Security systems № 2 (56), 2004
The number of publications on this topic does not decrease for several years. This is due to the rapid development of the RFID technology itself, the development of new frequency bands, the regular appearance of new products and new applications, where the technology of contactless identification (or radio frequency identification - Radio Frequency IDentification) allows us to solve problems previously impossible for hardware and software.
From chaos to order, or background
The radio frequency identification technology came into being about 20 years ago and the entire period was shaped at a pace ahead of computer technology. Particularly intense RFID has been improved in the last 5-7 years. This can be explained by two factors: firstly, the development of microelectronics allowed to implement many ideas previously unavailable for purely technological reasons, and secondly, there were standards, the use of which ensured compatibility of technical solutions from different manufacturers. Before considering the specific issues of the use of contactless identification in various areas of human activity, let us dwell on the general principles of RFID systems and regulatory documents that define and will determine in the near future the course of design ideas.
For those who are not familiar with RFID technology, briefly outline its essence. Physical principles (at least for most frequency ranges) resemble the operation of a transformer or a system of connected circuits. As you know, if you take two coils and place them not very far from each other, then they will have mutual influence on each other.
The reader contains a high frequency generator G, which powers the reader antenna Lc. Due to the presence of electromagnetic coupling M between the reader antenna and the identifier (card) antenna LK, an alternating voltage is induced in the latter, the value of which depends on the design and the distance between the card and the reader. The induced voltage is used to power the chip of the DK card through a rectifier formed by a diode VDp and a filter capacitor Cf. The chip of the DK card modulates the voltage in the IK antenna by shunting it with the Ksh resistor. Due to the communication of antennas, modulation appears in the antenna of the Lc reader, is detected by the VDd diode and is fed to the microcircuit by reading Dc, which decrypts the card code and sends it to the controller via the Int interface. By this principle, the first passive R / O (Read Only - read only) worked. Proximity-cards and readers. Then identifiers were created that can not only transfer information to the reader, but also receive it for programming purposes (writing information to non-volatile memory). From the point of view of the basic principles of building an RFID system, a modulator appeared in the reader, which modulated the carrier radiated by the reader, and in the card — a detector and reprogrammable nonvolatile memory, into which the information transmitted by the reader was recorded (Fig. 2). Identifiers (cards) with this technology are already called R / W (Read Write), that is, "read and write."
Fig. 2. RFID system with Read / Write technology
The first industrial RFID systems were located in the 125 kHz frequency range. But with the growing demand for the amount of information transmitted in a short time, higher frequency systems were developed, in particular, operating in the 13.56 MHz range.
Regardless of the frequency range and coding method, the design of RFID cards is about the same
From the principle of the card-reader pair, the conclusion is unequivocal: the longer the reading range we want to ensure, the larger the reader will be and the higher the radiated power should be. For a rough estimate of the potential range of the passive system of the 125 kHz and 13.56 MHz RFID ranges, one can take into account the fact that the maximum reading range of the card code is equal to the diagonal of the reader antenna. If they convince you otherwise, do not believe!
Frequencies and standards
In order to be meaningful to all subsequent material, the frequency ranges of the RFID systems and the basic standards, to which almost all modern developments in this field are subject, should be considered. Let's start with the frequencies. Today, RFID has "occupied" four frequency bands: 125 kHz, 13.56 MHz, 800 ... 900 MHz, and 2.45 GHz. Immediately it should be noted that the range of 800 ... 900 MHz is used much less frequently than the other three, so we will not dwell on it in more detail.
What explains the choice of these frequencies? Yes, the fact that it is precisely these values that take the "holes" in the frequency schedules that were scored today for the most varied communication systems for military and broadcasting purposes. As a matter of fact, these are the frequencies for which most countries are allowed to conduct commercial development without obtaining permission to use the frequency. For example, we note that the range of 2.45 GHz is the frequencies on which Bluetooth and Wireless LAN operate, that is, wireless networks for household purposes. Naturally, quite specific features are inherent in each of the frequency ranges of RFID systems.
