Patent Document

CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to foreign French patent application No. FR 1102965, filed on Sep. 30, 2011, the disclosure of which is incorporated by reference in its entirety. 
     FIELD OF THE INVENTION 
     The present invention relates to a method for guaranteeing with a high level of reliability the continuity of the communications operated from a fourth-generation (4G) mobile terminal linked to a level-3 interconnection network, in the terminology defined by the OSI (“Open Systems Interconnection”). The invention applies notably to the mobility of mobile terminals in a context which is highly intolerant to faults, for example for networks used by military forces, public bodies, or civil agents such as the police, fire brigade or civil security. In particular, the invention may be implemented in networks liable to experience breaks in communication links in the interconnection network. 
     BACKGROUND 
     The computerized networks used by fourth-generation (4G) mobile terminals comprise radio sub-networks, sometimes designated by the initials RAN for “Radio Access Network”, which are hooked up to an interconnection network, also called a CSN (“Connectivity Service Network”), the CSN being linkable to the Internet. A fourth-generation mobile terminal is identified by an IP (“Internet Protocol”) address which allows it to receive and to send data across the whole of the computerized network. The mobile terminal is under the coverage of an antennal station, also called a base station. A RAN is formed of a set of base stations whose coverages supplement one another so as to cover a territory. Between the mobile terminal and the base station, the data are transmitted in the form of radioelectric waves and then the base station transmits the data, generally via optical fibers or cables, to a gateway interfacing between a RAN and a CSN. As a general rule, several base stations are controlled by one and the same control station. Such a control station fulfills several roles, notably the filtering of packets, the management of service quality, the authentication of users, the control of the base stations. 
     When the mobile terminal moves and exits the zone covered by a first base station under the coverage of which it was situated, the communications are ensured by a second base station whose coverage is adjacent to the first. If the second base station is connected to the same control station as the first base station, one speaks of micro-mobility. In the case where the mobile terminal moves to a second base station which is connected to a control station different from the first control station, the term macro-mobility is employed. The present invention deals more particularly with problems of continuity of communications within the framework of the macro-mobility of mobile terminals. 
     A protocol, called MIP for “Mobile IP”, is known for managing the macro-mobility of mobile terminals on WiMax networks. MIP relies on the HA (“Home Agent”) and FA (“Foreign Agent”) functions which are software modules executed by routers at the level of the IP network layer (layer 3). The HA function makes it possible to receive and to steer data packets intended for the mobile terminal, including when the latter leaves its initial gateway. The FA function is executed by a router for relaying the data packets up to the mobile terminal. The HA function is an anchoring point for the mobile terminal in the CSN and this anchoring point persists as long as the mobile terminal is under the coverage of a RAN, whatever the base station to which it is connected. The HA function is generally executed by a single router of the CSN and thus constitutes a significant point of weakness. Hence, if a movement of the mobile terminal entails a change of control station, and if the router executing the HA function is not reachable from the new gateway—for example, if several communication links are cut—, the communication may not be maintained, despite the optional duplication mechanisms implemented. Hence, although the MIP protocol can constitute a solution in a centralized environment, it is not suited to networks with strong reliability constraints, such as for example private mobile radio or PMR. 
     A technique described in the French application published under the number FR2953357 has already been proposed for solving the problems of macro-mobility in a context of high reliability. However, this technique is less optimized when the interconnection network operates at the level of the IP layer, that is to say when the network uses IP routers rather than level-2 Ethernet switches. 
