Abstract:
A device for distributing information and its fault management process which improves the robustness of a network, such as in an aircraft, a boat, or a train. The device or process includes one or more splitters of which an upstream input output is linked to a first end of a chain including stations, at lower overdimensioning cost, and with dynamic management of a fault which renders it transparent or of very short duration. A second end of the chain is linked to a downstream input output of another splitter and a fault management procedure is implemented activating or otherwise one of the ends of a chain depending on the nature and the conditions of the fault. Preferably, addresses of the elements of the network can reflect its topology and facilitate the shunting of the addresses of the stations between splitters.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The subject of the present invention is a network for distributing information and its fault management process. It is mainly usable in the field of telecommunications, especially in aeronautics. This invention could equally well be applied to ground-based telecommunications networks or in any other field such as the marine sector. The aim of the invention is to allow an increase in a reliability of such a network. With the invention, one increases a fault resistance of a network. The fault management process associated with the invention makes it possible to increase the stability whilst increasing the robustness of the network (load holding), thereby making it possible to reduce a risk of collapse thereof (severing of transmissions for some of the users of this network). 
   2. Discussion of the Background 
   At present, IFE type (In Flight Entertainment) telecommunications networks are found in aircraft cabins. These telecommunications networks makes it possible to offer passengers intangible services such as video on demand, music, television, Internet connection or more generally to send inquiries to a central unit without moving. This central unit has the job of supplying data, associated with a corresponding function across the network. A function can thus be a broadcast on a terminal available to a passenger of a requested program, a telephone call, an order for a product (drink, purchasing of duty-free goods) or any other function able to travel over a telecommunications network. One method of construction commonly employed to construct such a distribution network consists in adopting a star topology with several levels, in particular according to ARINC standard 628 part 4A. In such a construction, a first level consists of a central unit to which are linked, according to a point-to-point mode, information splitting devices. Each information splitting device comprises several inputs/outputs. An input/output is linked to a terminal by way of a bus. There are as many terminals as passengers. 
   Such a construction exhibits problems. In practice, the airline companies and more generally the operators of such networks are very sensitive to the availability rates of these items of equipment, which condition the degree to which their lines and their apparatuses are frequented. Thus, upon a fault of the network between a terminal and a splitting device, access to the services from this terminal is impossible. Moreover, if the fault occurs in a splitting device, then all the terminals which are linked to this device are inaccessible. Generally, faults in a network arise out of very harsh climatic and environmental conditions (vibrations, shocks) to which elements of the network are subjected. In practice, in most cases such a network is constructed from hardware which is envisaged mainly for ground-based use. In this ground-based use, one generally has stable climatic conditions. By contrast, with an aircraft, the climatic conditions are highly unstable. Thus, before a departure, during an aircraft parking phase, one may have a temperature of greater than +60° C. During a flight the temperature decreases to a value of the order of −50° C. On-landing the temperature may be +40° for example. These sizeable temperature variations are detrimental to correct operation of the constituent hardware of the network. This results in a possibly high fault rate. 
   Moreover, upon a fault, users of these terminals are generally moved to other terminals accessible from the central unit. Such a movement has the effect of creating an imbalance of the aircraft. This imbalance is generally compensated for through an increase in the speed of one of the engines of the craft, resulting in an increase in fuel consumption. 
   A common solution for testing the correct operation of terminals consists in bringing in tester users whose job is to test the terminals before each use of the aircraft. However, this solution is very unwieldy to implement by reason of a sizeable number of tester users which it requires and of the time for which the aircraft is grounded. This entails an increase in a maintenance cost of such a network. This solution can be applied only when the craft is on the ground. That is to say one does not intervene at the time the fault occurs but afterwards. Moreover, this checking operation merely has the aim of cataloging the terminals or splitting devices which are nonoperational. 
   SUMMARY OF THE INVENTION 
   The present invention proposes to remedy these problems by proposing an information distribution device for which a redundancy is created. This redundancy makes it possible to supply a terminal with two different influxes of information. Consequently, when one information influx is blocked, then one immediately switches an access path so as to use the other information influx. In this case a fault in an information splitting device is transparent or at the very least of short duration for a terminal linked to this splitting device. It is thus possible to increase the robustness of this network to faults. One thus avoids depriving the users of the system or moving them and creating imbalances in the craft. To do this, one uses an information splitting device of which one manner of operation is obtained for a bit rate below an allowable maximum bit rate. With this margin of available bit rate it is therefore possible to create a redundancy. When a splitting device develops a fault all the information which was destined for it is sent to a neighboring information splitting device. This neighboring information splitting device makes it possible to obtain the second information influx of a terminal. This picking up of the bit rate by this neighboring information splitting device is manifested as an increase in its bit rate. However, this surge in bit rate is easily absorbed since a splitting device possesses a bit rate margin. 
