Patent Publication Number: US-2022223051-A1

Title: Secure transfer system for digital aircraft data, data producer system, data consumer system, and related transfer method

Description:
The present disclosure relates to an aircraft data transfer system, comprising:
         a data producer system able to produce first data at successive moments;   at least one data consumer system able to receive and use the data produced by the data producer system;   with at least one of the data producer system and the or each data consumer system being installed in an aircraft;   a data transfer link between the data producer system and the or each data consumer system.       

     The data transfer system is intended to ensure secure transfer of digital data within an aircraft, from the aircraft to at least one data consumer system external to the aircraft (such as at least one other aircraft, a ground control station in the case of a UAV, etc.), and/or from a data producer system external to the aircraft (such as another aircraft, a mission preparation system, a data download system, etc.) to the aircraft. 
     It is intended to be used in particular between a data producer system and one or more data consumer systems located at a distance from each other. The data transfer system can also be used within the same aircraft system, in particular between different computers of the same aircraft system, between an application producing equipment such as a sensor and one or more computers, between several application layers of the same computer, or between different cores of the same processor. 
     The transfer system is intended to be implemented independently of the number of digital data transfer links between each data producer system and each data consumer system and independently of the nature of these links in the overall data transmission architecture. 
     BACKGROUND 
     In an aircraft, many aircraft systems are able to generate functional data. This is the case with systems for measuring sensor parameters, for example, with the parameters internal or external to the aircraft or avionics systems, in particular aircraft piloting, control or guidance systems. 
     The functional data is aircraft parameter measurements, aircraft system control commands or aircraft systems tracking data, for example. 
     The functional data is often used by other aircraft systems, or by other application layers of the same aircraft system. They are therefore transferred between a data producer system and at least one data consumer system that may be located remotely, or even outside the aircraft or in other application layers. 
     Digital data transfer protocols are widely used within aircraft. 
     Some “low-level” protocols transmit digital data in the form of data words, each of which contains protocol management and functional information. This is the case with the ARINC429 protocol, for example. 
     More advanced protocols, such as the GAM-T-101 protocol (Digibus), the MIL-STD-1553B protocol, or the Ethernet protocol and its so-called deterministic derivatives (ARINC664 Part 7 or SAE AS6802 or TSN) implement messages that enable the atomic transmission of several data words, some of which contain exclusively protocol management information and others exclusively functional information. 
     In all cases, ensuring the integrity of the transmitted data is essential in the aeronautical field. 
     In the example of the ARINC429 protocol, the functional data is transmitted as a sequence of bits via ARINC429 words, whose size is limited to 32 bits. 
     Each ARINC429 word includes a data identifier (ID) called a label, an identifier of the source that issued the data (SDI), the functional data (D), and a validity identifier of the data issued by the producer system (SSM). 
     The ARINC429 protocol facilitates the transmission of so-called functional data in the form of data labeling and, in the following, as commonly used, “label” may refer to the full ARINC429 word. 
     To verify functional data integrity after transmission, each ARINC429 word also includes a parity check bit (P), resulting from the binary sum of the functional data bits. The parity bit is calculated when encoding the data into ARINC429 words in the data transmission layer. 
     When retrieving the data contained within an ARINC429 word, the consumer system is able to recalculate the parity of the functional data it receives and verify that it is identical to the parity bit contained in the ARINC429 word. 
     Nevertheless, this integrity test is often an insufficient indicator for seeing an integrity loss in functional data packet during the transfer. 
     Thus, if two bits of the functional data packet are erroneous, the sum may yield a parity corresponding to that of the check bit, while the functional data packet has been doubly corrupted. 
     Such a protocol can therefore be used when the data being transmitted is less important or not critical to the safety and operation of the aircraft. 
     If the data is important or critical, it is necessary to use redundant producing systems that elaborate data in parallel and transmit them in parallel to one or more consuming systems. The consuming system(s) receive(s) the data from the different producing systems and applies selection or voting strategies to ensure data integrity. Such a solution is safe, but increases the mass, the cost and the complexity of the aircraft. 
     SUMMARY 
     One object of the present disclosure is to provide an aircraft data transfer system between a functional data producer system and at least one system consuming such data that is simple and easily adaptable to an existing data transfer protocol, and which is sufficiently secure in the event that an error in this data could have catastrophic consequences. 