     SUMMARY OF THE INVENTION 
     An aim of the invention is to propose a reliable method for ensuring the continuity of communication of a mobile terminal moving around in a network comprising control stations linked by an interconnection network operating at the level of layer 3 of the ISO model. For this purpose, the subject of the invention is a method for guaranteeing the continuity of the communications operated from a fourth-generation mobile terminal connected to a radio network provided with several base stations with which said terminal is able to communicate, a base station being affiliated to a controller from among several controllers linking said radio network to an interconnection network comprising routers operating at the level of the IP layer, at least one controller being connected to at least one gateway configured to encapsulate in IP packets all the IP packets arising from the radio network before broadcasting them over the interconnection network, the method comprising at least the following steps:
         when a mobile terminal connects to a base station affiliated to a controller to which the mobile terminal was not affiliated hitherto, said controller transmits to a gateway a level-2 message comprising at least the IP address of the mobile terminal,   said gateway creates, at the level of the IP layer, a message comprising the IP address of said mobile terminal and the IP address of said gateway in the interconnection network,   said IP message is broadcast by said gateway to destination gateways associated with the other controllers, each of said destination gateways storing a correspondence between the IP address of the mobile terminal and the IP address of the sending gateway associated with the controller to which the mobile terminal is affiliated.       

     The method according to the invention makes it possible to permanently update the mobility management function, which is distributed throughout the network. Thus, if a control station becomes defective, only the sub-network of radio stations becomes inoperative, the other sub-networks of radio stations not being affected due to the fact that the mobility management function is ensured by each controller independently of one another. According to one embodiment, the level-2 message sent by the controller comprises a correspondence between the IP address of the mobile terminal and the level-2 address of said controller. 
     According to an implementation of the method according to the invention, the gateways are accessible through at least two different IP addresses, a first IP address being known to the nodes of the radio network, a second IP address being known to the nodes of the interconnection network, in which all the IP packets arising from a controller are systematically transmitted to the gateway with which it is associated, said gateway encapsulating said IP packets in other IP packets whose destination IP address is the second address of a destination gateway. 
     Thus in a communication between two mobile terminals each connected to a different base station, the stations being affiliated to different controllers, all the IP data packets transmitted by the first mobile terminal are encapsulated by IP packets of higher level in the gateway associated with the first controller, these IP packets of higher level being transmitted to the gateway associated with the controller to which the second mobile terminal is affiliated. The IP packets are thereafter de-encapsulated, in such a way that the gateway associated with the second controller transmits the initial IP packets to the second controller, and then to the second mobile terminal. 
     According to an implementation of the method according to the invention, each controller is connected to a gateway specific to this controller. This gateway makes it possible at one and the same time to ensure the function for signaling the mobility of the mobile terminals, and also the function for guiding the IP data traveling through the interconnection network. 
     According to an implementation of the method according to the invention, each gateway is connected to the controller at which it is located by way of at least one IP router included in the interconnection network, said router being configured to systematically guide all the IP packets or level-2 messages to said gateway, prior to their encapsulation and broadcasting in the interconnection network. 
     According to an implementation of the method according to the invention, after the encapsulation step, all the packets arising from a first gateway connected to a first controller travel via the interconnection network and are then transmitted to a second gateway connected to a second controller, this second gateway removing the IP capsule added to the IP packets by the first gateway before transmitting said IP packets to the second controller. 
     According to an implementation of the method according to the invention, the first IP address known to the nodes of the radio network is the same for all the gateways. The presence of these two IP addresses allows the mobile terminal to keep just a single IP address. 
     According to an implementation of the method according to the invention, when a router sends an ARP request to the interconnection network, the first router of the interconnection network receiving said request transmits it to a gateway, said gateway transmitting a response to said router, the response comprising the level-2 address of said gateway. 
     The subject of the invention is also a system for guaranteeing the continuity of the communications operated from a fourth-generation mobile terminal connected to a radio network provided with several base stations with which said terminal is able to communicate, said radio network being linked by controllers to an interconnection network comprising routers operating at the level of the IP layer, wherein each controller is connected to at least one gateway configured to encapsulate in IP packets all the IP packets arising from the radio network before broadcasting them over the interconnection network. 
     According to one embodiment of the system according to the invention, at least one gateway is connected via a data bus to the controller with which it is associated. 