   The process of the invention actually makes it possible to limit the overdimensioning which produces this bit rate margin and to even out the bit rates of all the information splitting devices by splitting a bit rate surge applied to the neighboring information splitting devices. A consequence of this evening out of the bit rates is to increase a bit rate in the splitting devices by a lesser factor as compared with the nominal bit rate. With the process of the invention, a surge is applied to all the information splitting devices but this surge may be 50%, 33%, or 25% of the nominal bit rate, instead of 100% were all the surge to be shunted to the neighboring information splitting device. 
   The invention therefore relates to a network for distributing information, between a central unit and stations, comprising information splitting devices with inputs/outputs connected on the one hand to the central unit and on the other hand to the stations, an interface device in each station, characterized in that the interface device of each station is linked to a first splitting device and to a second splitting device. 
   It also relates to a process for splitting the effects of a fault in a network for distributing information among terminals 
   characterized in that
         N splitting devices are linked, according to a star topology, to a central unit with the aid of transport means over each of which a primary stream travels, to a splitting device of rank m there corresponds a primary stream FP m ,   the splitting devices are furnished with first inputs/outputs A 1  to A i  and with second inputs/outputs B 1  to B j ,   the first inputs/outputs A 1  to A i  of a splitting device K are linked by buses K 1  to K i  to the second inputs/outputs B 1  to B i  of a consecutive splitting device K+1, with 1≦K≦N,   terminals are linked in cascade to each bus K 1  to K i ,   the first inputs/outputs A 1  to A i  of the splitting devices  1  to N are activated,   upon a fault between a terminal linked by a splitting device K to the central unit, a first input/output A 1  to A i  of the splitting device K is deactivated,   a second input/output B 1  to B i  of the splitting device K+1 is activated.       

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be better understood on reading the following description and on examining the figures which accompany it. These are presented merely by way of a wholly nonlimiting indication of the invention. The figures show: 
       FIG. 1 : a representation of the device of the invention; 
       FIG. 2 : a representation portraying a first solution for managing a fault of an ADB with the process of the invention; 
       FIGS. 3 and 4 : representations portraying a second and a third solution for managing faults with the process of the invention faced with one fault and then two faults respectively; 
       FIG. 5 : a description, in algorithm form, of the process of the invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  shows a diagrammatic representation of a network  1  for distributing information in an aircraft  2 . It would be quite possible to have this network  1  according to the invention in a boat, a train or elsewhere. This network  1  comprises stations. A station essentially comprises a communications terminal and a communications interface device for one or more users. The communication terminal conventionally comprises a monitor, a keyboard and more generally multimedia means including a microphone and a loudspeaker. So as not to overburden the description, in one example, a number of stations restricted to sixteen, stations  3  to  18 , has been used. This example does not constitute a limitation of the invention. In practice, such a network  1  for an aircraft can in reality comprise more than 500 stations (or fewer). The main function of these stations  3  to  18  is to receive information from a central unit  19 . The function of this central unit  19  is to produce and monitor information exchanges over the network  1 . This may involve a video-on-demand server, an encoder transforming images from a camera for example or any other means making it possible to supply information. The network  1  furthermore comprises intermediate load splitter nodes or information splitting devices  20 ,  21  and  22  which will subsequently be referred to as ADBs (Area Distribution Boxes)  20  to  22 . Each ADB  20  to  22  comprises upstream inputs/outputs and downstream inputs/outputs. The ADBs  20  to  22  are linked on the one hand to the central unit  19  and on the other hand to the stations  3  to  18 . 
   More precisely, each station  3  to  18  comprises interface devices  23  to  38  respectively. Thus, an ADB  20  to  22  effects a link between the central unit  19  and interface devices  23  to  38 . In the invention, an interface device  23  to  38  is linked on the one hand to a first ADB  20  to  22  and on the other hand to a second ADB  20  to  22  which is different from the first ADB. Thus, an interface device  23  to  38  possesses two paths or means of access to the central unit  19 . These accesses are complementary, that is to say when an interface device  23  to  38  is using one access path the other access path is deactivated. 
