Patent EA-002003-B1 (EA)
Title: Method for managing collisions in a contactless data exchanging system
Inventor: MARDINIAN GREGORY
IPC: G06K17/00
Filed: 1998-01-26
Abstract: 1. A method of controlling collisions in a system of interchanging data between badge-type portable objects and at least one data transceiver terminal suitable for co-operating with a plurality of badges simultaneously present in the field of action of the terminal, which method is characterized by the following successive steps: a) the terminal transmits, to said plurality of badges, general call messages (GENCALL), each including a collision probability modulation parameter (PMP); b) each badge of said plurality of badges conditionally transmits, to the terminal, a general call response message (GENREP), said message being transmitted with a transmission probability of less than 100% as a function of the value taken by the probability modulation parameter (PMP), each general call response message (GENREP) 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 said message and a message transmitted by another badge of said plurality of badges, a specific data communication link is established between the terminal and the badge which has transmitted the message at step (b), 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. 2. The method of claim 1, in which the probability of a general call response message being transmitted is modified on each new consecutive iteration in step c) or d). 3. The method of claim 2, in which the probability of a general call response message being transmitted is decreased in the event of a collision on the new iteration following step d). 4. A method of claim 1, in which, 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 (PMP) is transmitted by the terminal so as to define a probability of less than 100% for transmission of a general call response message (GENREP) by each badge likely to reply. 5. The method of claim 4, in which, when a badge receives the probability modulation parameter (PMP), the badge produces a pseudo-random value and compares said pseudo-random value with the received probability modulation parameter. 6. The method of claim 4, in which: - the probability modulation parameter comprises a first field containing fixed data representative of a predetermined type of terminal, and a second field containing specific data for modulating probability; - in step b), the fixed data is subjected to prior comparison with corresponding data contained in the badge; and - the conditional transmission of step b) and the conditional transmission of step c) are performed only if the prior comparison of step b) has also given rise to a matching result. 7. The method of claim 1, in which in step c), establishing the specific link includes: - generating a control message (COM) comprising both the identifier specific to the badge as received by the terminal in response to the general call, and an unambiguous communications channel number; - sending said control message from the terminal to the badge; and then - continuing the data interchange on the channel allocated in this way. 8. The method of claim 7, in which, for a badge that might be found simultaneously in the fields of action of a plurality of terminals, the channel number includes a data item specific to the terminal which has established a link with the badge, said data item differing between adjacent terminals. 9. The method of claim 1, in which: - the probability modulation parameter is a parameter that is modifiable dynamically from one general call message to the next; - the terminal includes means operative in step d) to discriminate between the absence of any general call response message being received and a non-compliant message being received, particularly in the event of collision between messages transmitted by two different badges; - in step d), in the presence of a non-complaint message, the probability modulation parameter is modified before returning to step a), with the modification being in a direction that decreases the probability of a matching result being obtained in the subsequent step b); and - in step d), in the absence of the terminal receiving a general call response message, the probability modulation parameter is modified prior to returning to step a) in a direction that increases the probability of obtaining a matching result in the subsequent step b). 10. The method of claim 1, in which, after each comparison of step b), the badge counts the number of consecutive non-matching results produced in response to the same series of general call messages transmitted by the terminal, and forces a response message to be transmitted to the general call whenever the corresponding count has reached a predetermined threshold, even when the protocol defined by the probability modulation parameter requires a random response.