     According to one embodiment of the system according to the invention, at least one gateway is connected by way of at least one router to the controller with which it is associated. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other characteristics will become apparent on reading the nonlimiting detailed description given by way of example which follows in relation to appended drawings which represent: 
         FIG. 1 , a diagram presenting an exemplary network on which the method according to the invention may be implemented; 
         FIG. 2 , a diagram illustrating an exemplary path taken by data in a network in which the method according to the invention is implemented; 
         FIG. 3 , an example of macro-mobility handled by virtue of the method according to the invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a view presenting an exemplary network on which the method according to the invention may be implemented. A 4 G communications network  100  comprises a radio network  110  called a RAN subsequently, and an interconnection network  120  formed, for example, of cabled communications links or RF beam radio links or the like. In the example, the interconnection network  120  is linked to the Internet network  140  by way of a router  121 . The RAN  110  comprises several radio base stations  111  spread out to cover a territory. Thus, the 4 G communications network  100  allows mobile terminals  151  present in this territory to communicate with other terminals  152  connected to the 4 G network  100  via other base stations. A mobile terminal  151 ,  152  is, for example, a telephone, a laptop computer or any other roaming apparatus able to communicate via the 4 G network  100 . 
     Several radio base station controllers  113   a ,  113   b ,  113   c ,  113   d , also called more simply “controllers” subsequently, make it possible notably to manage the problems of macro-mobility of the mobile terminals, service quality, and authentication of users. Each of these controllers  113   a ,  113   b ,  113   c ,  113   d  controls one or more radio base stations  111 . The functions traditionally allotted to the “Home agent” in the case of the use of the MIP (Mobile IP) protocol are, within the framework of the invention, distributed at the level of the base controllers  113   a ,  113   b ,  113   c , notably so as to avoid the vulnerability of a centralized system. Furthermore, the controllers  113   a ,  113   b ,  113   c ,  113   d  are linked together by a network of routers  131 ,  132 ,  133 ,  134 ,  135  operating at the level of the IP layer. 
     Hence, when a new mobile terminal  151  connects to a base station  111  affiliated to a controller  113   a  to which this terminal  151  was not hitherto affiliated—stated otherwise, the mobile terminal was not connected to a base station affiliated to this controller  113   a —, the other controllers  113   b ,  113   c ,  113   d  must be advised of the arrival of the new mobile terminal  151 . In the example, the controller  113   a  is designed to be hooked up to an interconnection network comprising switches operating at level 2 of the ISO layer, although in the invention, this controller is hooked up to a network of routers  131 ,  132 ,  133 ,  134 ,  135  operating at level 3. The base controller  113   a  to which the new mobile terminal  151  is affiliated is configured to send a message to advise the other controllers  113   b ,  113   c ,  113   d  of the arrival of the mobile terminal  151 . According to one mode of implementation of the method according to the invention, the message is of “Gratuitous ARP” or GARP type. This message comprises the IP address of the mobile terminal and the level-2 address of the controller  113   a , which in the case of an Ethernet network is an MAC address. In the implementation of the invention, the interconnection network  120  being level 3, the network cannot propagate the GARP message automatically to the other controllers  113   b ,  113   c ,  113   d . To allow the propagation of this message, gateways  141 ,  142 ,  143 ,  144  operating at the level of the IP layer are interfaced between each controller  113   a ,  113   b ,  113   c ,  113   d  and the interconnection network  120 . The gateway  141 ,  142 ,  143 ,  144  is accessible to the controller  113   a ,  113   b ,  113   c ,  113   d  via one or more routers  131 ,  132 ,  133 ,  134  of the interconnection network  120 . The GARP message sent by the controller  113   a  with which the mobile terminal is newly affiliated is redirected to the gateway  141  which is associated with this controller  113   a . This level-2 GARP message can thus be