Patent Application
20090274056
The field of the invention is that of routing in a telecommunications network.
The invention has a particular application to ad hoc telecommunications networks, although this is not limiting on the invention.
Such networks consist of mobile and/or fixed nodes having the property of automatically and dynamically building a network able to route packets from any node of the network to any other node provided that a radio link can be set up between a node and its neighbors.
Packets are transmitted between the source node and the destination node either directly if the destination node is in the connectivity area of the source node or via intermediate neighbor nodes serving as routers if the destination node is out of range of a source.
The nodes are therefore responsible for continuously setting up and maintaining the connectivity of the network.
They do this using ad hoc routing protocols.
Generally speaking, there are two major families of routing protocols, i.e. proactive protocols and reactive protocols.
The invention relates to a hybrid protocol using both techniques.
In a proactive routing protocol, there is a mechanism for periodic exchange of control messages so that the topology of the network can be constructed in a distributed manner by each of its nodes.
This is done by broadcasting control messages to inform all nodes of the evolution of the network topology. Each node periodically updates its routing table on reception of these control messages.
Proactive routing protocols provide very good visibility of the network topology and immediate availability of routes.
In a proactive routing protocol, routes are generally calculated by a shortest path algorithm using a metric defined on the basis of predetermined constraints, for example number of hops, quality of service (QoS), etc.
Consequently, proactive routing protocols are not suitable if these constraints vary, because the precalculated routes are based on hypotheses that are no longer correct.
In contrast to a proactive approach, in a reactive routing protocol, a source node calculates routes to a destination node only at the request of an application seeking to send a packet to the destination node.
The route is calculated by broadcasting a query throughout the network. The response to this broadcast query enables the source node to obtain topological information relating to the route.
Thus in a reactive protocol routes are created on demand. These routing protocols are therefore very suitable for networks in which constraints are liable to vary in time.
In contrast, reactive protocols have the major drawback that they incur a high cost through flooding the network when discovering routes, in contrast to proactive routing protocols, which maintain a view of the network topology.
Generally speaking, the invention is directed to a new method of identifying routes suitable for networks in which constraints taken into account to calculate routes are liable to vary in time.
To be more precise, the invention relates to a method of identifying a route satisfying a first constraint between a source node and a destination node in a telecommunications network in which a routing table and a topology table are established using a proactive protocol (PRP).
The method includes a step of testing a route determined using the routing table established using a proactive routing protocol.
In the event of failure of said test step, the method includes:
In a correlated way, the invention relates to a device for identifying a route satisfying a first constraint between a source node and a destination node in a telecommunications network.
This device includes:
Generally speaking, the invention is therefore aimed at a method and a device for implementing a reactive protocol to search for new routes if routes previously established by a proactive protocol do not find a route that satisfies some given constraint identified between a source node and a destination node.
In other words, so as not to broadcast unnecessary queries, routes are looked for a priori among those identified by the proactive protocol.
However, if the routes identified by the proactive protocol do not satisfy the constraint, a reactive protocol is triggered to identify new routes.
The search for new routes uses a topology table established by the proactive protocol, so that broadcast queries do not flood the network, but follow only a limited number of potential routes, which is highly advantageous.
In one particular implementation of the invention, the potential routes are calculated using a graph search algorithm, for example a BBS or DFS algorithm, as described in “Introduction to Algorithms” by Thomas H. Cormen, Charles E. Leiserson, Ronald L. Rivest, and Clifford Stein, Second Edition, ISBN: 0262032937.
In one particular implementation of the invention, during the search step, a search query is broadcast on the potential routes using a source routing mechanism so that, when it reaches the destination node, the query contains all the references of all the nodes on the route.
In a source routing mechanism, the identities of the nodes are inserted into queries as they progress through the network.
In one particular implementation of the invention, the proactive routing protocol establishes the routing table taking account of a second constraint different from the first-mentioned constraint.
To limit the complexity of the proactive routing protocol, the number of second constraints is preferably limited to three.
In this particular implementation of the invention, the reactive routing protocol is used when searching for a route that must satisfy at least the first constraint.
In one particular implementation of the invention, the second constraints taken into account by the proactive protocol to construct the routing tables are quality of service metrics, for example:
The invention is especially beneficial if the network nodes are mobile telecommunication devices.
In this implementation, the first constraint, which triggers use of the reactive protocol, can for example be a constraint that is satisfied when the cumulative energy available in the mobile devices constituting a route (in other words the length of time they have left to run) exceeds a predetermined minimum energy level.
In one particular implementation of the invention, the reactive routing protocol identifies a number of possible routes between the source node and the destination node. A selection step then uses a metric taking into account the number of hops between the source node and the destination node to select the best route from the possible routes.
In one particular implementation, the steps of the route identification method are determined by computer program instructions.