Consequently, for each of the ranges, their own methods of encoding signals in a pair of "reader - card", their transmission speeds and collision resolution algorithms are used. The anti-collision mechanism is used so that while there are several identifiers in the reader field, you can choose only one for the dialogue that is needed at a given time.
In the old Proximity systems without such a mechanism, the simultaneous presentation of two or more cards to the reader resulted in none of them being read. From further material it will become clear to us that many modern applications based on RFID technology simply could not function without this tool.
But back to the standards, because it is unification and standardization that have always been the engines that allowed private solutions to integrate into the world economy. Immediately, we note that standardization is not an event, but a process that goes along with the development of technology, but once rooted, standards act for quite a long time (for fairness, we can say that from a certain point in time, unfortunately, they become the brakes of progress ).
So, for each of the mentioned frequency ranges, there are different standards with their degree of development. Their most common characteristics are more conveniently presented in tabular form (see table).
The table does not mention the range of 800 ... 900 MHz due to the fact that it is used quite rarely and the author does not know the applicable standards for this range.
Paradoxically, but today there is a huge amount of Proximity-cards in circulation, which do not correspond to any of the standards reviewed. They were simply designed and put into circulation before standardization touched the RFID field. Nevertheless, in the access control systems (ACS), it is these cards that still occupy the main positions, so we briefly discuss their characteristics. Immediately, we note that almost all of them operate in the good old 125 kHz range, for which even 15 years ago the technical implementation was quite affordable.
The “non-standard” solutions that are considered in today's approaches have for many years been and in part remain the “de facto” standards today.
Indala (a division of Motorola) is historically one of the first serial manufacturers of Proximity cards and readers for access control systems.
Cards have a fixed internal card code length of 35 bits, while 26-bit readers “cut off” the extra part of the card's code when converted to Wiegand format, while reader readers with a longer code length, for example Wiegand 44 (differently referred to as AMicro) “dilute” the output code with bits that have a constant value. The type (dimension) of the output code of Indala is determined by the reader. Indala identifiers use amplitude modulation of the carrier divided in half, and the circuit implementation of the demodulator in the reader for them is one of the most complex.
Unlike Indala, any HID reader works in all formats (Wiegand 26, Wiegand 35, Wiegand 37 and so on). Here the format of the output code is defined in the map, which has a constant code length of 64 bits. Depending on the value of the control bits at the beginning of the code, the reader automatically selects the mode in which it will generate the output code. HID identifiers use frequency modulation of the subcarrier, which provides a sufficiently high noise immunity and allows stable operation even at ranges up to 80-100 cm.
EM Marin Cards
Most likely, if you exclude America, where HID and Indala occupy most of the Proximity market for security systems, this is the most popular card type in the world. The fact is that HID and Indala patented their decisions and vigilantly ensured that their rights were not violated. The format of the Swiss company EM Marin turned out to be open, easily realized technically due to the simplest amplitude modulation, and today identifiers of dozens of card and key fob manufacturers are made in this format. Even in Russia, maps of this format have been issued for about 10 years. Unlike the identifiers Indala and Nude, in the Marin EM the information length of the code is 5 bytes (40 bits), so when converting to the most popular 26-bit Wiegand, the high bits of the code are always cut off. In the EM Marine format, a more serious protection against read errors has been implemented, due to which, even with a less robust amplitude modulation, the accuracy of the “ID-reader” pair is quite high.
Identifiers from other manufacturers
Among other manufacturers, Texas Instruments is the first to mention. Its Tiris (125 kHz) technology is noteworthy because it has been the first to use “pumping” technology for RFID. The bottom line is that the Tiris identifier does not operate in the continuous emission mode of the code, as all the systems discussed above, but accumulates for some time the energy emitted by the reader, then sends a relatively short message with its code to the reader, and then goes into accumulation mode again. This solution allows you to get a greater range with good energy performance. Texas Instruments occupies a relatively strong position in the 13.56 MHz range (Taglt system).
