FR2939585A1 - Network coding method for wireless mesh communication network, involves constructing reconstituted result packet using all or part of redundant packets along modified network coding scheme, and transmitting reconstituted result packet - Google Patents

Abstract

The method involves detecting whether a used data packet is missing or the used data packet is received in a modified manner such that linear combination of source packets forming the modified used packet is not conformed to a predetermined network coding scheme. A reconstituted result packet is constructed using all or part of redundant packets along a modified network coding scheme in case of positive detection. The reconstituted result packet is transmitted. Independent claims are also included for the following: (1) a computer program product comprising program code instructions for implementing a network coding method (2) a storage medium for storing a computer program comprising instructions for implementing a network coding method (3) a relay node comprised in a mesh network and participating in a network coding method (4) a destination node comprised in a mesh network and participating in a network coding method.

Description

A method of network coding robust to transmission losses in a mesh network, relay node, destination node, computer program product and corresponding storage means. FIELD OF THE INVENTION The field of the invention is that of meshed communication networks. More specifically, the invention relates to a network coding technique adapted to the transmission of data packets, in a mesh communication network, from source nodes to destination nodes, via relay nodes retransmitting packets. data from the source nodes and destined for the destination nodes. In accordance with the general principle of network coding, the messages retransmitted by the relay nodes are linear combinations of the messages received by these relay nodes, which makes it possible to increase the data rate and make best use of the network capacity. The invention applies in particular, but not exclusively, in the context of a wireless mesh network. 2. TECHNOLOGICAL BACKGROUND 2.1 Context and problem In the remainder of this document, we will focus on the problem existing in the context of network coding in a wireless mesh network, which the inventors of this patent application. The invention is of course not limited to this particular context of application, but is of interest in all cases of network coding adapted to the transmission, via relay nodes, of data packets from source nodes to destination nodes. , in a mesh communication network.

A wireless mesh communication network consists of a set S of M source nodes and a set Dt of N destination nodes. The transmissions considered are called multicast (the term multicast is sometimes also used), that is to say that each data packet sent by a source node is intended for all the destination nodes of Dt. The other nodes being relay nodes, used to retransmit source packets sent by the source nodes. The topology of the network is assumed to be known, that is to say the set of radio qualities (such as the radio power received) between two communicating nodes is known. The relay nodes conventionally have the function of retransmitting one of the packets they have received as input (routing function). As part of a network coding, these relay nodes have a new feature: they have the ability to issue a resulting packet that is a combination of packets received as input. The concept of network coding was introduced by R. Ahlswede and co. (see article: R. Ahlswede, N. Cai, S. YR Li and RW Yeung "Network Information Flow" IEEE Transactions on Information Theory Vol 46, No. 4, pp 1204-1216, July 2000) and very quickly, many articles have demonstrated the contribution of this new concept, especially in terms of bandwidth gain and in terms of network capacity utilization. In addition, it has been shown that in a multicast framework (each message sent by a source node is intended for all the nodes of Dt), the linear combinations in a sufficiently large body Fq give an optimal result (in terms of gain ). In the remainder of the present invention, the term network coding scheme is defined as: • the relay nodes combining the input packets to generate a combined packet called the resulting packet; For these relay nodes, the input packets to be combined among the set of packets received; • Each input packet to combine is associated with a coefficient in Fq. These relay nodes and these packets (corresponding to links) participating in the network coding are considered useful, the others are considered redundant. The destination nodes then receive a plurality of packets (the resulting packets from the relay nodes) which are a linear combination of the source packets (packets sent by the source nodes). In the context of the invention, the coding scheme used is an optimized coding scheme (described for example in patent application US2005 / 0010675) reducing the number of relay nodes and of useful packets participating in the network coding scheme in order to decrease the number of operations and therefore the CPU time. This scheme is determined at the initialization of the system taking into account the topology of the system.

The resulting packets emitted by the relay nodes are therefore predetermined (more precisely their associated linear combination). But the wireless network is sensitive to masking caused for example by a living being crossing the field of transmission. The loss of links caused by these masks can temporarily modify the topology of the initial network, topology on which the predetermined network coding scheme has been calculated. It is then no longer valid during this period, which can lead to the inability of the destination nodes to decode all the source packets because they are all linked together in the packets received.

It is therefore necessary to increase the robustness of the network coding scheme against these link losses (measured in number of source packets decodable by the destination nodes). 2.2 Prerequisites and Limits of the State of the Art 2.2.1 The body Fe q = 2s The set of operations carried out in the present invention is carried out in a finite body denoted Fq, consisting of q elements. In the context of the invention, the information sequences sent in the network are in a container called package, which has a fixed size. s consecutive bits of a packet constitute a symbol on the body Fq with q = 2s. For example, a 1400-byte packet contains 1400 symbols if the body size is 28 or contains 700 symbols if the body size is 216. The addition, multiplication, and division operations are then performed symbol by symbol:> Addition + is done bit by bit with the classical function XOR - for multiplication. a symbol of s bits, b0 ... bs_1, is transformed into a polynomial: bo + b1X + .. bs-1Xs-1 • An irreducible polynomial on Fq is chosen. The multiplication between two symbols is then obtained by the classical multiplication between two polynomials modulo the irreducible polynomial. - The division is calculated from the Euclidean algorithm. Implementations are given for example in the document N.R. Wagner. The laws of Cryptography with Java Code, available on the Internet at: http://www.cs.utsa.edu/ùwagner/laws/FFM.html.

Therefore, in the remainder of the present description, when Y = ai.Ar is written with A1 ... A1, 1 input packets and a1, .., a1 coefficients in Fq, these = 1

operations are actually performed on all the symbols of the packet: yk = a, .a; `where yk and 1 = 1

ak are respectively the kth symbol of packets Y and A and the operations + and. take place in Fq.

2.2.2 Network coding

2.2.2.1 Introduction

At a relay node, the linear combination associated with a resulting packet can be seen at two levels, a local level and a global level. To explain these two notions, suppose that a network coding scheme is predetermined for all the nodes and situate us in a useful relay node (also called combining). This relay node Rj must then combine a set of received packets M1, M2, ..., M1 among the received packets. The resulting packet is then equal to y, = a? Mi with 1 = 1 a1 ', az, ..., a1' belonging to the finite field Fq. This linear combination y, = a? Mi is the 1 = 1 local representation of the network coding, hereinafter called local combination. But the packets to combine received as input can be themselves also resulting packets from other relays. Let us denote by X1, ... XM the source packets sent by the M source nodes. It is then clear that each packet flowing in the network is a linear combination of X1, ... XM packets. The previously quoted packet y can be m also thus represented in the following way: y, = g X; . This representation is the 1 = 1 global representation of the network coding. It takes into account that each packet received M; is also a resulting package that combines source packages. It is called thereafter global combination. It is clear that the coefficients g3 are recursively deduced from the coefficients of a1.

Notation: the received packets intervening in a local combination are called later the packets associated with the local combination. Source packets that have a nonzero coefficient in a global combination are subsequently called the packets associated with the global combination. Note: Only one global combination can be associated with a given locale, but not vice versa. Indeed, to a global combination, we can associate several local combinations by playing on the choice of received packets M; to combine. In the context of the invention, each relay node stores the global representation of each packet received as input in a matrix, called relay matrix R = (r1,). Each line represents a received packet, each column a source packet, and each element of the matrix r1i then represents the coefficient of the source packet j of the overall linear combination of the received packet i. The destination node di which receives the packets Y1, Y2,..., Y1 'also stores the global representation of each input packet received in a matrix, called destination matrix D = (d1i). Each line represents a received packet and each column a source packet, and each element of the matrix d1i represents the coefficient of the source packet j of the overall linear combination of the received packet Y1. The destination node must then solve the linear system constituted by the set of global linear combinations m: Yi = of Xi j = 1 ... 1 ', ie from a matrix point of view, Y = DX with D la 1 = 1 destination matrix, Y the column vector of the received packets and X the column vector of the source packets. The resolution of this linear system is explained in the following section. It should be noted that if the rank of D (in the sense of the linear algebra) is equal to the number of source nodes M then the M source packages are decodable. Otherwise, it is necessary to study more precisely the destination matrix to determine the number of decodable source packets. 2.2.2.2 Construction of a network coding In a mesh network consisting of M sources and N destination nodes, the preliminary step before building a network coding scheme is to ensure that there is a network coding scheme seen the topology of the network. This phase is based on graph theory concepts and in particular on a use of the Ford-Fulkerson algorithm (see "Maximum Flow through a Network" Canadian Journal of Mathematics, 8, pp 399-404, 1956).

The network is modeled by a graph G = (V, E), V being a finite set of K nodes (called vertices in English), V = {xi,. . . , xK}, and E V V x V a finite set of edges that connect the nodes. In the context of the invention, the set E is an ordered set of pairs (xi, xi), with x; and xi belonging to v, these pairs being called connected edges, arcs or links (called edges in English). A node of V models a communicating entity in the network (computers, loudspeakers, amplifier) while an arc (xi, xi) indicates for example a sufficiently good radio communication (that is to say above a certain threshold of radio power received) between the two communicating entities x; and xi.