     To this end, the present disclosure provide a data transfer system as defined above, where the data producer system is configured to generate, at each data generating, a data capsule, each data capsule comprising an identifier of the data producer, a refreshment indicator updated at each data production, possibly a functional data packet, and a result of the data integrity check relating to at least the identifier, the refresh indicator and possibly the functional data packet; 
     each data capsule being able to being transferred to the or each data consumer system via the transfer link; 
     the or each data consumer system being able to retrieve each data capsule to extract the identifier, the refresh indicator, optionally the functional data packet, and the integrity check result, and to check the integrity of the data in the data capsule by establishing a new integrity check result from at least the identifier, the refresh indicator and optionally the functional data packet retrieved from the data capsule and by comparing the new integrity check result to the integrity check result retrieved from the data capsule, the data capsule being in an integrity state when the new integrity check result matches the integrity check result retrieved from the data capsule. 
     The system according to the present disclosure may include one or more of the following features, taken alone or in any technically feasible combination:
         each data capsule comprises a functional data packet, the integrity check result of the capsule data relating to at least the identifier, the refresh indicator and the functional data packet, the or each data consumer system being able to extract the functional data packet and to check the data integrity of the data capsule by establishing a new integrity check result from at least the identifier, the refresh indicator and the functional data packet;   the or each data consumer system is able to compare the identifier received in each data capsule with a predefined identifier, on a semantic or a mapping table basis, to deduce at least one content characteristic of the data capsule;   the data producer system issues data capsules of fixed size, the or each data consumer system being able to determine the size of the data capsule, the size and/or the position of the identifier, the refresh indicator, possibly the functional data packet or/and the integrity check result in the data capsule from a semantic or a mapping table linking the predefined identifier to the size of the data capsule, to the size and/or the position of the identifier, the refresh indicator, possibly the functional data packet and/or the integrity check result in the data capsule;   for each data capsule received, the or each data consumer system is able to determine a refresh increment from the refresh indicator of the data capsule and from the refresh indicator of a previously received data capsule, the or each data consumer system being able to determine a data capsule refresh state, between an expected refresh state and an inadequate refresh state, depending on the refresh increment and depending on the capsule production frequency, the capsules transfer and/or receiving frequency;   from the capsule identifier, the or each data consumer system is able to verify that the data producer system that issued the data capsule is the data producer system expected to issue the data capsule, the or each data consumer system being able to put the data capsule into a valid state that allows the use of the data it contains when the data capsule comes from the expected data producer system, is in the integrity state, and is in the expected refresh state;   the or each data consumer system is able to put into the invalid state a data capsule that does not come from the expected data producer system, which is not in the integrity state or which is not in the expected refresh state and to keep data capsules that follow a data capsule placed in the invalid state in the invalid state, the or each data consumer system being able to return at least one data capsule following a plurality of capsules held in the invalid state to the valid state when the data capsules held in the invalid state come from the expected data producer system, are in the integrity state and in the expected refresh state for a predefined reset period;   the transfer link implements a word generating transfer protocol, with each data capsule being transmitted by using a plurality of words;   the plurality of words contains:   at least one identification word comprising the identifier and/or the refresh indicator;   optionally at least one functional data word containing functional data of the functional data packet;   at least one integrity check word, containing the integrity check;   the plurality of words comprises at least two distinct integrity check words, each integrity check word containing a part of the integrity check result;   the data producer system is able to produce the integrity check result by adopting a memory representation from the data to be distributed in the words intended for transferring the data capsule, with the or each data consumer system able to reconstitute the memory representation processed by the data producer system from the data retrieved from the data capsule words; and wherein optionally each word includes transfer protocol characteristic fields, at least some of the data in the transfer protocol characteristic fields being substituted or deleted in the memory representation;   the capsule transfer frequency or the capsule receiving frequency is higher than the capsule generation frequency, the data consumer system being able to keep at least one data capsule in an expected refresh state even if the refresh increment is zero;   if the capsule processing frequency or the capsule transfer frequency is greater than the capsule receiving frequency, the data consumer system is able to place at least one data capsule in an expected refresh state even if the refresh increment is greater than 1;   the integrity check result is a checksum or cyclic redundancy code.       

