Contactless Technologies
The smart card industry has developed three
ranges of smart cards that operate without metallic contacts: Close-coupled,
Proximity and Vicinity.
Proximity cards, which typically operate at
up to 10 centimeters, are currently the focus of the most intense industry
activity, as the cards are making headway in replacing the 1.5 billion
mag stripe cards used for transit ticketing each year (The Nilson
Report, January, 1997, Issue 635). Vicinity cards, which can be
expected to operate at a meter and even more, are receiving attention
because of their potential for replacing other RFID technologies, as
well as bar codes, in access control and materials tracking.
Close-coupled cards, which have no metallic
contacts but which must be physically touched to the reader, are now
receiving renewed interest, as technological innovations have made these
cards a cost-effective alternative to contact cards for a range of applications.
This article will review the features and applications for new, close-coupled
technology.
History of close-coupled cards
Close-coupled cards provide many of the advantages
of other contactless cards, including high durability for cards as well
as for the card readers, because the readers have no moving parts and
are therefore virtually maintenance-free. In the 1980's, AT&T invested
considerable resources to develop capacitive, close-coupled cards. The
AT&T approach was included in the original ISO 10536 standard for
contactless chip cards.
In the ATT configuration (now owned by Lucent),
a coil is used in the card to receive unmodulated carrier energy for
powering the card, and multiple pairs of capacitive plates are used
for data transmission and reception. The close-coupling provides an
efficient transfer medium, with minimal energy being wasted or radiated.
However, the configuration has several shortcomings. First, the combination
of coils and capacitive interfaces leads to higher production costs.
The interface also lacks the convenience of other contactless modes
in that cards must be inserted in a slot, and cannot be swiped or touched
to the reader.
Recent advances - ISO 7816 transparency
In order to achieve the benefits of close-coupling
in a cost-effective manner, Ambient has invented a capacitive interface
(see Figure 1) which uses only one pair of capacitive plates, and which
combines power, data and all other ISO 7816 signals together. The capacitive
geometry also permits the card to be swiped or touched to the reader.
The first implementation of the Ambient technology
provides up to 60 milliamps of current at 5 volts, enough for virtually
any chip likely to be put into a smart card. It further provides the
3.57 MHz standard clock, as well as a 13.56 MHz clock for high speed
processors. Data rates can vary over the entire range specified by ISO
7816, namely 9.6 to 76.8 kBaud, and well beyond. Reset signals are generated
at power up and whenever the reader asserts the Reset line.
This close-coupled interface is thus a transparent
link to standard, contact smart card chips. The interface chip is also
equipped with contact inputs, for use in a "Combi" card. Thus,
the introduction of such contactless cards can be gradual and low risk,
continuing to also utilize contact readers which may already be in place.
Contactless large memory cards and other future directions
The large currents and high data rates available from
this close-coupled interface are sufficient to power multi-megabyte,
FLASH memories. The basic interface design also lends itself to supporting
certain other kinds of asynchronous serial interfaces, such as I2C and
other standards. This interface can also be useful for geometries and
for products other than smart cards, utilizing the power transfer and
high speed bi-directional data transfer in harsh environments.
Advantages of close coupling
Power and Data Rates: Capacitive, close-coupling emits very little radio frequency radiation.
The coupling is based on an electrostatic field, which is much easier
to shield than a magnetic field. Consequently, capacitive cards do not
have the power and data rate limitations that are imposed on proximity
cards because of legal limitations on stray radiation. The capacitive
interface can provide the power necessary for the most secure, large
memory processors available for contact cards. By comparison, other
contactless technologies lag behind contact chips because of the delay
in creating lower power chip designs.
Hacker security: The convenience of the
proximity cards for transportation applications has been proven. However,
issuers of these cards are faced with a security dilemma, especially
if they want to issue the cards for a wider range of applications. If
the cards are programmed to become permanently inactive after a number
of incorrect access attempts, then malicious individuals may use hidden
transmitters to repeatedly attempt access to cards at a distance, causing
the cards to "self-destruct."
If the chips omit this feature, they become
vulnerable to hacking, as the possibility of an unlimited number of
access attempts facilitates a cryptographic attack. Capacitive, close-coupled
cards are not accessible from any distance, and so are immune to remote
jamming. Although the capacity to authorize transactions at a distance
may be advantageous for high volume transit, the remote capability is
a disadvantage for broader payment systems.
Summary
The new close-coupling technology allows continued
use of proven contact card technology, including communications protocols,
chip masks, security layers, and applications. It provides an economical
migration path towards enjoying the benefits of low reader maintenance,
potentially long card lifetime, and usage of cards in hostile environments,
without encountering some limitations of magnetically coupled cards.