In the present document, the term “computer” is to be understood in a broad sense, and refers to any data processing device and in particular to a cellular telephone.
Consequently, the invention is also directed to a computer program on an information medium, which program can be executed in a route identification device or more generally in a computer and includes instructions adapted to execute the steps of the above route identification method.
This program can use any programming language and take the form of source code, object code or an intermediate code between source code and object code, such as a partially compiled form, or any other desirable form.
The invention is also directed to a computer-readable information medium containing instructions of the above computer program.
The information medium can be any entity or device able to store the program. For example, the information medium can include storage means, such as a ROM, for example a CD ROM or a microelectronic circuit ROM, or magnetic storage means, such as a diskette (floppy disk) or a hard disk.
The information medium can be a transmissible medium such as an electrical or optical signal, which can be routed via an electrical or optical cable, by radio or by other means. The program of the invention can in particular be downloaded over an Internet-type network.
Alternatively, the information medium can be an integrated circuit into which the program is incorporated, the circuit being adapted to execute the method in question or to be used in its execution.
The invention is also directed to a telecommunications network including nodes adapted to execute the above route identification method.
Other features and advantages of the present invention emerge from the description given with reference to the appendices and to the drawings, which illustrate a non-limiting implementation of the invention, and in which:
Appendix 1 is a listing in pseudo-code of a computer program for implementing the known Dijkstra algorithm;
Appendices 2A and 2B are listings in pseudo-code of a modified Dijkstra algorithm used in one particular implementation of the invention;
This network consists of a set of nodes 10.
These nodes can be identical or not. For example, they can all be mobile devices.
In the remainder of this document:
A single line or a double line between two nodes 10 represents the fact that those two nodes are neighbors and that communication by radio can be established between them to route a packet.
The Dijkstra shortest path algorithm is described below with reference to Appendix 1, and can be used by a proactive routing protocol to establish a routing table TR for the ad hoc network 1 from a topology table TT.
In this known algorithm:
According to predefined metrics, for example the number of hops, the Dijkstra algorithm finds the shortest path between a given source node s and any node v of the graph G representing the ad hoc network 1.
The Dijkstra algorithm uses the relaxation technique.
To be more precise, for each node v there is an attribute d[v] that is an upper bound for a shortest path between the source s and v.
This attribute d[v] is called the “shortest path estimate”.
The estimates and predecessors (π[.]) are initialized by the “Initialisation” procedure in Appendix 1.
The “Relax” procedure is used for the relaxation process.
Using the notation of Appendix 1, the set E contains a list of the nodes forming the shortest path (in terms of a chosen routing metric) between the source s and the destination d. The element of E that is one hop away from s is the “next router” toward the destination node d in the routing table TR.
The routing table TR shown in
It includes a first column including all the nodes a, b, c, d, D of the ad hoc network 1 and a second column including, against a node v, the next router PR(V) after S in the shortest path to v.
In
Accordingly, to reach D from S, the shortest path passes first through the intermediate node a, which is why the intermediate node a is associated with the destination node D in the routing table TR.
The Dijkstra algorithm described previously with reference to Appendix 1 uses a metric representing the number of hops between the source node S and the destination node D.
A modified Dijkstra algorithm is described below with reference to appendices 2A and 2B, this algorithm using, instead of the “number of hops” metric, a metric that consists of three primary quality of service metrics:
The loss rate metric is multiplicative and can be transformed into an additive metric as described below:
P
loss(P)=1−((qij)(qjk), . . . , (qit))
Assuming that the logarithmic probability plsij of success of the link (i, j) is defined by plsij=−log qif, then the logarithmic probability of success on the path P is:
Assuming that the probability of loss on a given route R is less than 0.9, then 0.1<pls(R)<1.
Assume also that the delay assumes integer values. If the millisecond unit gives real values, then the microsecond unit is used to calculate the delays.
A method of searching for the shortest path satisfying the three QoS metrics, if such a path exists, is described below.
G=(N, A) is the graph representing the ad hoc network 1, including N nodes and A arcs (links). Each arc is assigned two real values:
Consider a route R=(s, k, i, . . . , r, d). The available bandwidth on R is min[bsk, bki, . . . , brd] and the length of R is msk+mki+ . . . , +mrd.
The problem is to find a route R such that the available bandwidth on R is greater than B and the length of R is less than M.
If R is a route from the vertex s to the vertex d including y hops, and if plsij<P/y for each link (i, j) belonging to the route R, then pls(R)<P. However, the value of y is unknown during the search for a route. The idea is therefore to restrict plsij to P/C, where C is the length (in terms of the number of hops) of the shortest route between s and d (that is an optimistic hypothesis).
Two counters are maintained:
Thus the maximum logarithmic probability of success that can be added on each step is RPi/NCi instead of P/C.
The modified Dijkstra algorithm of Appendices 2A and 2B solves the problem stated above by assuming that the vertex s is the source node and that vertex d is the destination node.
