Classifications

• • G—PHYSICS
  • G06—COMPUTING; CALCULATING OR COUNTING
  • G06F—ELECTRIC DIGITAL DATA PROCESSING
  • G06F13/00—Interconnection of, or transfer of information or other signals between, memories, input/output devices or central processing units
  • G06F13/14—Handling requests for interconnection or transfer
  • G06F13/36—Handling requests for interconnection or transfer for access to common bus or bus system
  • G06F13/368—Handling requests for interconnection or transfer for access to common bus or bus system with decentralised access control
  • G06F13/376—Handling requests for interconnection or transfer for access to common bus or bus system with decentralised access control using a contention resolving method, e.g. collision detection, collision avoidance

Definitions

• the present invention relates to systems in which several elements can be identified simultaneously by a host computer or an equivalent device responsible for identifying the elements, thus resulting in a collision between several elements, and relates in particular to an anti-collision method of elements to be identified by a host computer.
• a host computer or equivalent device is responsible for identifying elements each having an identification number and which is in a certain relationship with the host computer. This is the case when the elements are terminals belonging to the same data transmission network (relation belonging to the same network), but also when the elements are contactless objects allowing access to an access area checked when identified by a reader (proximity relationship to the reader).
• the "root" of the tree gives rise to two “branches” each ending in a "node” corresponding to the logical value 1 or 0 of a first bit of the identification number, for example the bit strongest weight.
• These two nodes each give birth to two new branches each ending in a node corresponding to the value 1 or 0 of another bit of the identification number, and so on until the last bit of the identification number knowing that the tree comprises as many generations as there are bits for coding the identification numbers of the cards.
• the anti-collision methods based on tree iterative algorithms have the drawback of being slow in that the exploration of the whole tree representing the identification numbers of the cards is all the longer that the identification number is coded on a large number of bits.
• a large number of bits must be provided for coding the identification numbers, whereas in most cases only a very small number of cards will be present simultaneously in the area covered by the reader.
• the object of the invention is to carry out an anti-collision method which is not based on the bit-by-bit comparison of the identification numbers and therefore does not require the tracking of the bit which caused the collision, but which uses the positioning of each identification number relative to a numerical value of identification attempt transmitted by the host computer using an algorithm allowing the rapid resolution of the collision.
• the present invention therefore relates to a method of anti-collision by a host computer of elements being in a determined relationship with the host computer allowing the identification of the elements by the latter, each of the elements having an identification number included. between 0 and a maximum value (MAX).
• the method comprises the following steps: a) transmission by the host computer of a request instruction capable of being received and recognized by any element in the determined relationship with the host computer, b) when at least one element is in the determined relationship with the host computer, transmission by the latter of an anti-collision instruction comprising a numerical value of attempt to identify, c) response by any element being in the determined relationship with the host computer and having an identification number less than or equal to the numerical value of temptation to identify, and d) detection of a collision between several elements when the host computer receives several responses and in this case, transmission of a anti-collision instruction comprising a numerical value of attempt to identify defined according to a determined algorithm, or selection of the element by its number identification when there is only one answer
• the elements are contactless electronic cards such as access cards to a controlled access area, the identification of which is carried out by passing the card through a determined area in front of a reader playing the role of the host computer.
• the method of the invention can be implemented in any system in which several elements are in a determined relationship with a host computer. However, in the following description, the method is described in a preferred embodiment where the elements are contactless cards communicating with a reader when they are in a space defined relative to the reader.
• any card which is in the space defined in relation to the reader expects to receive a request from the reader, this request being transmitted cyclically by said reader (for example every 5 ms ). Consequently, each card verifies that it receives the request (step 10), this step being looped back on itself until the request is received.