Moreover, a path between a node x1 and a node xp is called an ordered set of nodes {x1, x2, .., xp} such that (xi, xi + 1) E E for i = 1,. . . , (pùl). A path can also be written {(x1, x2), .., (x1, x1 + 1), ..., (xp_1, xp)} with (xk, xk + 1) E for k = 1,. .., (pùl). We call k disjoint paths, k sets that do not have two consecutive nodes in common, either from a graphical point of view, two arcs in common. Let T be a set of k disjointed paths. We then define: 1. Let (x, y) EE (a link of the graph), (x, y) AND if and only if there exists a path of T which contains (x, y) 2. Let x EV ( a vertex of the graph), x AND if and only if there exists a path of T that contains x. We call two disjoint paths between a set S and a node d, two ordered sets of nodes {x1, x2, .., xp, d} and {y1, y2, .., yq, d} such that:> (xi , xi + 1) EE for i = 1, ..., (pù1) and (xp, d) EE - (y, y + 1) EE for j = 1, ..., (qù1) and (yq, d) EE - x1 ES, Y1 ES and x1 ≠ Y1 - `di = 1 .. (p -1),` dj = 1 .. (q -1), (xi, x1 + 1) ~ (y; y; +1) and (xp, d) ~ (Yq, d) The definition is generalized very easily for a set of k disjoint paths. From these notions of graph theory, the state of the art gives the following condition: Let S be a set of M source nodes, if there exist M disjoint paths between S and each destination node of Dt, then the M source nodes are able to multicast to Dt if the relay nodes are allowed to linearly combine their incoming packets on a sufficiently large finite field. This result is a direct consequence of the aforementioned article by R. Ahlswede and co. The invention is placed in the context where this condition is verified at the start of the system. On the other hand, during the life of the system, this condition may no longer be respected because of temporary loss of links. In this case, no network coding scheme exists for decoding the M source packets. The destination nodes can no longer decode M source packages. In the state of the art, there are mainly two approaches for the construction of a network coding scheme according to the knowledge or not of the initial topology of the network: for a known topology, a first deterministic diagram of construction of Matrix-based coefficients are given in the following article: R. Koetter, M. Medard, "An Algebraic Approach to Network Coding" ACM Transactions on Networking, Vol 11, Nos., October 2003. "The Network Coding Scheme obtained according to this first approach is called a deterministic coding scheme: - For an unknown topology, an on-the-fly construction of the randomly drawn coefficients is given in the following article: "Randomized distributed network coding" - Michelle Effros, Tracey Ho, David Karger, Ralf Koetter, Muriel Medard 20050152391. The network coding scheme obtained according to this second approach is called a random coding scheme. As a first approach, the coefficients are calculated once and for all at the start of the system and thus during the lifetime of the system. Destination nodes and relay nodes have inexpensive operations because they know exactly what they will receive and transmit. In the second approach, the relay nodes randomly pull the coefficients of the resulting packet to be generated, which requires additional processing time. In addition, they are obliged to send the randomly drawn coefficients in the resulting packet so that the other nodes, and more especially the destination nodes, know the linear combinations to decode. But this signaling is even larger than the body is large (each coefficient will be encoded on a larger number of bits), and it is as much lost bandwidth. But in this second approach, the larger the body, the greater the chances of successful decoding of source packages. Hence the contradiction. In the first approach, the decoding is sure to succeed because the construction made at initialization ensures that this is the case. The first approach is therefore more efficient than the second in terms of gain in bandwidth and computing time. On the other hand, the second approach is more efficient than the first on the following points: • it does not require knowledge of the topology of the network. In the context of the present invention, it is assumed that there is a means of recovering the topology of the network, so this disadvantage of the first approach is not taken into account; • it is robust to link losses, unlike the first approach. Indeed, since it generates a resulting packet with a different linear combination with each send, the loss of link is transparent for it (although not guaranteeing the decoding). It indicates a zero coefficient for the packet that was to arrive by this link.

The first approach is, however, strongly affected by link losses because each loss of link modifies the linear combination of the relay node (combination initially fixed) and only the relay node observing this loss is aware of it. The other nodes and especially the destination nodes are not aware of this modification of the topology and believe that everything is happening as usual. The decoding can then be catastrophic and influences all the source packets to be decoded since they are all linked by the linear combinations (unlike a routing where only a source packet would be reached). Considering in particular that the advantages of the first approach are essential in high speed applications as targeted by the invention, it is assumed in the following description that it is the first approach that is used to build a network coding scheme based knowledge of the initial topology of the network. In practice, the method of constructing the commonly used predetermined network coding scheme (according to the first approach) (hereinafter referred to as the optimized version) is that which limits the number of nodes and links participating in the network coding using only the paths disjoint from the existence condition of network encoding: only nodes and links of disjoint paths are useful for building the network encoding scheme. This optimized version is described in particular in the aforementioned US2005 / 0010675 patent application. From this observation, we call a node (respectively link) useful, a node (respectively link) which belongs to the set of disjoint paths and, we call node (respectively link) redundant, a node (respectively link) which n ' does not belong to the set of disjoint paths. This optimized version thus makes it possible to limit the number of operations. But in a wireless LAN, it is likely that these redundant links are numerous, because every time a node sends a packet, all other nodes in the network are likely to receive this packet. This optimized version of the network coding construction is for example used in the context of the present invention. It severely limits operations because only the nodes and links considered useful are used, which reduces the number of operations in the body at the relay node and the destination node. For example, a relay node no longer needs to combine the plurality of packets it receives but only a subset of packets. Same for the destination node that only needs a subset of the packets received to decode the source packets.

With this optimized version, at first a network coding scheme is constructed which takes into account only the relay nodes and useful links, according to the following definitions: the relay nodes combining the input packets are the useful relay nodes; for these useful relay nodes, the input packets to be combined among all the packets received are the useful packets; each input packet to combine is associated with a coefficient in Fq.

The resulting packets are thus predetermined. In the same way, the destination node then constructs a sub-matrix of the destination matrix D, called the coding matrix and noted C, which only takes into account the useful packets (lines are therefore deleted from D). From C, a square submatrix of dimension (M, M) (the optimized schema assures this property), the destination node calculates the inverse of this one C-1 (which exists by construction of the schema) and decodes thus the source packets: X = C-1Y, with X the column vector of the source packets and Y the column vector of the packets received. 2.2.3 Resolution of a linear system Let a linear system of n equations with p unknowns in Fq: a11 31 + + alp ~ p = bl an1131 + + anpI3p = bn With [(31, ... (3p] the p unknowns of the system, ale, i = 1..n, j = 1..p the coefficients of the system and [b1, ... bn] the constants of the system To solve this system, we use for example the pivot method Gauss, also called Gauss-Jordan elimination, which is known to those skilled in the art Other matrix resolutions are possible to solve the system of equation: LU decomposition, QR decomposition, Cholesky method. In order to apply the Gauss method, the following matrix of dimension (n, p + 1) is associated with the system: ## EQU1 ## On this matrix, the Gauss-Jordan algorithm realizes elementary operations line by line in order to obtain a matrix of the form: ## EQU1 ##

o 0 bn The system is said to be compatible if it admits at least one solution. The compatibility depends on the value of r with respect to the values of n and of p: 1. In the case where r is strictly smaller than n and p, two cases occur: - The coefficients br + 1, ..., bn are void. In this case, the system is compatible and there exists an infinity of solution for the [(31, ... I. The Il,. • • Rr] are the main unknowns, the others [(3r + i, ... (3p) are the secondary unknowns To solve the system, one fixes then the [(3r + 1, ... (3p] which become parameters and one determines the principal unknowns by solving the linear system starting with Rr then ( 3r_1, ... (31. - There are coefficients br + 1, ..., bn not null The system is incompatible 2. In the case where r is equal to n and strictly inferior to p, there exists a infinity of solution for the [(31, ... I. The [(31, ... (3n] are the main unknowns, the others [(3n + 1, ... Rp] are the secondary unknowns. In the case where r is equal to p and strictly less than n, two cases occur: • The coefficients br + 1, ..., bn are zero The system admits a solution p 1 = b1, .., (3p = bp - Otherwise the system is incompatible 4. If r is equal to p and n, the system admits a unique solution: (31 = b1, .., (3p = bp. 2.2.4 Limits of the state of the art In the state of the art, the management of packet losses in a network coding scheme is mainly managed by the random coding scheme (second approach described above), This makes it (unlike the deterministic coding scheme according to the first approach) robust to link losses (as detailed previously). Indeed, with the random coding scheme, the relay nodes systematically perform expensive operations to combine packets with or without loss of packets. But, considering that the advantages of the first approach (in terms of bandwidth gain and calculation time) are essential in high bit rate applications (as detailed before), the present invention is placed in the context where it is this first approach that is used (network coding scheme based on knowledge of the initial topology of the network). In this case, packet loss management consists of looking for robust network coding schemes as soon as they are built. This is to ensure that the destination matrices are still able to decode all the source packets sent by the source nodes for a predefined set of broken links. But the relay nodes are obliged to use all the packets they receive, which imposes at the relay nodes a significant and especially systematic calculation time. OBJECTIVES OF THE INVENTION The invention, in at least one embodiment, has the particular objective of overcoming these various disadvantages of the state of the art. More specifically, in at least one embodiment of the invention, one objective is to provide a network coding technique adapted to the transmission of data packets in a meshed communication network, this technique making it possible to optimize loss management. of packets (that is, loss of links) in the case where a deterministic network coding scheme (obtained according to the above-mentioned first approach) is used. At least one embodiment of the invention also aims to provide such a technique that is transparent to the source nodes.