This appendix uses the following variables and notation:
An identification device 20 of the invention is described below with reference to
In the example described here, the device 20 is incorporated into a mobile telephone 10 constituting a node of the ad hoc network 1.
In the example described here, the identification device 20 of the invention includes conventional processing means, i.e. a processor 31, a read-only memory (ROM) 33, and a random-access memory (RAM) 312.
In the example described here, a computer program PG of the invention is stored in the read-only memory 33.
This program includes instructions for executing the steps of the route identification method of the invention, of which
In the implementation described here, the computer program PG includes a module PRP adapted to implement the proactive routing protocol PRP based on the modified Dijkstra algorithm described above with reference to Appendices 2A and 2B.
In particular, the proactive routing module PRP establishes and maintains a routing table TR and a topology table TT which, in the example described here, are stored in a non-volatile rewritable memory 34 of the identification device 20 of the invention.
Using the notation of Appendix 1B, the set L contains a list of the nodes forming the shortest path between the source s and the destination d. The element of L to reach the destination node d that is one hop away from the source s is the “next router” in the routing table TR.
The main steps of one particular implementation of the identification method of the invention are described below with reference to
In this context the routing table TR and the topology table TT have already been established by the proactive routing module PRP.
Accordingly, the topology table TT represents a topology of the ad hoc network taking into account the three constraints consisting of the delay metric (C1), the loss rate metric (C2), and the available bandwidth metric (C3).
It is assumed in this example that a new connection that requires a fourth constraint C4 to be satisfied is to be set up between the source node S and the destination node D.
In the example described here, this constraint C4 is satisfied if the cumulative available energy ei of the mobile devices 10 constituting the route R between the source node S and the destination node D is greater than a predetermined minimum energy level E.
During a first test step E10 of the method, whether the route between the source node S and the destination node D corresponding to the shortest path defined by the routing table TR satisfies this fourth constraint is verified.
The source node S sends a query on the route defined in the routing table TR, for example in source-routing mode. According to the known source-routing mechanisms, as this query crosses the intermediate nodes toward the destination node D, whether this route satisfies the fourth constraint C4 is verified.
When the destination node is reached, whether the fourth constraint is satisfied is verified.
In the example described here, this amounts to verifying if the sum of the energies available in the mobile telephones S, a, c, and D is greater than the minimum energy E.
If so, a positive response is sent to the source node S and the result of the test E10 is positive. That test is then followed by a step E20 during which a connection is set up between the source node S and the destination node D along the route R defined by the routing table TR.
If not, a negative response is sent to the source node S and the result of the test E10 is negative. A reactive routing algorithm is then triggered to identify new routes that can satisfy all the constraints.
To this end, during a step E30, the node S determines a certain number of potential routes Rk from the topology table TT kept up-to-date by the proactive routing module PRP using a graph search algorithm.
The graph search algorithms used can be of the BFS (Breadth First Search) or DFS (Depth First Search) type, as described in “Introduction to Algorithms” by Thomas H. Cormen, Charles E. Leiserson, Ronald L. Rivest, and Clifford Stein, Second Edition, ISBN: 0262032937.
In the example described here, these potential routes Rk are stored in the random-access memory 32 of the identification device of the invention.
This step E30 of selecting potential routes Rk is followed by a step E40 during which the source node S sends a query on each of the routes Rk, for example in source-routing mode.
According to the known source-routing mechanisms, whether this route satisfies the fourth constraint C4 is verified as this query crosses the intermediate nodes toward the destination node D.
Because this is an additive constraint, it is typical to establish the cumulative sum of the energies ei available in all the nodes that the query crosses.
When the destination node D is reached, whether the energy of this path is greater than the required minimum value E is verified.
If so, a positive response is sent to the source node S.
If not, a negative response is sent to the source node S.
This step E40 of broadcasting the query is followed by a step E50 during which the source node S verifies whether any potential routes satisfy the fourth constraint as a function of the responses) received in the preceding step.
If so, the result of the test E50 is positive. That test is then followed by a step E60 during which the satisfactory route comprising the fewest hops is chosen.
Then, during a step E70, the connection between the source node S and the destination node D is established along that route.
If none of the potential routes Rk satisfies the fourth constraint, the result of the test E50 is negative.
That test is then followed by a step E80 during which a query is broadcast over the entire ad hoc network 1 to search for routes that satisfy the constraint C4.
In the example described above, the reactive routing algorithm is used to search for a route if a new constraint has to be taken into account (here the fourth constraint).
In other embodiments of the invention, reactive routing can be used at the initiative of the routing application if the proactive routing algorithm does not constitute or no longer constitutes a satisfactory solution for identifying routes, even if the constraints taken into account for calculating routes are unchanged.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/FR07/51028 | 3/28/2007 | WO | 00 | 10/3/2008 |
