Experts of UNDERSTANDING RFID TECHNOLOGY
This chapter presents a technical introduction to the RFID, the Electronic Product Code (EPC), and the Object Name Service (ONS). It then looks at two speciﬁc RFID applications that have been ﬁelded over the past ten years.
Most histories of RFID trace the technology back to the radio-based identiﬁcation system used by Allied bombers during World War II. Because bombers could be shot down by German anti-aircraft artillery, they had a strong incentive to ﬂy bombing missions at night because planes were harder for gunners on the ground to target and shoot down. Of course, the Germans also took advantage of the cover that darkness provided. Early Identiﬁcation Friend or Foe (IFF) systems made it possible for Allied ﬁghters and anti-aircraft systems to distinguish their own returning bombers from aircraft sent by the enemy. These systems, and their descendants today, send coded identiﬁcation signals by radio: An aircraft that sends the correct signal is deemed to be a friend, and the rest are foe. Thus, radio frequency identiﬁcation was born.
Shortly after the war, an engineer named Harry Stockman realized that it is possible to power a mobile transmitter completely from the strength of a received radio signal. His published paper “Communication by Means of Reﬂected Power” in the Proceedings of the IRE2 introduced the concept of passive RFID systems.
Work on RFID systems as we know them began in earnest in the 1970s. In 1972, Kriofsky and Kaplan ﬁled a patent application for an “inductively coupled transmitter-responder arrangement.”3 This system used separate coils for receiving power and transmitting the return signal. In 1979, Beigel ﬁled a new application for an “identiﬁcation device” that combined the two antennas; many consider his application by to be the landmark RFID application because it emphasized the potentially small size of RFID devices.
In the 1970s, a group of scientists at the Lawrence Livermore Laboratory (LLL) realized that a handheld receiver stimulated by RF power could send back a coded radio signal. Such a system could be connected to a simple computer and used to control access to a secure facility. They developed this system for controlling access to sensitive materials at nuclear weapons sites.
Today we would call this Livermore system an example of security through obscurity: What made the system secure was that nobody else had a radio capable of receiving the stimulating radio signal and sending back the properly coded response. But at the time it was one of the most secure access control systems available. The scientists left LLL a few years later and created their own company to commercialize the technology. This system ultimately became one of the ﬁrst building entry systems based on proximity technology and the ﬁrst commercial use of RFID.
The Elements of an RFID System
RFID systems fundamentally consist of four elements: the RFID tags themselves, the RFID readers, the antennas and choice of radio characteristics, and the computer network (if any) that is used to connect the readers.
The tag is the basic building block of RFID. Each tag consists of an antenna and a small silicon chip that contains a radio receiver, a radio modulator for sending a response back to the reader, control logic, some amount of memory, and a power system. The power system can be completely powered by the incoming RF signal, in which case the tag is known as a passive tag. Alternatively, the tag’s power system can have a battery, in which case the tag is known as an active tag.
The primary advantages of active tags are their reading range and reliability. With the proper antenna on the reader and the tag, a 915MHz tag can be read from a distance of 100 feet or more. The tags also tend to be more reliable because they do not need a continuous radio signal to power their electronics.
Passive tags, on the other hand, can be much smaller and cheaper than active ones because they don’t have batteries. Another advantage is their longer shelf life: Whereas an active tag’s batteries may last only a few years, a passive tag could in principle be read many decades after the chip was manufactured.
Between the active and the passive tags are the semi-passive tags. These tags have a battery, like active tags, but still use the reader’s power to transmit a message back to the RFID reader using a technique known as backscatter. These tags thus have the read reliability of an active tag but the read range of a passive tag. They also have a longer shelf life than a tag that is fully active
Tags come in all shapes and sizes. The smallest tag that has ever been produced is the Hitachi mu-chip, which is less than 0.4mm on a side. Designed to be embedded in a piece of paper and used for tracking documents printed in an ofﬁce environment, the mu-chip can be read only at a distance of a few centimeters. Of course, the mu-chip is a passive tag. With a larger antenna it could have a signiﬁcantly longer reading range, but that would defeat its purpose.
Other small tags are the implantable tags the size of a grain of rice manufactured by VeriChip. Like the mu-chip, these passive tags have a very limited reading range; their intended application is to give machine-readable serial numbers to people. The company says that the chips can be used to authenticate people in high-security environments—unlike passwords, the implanted chips can’t be easily shared—and in hospitals, where staff occasionally mix up patients and give them the wrong treatments. Implantable chips might also work to identify wandering Alzheimer’s patients who go out without any identiﬁcation or cognizance of their location or destination. We’ll come back to the topic of implantable chips later in this chapter.
RFID tags can also be quite large. The semipassive RFID tag used in the FastLane and E-ZPass electronic toll collection systems is the size of a paperback book and includes an antenna and a ﬁve-year battery. The battery gives the system a longer read range and also makes reads more reliable—at least until the battery dies. In practice, the instrumented toll crossings have a large light that ﬂashes green if the tag is read successfully, red if no tag is detected, and amber or yellow if the tag cannot be read properly. When the light ﬂashes amber, the driver is supposed to call the program’s administrator and arrange to have the tag sent in for service.
RFID tags can be promiscuous, in which case they will communicate with any reader. Alternatively, they can be secure, requiring that the reader provide a password or other kind of authentication credential before the tags respond. The vast majority of RFID tags that have been deployed are promiscuous. Not only are these tags cheaper, but the systems also are much easier to manage. Systems that employ passwords or encryption codes require that the codes be distributed in advance and properly controlled. This is an exceedingly difﬁcult management problem.
The simplest RFID chips contain only a serial number—think of this as a 64bit or 96-bit block of read-only storage. Although the serial number can be burned into the chip by the manufacturer, it is also common for the chips to be programmed in the ﬁeld by the end user. Some chips will accept only a single serial number, while other chips allow the serial number to be changed after it is burned in. More sophisticated RFID chips can contain read-write memory that can be programmed by a reader. Chips can also have sensors, an example of which is an air pressure sensor to monitor the inﬂation of a tire. The chips might store the results of the sensor in a piece of read-write memory or simply report the sensor’s reading to the RFID reader. Chips can also have a selfdestruct, or “kill” feature. This is a special code that, when received by the chip, causes the chip to no longer respond to commands. For ﬁnancial applications, the full capabilities of smart cards have been combined with the wireless protocols and passive powering used in RFID. The result is a class of high-capability RFID tags also called contactless smart cards.
RFID tags can interfere with each other. When multiple tags are present in a reader’s ﬁeld, the reader may be unable to decipher the signals from the tags. For many applications, such as raising the gate in a parking lot, this is not a problem. The systems are optimized so that only one tag is within range at a time. However, for other applications, reading multiple tags at once is essential. For these applications, the tags need to support either an anticollision protocol or, more commonly, a singulation protocol. A singulation protocol allows a reader to determine that multiple tags are visible and to iterate through the tags, getting them to take turns responding so that each may be read without interference from the others.
Electronic Product Code (EPC) tags are a special kind of tag that follows the EPC standard developed by the MIT Auto-ID Center and is now managed by the trade organization EPCglobal. Sanjay Sarma, cofounder of the Auto-ID Center, discusses the history of the EPC standard in Chapter 3.
EPCglobal has deﬁned a series of RFID tag “classes” and “generations” of RFID devices (see Tables 2.1 and 2.2).
Table 2.1 EPC RFID Classes
Table 2.2 EPC RFID Chip Generations