   A possible bit rate of a link between an ADB and a station makes it possible, in accordance with ARINC standard 628 part 4A in the case of networks in aeronautics, to have several stations on the link. To do this, several interface devices are linked in cascade by virtue of a bus, or a chain, one end of which is linked to the first ADB and another end of which to the second ADB. A chain is therefore a bus to which stations are linked in cascade (or in series). That is to say an output of a station is linked to an input of a following station. Hereinbelow, the term bus will be used to speak either of a bus or of a chain and the term cascade to speak of a cascaded or serial link. Thus, for example, the interface devices  23  and  24  are linked in cascade with a bus  39  a first end of which is linked to an upstream input/output  40  of the ADB  20  and a second end of which is linked to a downstream input/output  41  of the ADB  21 . 
   An interface device such as the interface device  23  preferably comprises a means for detecting a fault relating to a problem on a link to which it is linked. Such a means of detection makes it possible to detect a fault between the interface device in which it is located and the upstream input/output to which the interface device is linked. Thus, if the means for detecting a fault of the interface device  24  detects a fault, this signifies that the link between the input/output  40  and the station  4  is broken. Then, according to the invention, the communication between the station  4  and the central unit  19  will be done by way of the ADB  21  by activating the input/output  41  and by deactivating the input/output  40 . 
   In order for the central unit  19  to be informed of a fault, the fault-detection means of the interface device  24  comprises in a preferred example means for mutual acknowledgement with the central unit  19 . In such mutual acknowledgement, the central unit  19  and the interface device periodically send one another protocol messages, the aim of which is simply to inform one another reciprocally regarding their correct availability. Should the interface device  23  be faulty, it will not be able to acknowledge a request originating from the central unit  19 . The input/output  40  can then no longer serve as information influx for the station  3 . Hence, the central unit  19  diverts a request to the station  3  by way of the ADB  20  into a request to the station  3  by way of the ADB  21  by using the input/output  41 . If in this case the station  3  still does not acknowledge the request of the central unit  19 , then this station will be regarded as defective and will therefore have to be deactivated by the central unit  19 . More generally, the paths using defective splitters are invalid, given that the interface devices  23  and  24  are linked in cascade. If the device  23  develops a fault, the input/output  40  can no longer be used to send information to the interface device  24 . Thus, even after having deactivated the station  3 , the central unit  19  can only communicate with the station  4  by way of the downstream input/output  41  of the ADB  21 . Through their organization, the protocol exchanges allow the central unit to determine whether a terminal is faulty, if its interface is faulty, or if the whole ADB is faulty. Transmission diversions are organized accordingly. The diversions are performed in a physical form (by switching circuits of the central unit) or in a functional form (by addressing the ADBs and their activated inputs/outputs so as to link terminals). 
   In order to carry out management of the inputs/outputs, the central unit  19  comprises a microprocessor  42 , a management program  43  in a program memory  44 , a data memory  45  and also an information memory  46 , all these elements being linked by a bus  47 . Thus, when the central unit  19  does not receive an acknowledgement from a station with which it wishes to communicate then the management program  43  commands the microprocessor  42  to select the ADB  21 . The input/output  41  is activated in the ADB  21  so that information originating from the information memory  46  can be sent to the station  4 . A main function of the information memory  46  is to be used as data server. In a variant there are several information memories such as  46  each possibly monitored by a microprocessor. Thus, the types of services offered and the amount of information available (programs) are increased and/or one ensures redundancy of a data server. Each station  3  to  18 , each input/output and each ADB is identified by an address. The management program  43  stores in the data memory  45  all the addresses of the defective stations  3  to  18 . 
   The central unit  19  is nevertheless not limited to such management operations. In a variant it could comprise an interface device (not represented) plugged into the bus  47 . Additional communication means such as an antenna could thus be connected up to this interface device as could means used as additional information source such as for example a camera, the information from which would be transmitted by way of the central unit  19 . 
   The network  1  furthermore comprises a device  48  for switching from a first ADB to a second ADB. In a preferred example this switching device  48  is in the central unit  19 . The central unit  19  furthermore comprises an interface device  49  between the information memory  46  and the switching device  48 . This interface device  49  taps off, when ordered by the microprocessor  42  by way of the bus  47 , information from the information memory  46  and supplies it to the switching device  48 . The switching device  48  is commanded by the microprocessor  42  by way of the bus  47  as a function of the address of the ADB for which the information is destined. Thus, the microprocessor  42  commands the switching device  48  so that the information tapped off by the interface device  49  is sent to the input/output  41  of the ADB  21  rather than to the input/output  40  of the ADB  20 . The switch or switches comprise switching tables with the addresses of the elements of the network. These switching tables make it possible to steer an incoming or outgoing information item to the corresponding ADB. 