In the 125 kHz range, Philips HITAG identifiers are well known. The most common identifiers HITAG, however, like Tiris, received in the field of automotive security systems - in immobilizers. Check Point IDs were among the first to operate in the 13.56 MHz band. Check Point readers, unlike other manufacturers' readers with the Wiegand format, have their own Wiegand 33 format, which differs both in terms of time characteristics and the principle of code generation. In Russia, quite large access control systems are presented using readers and cards of this type. There are also original identifier formats for British companies Cotag and RAS in the low-frequency range (125 kHz for Cotag and 135 kHz for RAS).
Using RFID in ACS
Access systems are one of the first truly massive applications of RFID technology. This is explained, apparently, by two factors: first, the ease of implementation of the technology itself as applied to the ACS (it is enough to use R / 0 identifiers only for reading with a small - three or four bytes - code length); secondly, unsurpassed convenience in comparison with any other types of identifiers: contact, with a magnetic strip, Wiegand (not to be confused with the format of the code transfer by readers!).
Proximity card can be read by the reader even through a wallet or purse and at the same time can play the role of a photo pass or badge. In addition, compared to the pre-dominant magnetic cards of magnetic cards, today's Proximity cards have a higher level of copy protection and counterfeiting. We can say that magnetic cards "survived" only where they really provide an advantage - for example, access to ATMs at night. By the way, in this respect America turned out to be the most conservative country — it is the region where the greatest number of old systems on magnetic cards still exist.
The appearance of the Proximity technology in the access control system (literally “close”, “near”) caused a natural desire to increase the range of reading the card code. As a result, the Hands free system was born. But since it is impossible to deceive nature, in order to increase the range, we had to equip identifiers (cards) with a compact lithium battery. But now for the power supply of the microcircuit of the card the high radiation power of the reader was not required, and the range of such systems exceeded one meter. Historically, the English company Cotag has become the leader. Her active cards have a lifespan of at least five years.
The technology of active identifiers has been further developed in vehicle identification systems. Such an identifier (no longer in the form of a card, but in the form of a small block attached to the car body and receiving power from its on-board network) worked, as a rule, with a reader whose antenna was a wire loop, closed into a roadway.
The idea of identification continued to advance in systems in the 2.5 GHz band. In this range, the linear dimensions of the antennas are quite small, and the reader, even the size of a shoe box, can easily overcome the meter barrier on passive identifiers, and with the active one, the typical value of the maximum read range becomes 10 m. Examples of such systems known in Russia are Tag-Master (Sweden) and Nedap (Holland). It should be remembered that it is not safe to be in the field of operation of a long-range microwave range reader for a long time (this applies even to power of about 100 mW). Another advantage arising from the physical properties of this frequency range is that the reader’s pattern is clearly pronounced in space, which increases its efficiency from an energy point of view, and when using nearby passages (passages) it reduces the mutual influence of nearby readers.
The appearance of rewritable (R / W) cards opens up a number of new possibilities with regard to the ACS. This, for example, is the organization of a global anti-passback, even in the absence of communication between controllers serving the different points of passage of one of its areas. Other interesting perspectives are related to the fact that it is easy to write the biometric characteristics of a person (say, fingerprints) and use them on objects of heightened secrecy. Such a solution is being marketed by BioScript. In principle, it is realistic to record a color photograph of its owner on the card and use it at the points of passage for video verification. Unfortunately, the further development of the rewriting potential of the information stored in the card is limited by the fact that today there are practically no access controllers for access control systems that allow working directly with R / W cards. This also applies to the commonly used exchange protocol with Wiegand readers, and the controller's database structures, and its operation logic. All this is due to the large inertia in the development of technical security equipment - the world leaders of the ACS industry do not update their product lines for 10-15 years.
... and further
In the material presented to your attention, we touched upon the basics of RFID technology, gave an overview of the frequency bands used, standards that have emerged in this area in recent years, and also talked about the features of using RFID technology in security systems (ACS). In the near future will be considered issues not covered in this article. It will be a question of such a new direction as contactless and multi-interface Smart-cards and "smart" tags.