     The present disclosure also provides a data producer system for an aircraft data transfer system, the data producer system being able to produce data at successive moments; 
     the data producer system is configured to generate a data capsule at each data production, each data capsule comprising an identifier of the data producer, a refresh indicator updated at each data production, possibly a functional data packet, and a result of the data integrity check relating to the identifier, the refresh indicator and possibly the functional data packet; 
     each data capsule being able to being transferred to a data consumer system via a transfer link. 
     The present disclosure also relates to a data consumer system intended to be used in an aircraft data transfer system, the or each data consumer system being able to receive data capsules, each data capsule comprising an identifier of the data producer, a refresh indicator updated at each data production, possibly a functional data packet, and a result of the data integrity check relating to at least the identifier, the refresh indicator and possibly the data packet; 
     each data capsule being able to being transferred to the data consumer system via the transfer link; 
     the or each data consumer system being able to retrieve each data capsule to extract the identifier, the refresh indicator, possibly the functional data packet, and the integrity check result, and to check the integrity of the data of the data capsule by establishing a new integrity check result from at least the identifier, the refresh indicator and optionally the functional data packet retrieved from the data capsule and by comparing the new integrity check result to the integrity check result retrieved from the data capsule, the data capsule being in an integrity state when the new integrity check result matches the integrity check result retrieved from the data capsule. 
     The present disclosure also relates to an aircraft data transfer method comprising the following steps:
         processing of data by a data producer system, at successive moments;   transferring the data by a data transfer link between the data producer system and the or each data consumer system;   receiving and, after integrity check, using by the or each data consumer system, data produced by the data producer system;       

     at least one of the data producer system and the or each data consumer system being on board an aircraft; 
     and further including the following steps:
         the data producer system generating a data capsule at each data production, each data capsule comprising an identifier of the data producer, a refresh indicator updated at each data production, possibly a functional data packet, and a result of the data integrity check relating to the identifier, the refresh indicator and possibly the functional data packet;   transferring each data capsule to the data consumer system via the transfer link;   the or each data consumer system retrieving each data capsule, to extract the identifier, the refresh indicator, possibly the functional data packet and the integrity check result, and   the or each data consumer system checking the integrity of the data of the data capsule by establishing a new integrity check result from at least the identifier, the refresh indicator and possibly the functional data packet retrieved from the data capsule and by comparing the new integrity check result to the integrity check result retrieved from the data capsule;   assigning the data capsule an integrity state when the new integrity check result matches the integrity check result retrieved from the data capsule.       

     The method according to the present disclosure may comprise one or more of the following features, taken alone or in any technically possible combination:
         the method comprises the or each data consumer system determining a refresh increment for each data capsule received, from the refresh indicator of the data capsule and from the refresh indicator of a previously received data capsule and the or each data consumer system determining a data capsule refresh state, the between an expected refresh state and an inadequate refresh state, depending on the refresh increment, the capsule generation frequency, the capsule transfer frequency and/or the capsule receiving frequency;   the method comprises the or each data consuming system verifying from the capsule identifier that the data producer system that issued the data capsule is the data producer system expected to issue the data capsule, and placing the data capsule in a valid state for use of the data contained therein when the data capsule is from the expected data producing system, is in the integrity state and is in the expected refresh state.       

    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The present disclosure will be better understood upon reading the following description, given only by way of example, and made with reference to the appended drawings, in which: 
         FIG. 1  is a schematic representation of an aircraft data transfer system according to the present disclosure; 
         FIG. 2  is a schematic representation of the contents of a capsule used to transfer data within the transfer system of  FIG. 1 ; 
         FIG. 3  is a schematic representation showing the structure of a capsule transmitted via a transmission protocol using words, in an example where the identifier and the refresh indicator are placed in the same word; 
         FIG. 4  is a detailed view of a word representation of the ARINC429 protocol; 
         FIG. 5  is a view of a memory representation of a sequence of words forming a capsule, in order to implement an integrity test; AND 
         FIG. 6  is a view analogous to  FIG. 1  of an alternative data transfer system according to the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     A first data transfer system  10  according to the present disclosure within an aircraft  12  is illustrated schematically in  FIG. 1 . 
     The aircraft  12  is a business aircraft, a military aircraft, a passenger or cargo aircraft, or aerial drone vehicle, for example. 
     The data transferred by the system  10  is functional data produced or received in the aircraft, such as results of physical measurements of aircraft sensors, control command data of aircraft systems, status tracking data of aircraft systems or avionics data. 
     This functional data is transported and encoded in binary form as bit strings. 