At least one embodiment of the invention also aims to provide such a technique that is transparent to the destination nodes, if it is possible, or which minimizes the impact on the network capacity (ie ie, the number of decodable source packets by all the destination nodes) if such transparency for the destination nodes is not possible. Another objective of at least one embodiment of the invention is to provide such a technique which is simple to implement and inexpensive. SUMMARY OF THE INVENTION In a particular embodiment of the invention, there is provided a method of network coding in a mesh network comprising at least one relay node which, according to a predetermined network coding scheme, expects receiving as input a plurality of data packets and outputting a predetermined resultant packet obtained according to a first predetermined linear combination of a portion of said plurality of data packets, the data packets belonging to said party being said to be useful packets and the data packets not belonging to said part being said redundant packet, each of the expected data packets being formed, according to said predetermined network coding scheme, by a linear combination of one or more source packets, the packets sources being issued by source nodes. Said method is implemented by at least one relay node and comprises the steps of: - detecting whether at least one useful packet is missing or has been received modified so that the linear combination of the source packet (s) forming said modified useful packet does not conform to the predetermined network coding scheme; in the case of a positive detection, constructing a reconstructed resultant packet using, in accordance with a modified network coding scheme, all or part of the redundant packets; - transmit the resulting reconstituted packet. Thus, this particular embodiment of the invention is based on a completely new and inventive approach of using, at the relay node level, received packets links not included in the predetermined network coding scheme, called redundant packets ( packets not useful), and thus normally not used by the relay nodes (not taken into account in the resulting packet) and to perform new operations with these redundant packets to limit the impact of the loss of link. In other words, at the relay node level, the key idea is to use the redundant packets if the loss of a link touches the reception of a useful packet. Thus, when a link loss is detected (detection of the impossibility of generating the predetermined predetermined packet), an alternative and provisional network coding scheme (modified network coding scheme) different from the predetermined coding scheme is used. This makes it possible to limit the impact of this temporary change of network coding scheme (to be in the best case transparent for the destination nodes). Advantageously, said step of constructing a reconstructed resultant packet comprises a step of determining a second linear combination of redundant and useful packets, from a plurality of data packets actually received by said given relay node, to obtain a packet. resulting identical reconstituted to the predetermined resulting data packet to be generated, if said second linear combination of redundant and useful packets exists.

In other words, if it is possible, the value of the coefficients associated with the redundant and useful packets is chosen such that the resulting reconstructed packet sent is exactly the predetermined resulting packet. In this way, the operation performed in the given relay node (sending the reconstructed resultant packet instead of the predetermined resulting packet) is transparent to all other downstream relay nodes, as well as to the destination nodes. Advantageously, said step of determining the second linear combination of redundant and useful packets is performed by testing whether each missing detected useful packet can be replaced by a redundant packet actually received by said given relay node.

Thus, the transparent construction operation of the resulting reconstituted packet is quick and inexpensive. According to an advantageous characteristic, said step of determining the second linear combination of redundant and useful packets comprises steps of: - determining a third predetermined linear combination of source packets, associated with said first predetermined linear combination of useful packets, and allowing obtaining said predetermined predetermined packet; constructing a linear system A (3 = g, where (3 is a column vector of the coefficients of said second linear combination of redundant and useful packets to be determined, where g is a column vector of the coefficients of said third predetermined linear combination of source packages , and where A is a matrix of dimension (M, 1), with M the number of source packets and 1 the number of packets actually received by said given relay node, the matrix A being defined as follows: * each line of matrix A represents a source packet, * each column of matrix A represents a packet actually received, * each element aii of matrix A represents the coefficient of source packet i of a fourth linear combination of source packages making it possible to obtain the packet actually received j - solving said linear system by a predetermined method - if the linear system admits a solution, determining said uxth linear combination of redundant and useful packets, depending on the column vector (3 of said solution. Thus, the transparent operation of construction of the reconstituted resulting packet makes it possible to end in a systematic way if the linear system considered admits a solution. Advantageously, if said second linear combination of redundant and useful packets does not exist, said step of constructing a reconstructed resulting packet comprises the steps of: a) constructing a resulting packet containing source packets not contained in the packet predetermined result; b) obtaining relative information, for at least one other node participating in the predetermined network coding scheme, to a modified network coding scheme that would result from sending the reconstructed result packet constructed in step a); C) depending on the information obtained in step b), selecting the reconstructed resultant packet constructed in step a) or repeating step a) to construct another resulting packet. In this way, the operation performed in the given relay node (sending the reconstructed resultant packet instead of the predetermined resulting packet) is not transparent (for all other downstream relay nodes, as well as for the destination nodes), but it has no impact on the capacity of the network (ie the number of source packets decodable by all the destination nodes). In other words, if a transparent construct is not feasible, the relay node tries to build a resulting packet that has no influence on the network, that is, that does not decrease the number source packages that can decode the destination nodes. This construction is also based on the use of redundant packets. On the other hand, in this case, the relay node must notify the other nodes that the resulting packet that it sends is not based on the predetermined linear combination.

Advantageously, step a) comprises the steps of: i) determining a fifth predetermined linear combination of source packets, to obtain said resulting packet containing source packets not contained in the predetermined resulting packet; ii) constructing a linear system A (3 = g, where (3 is a column vector of the coefficients of said second linear combination of redundant and useful packets to be determined, where g is a column vector of the coefficients of said fifth predetermined linear combination of packets sources, and where A is a matrix of dimension (M, 1), with M the number of source packets and 1 the number of packets actually received by said given relay node, the matrix A being defined as follows: * each line of matrix A represents a source packet, * each column of matrix A represents a packet actually received, * each element aii of matrix A represents the coefficient of the source packet i of a sixth linear combination of source packages making it possible to obtain the package actually received; iii) resolving said linear system by a predetermined method; iv) if the linear system admits a solution, determining adite second linear combination of redundant and useful packets, depending on the column vector (3 of said solution, if not reiteration of step i) with another fifth predetermined linear combination of source packets.

Thus, the operation (not transparent but without impact) of construction of the resulting reconstituted package allows to achieve systematic if one of the linear systems, considered during successive iterations, admits a solution. Advantageously, in step b), said information relating to the modified network coding scheme, which would result from sending the reconstituted resulting packet constructed in step a), is the sum of the number of decodable source packets by the set of destination nodes of the network if the resulting reconstructed packet constructed in step a) is assumed to be sent. In step c, the reconstructed resulting packet constructed in step a) is selected if said information relating to the modified network coding scheme is equal to the number of source nodes in the network multiplied by the number of destination nodes in the network. Thus, the information test (relative to the modified network coding scheme) makes it possible to ensure that the (non-transparent) operation has no impact on the capacity of the network (that is to say the number of source packets). decodable by the set of destination nodes).

Advantageously, if said second linear combination of redundant and useful packets does not exist, said step of constructing a reconstituted resulting packet comprises steps of: - constructing a first resulting packet, different from the predetermined resulting packet but containing only source packets included in the predetermined resulting packet; obtaining a first relative information, for at least one other node participating in the predetermined network coding scheme, to a modified network coding scheme that would result from sending the first resulting packet, the first resulting packet being associated with said first information on the modified network coding scheme; - among the packet or packets not selected in step c) and containing source packets not contained in the predetermined resulting packet, selecting a second resulting packet, based on said information, obtained in step b) for each packet not selected in step c), on the modified network coding scheme, the second resulting packet being associated with a second piece of information relating to the modified network coding scheme; selecting between the first and second resulting packets, according to the first and second information relating to the modified network coding scheme.

