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 formass-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 1m.
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 (10536, 11443 and 15693).
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:
- energy transfer to the card for powering the integrated circuit
- clock signal transfer
- data transfer to the smart card
- data transfer from the 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 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
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 100mW.The distance fromthe terminal is thus even more restricted.
Independent of their range and power consumption, all cards that employ inductive coupling 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.
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<135kHz and 13.56MHz, 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 voltage Ui in 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 capacitor CT in 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.
- Coil LC and capacitor C1 in the card also form a resonant circuit with the same resonant frequency.The voltage induced in the card is proportional to the signal frequency, thenumber of windings of coil LC and the enclosed area of the coil. This means that the number of turns needed for the coil drops with increasing signal frequency. At 125kHz, itis100 to1000 turns, while at 13.56 MHz it is only 3 to 10
For transferring data from the terminal to the card, all known digital modulation techniques can be used. The most commonly used techniques are:
- ASK (amplitude-shift keying)
- FSK (frequency-shift keying)
- PSK (phase-shift keying).
ASK and PSK are usually used,since these are especially easy to demodulate
In the other direction, from the smart card to the terminal, a type of amplitude modulation is used.
- It is generated by using the data signal to digitally alter a load in the card (load modulation).
- If a smart card tuned to the resonant frequency of the terminal is brought into the near ﬁeld of the terminal , it draws energy from this ﬁeld as previously described.
- This causes the current I0 in the coupling coil of the terminal to increase, which can be detected as an increased voltage drop across an internal resistor Ri. The smart card can thus vary (amplitude modulate) the voltage U0 in the terminal by varying the load on its coil, for example by switching the load resistor R2 in to and out of the circuit as shown in Figure .
- If the switching of resistor R2 is controlled by the data signal, the data can be detected and evaluated in the terminal.
Due to the low degree of coupling between the coils in the terminal and the card, the voltage variations induced in the terminal by load modulation are very small. In practice, the amplitude of the usable signal is only a few millivolts. This can only be detected using sophisticated circuitry, since it is overlaid by the signiﬁcantly larger signal (around 80 dB) transmitted by the terminal.However,if a subcarrier frequency is employed with a frequency of fs, the received data signal appears in the terminal as two sidebands at the frequencies fc ± fs. These can be isolated from the signiﬁcantly stronger terminal signal by ﬁltering with a bandpass ﬁlter and then ampliﬁed. After this, they can readily be demodulated. The disadvantage of modulation with a subcarrier is that it requires signiﬁcantly more bandwidth than direct modulation. It can thus only be used in a limited number of frequency bands.
If the distance between the card and the terminal is very small, it is possible to transfer data using capacitive coupling. With this type of coupling, conductive surfaces are incorporated into the card body and the terminal such that they act as the plates of a capacitor when the
cardisinsertedintheterminalorplacedontheterminal.Thecapacitancethatcanbeobtained essentially depends on the sizes of the coupling surfaces and their separation. The maximum size is thus limited by the dimensions of the card,while the minimum separation is determined by the insulation required between the coupling surfaces . With an acceptable level of cost and effort, a usable capacitance of several tens of picofarads can be obtained. This is insufﬁcient for transferring enough energy to power a microprocessor. Consequently, this method is used only for data transmission,with the operating power being transferred inductively. This mixed method has been standardized in ISO/IEC 10 536 for ‘close coupling cards
When contactless cards are used, there is always a possibility that two or more cards may be located in the range of a terminal at the same time. This is especially true for systems with large effective ranges, but it can even happen with systems with relatively small ranges – for
instance, two cards might be lying on top of each other and thus be activated concurrently by the terminal. All cards within range of a particular terminal will attempt to respond to commands from the terminal. However, simultaneous data transmissions will unavoidably cause interference and loss of data if suitable countermeasures are not taken. The technical methods used to ensure interference-free data exchanges with multiple cards with in the effective range of a card terminal are called collision-avoidance methods or anticollision methods.
Exchanging data between many mobile units and abase station is a frequently encountered situation in communications engineering, and it is referred to as ‘multiple access’. A typical
example is a mobile telephone network, in which all users located in a particular radio cellcon currently access a single base station.Numerous methods have been developed to allow the signals of the individual users to be distinguished from each other.
The present state of standardization
Given the many different techniques used by various manufacturers, standardization (which was initiated in 1988 by ISO/IEC) proved to be difﬁcult and time consuming,as was expected.The responsible working group had the task of deﬁning a standard for contactless cards that is largely compatible with other standards for identiﬁcation cards.This means that a contactless card can also have other functional components, such as a magnetic stripe, embossing and chip contacts. This allows contactless cards to also be used in existing systems that employ other technologies. As already described, the technical options for transferring energy and data without using contacts essentially depend on the desired distance between the card and the terminal for reading and writing data. It was therefore not possible to create a single standard that provides a single technical solution to all the requirements arising from various applications
Presently,three different standards describing three different reading ranges have been completed. Each of these standards in turn permits various technical solutions,since the members of the standardization committee could not agree on a single solution. In order to achieve interoperability among the various options, card terminals must support all of these options.
Standardization started with ‘close-coupling’ cards (ISO/IEC 10536), since the microprocessors available at that time had relatively high power consumption, making energy transfer overarelativelylargedistanceimpossible.Theessentialpartsofthisstandardhavebeencompletedand approved and are described in the following section.In use,this type of card offers only minimal advantages compared with normal contact-type cards, since it must be inserted into a terminal or at least precisely placed on a surface of a card terminal. Furthermore, the structure of the card is complex, which results in high manufacturing costs. Consequently,up to now this type of system has hardly established a signiﬁcant position in the market.
