The present invention concerns a communication protocol between one or more transceiver units and transponders or transceivers. The transponders or transceivers respond to a transceiver unit by entering its field of action and interacting with the latter in accordance with a given protocol, associated with a defined coding of the commands and data exchanged between said transceiver unit (hereinafter  less than  less than communication unit  greater than  greater than ) and said transponders or transceivers (hereinafter  less than  less than transponders  greater than  greater than ).
Usually, the encoding of the commands and exchanged data is performed in accordance with a binary mode, i.e. a succession of logic bits  less than  less than 1 greater than  greater than  and  less than  less than 0 greater than  greater than . The transmission of these logic bits is effected by means of an electromagnetic coding signal known to those skilled in the art. For example, these bits are transmitted by modulation or non-modulation of the carrier wave. For example, Manchester, Miller and differential bi-phase types of coding are known, which are applied to amplitude modulation (ASK: amplitude shift keying), phase modulation (PSK: phase shift keying), frequency modulation (FSK: frequency shift keying) and also others known to those skilled in the art.
The communication protocol defines the exchange of commands and data or information between one or more communication units and one or more transponders.
Certain communication protocols include data identification or transmission procedures composed of very long bit sequences formed of random bits and bits representing functions calculated using given algorithms from said random bit sequences and, possibly, from certain public or secret keys. There is a non-negligible probability that, during transmission of the aforementioned bit sequences, a communication unit generates a sequence corresponding to a valid system command during transmission of data or information other than a command. Such probable situations cause poor operation of the system when several transponders enter the field of action of a communication unit in a same time
Moreover, it is possible for transponders associated with different systems to enter the field of action of a communication unit belonging to a particular system. In such case, it is necessary to prevent the unit entering into communication with said transponders, which would cause undesired responses and disrupt the desired data exchanges. More precisely, a communication unit simultaneously receives in its field of action a transponder associated with the system of said unit and another transponder associated with another system. If the commands and data exchanges are performed in accordance with the same coding and the same coding time structure, at least a portion of the interrogation signal may correspond to a command actuating the transponder belonging to said other system, the latter starting to modulate. This creates disruption and confusion in the receiving part of said communication unit, which prevents proper reading of the transponder associated with the system of said unit.
Even in the event that the transponders include an anti-collision protocol, the risk of interaction between a communication unit and transponders not associated with the system of said unit remains since there exists a multitude of different protocols.
In order to overcome the aforementioned problems and prevent transponders interacting or exchanging data with a communication unit belonging to a different system, the present invention proposes a communication protocol between at least one communication unit and transponders which are associated with the communication system of such unit and arranged for exchanging coded data with such unit, wherein said communication unit sends at least one initial command to establish communication with said transponders entering its field of action, this initial command having at least partially a coding with a specific time structure which is different from the basic time structure used for encoding data other than said at least one command.
As a result of the features of the protocol according to the invention, the portion of the initial command coding which has a different specific time structure from the basic time structure used for the coding of exchanged data, in particular data other than commands, prevents a random bit distribution sequence for any data exchange being confused with a command from a communication unit used to establish interaction with transponders. Thus, no transponder will be actuated by a coded data exchange between another transponder and a communication unit as long as all the transponders present in the field of action of the communication unit are arranged so as to interact with said unit following reception of a command signal having at least partially a coding with a specific time structure according to the present invention.
The present invention thus provides a reliable and efficient solution in the event of a consensus between the different manufacturers and suppliers of communication units and transponders at least as regards systems or applications which are similar or have a non-negligible probability of momentarily cohabiting in the same place or space.
It was mentioned hereinbefore that at least the initial command had at least in part a coding with a specific time structure, but several variants are possible and can be implemented by those skilled in the art without any particular difficulty. In particular, it is possible for all the commands to have at least partially a coding with a specific time structure according to the present invention. It is also possible for the initial command or all the commands to have only one initial bit and possibly a final bit in accordance with the specific time structure or to have all the bits coded with this specific time structure. In a particular implementation, the initial command and the final command of a data exchange between a communication unit and the transponders which are associated therewith have, at least partially, a coding with a different specific time structure from the basic time structure. In a preferred implementation, at least each first bit or initial bit of every command is generated on the basis of the specific time structure according to the invention.
It will be noted that it is also possible at least in the case of active transponders, for the commands and/or at least an initial response sent to a unit to be generated in accordance with a coding having a specific time structure according to the present invention. Those skilled in the art who understand the present invention will know how to apply it in an appropriate and efficient manner according to the various systems and protocols present without thereby departing from the scope of the present invention.
According to a preferred implementation of the protocol according to the invention, said specific time structure has a greater coding time period than the coding time period used by the basic time structure. In particular, during at least one coding time period, this specific time structure has a characteristic electric value which is constant over a time interval greater than any length of time or duration of said characteristic electric value able to appear in the coding(s) provided with said basic time structure.
As a result of this latter feature, the specific time structure provided for said at least one initial command cannot in any event correspond to the coding of any data with said basic time structure.