   In a variant, the switching obtained with the switching device  48  is carried out by a switch, or a set of switches, operating according to the Ethernet standard. In this case the interface device  49  has the job of shaping according to this Ethernet standard the information emanating from the information memory  46 . Depending on the faults catalogued by the management program  43  and stored in the data memory  45 , the microprocessor  42  modifies the values of the addresses in the switching tables of the switch or switches. The definition of the addresses, which themselves consist of one or several fields, makes it possible to reflect the topology of the network and to act on the modification of a field (for example, the ADB number) so as to shunt all the stations from one ADB to another. 
   In a preferred example, a transmission of information between an ADB and a station is done by means of a bus such as the bus  39  constructed with a cable having two twisted conductors. Such cables are sufficient to transmit information with a bit rate of the order of 100 Mbits/s. It would be quite possible to use any other type of medium such as in particular a coaxial cable or an optical fiber. Choosing a cable with two twisted conductors leads to an inexpensive solution. A link  50 ,  51  or  52  between the central unit  19  and the ADB  20 ,  21  or  22  respectively is constructed with an optical fiber. This link  50 ,  51  or  52  could equally well be constructed with any other means provided that this means permits information transmission at bit rates of the order of 800 Mbits/s. 
   The network  1  furthermore comprises special interface devices  53  and  54 . Each special interface device serves to plug in a special terminal. A special terminal allows the execution of functions which differ, or are additional to those permitted to a normal terminal. In an aircraft, a special terminal is made available to a hostess or to a steward. Each ADB  20 ,  21  or  22  furthermore comprises an additional downstream input/output  55 ,  56  or  57  and an additional upstream input/output  58 ,  59  or  60  respectively. Hereinbelow, the term CCC  53  or CCC  54  (Common Cabin Console) will be used to designate the special interface device  53  or the special interface device  54  respectively. The CCC  53  is linked on the one hand, by a link, to the input/output  55  of the ADB  20  and on the other hand, according to the invention, by another link to the input/output  59  of the ADB  21 . Likewise, the CCC  54  is linked on the one hand, by a link, to the input/output  56  of the ADB  21  and on the other hand, by another link, to the input/output  60 . The CCC  53  or  54  receives requests emanating from the stations  3  to  10  or from the stations  11  to  18  respectively. 
   A station, for example the station  3 , comprises a terminal  61  linked to the interface device  23  by way of a data bus  62 , this bus  62  being managed by the interface device  23 . The terminal  61  can take all possible forms. That is to say it can consist of a screen with a keyboard or else a touch screen or furthermore comprise a telephone or any other means of communication. In this example the terminal  61  consists of a screen and a keyboard. Thus, a user using this terminal  61  makes a request to a user linked to the CCC  53 , or  54 . To do this, the request is firstly transmitted from the station  61  to the interface device  23  via the bus  62 . This request is then transmitted from the interface device  23  to the central unit  19 . It is processed by the management program  43 . The management program  43  which has recognized a request relating to a CCC, in particular the CCC  53 , commands the microprocessor  42  accordingly. The microprocessor  42  sends the request to the CCC  53  by way of the ADB  20 . Should a fault occur between the input/output  55  and the switching device  48 , the request is then transmitted to the CCC  53  by way of the input/output  59  of the ADB  21 . Should several stations wish to communicate together, the same information routing procedure as before is carried out. In normal operation, that is to say fault-free, and in one example, only the upstream inputs/outputs of an ADB  20 ,  21  or  22  are active. Thus, in a preferred example, a nominal bit rate of each ADB  20 ,  21  or  22  is equal to half a maximum bit rate which may travel through this ADB. This maximum bit rate is in particular reached when the upstream inputs/outputs and the downstream inputs/outputs are simultaneously active. This allows an ADB to be able to absorb a surge caused by a fault on a neighboring ADB or on a part of a link of a bus. 