In this way, the operation performed in the given relay node (sending the resulting reconstructed packet instead of the predetermined resulting packet) is not transparent and has an impact on the network capacity, but this impact is minimized. In other words, if no construction, which does not have an impact on the network, is possible, the relay node builds the packet that has the least impact on the network, that is, say the one that degrades the network the least (always in number of decodable source packets for all the destination nodes). In this case, the relay node must also notify the other nodes that the resulting packet that it sends is not based on the predetermined linear combination. Advantageously, if said second linear combination of redundant and useful packets does not exist, said step of constructing a reconstructed resultant packet comprises a step of: - constructing a resulting packet, different from the predetermined resulting packet but containing only packets sources included in the resulting predetermined packet. In this variant, if a transparent operation is not possible, the given relay node directly performs an operation that has an impact on the capacity of the network, but this impact is minimized. In this case, the relay node must also notify the other nodes that the resulting packet that it sends is not based on the predetermined linear combination. Advantageously, if the resulting reconstructed packet is not identical to the predetermined resulting data packet, the method comprises an additional step of transmitting a notification identifying the differences between the linear combination of source packets representative of said predetermined resulting packet, and the linear combination. source packet representative of said reconstructed resultant packet. Thus, all the other nodes (relay or destination) which are located downstream of the given relay node and which receive the reconstituted resulting packet as well as the corresponding notification, are able to determine how the network coding scheme has been modified (temporarily) in the relay node given. In another embodiment, the invention relates to a network coding method in a mesh network comprising at least one destination node which, according to a predetermined network coding scheme, expects to receive as input a plurality of data packets and should output a plurality of decoded source packets each obtained in a predetermined linear combination of a portion of said plurality of data packets, the data packets belonging to said portion being said to be useful packets and the non-owned data packets said portion being said redundant packet, each of the expected data packets being formed, according to said predetermined network coding scheme, by a linear combination of one or more source packets, the source packets being transmitted by source nodes. The method is implemented by at least one destination node and comprises the steps of: - detecting whether at least one useful packet is missing or received modified so that the linear combination of the source packet (s) forming said modified useful packet does not conform to the predetermined network coding scheme; in the case of a positive detection, decoding the received useful data packets, according to a modified network coding scheme, to obtain at least one decoded source packet. Thus, this particular embodiment of the invention is based on a completely new and inventive approach of using, at the destination node level, received packets links not included in the predetermined network coding scheme, called redundant packets ( not useful packets), and thus normally not used by the destination nodes (not taken into account in obtaining decoded source packets) and to perform new operations with these redundant packets to limit the impact of the loss of link. In other words, at the destination node level, the key idea is to use the redundant packets if the loss of a link touches the reception of a useful packet. Thus, upon detecting a loss of link (detection of the impossibility of generating the predetermined decoded source packet), an alternative and provisional network coding scheme (modified network coding scheme) different from the predetermined coding scheme is used. This makes it possible to limit the impact of this temporary change of network coding scheme. Advantageously, said step of detecting whether at least one useful packet is missing or has been received modified comprises a step consisting of, for at least one useful packet actually received: detecting a received notification associated with said useful packet actually received , said notification identifying the differences between the linear combination of source packets representative of the expected useful packet input, and the linear combination of source packets representative of said useful packet actually received. Thus, the destination node modifies its predetermined coding scheme on receipt of a notification associated with a packet received as input, in order to take into account the new combination of this received packet, to quantify its impact on the predetermined coding scheme, and modify it to decode a maximum of source packages. In another embodiment, the invention relates to a computer program product downloadable from a communication network and / or recorded on a computer readable medium and / or executable by a processor. This computer program product includes program code instructions for carrying out the aforesaid method (in any one of its various embodiments), when said program is run on a computer. In another embodiment, the invention relates to a computer readable storage means, possibly totally or partially removable, storing a computer program comprising a set of instructions executable by a computer to implement the aforementioned method (in any of its different embodiments). In another embodiment, the invention relates to a relay node included in a mesh network and participating in a network coding method in said mesh network, said relay node, according to a predetermined network coding scheme, expecting to receive in inputting a plurality of data packets and outputting a predetermined resultant packet obtained according to a first predetermined linear combination of a portion of said plurality of data packets, the data packets belonging to said portion being said useful packets and the packets of data not belonging to said part being said to be redundant packet, each of the expected data packets being formed, according to said predetermined network coding scheme, by a linear combination of one or more source packets, the source packets being transmitted by source nodes. Said relay node comprises: detection means, making it possible to detect whether at least one useful packet is missing or has been received modified so that the linear combination of the source packet or packets forming said modified useful packet does not comply the predetermined network coding scheme; - building means, activated in the case of a positive detection, for constructing a reconstructed resultant packet using, in accordance with a modified network coding scheme, all or part of the redundant packets; transmission means for transmitting the resulting reconstituted packet. In another embodiment, the invention relates to a destination node included in a mesh network and participating in a network coding method in said mesh network, said destination node, according to a predetermined network coding scheme, expecting to receive in inputting a plurality of data packets and having to output a plurality of decoded source packets each obtained in a predetermined linear combination of a portion of said plurality of data packets, the data packets belonging to said portion being said to be useful packets and the data packets not belonging to said part being said to be redundant packet, each of the expected data packets being formed, according to said predetermined network coding scheme, by a linear combination of one or more source packets, the source packets being issued by source nodes. Said destination node comprises: detection means, for detecting whether at least one useful packet is missing or has been received modified so that the linear combination of the source packet (s) forming said modified useful packet does not comply the predetermined network coding scheme; and decoding means, activated in case of positive detection, for decoding the received useful data packets, according to a modified network coding scheme, to obtain at least one decoded source packet.

More generally, the relay node and the destination node according to the invention comprise means for implementing the methods as described previously (in any one of their various embodiments). 5. LIST OF FIGURES Other features and advantages of the invention will appear on reading the following description, given by way of indicative and nonlimiting example, and the appended drawings, in which: FIG. example of a wireless mesh network in which the network coding technique of the invention can be implemented; FIG. 2 represents the architecture of a device (relay node or destination node) according to a particular embodiment of the invention; FIG. 3 describes the structure of a packet notifying a modification of the predetermined coding scheme, according to a particular embodiment of the invention; FIGS. 4a, 4b and 4c describe algorithms, implemented in a central node, a relay node and a destination node respectively, of construction of a predetermined network coding scheme, at the start of the system; FIG. 5 presents a flowchart of a network coding method according to a particular embodiment of the invention, implemented by a relay node to construct and transmit a resulting packet; FIG. 6 describes an algorithm for reconstituting a local combination, corresponding to step 545 of FIG. 5; Figure 7 depicts an algorithm for determining a new local combination from a global combination, corresponding to step 635 of Figure 6; FIG. 8 describes an algorithm for assigning a confidence index to a local linear combination, corresponding to step 740 of FIG. 7; FIG. 9 describes an algorithm for selecting the overall linear combination that has the least impact on the network, corresponding to step 765 of FIG. 7; FIG. 10 describes an algorithm for reconstituting a local combination associated with a global combination closest to another global combination, corresponding to step 905 of FIG. 9; FIG. 11 presents a flowchart of a network coding method according to a particular embodiment of the invention, implemented by a destination node for decoding the source packets (that is to say obtaining decoded source packets) ); and FIG. 12, corresponding to step 810 of FIG. 8 or step 1145 of FIG. 11, describes an algorithm for determining the number of decodable source packets, and consequently for determining the associated decoding matrix. . 6. DETAILED DESCRIPTION In all the figures of this document, the elements and identical steps are designated by the same numerical reference. FIG. 1 represents an example of a wireless mesh network in which the network coding technique of the invention can be implemented. The medium access protocol is of the TDMA (Time Division Multiple Access) type, where during a fixed period, called superframe, each node of the network has a unique access to medium during a fixed speaking time called time slot (or slot in English). This network is composed of three source nodes, named respectively NS 1, NS 2 and NS 3, and six nodes which are both relay nodes and destination nodes, named NR / D 1,..., NR / D 6. Each communication link is represented by an arrow indicating the direction of the transmission.

The four full-line communications links (as for example between the nodes NS1 and NR / D1) represent packets consisting solely of a source packet from the node NS 1. The three links of communications in small dots (as for example between the nodes NS2 and NR / D4) represent packets consisting solely of a source packet from the node NS2. The two large dotted communications links (such as between the NS3 and NR / D2 nodes) represent packets consisting solely of a source packet from the NS3 node.

The other five communication links (for example between the nodes NR / D1 and NR / D3) represent packets which have undergone network coding and therefore contain a linear combination of at least two source packets. Communications links are not represented which are either victims of permanent masking or not usable due to the TDMA protocol and the successive speech times given to each node, the source nodes emitting first in the order of their numbering and then all the relay nodes also in the order of their numbering. FIG. 2 represents the architecture of a device (relay node or destination node) according to a particular embodiment of the invention. This device is represented by the block 210, blocks 230 (application) and 240 (Radio Frequency transmitter / receiver) corresponding to the environment of this device. The Man / Machine interface 212 allows the user to give the graph representing the network in order to calculate the reference network coding scheme with the Ford-Fulkerson algorithm. This network coding scheme will then be shared between all the nodes of the network. The data memory 216 makes it possible to store the different copies of the data transmitted by the source nodes and the different relay nodes. This data is received by the Radio Frequency transmitter / receiver 240 and passes through the radio packet receiver 215. This memory 216 also makes it possible to store the matrices of the various nodes involved in the network coding process.