     With reference to  FIG. 1 , the transfer system  10  includes a data producer system  14 , at least one data consumer system  16  and a data transfer link  18  between the data producer system  14  and the or each data consumer system  16 . 
     The data producer system  14  and the data consumer system  16  are present in two respective separate aircraft systems of the aircraft  12 , for example. The data transfer link  18  is then a transmission link present in the aircraft  12 , such as a physical data transmission link through a cable network. 
     In a variant, at least one data consumer system  16  is external to the aircraft  12 , located in another aircraft, for example, a ground control station in the case of a UAV, etc. 
     In another variant, the data producer system  14  is external to the aircraft  12 . It is located in another aircraft, for example, in a mission preparation system, in a data downloading system. 
     In the latter two cases, the data transfer link  18  includes at least one wireless data transmission link from the aircraft  12  and/or to the aircraft  12 . 
     The data producer system  14  and the data consumer system  16  then each comprise at least one computing resource consisting of at least a processor and at least a memory containing software modules able to execute by the processor. 
     In a variant, the systems  14 ,  16  are made at least partially in the form of programmable logic components, or in the form of dedicated integrated circuits. 
     In one variant (not shown), the data producer system  14  and the data consumer system  16  are located within the same aircraft system, or even located together in the same aircraft system&#39;s computing resource at two separate application layers of the same aircraft computing resource, for example, or in two different cores of the same processor. 
     The data producer system  14  is able to produce functional data at successive moments by generating them itself or by receiving them from one functional data source. 
     According to the present disclosure, the data producer system  14  is then able to transmit the processed data to the data consumer system  16  through the transfer link  18 , encapsulating the first functional data processed at successive moments in successive capsules  40 , the content and structure of which are illustrated in  FIGS. 2 to 3  respectively. 
     As illustrated in  FIG. 2 , each capsule  40  includes a capsule identifier  42 , a refresh indicator  44 , updated by the first data producer system  14  at each processing of functional data, independent of the data transmission frequency in the transfer link  18 , a functional data packet  46  developed by the first data producer system  14  during a functional data production and an integrity check result  48 , computed by identifier  42 , the refresh indicator  44  and from the functional data packet  46  of the capsule  40 . 
     The capsule identifier  42  is a bit string for identifying the data producer system  14 , for example, and/or an entity that produced the functional data within the data producer system  14 . In particular, from a semantic and/or from a mapping table, the capsule identifier  42  allows the or each data consumer system  16  to identify the data producer and functional content of the data packet, and thus implement an authentication check to ensure that the data was indeed produced by the expected data producer system  14 . 
     The capsule identifier  42  makes it possible to bijectively associate the capsule  40  to a given transfer system  10  (implemented in an aircraft  12 , multiple aircraft, one or more aircraft, and means external to the aircraft  12 ), regardless of the number of transfer links  18  and the nature of those links in the overall transmission architecture. 
     It is thus possible, from the capsule identifier  42  associated with the capsule  40 , based on predefined semantics and/or a mapping table, to assign the capsule  40  to a given transfer system  10  including a data producer system  14  and at least one data consumer system  16 . 
     Further, if the capsules  40  generated by a data producer system  14  and/or an entity within the data producer system  14  are all of identical structures, the data consumer system  16  is able to identify the position of the data packet  46  within the capsule  40 , the length of the data packet  46  and the position of the integrity check result  48 , from the capsule identifier  42 . 
     The refresh indicator  44  is a bit string encoding a functional data production order number at each functional data production. The refresh indicator  44  is a counter or count-down, for example, generally updated individually at each functional data production. It makes it possible to associate a data refresh measurement to each transmitted capsule  40 . This measurement makes it possible to ensure that the functional data retrieved by the data consumer system  16  has been refreshed by the data producer system  14 , constituting a refresh control. 
     The data packet  46  is a bit string encoding the functional data generated during the functional data production associate with the number of the refresh indicator  44 . 
     The integrity check result  48  is a bit string encoding a check number calculated by mathematical processing, from a functional representation incorporating the identifier  42  of the capsule  40 , the refresh indicator  44  of the capsule  40 , and the functional data  46  of the capsule  40 . 
