Contactless bank cards
Peter Tomlinson
pwt at iosis.co.uk
Fri Nov 19 09:30:21 GMT 2010
Roland Perry wrote:
> ps A paywave and an Oyster presumably don't interfere, because
> Barclays have a card with both.
To labour the point a little, the interference discussed here has two
sources:
(1) When more than one card responds to the terminal's RF field . There
is a provision in the 14443 standard for anti-collision processing, but
the characteristics of the interface between card and terminal are such
that anti-collision may fail, or the terminal may not even implement the
function. (And the rule breaking blocker card would no doubt be designed
to start shouting continuously as soon as it has enough power to start
up, rather than simply indicate that its there and waiting.)
(2) Having two cards in very close proximity to each other (e.g. in a
closed wallet) affects the electrical characteristics of the RF
circuitry of both cards, so that data comms may well be compromised, or
it may even compromise the ability of the cards to collect enough power
to operate.
Where there is more than one application hosted on one card, then that
card will wait for an application select - and then only one app will
become active - or the card may be programmed to pre-select one app,
leaving the terminal to ask if it wants to see if another app is
present. (TfL is now in the position where it will soon have to cope
with 3 different on-card apps: Oyster, ITSO, EMV. Oyster and ITSO are
both just data areas, but in the DESFire cards currently being issued
these require app select; EMV is a microprocessor-based application.)
The carrier frequency used in the power transfer and data comms is 13.56
MHz, but the aerial coil in the card is NOT tuned to that frequency. It
is resonant at a rather higher frequency. The aerial coil in the
terminal is tuned to something very near 13.56 MHz - but the presence of
the card changes the exact resonant frequency of the terminal's aerial
coil. I don't know anyone who has been able to create an exact
mathematical model for this somewhat approximate technology. As I wrote
in a July 2003 paper, having (as expected) found that different
designers make different decisions about both card and terminal aerial
coil characteristics:
(I) have realised that those developing 14443 made unwarranted
assumptions about the circuitry used around the two coils involved:
card aerial coil and terminal aerial coil. One statement made to me
is that it is assumed that the card’s coil will be tuned to around
15 to 17 MHz, and the terminal’s coil to 13.56 MHz. Another
statement is that adjusting the tuning of the terminal’s aerial coil
changes the characteristics - you tune for maximum range but accept
a blind spot close to the terminal, or you tune for no blind spot
and the range reduces.
In practice it sems that card manufacturers try to make the resonant
frequency of the aerial coil as high as possible (over 20 MHz), while
maintaining power collecting performance. Some cards indeed use as large
an area of coil as is reliably possible from a durability point of view,
others have a very small coil (which is why we can have adequate
performance from very small form factors).
Peter
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