processed by the gateway  141  associated with the base controller  113   a  to which the new mobile terminal  151  is affiliated. The gateway  141  utilizes level-2 message to create an IP message so as to alert the other gateways  142 ,  143 ,  144  of the arrival of a new mobile terminal  151  at the controller  113   a  associated with this gateway  141 . The IP message propagates through the interconnection network  120  so as to reach the other gateways  142 ,  143 ,  144 , which store a correspondence between the IP address of the mobile terminal and the IP address of the gateway  141  with which the controller  113   a  of the mobile terminal is affiliated. Thus, it is not necessary to directly alert the other controllers  113   b ,  113   c ,  113   d  of the arrival of a new mobile terminal  151  affiliated to the first controller  113   a . It is the gateways  141 ,  142 ,  143 ,  144  which maintain this information necessary for proper management of the macro-mobility of the terminals  151 . In the case of a movement of a mobile terminal, previously affiliated to a first controller, to a base station affiliated to another controller, the correspondence, maintained by each of the gateways  141 ,  142 ,  143 ,  144 , between the IP address of the mobile terminal and the IP address of the gateway associated with the controller to which this mobile terminal is affiliated is modified by each of the gateways  141 ,  142 ,  143 ,  144 . Indeed, the IP address of the initial gateway is replaced with the IP address of the new gateway. Within this framework, the gateways play a role in signaling the mobility of the terminals. 
     The mobile terminals  151 ,  152  connected to the interconnection network  120  via the radio base stations  111  and the controllers  113   a ,  113   b ,  113   c ,  113   d  belong to the same IP sub-network, so that once a mobile terminal has been declared at the 4 G network  100 , and an IP address has been allocated to it, this IP address does not change, even in the case of macro-mobility of this terminal. 
     To allow the mobile terminals to pass from one controller to the other without changing IP address and without disturbing the communications, all the IP data packets transmitted through a controller  113   a ,  113   b ,  113   c ,  113   d  destined for another controller  113   a ,  113   b ,  113   c ,  113   d  are systematically encapsulated in other IP packets so as to be able to be propagated by the routers  131 ,  132 ,  133 ,  134 ,  135  of the interconnection network  120 . A known IP address of the interconnection network  120  is allocated to the IP packets encapsulating the IP data packets, so as to correctly route them up to the destination controller, that is to say up to the controller charged with transmitting the packets to the base station to which the destination mobile terminal is connected. The IP capsule which had been added before routing in the interconnection network is thereafter removed at the level of this destination controller, more exactly by the gateway associated with this destination controller. 
     This IP in IP encapsulation mechanism is advantageously implemented by a gateway  141 ,  142 ,  143 ,  144  such as described above, a gateway preferably being put in place for each controller  113   a ,  113   b ,  113   c ,  113   d . A gateway may be viewed, for the processing of IP data, as a means for encapsulating IP packets in other IP packets of higher level. Two IP addresses are allocated to each of the gateways  141 ,  142 ,  143 ,  144 : a first IP address viewed from the radio network  110 , and a second IP address viewed from the interconnection network  120 . The first IP address, viewed from the radio network  110 , is always the same, whatever the gateway  141 ,  142 ,  143 ,  144 ; stated otherwise, all the gateways have the same first IP address, which in  FIG. 1  is designated by “IP0”. The second IP address, specific to each gateway, allows the addressing of the data within the interconnection network  120 . 
     In the example of  FIG. 1 , a router  131 ,  132 ,  133 ,  134  is interfaced between each controller  113   a ,  113   b ,  113   c ,  113   d  and the gateway  141 ,  142 ,  143 ,  144  with which it is associated. This router  131 ,  132 ,  133 ,  134  is configured to systematically transfer all the packets received from the controller to which it is connected to the associated gateway. All the packets entering the interconnection network  120  via a controller  113   a ,  113   b ,  113   c ,  113   d  are therefore transmitted to the gateway associated with this controller, so as to perform the IP in IP encapsulation described above. 