The network coding packet management module, referenced 217, which is segmented into four sub-modules, manages, both at the relay nodes and the destination nodes, the network coding packets. The first sub-module 218 is a notification analyzer that makes it possible to know if the received packet corresponds to the expected packet or if coefficients related to the network coding have been modified. The second sub-module 219 allows it to update the network coding coefficients to be used either for retransmission or for decoding, in place of the reference coefficients stored in the data memory 216. The updating of these coefficients is performed after the analysis of a notification corresponding to a received packet. The third sub-module 220, encoded packet production, is used by the relay nodes. It makes it possible to generate the resulting package that must be sent. This resulting packet is a function (linear combination) of some received packets and their associated coefficients. Once produced, the resulting packet is stored in data memory to be transmitted by the radio packet transmitter 214. The fourth sub-module 221 is used during the decoding phase. It makes it possible to construct a decoding matrix according to the received packets and their associated coefficients (stored in the data memory), and to extract from this matrix the different source packets. Once decoded, the source packets are transmitted to the application 230 by the CPU 213. FIG. 3 describes the structure of a packet notifying a modification of the predetermined network coding scheme. In a stable regime where no packet loss is detected, a packet consists only of a given part 310, in which part the information transmitted by the source nodes or the relay nodes is contained. In the context of the present invention, this given part 310 is a linear combination of the source packets sent by the source nodes, linear combination predetermined at the initialization of the system. If the data portion 310 does not contain this predetermined linear combination of source packets (because of link loss), a notification portion 305 is added to indicate the coefficients of the linear combination that have been modified. It should therefore be noted that this notification part 305 is not systematically present in the packet. For each modified coefficient, information is introduced in the notification part 305. This information contains an identifier 315 (respectively 325) of the source packet associated with the coefficient, as well as the new value of the coefficient 320 (respectively 330). In the particular case where a source packet is missing, the value of the coefficient is set to zero. FIGS. 4a, 4b and 4c describe algorithms, implemented in a central node, a relay node and a destination node respectively, of construction of a predetermined network coding scheme, at the start of the system. The central node, which is one of the nodes present in FIG. 1 (for example one of the source nodes), has the function of constructing the predetermined network coding scheme and of distributing it to all the other nodes (as described in FIG. Figure 4a). On receipt of this network coding scheme by the other nodes of the network, the relay nodes store the latter (as described in FIG. 4b) and the destination nodes as well (as described in FIG. 4c). We now present, in relation with Figure 4a, the algorithm implemented in the central node. At system initialization (step 405), the central node retrieves (step 410) the topology of the network (i.e., recovers the quality of the communications between the nodes), the list of source nodes, and the list of the nodes. destination nodes. This topology can be known from the nodes of the network by construction, if the network is fixed and if the number of nodes is constant. It is also certain that this information can be introduced by the user after the installation of a system. Then, in step 415, the central node constructs the network graph G from the network topology. If the quality of a transmission between two entities is considered good, for example if the signal-on-noise ratio (in English Signal Noise Ratio) or the power of the signal received (in English Radio Signal Strength Indication) is higher than a given threshold ( depending on the type of radio used), the transmission is considered valid, if not invalid. The graph is then constructed in the following way: each communicating entity is modeled by a node of the graph and each communication considered valid between two entities is modeled by a link between two nodes (and therefore each communication considered invalid is not modeled in the graph). Then, in step 420, the central node determines the predetermined network coding scheme. The method is explained in the patent application US2005 / 0010675 and repeated in many articles of the state of the art. The steps are as follows:> Find the disjoint paths Uj between the set S of the source nodes and each destination node Dj; - Construction of the subgraph G 'of G containing only vertices and links belonging to at least one set Uj; - Determine the coefficients of the linear combinations in such a way that the set of destination matrices is of rank equal to the number of source nodes. Then, in step 425, the central node diffuses the coefficients according to a predetermined diffusion protocol. Several dissemination techniques are known to those skilled in the art for broadcasting information in a mesh network (for example data flood type in a mesh protocol, English flooding protocol). For a relay node (FIG. 4b), at the end of the diffusion step (425) by the central node, that is to say on reception (step 430) by the relay node of the coefficients of the linear combinations of the set of nodes of the network, the relay node keeps in memory its local combination cmb_loc_pred, which it will later use to build a predetermined resulting packet, and builds a relay matrix and a destination matrix (step 435). The relay matrix contains the global combinations of the resulting packets of the source nodes or relay nodes that had access to the medium, during the superframe, before the date of access to the medium of the relay node considered. The other packets, that is to say those sent by nodes which had access to the medium after the relay node considered, are not taken into account because they are not usable (because of the constraint that the combinations only occur with data sent during the current superframe). In addition, the lines corresponding to the packets involved in the combination cmb_loc_pred, that is to say the useful packets, are marked and are considered useful. The destination matrix contains the set of global combinations of the resulting packets received by the destination nodes. These two matrices are explained above in the section titled Network Encoding. For a destination node (FIG. 4c), at the end of the broadcasting step (425) by the central node, that is, at the reception (step 440) by the destination node of the coefficients of the linear combinations of the set of nodes of the network, the destination node constructs a destination matrix (step 445). From this destination matrix and from the Gauss Jordan algorithm, the destination node defines (step 450) a square submatrix of dimension (M, M) of D, denoted C, invertible, such that we have : Y '= CX, with Y' a subvector of the column vector Y of the packets received and X the vector of the source packets, the subvector Y 'incorporating only the received packets taken into account in C (the useful packets). From this equation, we deduce: X = C_ 'Y'. The destination node stores in memory the inverse of C, a matrix called a decoding matrix that it will use directly on receiving the packets. After this initialization phase, each source node starts sending the source packets on the network, each relay node performs its local combination and each destination node decodes the source packets from its decoding matrix. FIG. 5 presents a flowchart of a network coding method according to a particular embodiment of the invention, implemented by a relay node to construct and transmit a resulting packet. The input parameters are as follows:> A plurality of received combined packets, packets combining source packets of the current super frame; - The predetermined local combination of the relay node, noted cmb_loc_pred (which includes the predetermined subset of packets to be combined of the plurality of received packets); The relay matrix R of the global combinations of the packets received before access to the medium of this relay node. The output parameter is the resulting package, generated with notification if necessary (that is, if cmb_loc_pred could not be applied and a change was necessary).

This algorithm starts at a fixed date in the superframe (step 500), that is to say at a date preceding the access to the medium of the relay node, sufficient time to perform the following algorithm (at the expense of not take packets received if it is estimated that the period is too short between receiving a packet and sending the resulting packet). In step 505, unavailable user data packets are determined. This means is based for example on radio information, such as the signal-on-noise ratio (Signal Noise Ratio) or the signal strength (Radio Signal Strength Indication). If these variables exceed a predefined threshold, the packet is considered available, if not available. Then, in step 510, the composition of the available useful packets is determined. If the latter does not have a notification (305, FIG. 3), the source packet composition is given by the relay matrix; more precisely, by the line corresponding to the received packet. Otherwise, the notification is read and the new composition of the packet is determined by reading the changes indicated by the notification, relative to the predetermined overall combination. In step 515, a temporary relay matrix R '= r', i, valid only for the execution of this algorithm in the current super frame is created if a useful packet is considered unavailable in step 505 or if a useful package was modified in step 510.

It is filled in the following manner (step 515): initialization of the value of the coefficients of the temporary relay matrix to the value of the coefficients of the predetermined relay matrix; the lines corresponding to the unavailable packets, result of step 505, become null lines (all the coefficients of the line are set to zero); the lines corresponding to the modified available packets, result of step 510, are modified and take into account the modifications of the notification: a modified element r ', i means that the packet associated with this line has been modified; package i has been received with a notification. Column j corresponds to the identifier (315 or 325, FIG. 3) of the source packet indicated in the notification. The new value of r ', i corresponds to the coefficient (320 or 330, figure 3) indicated in the notification. In step 520, it is checked whether the temporary relay matrix R 'has been created or not. If yes, proceed to step 530, otherwise step 525.

In step 525, the predetermined resulting packet is constructed from the received useful packets and the local combination cmb_loc_pred. This predetermined resulting packet will be sent to the next access to the medium by the relay node. On the other hand, if the test of step 520 is positive, this indicates that a link loss has occurred and has hit a useful packet. The local combination cmb_loc_pred is therefore no longer valid, a new one has to be defined. For this, the redundant packets received are necessary. Initially, the algorithm determines whether the redundant packets have not been impacted by a loss of links, by executing the steps 530 and 535 on the redundant packets (steps identical to the steps 505 and 510 performed for the useful packets) . Then step 540 updates the temporary relay matrix R 'in the same manner as in step 515 performed for the useful packets. From the temporary relay matrix R ', in the next step 545, a new local combination cmb_loc_nouv is reconstructed, the associated received packets are selected and the differences between the global combinations associated with the local combinations cmb_loc_nouv and cmb_loc_pred are notified (if differences!). This step is explained below in detail in relation to FIG. 6. In step 550, the resulting packet is constructed from the selected received packets and the combination cmb_loc_nouv determined in step 545. If necessary, one builds also a notification (as described above in connection with Figure 3), to indicate the difference between the sent combination and the predetermined combination. This resulting packet and the notification (if there is one) are sent to the next access to the medium by the relay node. Note that if the local combination cmb_loc_nouv is the null combination, the relay node does not send a resulting packet. FIG. 6 corresponds to step 545 of FIG. 5 and describes the algorithm for reconstituting a local combination cmb_loc_nouv of a relay node affected by a loss of link.

The input parameters are as follows: - the predetermined local combination of the relay node, denoted cmb_loc_pred (which includes the predetermined subset of packets to be combined of the plurality of received packets); the relay matrices R and R 'of global combinations of the packets received before access to the medium of this relay node. The output parameter is the new combination cmb_loc_new, and the associated received packets. It is recalled that a local combination (linked to packets received) is associated with a single global combination (linked to the source packets). Note cmb_glob_pred the predetermined global combination associated with the predetermined local combination cmb_loc_pred. On the contrary, in a global combination it is possible to associate several local combinations. The object of the invention is then to find a new local combination cmb_loc_nouv corresponding to the predetermined global combination cmb_glob_pred (because the predetermined local combination cmb_loc_pred is no longer available). The principle is as follows: from a predetermined local combination cmb_loc_pred, one determines its predetermined global combination cmb_glob_pred and from this one determines a new local combination cmb loc nouv.

Step 605 checks whether each source package of cmb_glob_pred is at least in a received packet (a necessary condition to hope to reconstruct the global combination cmb_glob_pred with the received packets). For that, one checks that no column of the matrix R 'corresponding to a source package of cmb_glob_pred is null (all its null coefficients). If the test is positive (step 610), proceed to step 615 if not step 625. In step 615, it is checked whether rapid reconstitution of cmb_glob_pred is possible by a new local combination. For this, we compare the lines that have been modified between R and R '. For each line modified and considered useful for the matrix R, it is sought if this line of R is present in R '(necessarily at another line position because the line has been modified in R'). If so, the new line is called a successor line. If this test is positive (step 620), the reconstitution is transparent and a new local combination is assigned, cmb_loc_nouv (step 640). This new local combination is constructed in the following way: the values of the coefficients are not modified but the coefficients are no longer attributed to the same received packets: the packets corresponding to the modified lines in R 'are no longer taken into account and the corresponding coefficients packets of these last lines are now assigned to the packets of the substitute lines. If the test of step 620 is negative, proceed to step 635. In step 635, a local combination cmb_loc_nouv is reconstituted. First, we try to find a local combination corresponding to the predetermined overall combination. If it succeeds, the reconstruction is local and is transparent for the other nodes of the network. Otherwise, a notification is added to the resulting packet because the new local combination cmb_loc_nouv is a new global combination. This step 635 is described in detail below in connection with FIG. 7.