     Preferably, the integrity check result  48  is the result of a checksum, or cyclic redundancy code (CRC). The mathematical processing is an algorithm, for example, chosen based on the security objectives to be achieved, the minimum/maximum length of the data packet  46 , the identifier  42 , the refresh indicator  44 , and the reliability of the data link in terms of bit error rate (BER). Examples of the algorithms used are described in the United States Federal Aviation Administration document DOT/FA/TC-14/49, March 2015, available at: 
     https://www.faa.gov/aircraft/air_cert/design_approvals/air_software/media/TC-14-49.pdf. 
     In the example shown in  FIG. 1 , the data transfer link  18  is a link implementing a data transfer protocol using words  50  containing transmission protocol management information and functional information, as viewed from that protocol. 
     In a variant that will be described below, the data transfer link  18  is a link implementing a data transfer protocol using messages that make atomic transmission of multiple words  50  possible, some of which contain exclusively transmission protocol management information and others of which contain exclusively functional information, as seen from this protocol. 
     An example data transfer protocol using words  50  is shown in  FIG. 4 , which is advantageously implemented in an ARINC429 link. 
     Each word  50  is defined by a bit sequence, having a predefined number of bits. The word  50  for ARINC429, has thirty-two bits. As explained above, it is commonly referred to as the “ARINC429 word” or “label”. 
     The word  50  is broken down into separate fields. In the example shown in  FIG. 4 , the word  50  includes a field  52  for identifying the word (ID), and advantageously, a field  54  for identifying the receiver and/or subsystem that transmitted the data (optional SDI field in the ARINC429 standard). The word  50  also comprises a field  56  containing the data (D) carried by the word, a data validity (SSM) or sign/sense field (e.g. +/− or North/South, East/West), and a parity field (P)  60 . 
     Field  52  contains an identification of the word, with normalized semantics defined during word processing. 
     Field  54  (optional) is generally intended to define which data receiver is intended for the word, or which transmitter subsystem transmitted the data. 
     Field  56  contains the data to be transmitted by word  50 . In this example it has nineteen bits. 
     Field  58  contains an identifier that may be used to identify the validity of the data generated by the data producer system  14 , to may identify whether the data producer system  14  considers the data to be valid or whether it considers it to be no longer valid. 
     The parity field  60  is a binary sum calculation of all bits in the fields  52 ,  54 ,  56 , and  58  to determine the parity of the sum. 
     In the case where the transfer link  18  is a link implementing the words  50 , the capsule size  40  is generally larger than the field size  56  of the data carried by a word  50 . 
     As illustrated in  FIG. 3 , the capsule  40  is thus transmitted over a plurality of distinct words  50 A through  50 F by distributing the identifier  42 , refresh indicator  44 , functional data  46 , and integrity check result  48  into the fields  56  of multiple words  50 A through  50 F. 
     Preferably, at least a first identification word  50 A contains the identifier  42  of the capsule  40  and possibly, the refresh indicator  44 . Advantageously, the word  50 A contains both the identifier  42  of the capsule  40 , and the refresh indicator  44 . 
     In order to limit the bandwidth consumed, the identifier  42  of the capsule  40  is preferably formed by the field  52  identifying the word  50 A, and by the field  54  identifying the receiver and/or the subsystem that transmitted the data. This is possible by establishing a mapping table between each capsule  40  and the successive identifiers of the words  50 A to  50 F used to transmit the capsule  40 . 
     For example, if each capsule  40  corresponding to a functional data production is issued in N words  50 , the N successive identifiers of the N words  50  are associated with the capsule  40 . 
     The refresh indicator  40  is encoded in binary form in the field  56  of the word  50 A, using 16 of the 19 bits available in the field  56 , for example. 
     In a variant (not shown), the word  50 A contains the identifier  42  of the capsule  40 , in field  56 , for example, and another identification word (not shown) separate from the word  50 A contains the refresh indicator  44  in the field  56 . 
     Depending on the size of the functional data packet  46  produced at each functional data production, the functional data packet  46  is issued on at least one functional data word  50 B through  50 D, typically on multiple functional data words  50 B through  50 D. 
     The data in the functional data packet  46  is distributed over the fields  56  of the one or more functional data words  50 B through  50 D that contain only functional data. 
     The functional data words  50 B through  50 D do not contain the refresh indicator  44  or the result of the integrity test  48 . The identifier of each word  50 B through  50 D identifies to which capsule  40  the functional data contained in field  56  of word  50 B through  50 D belongs. 