     Advantageously, a gateway  141  is co-located with the router with which it is associated  131 . According to one mode of implementation of the method according to the invention, the controller and the gateway are physically integrated into the same machine, the gateway being for example connected to the controller by a network cable or a data bus. 
     Furthermore, certain gateways  145  of the interconnection network  120  can receive requests of ARP (Address Resolution Protocol) type sent from external routers  121  to this interconnection network  120 . These requests originate, for example, from external routers  121  charged with determining which MAC address in the interconnection network  120  is the one which corresponds to a determined IP address. These gateways  145  are configured to respond with their own MAC addresses; stated otherwise, to indicate to the external routers  121  that whatever the specified IP address whose corresponding level-2 party is sought, it is the gateway  145  which must receive all the packets so as to undertake an IP in IP encapsulation such as described above and then steer them to the correct gateways of the network. 
       FIG. 2  illustrates by a diagram an exemplary path taken by data in a network in which the method according to the invention is implemented. The figure illustrates with an arrowed line a path  201  followed by data between a first mobile terminal  151  and a second mobile terminal  152 , the mobile terminals  151 ,  152  having been registered beforehand with the controllers  113   a ,  113   b.    
     The IP data arising from the first mobile terminal  151  are transmitted to a base station  111   a , and then to the first controller  113   a  interfaced with the interconnection network  120 . A router  131  of the interconnection network receives the data; it is configured to steer the data packets automatically to the first gateway  141 . This gateway  141 , which comprises two IP addresses as described above, encapsulates the IP data packets in other IP packets of higher level. The destination IP address in the interconnection network  120  which is allocated to the IP packets of higher level is the IP address of the destination gateway  142  viewed from the interconnection network  120 . The IP packets of higher level are thus routed through the interconnection network  120  up to the destination gateway  142 , which decodes the previously encapsulated IP data packets, and transmits them to the second controller  113   b . The IP data are thereafter transmitted to a base station  111   b  and then to the second mobile terminal  152 . 
       FIG. 3  illustrates an example of macro-mobility handled by virtue of the method according to the invention. The example of  FIG. 3  again employs the network of  FIG. 2 , in which two mobile terminals  151 ,  152  communicate. 
     The second terminal  152  moves away from the base station  111   b  with which it was connected, until it connects to a base station  111   d  affiliated to a third controller  113   d  different from the previous controller. The second mobile terminal  152  is detected by the third controller  113   d . A level-2 message  302  is sent by this third controller  113   d  to the gateway  144  with which it is associated, which gateway sends an IP message  303  (represented dashed in  FIG. 3 ) destined for the other gateways  141 ,  142 ,  143  to advise them of the movement of the mobile terminal  152 . The tables of correspondences maintained by the gateways  141 ,  142 ,  143  are updated, so that the IP packets intended for the mobile terminal  152  are properly conveyed to the gateway  144  associated with the new controller  113   d  to which the second mobile terminal  152  is affiliated. 
     The path  301  followed by data between the first mobile terminal  151  and the second mobile terminal  152  after macro-mobility is illustrated by an arrowed solid line. 
     If one or more communication links are cut, then the meshed character of the network and the fact that each gateway has direct access to the correspondences between IP addresses of the mobile terminals and IP addresses of the gateways with which they are affiliated makes it possible to ensure the conveying of the data correctly. The mobility management function is decentralized. 
     An advantage of the method according to the invention is that it makes it possible to reuse controllers initially designed to operate at level 2 of the ISO layer. It is thus possible to realize a reliable 4 G network on the basis of a level-3 interconnection network. The method according to the invention does not require the implementation of particular functionalities on the mobile terminals. Standard mobile terminals can therefore benefit from the invention. Finally, the method according to the invention is compatible with the conventional IP routing solutions and relies only on functions implemented in the controllers between the radio access network and the interconnection network.

Technology Category: 5