In step 625, a new local combination cmb_loc_nouv associated with a global combination cmb_glob_cand closest to cmb_glob_pred is determined because the latter can not be reconstituted with the global combinations of all the packets received (test of step 610 negative ). Closest means: Containing the most associated source packages of cmb_glob_pred by banning source packages not contained in cmb_glob_pred. This step 625 is described in detail below in connection with FIG. 10. Then, proceed to step 630, in which the missing source packets contained in cmb_glob_pred and not contained in cmb_glob_cand are notified. These source packages will not be combined in the resulting package as originally intended. Then, end of the algorithm with step 640 which stores the local combination cmb loc nouv. FIG. 7 corresponds to step 635 of FIG. 6 and describes the algorithm for determining a new local combination from a global combination. The input parameters are:> the predetermined local combination of the relay node cmb_loc_pred; the predetermined global combination cmb_glob_pred given by the network coding scheme upon initialization; the relay matrix R 'of the global combinations of the packets received before access to the medium of this relay node. The output parameter is a new local combination cmb_loc_nouv relative to either cmb_glob_pred if it is possible, or to another global combination close to cmb_glob_pred (in terms of source packages). It should be noted that cmb_loc_nouv can not be equal to cmb_loc_pred because R 'is different from R. At first (step 705), the method selects only the 1 received packets which contain at least one source package associated with cmb_glob_pred. For this, the matrix R 'is analyzed line by line (a line representing a message received). On the current line tested i, it is analyzed whether there is at least one non-zero element r'1i with j corresponding to a column of a source package of cmb_glob_pred. Note the sub-matrix of R 'generated by this selection (R' minus the lines removed). In a variant, this step 705, which is a non-essential optimization, is omitted.

In step 710, from the received packets remaining at the end of step 705, a linear system is constructed to determine the coefficients to be assigned to these packets. Note (31, .., pi these coefficients with 1 the number of received packets remaining.) At first, we try to build a new local combination cmb_loc_nouv associated with cmb_glob_pred For each source package X1, it is necessary that the coefficient g1 associated to it in cmb_glob_pred is found in the sum of the packets received from cmb_loc_nouv, ie we must have the relation: = g ;,] = 1 for each source packet, so we have a linear system: A (3 = g with (3 the column vector of [(31, .., R1I, g = [gl, ••, gM] the column vector of cmb_glob_pred and A a matrix of dimension (M, 1) of which each coefficient aii is equal to r'i1 Remember that by definition gi is null (respectively non-zero) if the associated source package is not (respectively is) in cmb_glob_pred In step 715, we solve the linear system, created in l Step 710, by the method of Gauss-Jordan.

Then the test of step 720 verifies that the system is compatible or not (admits a solution or not) (see above the paragraph entitled Resolution of a linear system). If yes, proceed to step 725, otherwise step 760. In step 725, the values of [(31, .., (31] given by the Gauss-Jordan algorithm are then assigned. If the system admits an infinity of solutions, the value zero is assigned to the coefficients pi which are secondary unknowns, and then, in step 730, it is checked whether the value of the g1 of the linear system solved is zero for the non-source source packets. associated with cmb_glob_pred If this is the case, this means that the reconstitution of the packet has been transparent, we go to step 735. If the test of step 730 is negative, we go to step 740. This corresponds to the fact that we have found a local combination that is not associated with cmb_glob_pred (non-transparent reconstruction) Source packages have been added in this local combination that are not in cmb_glob_pred (the linear system of step 720 was not resolved at the first iteration).

In step 735, the new local combination cmb_loc_nouv given by definition by the values of [(31, .., (31] is kept in memory and the utility is indicated or not to the packets received or to receive (the associated packets non-zero coefficients are marked useful, others are marked redundant.) Then end of this algorithm.

In step 740, it is checked whether the added source packets have an impact or not on the network. This test is described in detail below in connection with FIG. 8. If this addition has no impact, step 745 is proceeded to if not at step 750. In step 745, the coefficients of the steps are notified. different source packets between the vector g and the global combination cmb_glob_pred (the different coefficients) then we go to step 735. In step 750, we memorize the local combination tested (storage of [(31, .., pl] ) and the overall combination tested (memorization of [g1, .., gM]) associated, as well as the confidence index related to this tested local combination (index also related to the overall combination tested, and given by the test of the step 740) in a list called Scand, then go to step 755.

In step 755, new values are given for the coefficients g; corresponding to the T source packages not associated with cmb_glob_pred (which have a null value in it). This step is associated with step 760 which checks whether values for g; have not been tested. Several implementations are possible for these two steps, for example: 1. One draws randomly the value of the T coefficients g; corresponding to the T unassociated packets on the body Fq in step 755 (this operation corresponds to a test) by verifying that at least one non-zero value is drawn. Then, in step 760, a maximum number of tries is imposed. If this number of maximum tests is crossed, the test is negative, if not positive. 2. One tests the set of possible values which amounts to testing (qT -1) possibilities. When this number is reached, it is decided in step 760 that the test is negative (otherwise it is positive). The implementation 1 allows to limit the processing time but is not necessarily optimal unlike the second because solutions can be forgotten. Implementation 2 can be very expensive in processing time, especially in the event of failure of the test of step 760. Step 765 selects a default local combination associated with a global combination that will impact the network as little as possible. This algorithm is described in detail below in connection with FIG. 9. This new local combination is stored in the variable cmb_loc_nouv (step 770), then step 775 in which the coefficients of the different source packets are notified between the vector g and the global combination cmb_glob_pred (the different coefficients). Step 775 corresponds to the end of this algorithm.

FIG. 8 describes an algorithm for assigning a confidence index to a local linear combination, corresponding to step 740 of FIG. 7. The confidence index measures the impact of the local combination to be sent on the network . At system initialization, the network schema was computed in such a way that all source packets sent in the network can be decoded by all destination nodes (note: the network topology is not a constraint in the present invention, it can even be oversized in the wireless context, which creates redundant received packets). So, if M is the number of source nodes (hence source packets) and if N is the number of destination nodes, then the number of source packets decoded in the network is equal to: M * N. This variable, number of packets sources decoded by the network, is the confidence index of the local combination. To measure this index, each relay node needs all the matrices of destination D; of all network destination nodes. The other input parameter is of course the tested local linear combination, called cmb_loc_test, but more specifically the associated global linear combination, denoted cmb_glob_test, which is different from cmb_glob_pred. The output parameter is the cmb_loc_test confidence index and therefore whether cmb_glob_test has an impact on the network or not. In step 805, the coefficients of the destination matrices that are impacted by the combination cmb_glob_test are modified. In other words, we modify the elements of each destination matrix involving the new coefficients of cmb_glob_test (modification of the line corresponding to the resulting packet to be sent by the relay node). In step 810, the confidence index of cmb_glob_test is calculated, that is to say the number of source packets decodable by the set of destination nodes. The number of source packets decodable by a destination node D ;, denoted Nb_src_pkt_dec (D;), is determined by the algorithm described hereinafter in detail, in relation with FIG. 12. The confidence index of cmb_glob_test, denoted ind_conf (cmb_glob_test), is then equal to: Ind_conf (cmb_glob_test) = ENb_src_pkt_dec (D;), i = 1..N This index is stored in step 815. In step 820, it is checked whether the confidence index cmb_glob_test is equal to m * N. If yes, cmb_glob_test has no impact on the network (step 825), and the test in step 740 of Figure 7 is positive. Otherwise cmb_glob_test has an impact on the network (step 830), and the test of step 740 of FIG. 7 is negative (at least one destination node will not be able to decode at least one source packet).

FIG. 9 describes the algorithm for selecting the overall linear combination that has the least impact on the network, corresponding to step 765 of FIG. 7.

The input parameters are: the predetermined global combination cmb_glob pred, the set Scand of the local and global combinations tested during the algorithm of FIG. 7 and rejected by the test of step 740 (FIG. 7).

The output parameter is the local linear combination associated with the global combination that has the least impact on the network. Since this algorithm is applied when the algorithm of FIG. 7 has found no global combination having no impact on the network, the local combination found necessarily has an impact. In step 905, a local combination cmb_loc_candl associated with a global combination cmb_glob_candl closest to cmb_glob_pred is searched. By closer, we mean containing the most associated source packages of cmb_glob_pred by banning source packages not contained in cmb_glob_pred. This algorithm is described in detail below, with reference to FIG. 10. In step 910, the cmb_glob_candl confidence index is determined using the algorithm described above in relation to FIG. In step 915, one selects among the set Scand constructed during the algorithm of FIG. 7 (in step 750), the overall combination cmb_glob_cand2 which has the highest confidence index (a conventional sort is performed). It should be noted that unlike the global combination cmb_glob_candl, cmb_glob_cand2 contains source packages of cmb_glob_pred but also source packages not contained in cmb_glob_pred. In step 920, the two confidence indices of cmb_glob_candl and cmb_glob_cand2 are compared. If the confidence index of global cmb_glob_cand2 is greater than that of cmb_glob_candl, then proceed to step 930, otherwise step 925.