     The integrity check result  48  of each capsule  40  is issued on at least one integrity check word  50 E,  50 F, advantageously on two integrity check words  50 E,  50 F, when it contains thirty-two bits, while the field  56  of each word  50 E,  50 F contains only 19 bits. 
     The capsule  40  thus created is then transferred via the transfer link  18  by spreading it over several words  50 A through  50 F, to accommodate the transfer protocol present in the aircraft  12 . 
     To this end, with reference to  FIG. 1 , the first data producer system  14 , includes a module  70  for producing functional data  46  and associating it with a capsule identifier  42   40  and with a refresh indicator  44 , a module  72  for calculating an integrity check result  48  established on the identifier  42 , the refresh indicator  44 , and the data packet  46  at each processing of functional data, and a module  74  for formatting the data for transfer via the transfer link  18 . 
     The data production module  70  is able to generate functional data as defined above or retrieving functional data applicable to other aircraft systems via an application layer. The functional data is generated at successive moments, on request or at a predefined functional data generation frequency. 
     The functional data is distinct from the transport data (such as word identifier) required to implement the transfer link  18 . 
     The data production module  70  is thus able to create a functional data packet  46  at each data production, of generally identical size for each data production. It is able to associate an identifier  42  with each functional data packet  46  processed, making it possible to identify the functional content of the functional data packet  46 , its producer, and the membership of the functional data packet  46  to a capsule  40 , using a mapping table. 
     As noted above, the identifier  42  further advantageously defines the size of the capsule  40 , the size of the data packet  46 , and the position of the integrity checksum  48  using the mapping table. 
     The data production module  70  is further able to relate a refresh indicator  44  to each functional data production, independently of the transmission frequency via the link  18 . The refresh indicator  44  ensures that the data transmitted via the transfer link  18  is indeed valid data that has been refreshed. The refresh indicator  44  is coded on a determined number of bits, between 8 and 32, for example. 
     In the case of an incremental counter, the refresh indicator  44  is able to reset to null when the refresh indicator maximum has been reached. 
     The integrity check result calculation module  72  is able to build a memory representation  80  of the identifier  42 , the refresh indicator  44 , and the data packet  46  at the application layer, independent of the transport layers. An example of a memory representation  80  is shown in  FIG. 5 , which corresponds to a transport in the form of words  50 . 
     With reference to  FIG. 5 , the bits intended for the identifier and/or refresh indicator word  50 A and the data words  50 B through  50 D on which the capsule  40  is transmitted are distributed as rows, with each row corresponding to a word (ARINC429 word, for example, in the case of an ARINC429 link) in a table having a number of columns corresponding to the number of bits in each word. 
     The bits specific to the transmission protocol in each word  50 A through  50 D are intentionally forced to a selected bit, such as a null bit, in the memory representation  80 . 
     For example, in the first row  82 A of the table in the memory representation  80  corresponding to the identifier and/or refresh indicator word  50 A, only the bits for the identifier  42  and the refresh indicator  44  are retained at their value. The same is true for each functional data transport word  50 B to  50 D corresponding to the lines  82 B to  82 D, in which the bits corresponding to the word identifier, the SDI field, and the parity calculation, are voluntarily forced to a chosen bit, a null bit in this case. 
     Only the functional data  46 , the identifier  42 , and the refresh indicator  44  are retained in the memory representation  80 . 
     Thus, the memory representation  80  makes it possible to test the integrity of the identifier  42  and refresh indicator  44 , using the first line  82 A of the representation, and also the integrity of the functional data packet  46  distributed on the lines  82 B through  82 D, by incorporating the data validity fields  58 . 
     Once this memory representation  80  is constructed, the integrity result calculator module  72  is able to implement a checksum calculation and/or cyclic redundancy code, as defined above. 
     For example, to ensure good reliability of the integrity check, the integrity calculation result  48  is obtained by a MIL-STD-1760 checksum, or by a CRC-32K/6.4 cyclic redundancy code. The check result is encoded as thirty-two bits, in this example. 
     The formatting module  74  is then able to emit each capsule  40  corresponding to a functional data production, in the form of a plurality of words  50 A to  50 F, as described previously, according to the chosen data transmission protocol. 