   For this purpose, the present invention proposes a process for splitting the effects of a fault within such a network  1 .  FIGS. 2 ,  3  and  4  show how the process of the invention manages bit rate surges due to a fault with an interface, with an ADB or with a fault between an ADB and the central unit. These diagrammatic figures portray only ADBs and the buses such as  39  to which the interface devices are linked. These  FIGS. 2 ,  3  and  4  portray only one direction of broadcasting of an information item emanating from the central unit on a bus. They illustrate that one of the two ADBs in charge of the bus. The buses, in  FIGS. 2 to 4  and for the sake of clarity, do not comprise any stations. 
     FIG. 2  shows, in the case of a fault with an ADB K−1, a first fault management solution of the process of the invention. One considers N ADBs linked according to a star topology to a central unit (not represented) with the aid of transport means over each of which a primary stream FP travels. A primary stream FP m  is made to correspond to an ADB of rank m. A splitting device is furnished with first inputs/outputs A 1  to A i  and with second inputs/outputs B 1  to B j . In a preferred example, the value 4 will be taken as the value of i. The first inputs/outputs A 1  to A i  of a splitting device K are therefore linked by buses K 1  to K i  to the second inputs/outputs B 1  to B i  of a consecutive ADB K+1, with K lying between values 1 to N inclusive. Terminals are linked in cascade to each bus K 1  to K i . In normal operation, that is to say fault-free operation, the first inputs/outputs A 1  to A i  of the ADBs  1  to N are activated. An input/output is furnished, for example, with a breaker device. In this case, when an input/output A 1 , for example, is active then the breaker device of the corresponding input/output B i  is open and thus prevents communication between the relevant bus and the input/output B i . The first inputs/outputs A 1  to A i  will be referred to hereinbelow as the upstream inputs/outputs and the second inputs/outputs B 1  to B i  will be referred to as the downstream inputs/outputs. 
   Should there be a fault with ADB K−1 or with the network feeding it, the upstream inputs/outputs of ADB K−1 of rank K−1 are deactivated with the aid of a microprocessor such as the microprocessor  42  ( FIG. 1 ). The downstream inputs/outputs of the ADB of rank K are activated with the microprocessor  42 . A consequence of this first solution of the process of the invention is to have a primary stream FP K  of which a bit rate is equal to the maximum bit rate which an ADB can support. 
   This first solution, which works, has the effect of creating an imbalance in the splitting of the primary streams. In practice, all the primary streams are at a nominal bit rate except the primary stream FP K  which is twice the bit rate of the nominal bit rate. This implies that the dimensioning of a bit rate of a stream must be at most twice the nominal bit rate if one wishes to serve the users, or less if one loses a part thereof. 
   An improvement to this first solution is shown in  FIG. 3 . Thus, in this second solution, upon a fault with the ADB of rank K, the microprocessor  42  commands the deactivation of all the upstream inputs/outputs of the ADBs of rank K to N. The microprocessor  42  activates all the downstream inputs/outputs of the ADBs of rank K+1 to N. In this case all the primary streams FP 1  to FP N  are of equivalent bit rate, equal to the nominal bit rate. 
   A third solution,  FIG. 4 , consists, should there be a fault with the ADB of rank K, in activating only some of the upstream inputs/outputs of the ADB of rank K+1. All the downstream inputs/outputs of the ADB K+1 are activated so as to serve the stations normally served by the ADB K. For example the ADB K+1 takes charge of only two of its upstream inputs/outputs. The other two buses, normally linked to the upstream inputs/outputs of the ADB K+1, are taken charge of by the downstream inputs/outputs of the ADB K+2. This split produces two results. Firstly, the nominal bit rate of the ADB K+2 (and hence of an ADB in general), need not be twice the actual need. In the example it need be only 50% higher. The increase in bit rate is related to the number of ADBs (here 2: the ADBs K+1 and K+2) which are involved in countering the fault with an ADB. Secondly, beyond this number of involved neighboring ADBs, the network can allow an additional fault, for example that with the ADB K+3. 
   The second solution will be preferred in the case of a single fault. The third solution is advantageous in the case where several faults occur, or else if the ADB at the downstream end of the chain has an active role in the normal mode (some of its upstream inputs/outputs are linked to stations by a bus) but with no redundancy. More generally one chooses the solution which is best tailored as a function of a desired maximum bit rate or according to a strategy, for example implementation in an automaton. 