• the card chip When the request has been received by the card chip, it waits for reception of an “anti-collision” instruction from the reader (step 12) and loops until the instruction has been received . On receipt of the anti-collision instruction, the chip verifies that its own identification number is less than or equal to a numerical value of attempt to identify contained in the instruction (step 14). If not, the process loops back to waiting for an anti-collision instruction. Note that all the cards that receive the request participate in the anti-collision process and that the cards connected after the sending of this request do not participate, this in order to prevent any disturbance by the new cards.
• the chip on the card responds to the reader by transmitting its identification number (step 16).
• the chip waits for a new instruction, this instruction possibly being a selection instruction or a new anti-collision instruction (step 18).
• this instruction possibly being a selection instruction or a new anti-collision instruction
• the process loops back to step 14 to verify that the identification number of the card is less than the numerical value of identification attempt contained in the new instruction. Note that the above process of looping back steps 14 and 16 can be repeated several times as described below with reference to FIG. 2.
• the basic principle of the invention is to check whether the identification number of the card is less than or equal to an identification attempt value, it would also be possible to check whether the identification number of the card is greater than or equal to an attempt to identify value without departing from the scope of the invention.
• the card receives a selection instruction, it means that it has been identified.
• the chip again transmits its identification number to the reader (step 20) and no longer participates in the anti-collision process.
• the anti-collision process implemented in the reader takes place as described below with reference to FIG. 2.
• the reader sends a request (step 30) to all the cards found in a space defined in relation to the reader.
• the reader waits for at least one response from a card (step 32) and loops until a response is not received.
• a MIN variable which designates the lower limit of the interval in which the identification numbers to be identified are located is initialized to 0
• VALID representing the numerical value of attempt identification is initialized to MAX which is the highest value that can take the identification number assigned to the card
• DERCOLL is a variable representing the last value of VALID used in the process is also initialized to MAX.
• an anti-collision instruction containing a numerical value of identification attempt is transmitted by the reader (step 36).
• the reader waits for a possible response (step 38). If a response is received, this means that there is at least one card in the defined space where communication can take place with the reader. The reader must then determine if there is a collision or if there is only one response, that is to say a single card in the defined space (step 40).
• the identification number of the card is recorded by the reader (step 46) so as to allow it to select the card as seen previously.
• the value of MIN is modified and brought to VALID
• step 48 the value of VALID is brought to the value of
• step 52 before looping the process back to the step of transmitting a new anti-collision instruction.
• this second algorithm makes it possible to bring the value of VALID up to the MAX value and to be able to identify cards whose identification numbers are in intervals close to MAX and which have not yet been detected because they collided with identification numbers located in lower intervals.
• step 54 it is first checked whether the value of VALID has reached MAX (step 54). If this is the case, it is certain that there are no more cards to identify and the process ends. Otherwise, a check is made to know if VALID is equal to DERCOLL (step 56). If this is the case, the value of DERCOLL is set equal to MAX (step 58). In both cases, the value of MIN is made equal to the value of VALID (step 60) and the value of VALID is modified taking into account a parameter M called the ascent speed which is greater than 1 ( step 62) as follows:
• VALID VALID + (DERCOLL - VALID) / M before the process is looped back to step 36 of transmission of a new anti-collision instruction. Note that thanks to the use of this third algorithm, the new value of VALID is necessarily greater than the previous one.
• the method according to the invention which is the subject of the description above uses the means of communication in accordance with standard ISO 14443. But unlike existing methods, it does not need to identification of the bit which generated the collision, the collision being deduced from the erroneous "checksum". It is not time dependent and can therefore be implemented in BASIC language on PC as well as "hardware" in the reader without impacting the functionality of the process. Whether in each element, for example a contactless card, or in the host computer or equivalent such as the contactless card reader, the method according to the invention is implemented by a computer program, in particular a computer program. 'computer.

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• Engineering & Computer Science (AREA)
• Theoretical Computer Science (AREA)
• Physics & Mathematics (AREA)
• General Engineering & Computer Science (AREA)
• General Physics & Mathematics (AREA)
• Mobile Radio Communication Systems (AREA)
• Computer And Data Communications (AREA)
• Credit Cards Or The Like (AREA)
• Small-Scale Networks (AREA)

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