     In particular in the case of an ARINC429 type link, the identifiers of the words  50 A to  50 F corresponding to a given capsule  40  are advantageously generated according to predefined semantics, so that if the words  50 A to  50 F are transmitted in disorder, or if other words  50 G not belonging to the capsule  40  are interposed between the words  50 A to  50 F of the capsule  40  in a discontinuous manner, the capsule  40  can be reconstituted by the data consumer system  16  based on the predefined semantics. 
     The data consumer system  16  includes a module  90  for receiving and processing each capsule  40 , a module for verifying the integrity  92  of the capsule  40 , and a module  94  for verifying the validity of the data of the capsule  40 . 
     The capsule reception module  90  is able to reconstitute each capsule  40 , in particular by identifying the various words  50 A to  50 F of the capsule  40  received via the transfer link  18  via their identification field  52 . 
     Once the capsule  40  is reconstituted, it is able to extract the identifier  42 , the refresh indicator  44  and the data packet  46  corresponding to the capsule  40 . 
     The capsule reception module  90  is able to perform an authentication check of the data producer system  14  by verifying whether the received data really comes from the expected data producer system  14 , from the identifier  42 . 
     Also from the capsule identifier  42 , and in the case of a capsule  40  of fixed size, the capsule reception module  90 , with the help of the same mapping table as that of the data producer system  14 , is able to know the size of the capsule  40  and the position of the refresh indicator  44 , the functional data packet  46  and the integrity check sum  48  within the capsule  40 . 
     The integrity check module  92  is able to reconstruct the same memory representation  80  as that constructed by the data producer system  14 , from the data retrieved from the capsule  40  that was transferred over the transfer link  18 . 
     It is able to apply the same algorithm as that implemented by calculation module of the integrity result  72  of the data producer system  14  to calculate a new integrity check result for the received data, including the identifier  42  of the capsule  40 , the refresh indicator  44  and the functional data packet  46 . 
     The integrity verification module  92  is able to determine that the capsule  40  comprising the identifier  42 , the refresh indicator  44  and the data packet  46  is in a integrity state if the new integrity check result calculated from the received data is identical to the integrity check result  48  calculated by the second data producer system  14 , extracted from the capsule  40 . 
     It is able to determine that the capsule  40 , including the identifier  42 , the refresh indicator  44  and the data packet  46  is in a corrupted state if the new integrity check result calculated from the received data is different from the integrity check result  48  calculated by the data producer system  14 . 
     Once the integrity of the refresh indicator  44  has been confirmed, the validity verification module  94  is able to evaluate the data refresh contained in the capsule  40  by performing a refresh check based on the refresh indicator  44 . 
     To this end, it is able to compare the refresh indicator  44  of each received capsule  40  with the counter  44  received from a previous capsule  40  to verify that the refresh indicator  44  has refreshed, by being incremented or decremented, for example, or by having followed an expected refresh trend. 
     The validity verification module  94  is able to calculate the refresh increment between the refresh indicator  44  of the received capsule  40  and the refresh indicator  44  of the capsule  40  received just before, for example. 
     In particular in the case of a counter or countdown, if the increment is unitary in absolute value, the validity verification module  94  is able to determine that the functional data present in the capsule  40  is indeed refreshed data and for placing the capsule  40  in an expected refresh state. 
     In the case of a frequency of transmission via the link  18  or of the data acquisition by the data consumer system  16  that is higher than the processing frequency of the functional data by the data producer system  14 , the validity verification module  94  is able to authorize a null refresh increment on a determined number of capsules  40 , calculated depending on the frequencies of processing, transmission or acquisition of the capsules  40 , for example. 
     Beyond the determined number of capsules  40 , if the refresh indicator  44  remains identical, it is able to pass the capsule  40  into an inadequate refresh state, because its data have not been refreshed. 
     Furthermore, when the processing and/or transmission frequency of the capsules  40  is higher than the acquisition frequency of the capsules  40 , the validity verification module  94  is able to authorize a refresh increment higher than one. 
     In any case, the validity verification module  94  is able to memorize the refresh indicator of each capsule  40  just received, to make an increment calculation possible when the next capsule  40  is received. 
     It is also able to take into account the setting to null of the refresh indicator  44 , when the latter reaches its maximum value. 
     Thus, depending on the increment calculated between the counters  44  of two successive capsules  40 , the validity verification module  94  is able to determine if the functional data present in the capsule  40  are indeed refreshed data that can be used by the consumer system  16 , and to place the capsule  40  in an expected refresh state. If, in contrast, the data is not refreshed or lacks intermediate data that has not been received, it is able to place the capsule  40  in an inadequate refresh state. 