   In this case with the process of the invention one determines how many ADBs are functioning between a defective ADB of rank K and a defective ADB of rank K±n. Thus, knowing a number of buses to be fed between these two ADBs, the program  63  ( FIG. 1 ) determines a number of upstream inputs/outputs and a number of downstream inputs/outputs to be activated for each of these functioning ADBs. The microprocessor  42  then activates the upstream inputs/outputs and the downstream inputs/outputs are determined. This last solution has the advantage of splitting a surge of bit rate of the faulty ADB or ADBs. This split makes it possible to even out the bit rates of the primary streams and thus simplify an operation of a central unit to which the ADBs are linked. 
     FIG. 5  illustrates in the form of an algorithm the various steps carried out by the process of the invention. A first step  64  corresponds to a waiting step of the process. During this step  64  the program  63  waits for the management program  43  to indicate that it has just detected an event, for example a fault. In this case the process of the invention increases by one unit a value in a register  65  for counting a number of faults in the central unit  19  ( FIG. 1 ). The process of the invention then carries out a step  66  of choosing a strategy. If shunting is chosen then the process of the invention instigates a step  67 . The defective ADB is located in this step  67 . That is to say a value of K or more precisely of the address K is sought. Once this has been carried out, the process initiates a step  68  in which it will command, by way of the microprocessor  42 , the deactivation of all the upstream inputs/outputs of the ADBs of rank K to N and the activation of all the downstream inputs/outputs of the ADBs of rank K+1 to N. The process of the invention therefore applies the second solution described earlier. The location of the defective ADB, that is to say the value of K, has been stored in the data memory  45 . 
   In the case where the test carried out in step  66  indicates a strategy of fault splitting around the defective ADB, then a step  69  is instigated instead of the step  67 . During this step  69  the defective ADB is located by searching for the value of the rank K±n of the faulty ADB. Once found, this value of K±n is saved in the data memory  45 . Next comes a step  70  during which the program  63  determines, as a function of the address of the ADB of rank K and of the ADB of rank K±n a number of upstream inputs/outputs and a number of downstream inputs/outputs to be activated for the ADBs which are operational. After this step  70 , begins a step  71  during which the microprocessor  42  commands the activation of the upstream inputs/outputs and of the downstream inputs/outputs thus determined. After the steps  68  or  71  the process of the invention returns to the waiting step  64 . 
   In this description of the various steps of the process of the invention, the events were regarded as being faults. In fact, it would be possible to have events of all sorts such as those related to maintenance of the network for example or any other function requiring disconnection of an ADB. That is to say, an ADB is deactivated so that it can be investigated. Thus, it is possible to have a first event relating to a fault and a second event relating to maintenance of an ADB or any other combination of events. 
   In a preferred example one considers an aircraft comprising 1000 stations. The stations are linked in cascade in groups of ten to a bus. The buses are linked as in the invention. That is to say an ADB can be found on either side of the ends of the bus. Thus, during normal operation, 40 stations are linked in cascade to the four upstream inputs/outputs of an ADB. Thus, 26 ADBs are used in such a network. In this case if one envisages a useful bit rate of the order of 10 Mbits/s per station then one must envisage a bit rate of 10×10=100 Mbits/s per bus. Thus an upstream or downstream input/output of an ADB must be able to supply information with a bit rate of the order of 100 Mbits/s. Knowing that a maximum bit rate is obtained when an ADB is operating with its upstream inputs/outputs and its downstream inputs/outputs active then the maximum bit rate is of the order of 8×100 Mbits/s=800 Mbits/s. However, in a preferred example the device of the invention is constructed so as not to have to dimension cables at 800 Mbits/s but on the contrary to be able to limit oneself to 500 Mbits/s. 
   During fault-free operation the primary stream has a bit rate of the order of 400 Mbits/s. When faults occur, a surge in bit rate of a primary stream will vary from 0%, in a case of a single fault, up to 100%, in the case where a single ADB is operational between two faulty ADBs. Conversely, in the case where two faulty ADBs are sufficiently far apart the bit rate surge applied to the various primary streams of the relevant ADBs reaches only 25%. The primary streams of the network  1  will therefore be substantially equivalent to within 25%. 
   In this preferred example stations are linked in cascade to a bus constructed in compliance with the IEEE 1394 standard. That is to say the buses are constructed from cables with two twisted conductors and a maximum bit rate flowing over these buses is of the order of 100 Mbits/s. This preferred example constitutes no limitation whatsoever of the invention. Moreover, the device of the invention and/or its process can be used in any network comprising at least two ADBs.