In step 925, the local linear combination cmb_loc_candl is then selected. In step 930, the local combination associated with cmb_glob_cand2, the local combination that was stored in Scand, is selected. FIG. 10 describes the local combination reconstruction algorithm associated with a global combination closest to the predetermined global combination, corresponding to step 905 of FIG. 9. By closer, is meant containing the most packets associated sources of cmb_glob_pred by banning source packages not contained in cmb_glob_pred. The input parameters are - the predetermined global combination cmb_glob pred. It consists of P associated source packets (recall: which corresponds to p non-zero coefficients among the M coefficients associated with the source packets). The relay matrix R 'of the global combinations of the packets received before access to the medium of this relay node. The output parameter is a local combination cmb_loc_cand and an associated global combination cmb_glob_cand, which is the closest global combination to cmb_glob_pred. In step 1010, the variable nb_src_pkt_to_del corresponding to the number of associated source packets to be removed from cmb_glob_pred is initialized to 1. In step 1015, the number of different global combinations is determined by deleting nb_src_pkt_to_del source packets. This value nb_test is equal to the following combination in the mathematical sense (with! Corresponding to the factorial operation): C (P, nb_src_pkt_to_del) = P! / (Nb_src_pkt_to_del)! (P - nb_src_pkt_to_del)! In step 1020, the variable iter is initialized to 1, a variable corresponding to a counter that tests at each iteration a new set of associated source packets to delete S_pkt (iter). Let cmb_glob_cand the global combination equal to cmb_glob_pred minus the source packages of S_pkt (iter) (the coefficients of these source packages being set to zero). Let g '= [g'l, .., g'M] be the column vector representing this global combination cmb_glob_cand. The goal now is to determine a local combination cmb_loc_cand associated with cmb_glob_cand. In step 1025, a linear system is then constructed from the relay matrix of R 'in order to determine the coefficients to be assigned to cmb_loc_cand. Note (3i, .., (31 these coefficients with 1 the number of packets received.) For each source packet X, the coefficient g ', associated with it in cmb_glob_cand, must be found in the sum of the packets received from cmb_loc_cand, that is to say we must have the relation: =, for each source package.] = 1 So we have a linear system: A (3 = g, with (3 the vector column of unknowns [R1, .., R1], g '= [g'l, ••, g'M] the column vector of cmb_glob_cand and A a matrix of dimension (M, 1) of which each coefficient aii is equal to r'i1 In a variant, an additional filtering step (not shown in FIG. 10) can be performed, similar to step 705 of FIG. 7. In step 1030, the linear system created in step 1025 is solved. by the method of Gauss-Jordan.

Then, in the test of step 1035, it is verified that the system is compatible or not (admits a solution or not). If yes, proceed to step 1040, otherwise step 1050. In step 1050, it is checked whether the variable iter is equal to Nb test, that is to say that all global combinations have been tested with a number of associated source packets deleted equal to nb_src_pkt. If the test of step 1050 is negative, proceed to step 1065, in which the variable iter is incremented and a new set of associated source packets to delete S_pkt (iter) is selected, then return to step 1025. If the test of step 1050 is positive, go to step 1055. In step 1055, it is checked whether the variable nb_src_pkt_to_del is equal to p, the number of associated source packets in cmb_glob_pred. If yes, we go to step 1070, this corresponds to the fact that there are no more source packages available to delete. In step 1070, then returns the local and global null combinations. If the test of step 1055 is negative, then the variable nb_src_pkt_to_del is incremented (step 1060) and one goes back to step 1015. In step 1040, the values of [(31, .., (31 ] If the system admits an infinity of solutions, the value of the coefficients pi associated with the secondary unknowns is set to 0. In step 1045, the local combination cmb_loc_cand associated with the column vector is memorized. [(31, .., (31] and the global combination cmb_cand_glob associated with the column vector g'MI.

FIG. 11 describes the decoding algorithm of a destination node (that is to say the algorithm of a network coding method according to a particular embodiment of the invention, implemented by a node destination to decode source packets and obtain decoded source packets).

This algorithm starts at the end of the superframe (step 1100), when all the relay nodes have access to the medium during the superframe. At this time, the algorithm determines the unavailable useful data packets (step 1105). This means is based for example on radio information, such as the signal-on-noise ratio (Signal Noise Ratio) or the signal strength (Radio Signal Strength Indication). If these variables exceed a predefined threshold, the packet is considered available, if not available. Then, in step 1110, the composition of the available useful packets is determined. If a packet has no notification, the source packet composition is given by the destination matrix, more precisely, by the line corresponding to the received packet. Otherwise, the notification is read and the new composition of the packet is determined by reading the changes from the overall combination of the destination matrix, indicated by the notification. In step 1115, a temporary destination matrix D '= d', i, valid only for the execution of this algorithm in the current superframe is created if a useful packet is considered unavailable at step 1105 or if a useful packet was modified in step 1110. It is filled in the following manner (step 1115): - initialization of the value of the coefficients of the temporary destination matrix to the value of the coefficients of the predetermined destination matrix; the lines corresponding to the unavailable packets, the result of step 1105, become null lines (all the coefficients of the line are set to zero); the lines corresponding to the modified available packets, result of the step 1110, are modified and take into account the modifications of the notification: a modified element of i means that the packet associated with this line i has been modified; column j corresponds to the identifier (315 or 325, FIG. 3) of the source packet indicated in the notification; the new value of d ', i is the new value of the coefficient given in the notification (320 or 330, Figure 3). In step 1120, it is checked whether the temporary destination matrix D 'has been created or not. If yes, proceed to step 1130, otherwise step 1125.

In step 1125, the source packets are decoded from the decode matrix (predetermined or modified from step 1145). On the other hand, if the test in step 1120 is positive, this indicates that a link loss has occurred and has hit a useful packet. The decoding matrix is no longer valid, a new one has to be calculated. For this, the redundant packets received are necessary. As a first step, the algorithm determines whether these redundant packets have not been impacted by a loss of links. Steps 1130 and 1135 (which are the same as steps 1105 and 1110) are then executed on the redundant packets. In step 1140, D is updated in the same way as step 1115. In step 1145, a temporary decoding matrix C-'prou which decodes will be recalculated from the temporary destination matrix the maximum of source package. This operation is described below in detail in relation to FIG. 12. This matrix C ''prou is valid only during the period of the current superframe. FIG. 12, corresponding to step 810 of FIG. 8 or step 1145 of FIG. 11, describes the algorithm that makes it possible to determine the number of decodable source packets and consequently to determine the associated decoding matrix. This algorithm is called: • at a relay node R; (at step 810 of FIG. 8), to determine the impact, on the destination nodes, of the sending of a new global combination by the relay node; And of course, at a destination node D; (at step 1145 of FIG. 11), when the latter must modify its decoding matrix. It should be noted that step 1260 is not implemented in the relay nodes (because these do not have the need to perform the decoding). The output parameter is therefore - at the relay node level, the number of decodable packets, nb_src_pkt_dec; at the destination node level, the new decoding matrix C-1 prov.

To determine the maximum number of decodable source packets, the algorithm tests whether the M source packets are decodable. If yes, the algorithm is complete, otherwise we test if (M-1) source packages are decodable. If yes, the algorithm is complete, otherwise we test if (M-2) source packages are decodable, and so on.

In step 1210, the number of source packets to be deleted, nb_src_pkt_to_del, is initialized to zero. In the test of step 1215, it is checked whether this number is strictly lower than M. If yes, we go to step 1220, otherwise to step 1235. In the latter case, this means that no packet source is not decodable, the variable nb_src_pkt_dec is initialized to zero and of course there is no decoding matrix, so the algorithm is complete (step 1280). In step 1220, the number of sets of source packets to be deleted is determined. This value nb_test is equal to the following combination in the mathematical sense (with! Corresponding to the factorial operation): C (M, nb_src_pkt_to_del) = M! / (nb_src_pkt_to_del)! (M - nb_src_pkt_to_del)! In this step 1220, the variable iter is also initialized to 1, which corresponds to a counter which tests at each iteration a new set of nb_src_pkt_to_delete associated source packets to be deleted. Step 1225 determines a set of nb_src_pkt_to_del source packages to delete, different set for each iteration iterating.

Step 1230 determines a new destination matrix D'i, sub-matrix of destination matrix D ;. The resulting packets containing the source packets to be deleted determined in step 1220 are not taken into account. So D '; corresponds to the destination matrix Di, minus the columns corresponding to the source packets to be deleted, and minus the lines which had a non-zero coefficient for the columns corresponding to the source packets to be deleted. Then, in step 1240, it is checked whether there remain source packets, that is to say if the matrix D'i still has lines. If no, we go to step 1265, if yes to step 1245. In step 1245, we search for the rank of the matrix D ', noted rand Di. For this, the Gauss-Jordan method is still used by triangulating the matrix D '. Let T be the triangulated matrix. The rank is the number of non-zero lines of T. Store in memory the matrix T '(consisting only of the non-zero lines of T) and the resulting packets associated with the lines of T'. In step 1250, it is checked whether the value of rank Di is equal to (M-nb_src_pkt_to_ del). If yes, all the source packages present in the matrix D '; are decodable, we go to step 1255. Otherwise the matrix T 'is deleted and we go to step 1265. In step 1255, we update nb_src_pkt_dec (D;) to rand Di. Then if the node is a relay node, the algorithm is complete (step 1280). Otherwise (that is, if the node is a destination node), step 1260 is passed before the end of the algorithm.

In step 1260, the matrix T 'is recovered, an inversion thereof is performed (T' is a square matrix of dimension (M-nb_src_pkt_to_del, M-nb_src_pkt_to_ del)) and the inverse matrix C- 'is stored. T'prou, C-'prou being the temporary decoding matrix. In addition, the packets stored in step 1245 are noted as the provisional useful packets.

In step 1265, it is checked whether all the sets of nb_src_pkt_to_del have been tested, that is to say if the variable iter is equal to Nb test. If yes, proceed to step 1275, otherwise step 1270. In step 1270, the variable iter is incremented and a new set of nb_src_pkt_to_del source packets to be deleted is determined. Then, we return to step 1230. In step 1275, we increment the number of source packets to delete nb_src_pkt_to_del then we return to the test step 1215.