Close-coupling cards: ISO/IEC 10536
In the ISO/IEC10536 standard for close-coupling cards,this application is designated as‘slot or surface operation’,which expresses the fact that in use the card must be inserted into a slot or laid on a marked surface of the terminal. The ISO/IEC 10536 standard , which bears the title‘Identiﬁcation Cards– Contactless Integrated Circuit(s)Cards’, consists of four parts:
- Part1: Physical characteristics
- Part2: Dimension and location of coupling areas
- Part3: Electronic signals and reset procedures
- Part4: Answer to reset and transmission protocols.
The term‘cards with remote coupling’encompasses smart cards that can transmit data over a distance of a few centimeters to approximately one meter from the terminal.This capability is of great signiﬁcance for all applications in which data should be exchanged between the card and the terminal without requiring the card user to take the card in his or her hand and insert it in to a terminal. Some sample applications are:
- access control
- vehicle identiﬁcation
- electronic driver’s licenses
- ski passes
- airline tickets
- electronic purses
- baggage identiﬁcation.
The variety of applications suggests that there are a large number of possible technical implementations. In the preparation of the standards, an attempt was made to limit the number of technical variants, with only mixed success. International standards ISO/IEC 14 443 and ISO/IEC 15 633 cover the ranges of up to 10 cmand 1 m, respectively.
Remote coupling cards :
- proximity coupling cards (PICC)
- ISO/IEC 14 443
- typical range 10 cm
- vicinity coupling cards (VICC)
- ISO/IEC 15 693
- typical range 1 m
Proximity integrated circuit(s) cards: ISO/IEC 14 443
The ISO/IEC 14 443 standard, which is titled ‘Identiﬁcation cards – Contactless integrated circuit(s) cards – Proximity cards’, describes the properties and operation modes of contactless smart cards with a range of approximately 10 cm. The amount of energy that can be transferred over this range is sufﬁcient to power a microprocessor. In order to allow this type of card to be used with existing infrastructures for contact-type cards, they often have contacts in addition to the coupling components for contactless operation, so that they can be used with or without contacts as desired. This type of card is called a ‘dual-interface card’ or ‘combi card’.
The ISO/IEC 14 443 standard consists of the following parts:
- Part 1: Physical characteristics
- Part 2: Radio frequency power and signal interface
- Part 3: Initialization and anticollision
- Part 4: Transmission protocol.
The physical characteristics of proximity cards, which are deﬁned in Part 1 of the ISO/IEC standard for proximity integrated circuit cards (PICCs), essentially correspond to the requirements speciﬁed for smart cards with contacts .It is to be expected that in use,proximity cards will be exposed to electromagnetic ﬁelds corresponding to those intended to be used for the operation of other types of cards that comply with standards such as ISO/IEC 10 536 or ISO/IEC 15 693. The cards must not suffer permanent damage as the result of exposure to such ﬁelds or the environmental stress of normal ambient electromagnetic ﬁelds. In order to ensure this, the standard speciﬁes maximum values for stresses due to alternating electric and magnetic ﬁelds that the cards must withstand without damage. It is the task of the semiconductor manufacturer to design the chips such that they meet these requirements.
Radio-frequency power and signal interface
Proximity cards work on the principle of inductive coupling. Operating power and data are both transfer red using an alternating magnetic ﬁeld generated by the card terminal. In the ISO/IEC 14443 standard,the card terminal is called a‘proximity coupling device’(PCD). For the sake of readability, in the following description the more general term ‘terminal’ and ‘PCD’ are used interchangeably
The transmission frequency of the PCD is set to fC =13.56MHz±7kHz, with a magnetic ﬁeld strength H of at least 1.5 A/m and at most 7.5 A/m (effective value). The typical ﬁeld strength versus distance is shown in below Figure .
The range of the system can be estimated from the ﬁeld strength of the PCD and the activation ﬁeld strength of a proximity card (PICC). With the typical ﬁeld strength curve shown in above Figure and an assumed PICC activation ﬁeld strength of 1.5 W/m, we obtain a range of approximately 10cm.
Signal and communication interface
Two different communication interfaces are deﬁned in the ISO/IEC 14 443 standard, which are designated Type A and Type B. The reason for standardizing two different methods was not technical, but rather that at the time that ISO/IEC 14 443 was being prepared, various designs from different manufacturers were already in existence. As is often the case with standardization, the differing interests of the persons involved made it impossible for them to agree on a single method, although that would have been technically desirable. The two methods mentioned above were agreed on as a compromise, and they were published as an international standard in June 2001.Even with the already existing methods,it is necessary for card terminals to support both methods in order to achieve full interoperability with all cards meeting the ISO/IEC 14 433 standard, since the cards generally support only one of the two standard methods.
While a terminal is waiting to detect a proximity card, it must periodically switch back and forth between the two communications methods.This allows it to recognize both Type-A and Type-B cards. Once the PCD has recognized a card, it continues to use the appropriate communications method until the card is deactivated by the terminal or leaves the working range of the terminal.
Type-A communications interface
With Type-A cards, data transmission takes place in both directions at a bit rate of fC/128 ≈106kbit/s).