Communications: electrical – Selective – Interrogation response
Reexamination Certificate
1999-11-22
2003-11-11
Lieu, Julie (Department: 2632)
Communications: electrical
Selective
Interrogation response
C340S010400, C340S572200, C340S572400, C340S010320
Reexamination Certificate
active
06646543
ABSTRACT:
FIELD
The invention relates to contactless communication between a portable object and a terminal.
BACKGROUND
Interchanging data without contact is well known; the applications of this technique include, in a non-limiting manner, controlling access and remote payment, e.g. access to and payment for public transport.
As shown in the example contactless communication system
1
in
FIG. 4
, each user is provided with a portable object
2
of the “contactless card” or “contactless badge” type which is an object capable of interchanging information with a fixed “terminal”
3
by bringing the badge close to the terminal so as to enable non-metallic mutual coupling to be established there between (with the term “terminal” by bringing the badge close to the terminal so as to enable non-metallic mutual coupling to be established there between (with the term “terminal” being commonly used to designate a data transceiver suitable for cooperating with portable objects).
SUMMARY
The invention relates to the particular situation in which a plurality of badges can be present simultaneously in the field of action of the terminal, and where there are no physical means, e.g. insertion into a slot, for isolating the arrival of a single object in the field.
Given that the signals transmitted by the various badges can overlap one another in time, the terminal must be provided with an “anti-collision” mechanism enabling it to detect such situations and to handle them so as to cause signals to be retransmitted by the badges.
Various algorithms have been proposed for this purpose. For example, European Patent Application No. 0 669 592 is based on accurate synchronization of the responses from badges that are present simultaneously in the field of action of the terminal. However, that requires the ability to achieve such synchronization, i.e. to cause all of the badges to transmit a signal at the same time in response to an interrogation message transmitted by the terminal. In practice, that technique requires a response mechanism to be integrated in each badge in the form of hard-wired logic.
It is not always possible to obtain such synchronization, particularly when using microprocessor badges which have response times that are not predictable with sufficient accuracy to be able to obtain the necessary degree of synchronization.
Under such circumstances, proposals have been made, e.g. in French Patent Application No. 2 666 187 A, to define a frame having a relatively large number of slots, with the response from the badge being transmitted asynchronously during a slot whose position within the frame is determined by selecting a random or pseudo-random number for each transmission. That technique solves the problem of lack of synchronization between badges and greatly reduces the probability of collision, with the probability decreasing with increasing number of time slots per frame.
Nevertheless, to be effective, that technique suffers from the drawback of requiring frames that are relatively long, given that the frames must have a large number of time slots, and that in addition the time slots must themselves be sufficiently large to enable all of the requisite information to be transmitted, particularly the complete identification number of the badge. This gives rise to a large and systemic increase in the duration of a transaction, and this applies independently of the number of badges actually present in the field of action of the terminal (since the duration of the frame is independent of the number, and is just as long in the most frequent case of only one badge being present as it is in the case of a large number of badges being present).
European Patent Application No. 0 702 324 A proposes another mechanism for handling collisions which provides that if a collision is detected, then the terminal sends a collision warning to the badge; on receiving this warning, each badge decides whether or not to send a message in reply, with the decision being random in nature and having a fixed probability of response, e.g. 50%. The number of responses received by the terminal, and consequently the risk of collision, is thus reduced after a first collision has been detected. If one or more collisions are still detected, the mechanism is reiterated until collisions have completely disappeared.
With such a mechanism, particularly when many badges are present in the field of action of the terminal, it will be understood that collisions can reoccur several times over and the number of reiterations can be large, thereby giving rise to a corresponding increase in the time required to identify all of the badges present.
One of the objects of the invention is to solve those various difficulties by proposing an asynchronous type of mechanism for handling collisions, i.e. a mechanism that is entirely suitable for use with microprocessor badges, which mechanism can easily be matched to the average number of badges actually or potentially present in the field of action of the terminal.
It is explained below that this adaptive matching under the control of the terminal can be static or dynamic. Static matching includes setting the terminal a priori to optimize the duration of the anti-collision algorithm as a function of the mean number of badges most likely to be found in the field of action of the terminal. Dynamic matching includes modifying the parameters of the algorithm during a given identification sequence, from one iteration to the next, as a function of the number of badges actually present in the field of action of the terminal.
Static or dynamic matching can be implemented independently or cumulatively.
It will also be observed that optimizing the process of collision handling (static or dynamic matching) is under the control of the terminal and thus independent of the badges, which can thus be used equally well in circumstances and environments that are very different.
Thus, the same badge can be used without modification of its own settings both with terminals having “handsfree” type readers (e.g. for passing through an inspection gate) where a large number of badges may be present simultaneously within the field of action of the terminal, typically as many as 8 to 10 badges simultaneously, and with terminals having “voluntary act” type readers, i.e. where the user is required to bring the badge into a read zone of small size or to press the badge thereagainst, as occurs, for example, when remotely paying for public transport (in which case the typical number of badges that can be present simultaneously is much smaller: close to 2, and only very rarely greater than 3).
SUMMARY
More precisely, the method of the invention comprises the following successive steps: a) the present terminal transmits, to said plurality of badges, general call messages, each including a collision probability modulation parameter; b) each badge conditionally transmits, to the terminal, a general call response message with a probability of less than 100% as a function of the collision probability modulation parameter, each general call response message containing an identifier specific to the badge; c) on the terminal receiving a general call response message, and in the absence of any collision between the message and a message transmitted by another badge, a specific data communication link is established between the terminal and the badge and data interchange is continued, returning to step a) for a new iteration; and d) otherwise, the method returns to step a) for a new iteration.
In various advantageous implementations:
the probability of a general call response message being transmitted is modified on each new consecutive iteration in step c) or d); in particular this probability is decreased on the next iteration following step d) in the event of collision;
at least within iterations following step d) in the event of a collision subsequent to first transmission of a general call message (GENCALL), the probability modulation parameter is transmitted by the terminal so as to define a probability of less than 100% for trans
Grieu Francois
Mardinian Gregory
Lieu Julie
Nixon & Vanderhye P.C.
Regie Autonome des Transports Parisiens
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