Claims (16)

REVENDICATIONS1. A network encoding method in a mesh network comprising at least one relay node which, according to a predetermined network coding scheme, expects to receive a plurality of data packets as input and is to output a predetermined resultant packet obtained according to a first predetermined linear combination of a part of said plurality of data packets, the data packets belonging to said part being said useful packets and the data packets not belonging to said part being said to be redundant packet, each of the expected data packets at the input being formed, according to said predetermined network coding scheme, by a linear combination of one or more source packets, the source packets being transmitted by source nodes, said method being implemented by at least one given relay node and being characterized in that it comprises the steps of: - detecting (505, 510, 515, 520) s i at least one useful packet is missing or has been received modified so that the linear combination of the source packet (s) forming said modified useful packet does not conform to the predetermined network coding scheme; in the case of a positive detection, constructing (530, 535, 540, 545) a resulting packet reconstructed using, in accordance with a modified network coding scheme, all or part of the redundant packets; and - transmitting (550) the resulting reconstituted packet. The method according to claim 1, characterized in that said step of constructing a reconstructed resultant packet comprises a step (615, 620, 640, 635, 735) of determining a second linear combination of redundant and useful packets, among a plurality of data packets actually received by said given relay node, for obtaining a reconstituted resulting packet identical to the predetermined resultant data packet to be generated, if said second linear combination of redundant and useful packets exists. A method according to claim 2, characterized in that said step of determining the second linear combination of redundant and useful packets is performed by testing (615, 620) whether each missing detected useful packet can be replaced by a redundant packet actually received by said given relay node. The method of claim 2, characterized in that said step of determining the second linear combination of redundant and useful packets comprises steps of: - determining (710) a third predetermined linear combination of source packets associated with said first predetermined linear combination of useful packets, and for obtaining said predetermined resultant packet; constructing (710) a linear system A (3 = g, where (3 is a column vector of the coefficients of said second linear combination of redundant and useful packets to be determined, where g is a column vector of the coefficients of said third predetermined linear combination source packets, and where A is a matrix of dimension (M, 1), with M the number of source packets and 1 the number of packets actually received by said given relay node, the matrix A being defined as follows: each line of the matrix A represents a source packet, * each column of the matrix A represents a packet actually received, * each element aii of the matrix A represents the coefficient of the source packet i of a fourth linear combination of source packages allowing 20 to obtain the packet actually received; - to solve (715) said linear system by a predetermined method; - if the linear system admits a solution (720), terminating (725, 730, 735) said second linear combination of redundant and useful packets, depending on the column vector (3 of said solution. 25 The method according to any one of claims 2 to 4, characterized in that, if said second linear combination of redundant and useful packets does not exist, said step of constructing a reconstructed resultant packet comprises steps of: a ) constructing (760, 755, 715, 720, 725, 730) a resulting packet containing source packets not contained in the predetermined resultant packet; 15b) obtaining (740) relative information, for at least one other node participating in the predetermined network coding scheme, to a modified network coding scheme that would result from sending the modified result packet constructed in step a); c) based on the information obtained in step b), selecting (735) the resulting modified packet constructed in step a) or repeating step a) to construct another resulting packet. The method of claim 5, characterized in that step a) comprises steps of: i) determining (755) a fifth predetermined linear combination of source packets, thereby obtaining said resultant packet containing non-source source packets contained in the predetermined resulting packet; ii) constructing (755) a linear system A (3 = g, where (3 is a column vector of the coefficients of said second linear combination of redundant and useful packets to be determined, where g is a column vector of the coefficients of said fifth linear combination predetermined set of source packets, and where A is a matrix of dimension (M, 1), with M the number of source packets and 1 the number of packets actually received by said given relay node, the matrix A being defined as follows: * each row of the matrix A represents a source packet, * each column of the matrix A represents a packet actually received, * each element aii of the matrix A represents the coefficient of the source packet i of a sixth linear combination of source packages allowing to obtain the packet actually received; iii) to solve (715) said linear system by a predetermined method; iv) if the linear system admits a solution (720), d determining (725) said second linear combination of redundant and useful packets, based on the column vector (3 of said solution, if not reiteration of step i) with another fifth predetermined linear combination of source packets. 7. Method according to any one of claims 5 and 6, characterized in that, in step b), said information relating to the modified network coding scheme, which would result from the sending of the resulting modified package built in step a), is the sum of the number of source packets decodable by the set of destination nodes of the network if the resulting modified packet constructed in step a) is assumed to be sent, and in that, in step c, the packet The modified resulting result constructed in step a) is selected if said information relating to the modified network encoding scheme is equal to the number of source nodes in the network multiplied by the number of destination nodes in the network. The method according to any one of claims 5 to 7, characterized in that, if said second linear combination of redundant and useful packets does not exist, said step of constructing a reconstituted resulting packet comprises steps of: constructing (905) a first resulting packet different from the predetermined resulting packet but containing only source packets included in the predetermined resulting packet; obtaining (910) first relative information, for at least one other node participating in the predetermined network coding scheme, to a modified network coding scheme that would result from sending the first resulting packet, the first resulting packet being associated with said first information on the modified network coding scheme; among the packet or packets not selected in step c) and containing source packets not contained in the predetermined resulting packet, selecting (915) a second resulting packet, according to said information, obtained at step b) for each packet not selected in step c), on the modified network coding scheme, the second resulting packet being associated with second information relating to the modified network coding scheme; and - selecting (920, 925, 930) between the first and second resulting packets, based on the first and second information relating to the modified network coding scheme. 9. A method according to any one of claims 2 to 4, characterized in that, if said second linear combination of redundant and useful packets does not exist, said step of constructing a reconstituted resulting packet comprises a step of constructing (625) a resulting packet different from the predetermined resulting packet but containing only source packets included in the predetermined resulting packet. The method of any one of claims 1 to 9, characterized in that, if the resulting reconstructed packet is not identical to the predetermined resultant data packet, the method comprises an additional step (630, 745, 775) of transmitting a notification (305) identifying differences between the linear combination of source packets representative of said predetermined resulting packet, and the linear combination of source packets representative of said reconstructed resultant packet. A method of network coding in a mesh network comprising at least one destination node which, according to a predetermined network coding scheme, expects to receive a plurality of data packets as input and to output a plurality of decoded source packets each obtained in a predetermined linear combination of a portion of said plurality of data packets, the data packets belonging to said portion being said to be useful packets and the data packets not belonging to said portion being said to be redundant packet, each of data packets expected in input being formed, according to said predetermined network coding scheme, by a linear combination of one or more source packets, the source packets being transmitted by source nodes, said method being implemented by at least one node given destination and being characterized in that it comprises steps of: - detecting (11 05, 1110, 1115, 1120) if at least one useful packet is missing or has been received modified so that the linear combination of the source packet (s) forming said modified useful packet does not conform to the predetermined network coding scheme ; and - in the case of a positive detection, decoding (1130, 1135, 1140, 1145, 1125) the received payload packets, according to a modified network coding scheme, to obtain at least one decoded source packet. 30 12. The method of claim 11, characterized in that said step of detecting whether at least one useful packet is missing or has been received modified comprises a step consisting of, for at least one useful packet actually received: detecting (1110) a received notification (305) associated with said actually received beneficial packet, said notification identifying the differences between the linear combination of source packets representative of the expected input packet, and the linear combination of source packets representative of said useful packet actually received . 13. Computer program product downloadable from a communication network and / or recorded on a computer readable medium and / or executable by a processor, characterized in that it comprises program code instructions for the implementation of the method according to at least one of claims 1 to 10 and / or the method according to at least one of claims 11 and 12, when said program is run on a computer. 14. A computer readable medium, possibly totally or partially removable, storing a computer program comprising a set of instructions executable by a computer to implement the method according to at least one of claims 1 to 10 and / or the process according to at least one of claims 11 and 12. A relay node included in a mesh network and participating in a network coding method in said mesh network, said relay node, according to a predetermined network coding scheme, expecting to receive as input a plurality of data packets and to generate at the output a predetermined predetermined packet obtained according to a first predetermined linear combination of a portion of said plurality of data packets, the data packets belonging to said party being said useful packets and the data packets not belonging to said party being said redundant packet, each of the expected data packets being formed, according to said predetermined network coding scheme, by a linear combination of one or more source packets, the source packets being transmitted by source nodes, said relay node being characterized in that it comprises: detection means, making it possible to detect whether at least ins a useful packet is missing or has been received modified so that the linear combination of the source packet (s) forming said modified useful packet does not conform to the predetermined network coding scheme; and - building means, activated in the case of a positive detection, for constructing a reconstructed resultant packet using, in accordance with a modified network coding scheme, all or part of the redundant packets; transmission means for transmitting the resulting reconstituted packet. A destination node included in a mesh network and participating in a network coding method in said mesh network, said destination node, according to a predetermined network coding scheme, expecting to receive as input a plurality of data packets and to generate at the output a plurality of decoded source packets each obtained in a predetermined linear combination of a portion of said plurality of data packets, the data packets belonging to said portion being said useful packets and the data packets not belonging to said part being said redundant packet, each of the data packets expected input being formed, according to said predetermined network coding scheme, by a linear combination of one or more source packets, the source packets being transmitted by source nodes, said destination node characterized in that it comprises: - detection means, allowing detecting whether at least one useful packet is missing or has been received modified so that the linear combination of the source packet (s) forming said modified useful packet does not conform to the predetermined network coding scheme; - Decoding means, activated in case of positive detection, for decoding the received payload data packets, according to a modified network coding scheme, to obtain at least one decoded source packet.