Figure 3.68 An example of a card with a chip, magnetic stripe, signature area and embossing (Source:
Giesecke&Devrient)
The position of the module within the card body is specified in the standard. The locations of the magnetic stripe area and the area reserved for embossing are also exactly specified (see ISO 7811). All three of these components may be present on a single card. However, in this case the following mutual relationships must be taken into account: (a) if only a chip and an embossing field are present, they may be located on the same side or on opposite sides of the card; (b) if a magnetic stripe is also present, it and the embossing area must be located on opposite sides of the card.
3.6 CONTACTLESS CARDS
As already described in Section 2.3.3, contactless cards do not require any electrical connection between the smart card and the card terminal in order to transfer energy and data over a short distance. The most important advantages of the contactless card technology are described in Section 2.3.3. In this section we examine the technology and operating principles of contactless cards in more detail. The techniques used with contactless cards for transferring energy and data are not new. They have been common knowledge for many years in radio-frequency identification (RFID) systems, which have been used for a variety of applications, such as animal implants and transponders for electronic anti-start systems for vehicles. There are many techniques for identifying persons or objects at short or even long distances based on radio techniques, and in particular on radar techniques. Among the large variety of technical possibilities, only a small number are suitable for use in smart cards in the ID-1 format (to which we restrict our attention), since all of the functional components must be housed in a flexible card that is only 0.76 mm thick. For instance, fitting flexible batteries into the card body remains an unsolved problem for mass-produced cards. Although flexible batteries with suitable thickness are now available, there is no experience with using such batteries in the field or in mass production. Consequently, we are still limited to passive techniques in which the energy to power the card must be extracted from the electromagnetic field of the card terminal. This limits the useful range to around 1 m.
To make it easier to understand the variety of techniques used, they can be classified according to various parameters. One possibility is to classify them according to the method used to transfer energy and data. The most commonly used methods are transmission using radio waves or microwaves, optical transmission, capacitive coupling and inductive coupling. Capacitive and inductive coupling are best suited to the flat shape of a smart card lacking an internal source of power. The systems presently available on the market utilize these methods exclusively, which are also the only ones considered in the relevant group of ISO/IEC standards (10 536, 11 443 and 15 693). Consequently, in this book we limit ourselves to these methods. Just as with contact-type smart cards, a system using contactless cards consists of at least two components, namely a card and a compatible terminal. The terminal can act as a reader or a reader/writer, according to the technology used. As a rule, the terminal includes an additional interface, via which it can communicate with a background system.
The following four functions are necessary to allow a contactless card to communicate with a terminal:
r
energy transfer to the card for powering the integrated circuitr
clock signal transferr
data transfer to the smart cardr
data transfer from the smart card.terminal contactlesssmart card power
clock data data
Figure 3.69 The necessary energy and data transfers between a terminal and a contactless smart card Many different concepts based on experience with RFID systems have been developed to satisfy these requirements. Most of them are specifically designed for particular applications. For instance, there is a considerable difference between systems where the cards are only a few millimeters away from the terminal in normal use and systems where the cards can by up to a meter away from the terminal. Naturally, when many different solutions specifically designed and optimized for particular applications are developed, they are inevitably mutually incompatible.
Inductive coupling
Inductive coupling is presently the most widely used technique for contactless smart cards. It can be used to transfer both energy and data. Various requirements and constraints, such as radio licensing regulations, have resulted in a variety of actual implementations.
3.6 Contactless Cards 95
Figure 3.70 Basic construction of a contactless smart card with inductive coupling
With some applications, such as access control, it is sufficient to only be able to read the data stored in the cards, which makes technically simple solutions possible. Due to their low power consumption (a few tens of microwatts), the usable range of such cards extends to approximately one meter. Their memory capacity is usually only several hundred bits. If data must also be written, the power consumption rises to more than 100 µW. As a consequence, the range is limited to around 10 cm in the writing mode, since licensing restrictions prevent the emitted power of the writing equipment from being arbitrarily increased. The power consumption of microprocessor cards is even greater and is typically 100 mW. The distance from the terminal is thus even more restricted.
Independent of their range and power consumption, all cards that employ inductive cou- pling work on the same principle. One or more coils (usually with large enclosed areas) are incorporated into the card body to act as coupling components for energy and data transfers, along with one or more chips.
Energy transfer
Almost without exception, contactless smart cards are used passively. This means that all of the energy needed for operating the chip in the smart card must be transferred from the reader to the card.
This energy transfer is based on the principle of a loosely coupled transformer. A strong high-frequency magnetic field is generated by a coil in the terminal in order to transfer the energy. The most commonly used frequencies are<135 kHz and 13.56 MHz, which correspond to wavelengths of 2400 m and 22 m, respectively. The wavelengths of the electromagnetic fields are thus several times greater than the distance from the card to the terminal, which means that the card is located in the near field of the terminal. This allows the loosely coupled transformer model to be used. If a contactless card is brought close to the terminal, a portion of the terminal’s magnetic field passes through the coil in the card and induces a voltageUiin this coil. This voltage is rectified to provide power to the chip. Since the coupling between the coils in the terminal and the card is very weak, the efficiency of this arrangement is very low. A high current level is thus required in the terminal coil to achieve the necessary field strength. This is achieved by connecting a capacitorCTin parallel with the coil LT, with the value of the capacitor chosen such that the coil and capacitor form a parallel-resonant network whose resonant frequency matches the frequency of the transfer signal.
UG
LT LC Ui
I0
Ri
CT
terminal smart card
~
C1 C2 chip
Figure 3.72 Using inductive coupling to supply energy to a smart card
CoilLCand capacitorC1in the card also form a resonant circuit with the same resonant frequency. The voltage induced in the card is proportional to the signal frequency, the number of windings of coilLCand the enclosed area of the coil. This means that the number of turns needed for the coil drops with increasing signal frequency. At 125 kHz, it is 100 to 1000 turns, while at 13.56 MHz it is only 3 to 10.
3.6 Contactless Cards 97