     When the capsule  40  originates from the expected data producer system  14 , is in the integrity state and in the expected refresh state, the functional data contained therein is considered valid and is then able to use by the consumer system  16 , or for transmission to another aircraft system for use. 
     When a capsule  40  does not originate from the expected data producer system  14 , or is in the corrupted state or an inadequate refresh state, the validity check module  94  is able to exclude the data from the capsule  40 , which is considered invalid. 
     The validity check module  94  is advantageously able to implement a reset phase during a given reset time, corresponding to the reinitialization of a computer or a transient failure ending, for example. 
     In this case, as soon as a new capsule  40  is received, originating from the expected data producer system  14  and being in the integrity state, the validity verification module  94  is able to memorizing the new value of the refresh indicator  44  of this capsule  40 . Then, it is able to retrieving the refresh indicator  40  of each new capsule  40  arriving, to determine a refresh increment. If the refresh increment reflects an expected refresh state, the validity check module  94  is able to switch the capsules  40  back to the valid state, after the predefined reset time. 
     In some cases, if the integrity or refresh failure affecting the data contained in the capsules  40  is varied, i.e. it occurs regularly or arbitrarily, the validity check module  94  is able to permanently stopping the reset and declaring the data transfer system  10  to be in default. 
     Thanks to the present disclosure just described, it is therefore possible to encapsulate functional data by securing it at the application level during each data production and not at the data transmission level, by integrating an identifier  42 , a refresh indicator  44 , and an integrity control result  48  in each capsule  40 , which takes into account the identifier  42 , the refresh indicator  44 , and the functional data without taking into account the transmission layers. 
     Functional data encapsulated in this manner is therefore highly secure, regardless of the transmission protocol(s) used. This prevents the loss or misplacement of this data from producing critical consequences. 
     Such capsules  40  can be transferred via a data transfer link  18  that uses words  50 , without affecting the syntax of the words  50 , and by distributing the contents of the capsule  40  over different words  50 A through  50 F. 
     Updating the refresh indicator  44  each time functional data is developed ensures that a data refresh has occurred, not simply that a new data transmission has occurred. 
     Thus, it is possible to transmit digital aircraft data in a very secure manner, without the risk of data loss or error. 
     In the variant shown in  FIG. 6 , the capsules  40  containing the data are passed through an intermediate system  100  between the data producer system  14  and the consumer system  16 . 
     A first data transfer link  18 A extends between the producer system  14  and the intermediate system  100 , and a second data transfer link  18 B extends between the intermediate system  100  and the consumer system  16 . 
     Optionally, the capsules  40  are transferred according to a first transfer protocol across the first link  18 A, and according to a second transfer protocol across the second link  18 B. 
     However, regardless of which transfer protocol is used on the respective links  18 A,  18 B, the identifier  42 , the refresh indicator  44 , the functional data packet  46 , and the integrity result  48  remain transmitted from the producer system  14  to the consumer system  16 , allowing data integrity and refresh control by the consumer system  16 . 
     In a variant, not shown, the capsule  40  is transmitted through a transmission protocol using messages consisting of a plurality of consecutive words  50 , typically transmitted in atomic form, one after the other. 
     Generally, at least one word  50  of each message just contains information for managing the transmission protocol. The capsule  40  is then distributed over several words  50  of the same message containing no transmission protocol management information, unlike the protocol described above, in which each word  50  contains transmission protocol management information. 
     One or more words of the same message then contain the capsule identifier  42  and the refresh indicator  44 , one or more words of the same message contain the functional data  46 , and one or more words of the same message contain the integrity check result  48 , calculated from the identifier  42 , refresh indicator  44  and data packet  46  of the capsule  40 . 
     In another variant (not shown), each capsule  40  is distributed across a plurality of messages by distributing capsule identifier  42 , refresh indicator  44 , functional data  46 , and integrity check result  48  across words of the plurality of messages. 
     In another variant (not shown), the capsule  40  lacks any functional data  46 . The capsule  40 , as such, comprising the capsule identifier  42 , the refresh indicator  44 , and the integrity check result  48  on the identifier  42  and on the refresh indicator  44  is then the equivalent of a signal for triggering or validating an action. The data transmitted by the capsule  40  is then the presence of the capsule  40 , or not.