are placed in books, CDs, and DVDs to prevent theft, track inventory, provide location, prevent mis-shelving, and speed up checkout.

• Animal Tracking Tags are usually placed on the animal's ear when tracking cattle. For household pets, tags are implanted under the skin.

• Amusement Parks Tags can track children as well as match them with their parents. Their wristbands with embedded tags are scanned at each attraction; therefore, you can identify where they were at what time.

• Ski Resorts Tags are embedded in ski passes.

• Airport Tags are used for baggage tracking and identification.

Semi-passive Tags

Semi-passive tags are also called battery-assisted passive tags; they are a hybrid of passive and active tags. (Occasionally they are referred to as "semi-active tags," which is not correct terminology; usually it's coming from what I call RFID pirates. The way to distinguish them is that they refer to R-F-I-D as Arrrr-fid!) Although semi-passive tags have a battery, they do not transmit an RF signal by themselves and they use passive communication methods (mostly passive backscatter and in a few cases inductive coupling) to talk to the reader. In semi-passive tags, the battery is used to power the chip's circuitry, as well as possible sensors and memory, but it does not assist with signal transmission. Semi-passive tags usually have longer read ranges than passive tags (up to 100 meters), but only because the chip needs a lot less power from the reader in order to wake up (it responds to a much weaker signal) since the additional power is supplied by the on-board battery.

Semi-passive tags vary a lot by size, capabilities, read range, casing, and of course price. (Their price is usually close to the price of an active tag.) Although they carry an on-board power source (which is usually very small, in the form of a battery used in watches or a printed battery), semi-passive tags can come in very small sizes that could compete with passive tags.

Semi-passive tags can operate on various frequencies, from 135 kHz to 2.4 GHz. Because the tag can detect different conditions and save the record even if it is not in the reader's field, it can be used for monitoring, activation, and deactivation of alarms, seals, thermostats, valves, and other devices and systems. Semi-passive tags are also used for temperature tracking in shipping trailers with environmental control (for instance, to avoid produce spoilage or thawing of frozen products) as well as for pressure, chemical, and tamper detection.

Table 4.1 provides an in-depth look at the pros and cons of the three types of tags I've discussed in this chapter.

Table 4.1: The Pros and Cons of Different Types of Tags
Open table as spreadsheet

Tag Types	Pros	Cons
Active tags	Long read ranges (up to a kilometer) Can beacon, and carry sensors and large memory Possible processing capabilities Work well in difficult environments Sturdy construction RTLS	Expensive (prices vary from $5 to $150) Relatively large size due to battery Lifetime limited by battery life
Passive tags	Cheap (prices start at 7 cents) Virtually unlimited lifetime (manufacturers claim unlimited reads and about 100,000 rewrites) Fast production in large quantities Fast and easy application with automatic label applicators	Short read ranges (up to 10 meters) Small memory Orientation sensitivity More susceptible to damage than active and semi-passive tags
Semi-passive tags	Relatively long read ranges (up to 100 meters) Can carry sensors and large memory Possible processing capabilities Sturdy construction	Relatively expensive (prices start at $5)