Abstract:
Rerouting of packet exchanges by a mobile terminal is controlled so as to be optimized on a real time basis to prevent network resources from being wasted by redundant routing. In an initial state a route of data from a mobile communication terminal M to a CN  8 , which is a communication partner, is a route R 5 . Then, an access router (AR)  72  acquires the number of hops of data received from the CN  8  by the mobile communication terminal M. As the mobile communication terminal M now performs a handover to under the command of the AR  73 , the route will change to a route R 9 . Then the AR  73  detects that the route becomes redundant by the fact that the number of hops acquired after the shift is greater than the pre-shift number of hops received from the AR  72 , and invokes control to reroute to a route R 7 , which provides the optimal routing.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method, a terminal and a router for detecting a trigger to rerouting, and more particularly to a method, a terminal and a router for detecting a trigger to rerouting for controlling an alteration of redundant routing. 
     2. Description of the Related Art 
     In the conventional packet exchange control system (GPRS) standardized under the 3rd Generation Partnership Project (3GPP), a manner of controlling the rerouting of a communicating mobile terminal when it has moved is differentiated with the type of radio channel between the mobile terminal and the base transceiver station during call. That is, available radio channels between the mobile terminal and the base transceiver station are classified into “dedicated channels” on which the volume of communication traffic is heavy and “common channels” on which the traffic volume is light. On the dedicated channel, the radio network controller (RNC) used at the time of initial establishment of communication is used as the anchor. And control to extend the route of data from the anchor (subscriber line extension) is performed. On the common channel, the Gateway GPRS Support Node (GGSN) is used as the anchor, from which routing is switched to the shortest cut from there to the mobile terminal (SRNS relocation (SRNS=Serving Radio Network Subsystem)) is performed. (Reference: 3G TS 25.832 “Manifestations of Handover and SRNS Relocation”.) This rerouting control is performed when the mobile terminal moves from one RNC to another (inter-RNC handover), but this rerouting control is not performed when the mobile terminal moves from one base transceiver station (BTS) to another in the territory of the same RNC (intra-RNC handover). In an intra-RNC handover, the route from the RNC to the BTS can be switched by soft handover, but that at a higher level than the RNC (between the RNC and the GGSN) cannot be switched. 
     When a mobile terminal performs an inter-RNC handover under the conventional rerouting control by GPRS, optimal rerouting can be accomplished if the radio channel is a common channel, but if it is a common channel, the subscriber line extension entails the occurrence of a redundant portion on the route, resulting in wasteful use of network resources. Moreover, as the choice between subscriber line extension and optimal rerouting solely relies on whether the radio channel is a dedicated channel or a common channel, even if an inter-RNC handover takes place, subscriber line extension may be chosen (if the volume of traffic drops in this state and a shift to a common channel occurs, a change to optimal routing will take place upon that shift), there is a disadvantage that the redundant routing resulting from the movement of the mobile unit cannot be optimized on a real time basis. Furthermore, although SRNS relocation is permitted under the 3GPP standard specifications even when operating on a dedicated channel, essentially SRNS relocation (or subscriber line extension) is a technique that is made feasible by the capability of the network to keep track of whether a mobile terminal has moved from one RNC to another. In a usual IP network, as there are many different topologies including a mesh structure and a tree structure or the like and the structure can usually be altered as desired, it is not realistic for the network to manage the structure, and it is impossible to apply the 3GPP specifications as they are. 
       FIGS. 7  illustrate how the conventional system works, namely how routing is altered where a mobile communication terminal M under GPRS shifts its position.  FIG. 7A  shows a case in which the radio channel between the mobile communication terminal M and the base transceiver station is a common channel, and  FIG. 7B , a case in which the radio channel between the mobile communication terminal M and the base transceiver station is a dedicated channel. A GGSN  1  in  FIG. 7A  is a gateway GPRS support node, positioned at the gateway to the network where there is a server or a terminal which is to become a communication partner  8  with the mobile communication terminal M. Communication between the mobile communication terminal M and the communication partner  8  takes place via this GGSN  1 . An SGSN  2  is a Serving GPRS Support Node (SGSN), which is connected to the GGSN  1  and is the switchboard nearest to the mobile communication terminal M. An RNC  3  is a radio network controller having functions to control radio resources and to control the handover when the mobile communication terminal M has shifted its position. BTSs  41  to  44  are base transceiver stations, and the mobile terminal carries out communication through connection to one or another of these BTSs. 
     Where the radio channel between the mobile communication terminal M and the base transceiver station is a common channel as shown in  FIG. 7A , the communication path between the mobile communication terminal M and its communication partner  8  is switched from a route R 1  to a route R 2 , which is the shortest cut, with the GGSN  1  as the starting point along with a shift, represented by an arrow Y 3 , of the mobile communication terminal M. However, where the radio channel is a dedicated channel as shown in  FIG. 7B , subscriber line extension takes place whereby, starting from an RNC  31  which was on the communication path when communication was begun, a route R 3  is extended toward an RNC  32  and a BTS 43 , which are the destinations of the shift, in the direction represented by an arrow Y 4 , of the mobile communication terminal M (a route R 4 ). This system is used to restrain any data loss that may arise when the communication path is switched by a handover, but a more redundant route shown in  FIG. 7B  will arise, compared with the optimal (shortest) route (the route R 2  in  FIG. 7A ). 
     Thus under GPRS, even at a timing at which rerouting for optimization is required, the rerouting method is selected solely dependent on the state of the radio channel, resulting in a disadvantage that routing cannot be optimized on a real time basis in response to a handover of the mobile communication terminal M. 
     Conceivably, this problem could be addressed by either (1) invoking the procedure of change-over to the optimal route upon every handover of the mobile terminal or (2) invoking the same upon an inter-RNC handover. 
     However, the method of (1) may invoke a wasteful procedure because an intra-RNC handover would need no optimization of routing (the handover would give rise to no redundant route). The method of (2) is unrealistic because managing the structure of a usual IP network, such as the one mentioned above, is difficult to manage and accordingly it is difficult to determine whether or not a given RNC is an “inter-RNC”. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a method, a terminal and a router for detecting a trigger to rerouting for providing on a real time basis a trigger to anchor router (hereinafter abbreviated to “AncR”) reselection by constantly comparing during communication the number of hops required for the routing of a data packet from the communication partner of a mobile terminal to the mobile terminal. 
     A method of detecting a trigger to rerouting is characterized by including a comparative step of comparing the respective numbers of hops arising at terminals in data transmission and reception arising between the terminals, and an optimizing step of achieving optimization, if the result of comparison at the comparative step indicates that the number of hops in later data transmission and reception is greater, by altering the routing so as to reduce the number of hops. 
     By comparing the numbers of hops on the route, any redundancy on the route can be detected according to an increase in the number of hops, and the route can be optimized on that basis. 
     In various embodiments discussed below, routing in the transmission and reception of the data is accomplished with at least one of a plurality of routers relaying the data serves as the anchor, if the number of hops in later data transmission and reception is found greater as the result of comparison at the comparative step, the router to serve as this anchor (anchor router) is altered at the optimizing step. 
     The numbers of hops between transmission terminal transmitting data and reception terminal receiving the data are compared at the comparative step, and the routing is altered at the optimizing step according to the result of comparison at the comparative step. 
     As the numbers of hops are compared and a rerouting instruction is issued on the part of a specific party to communication, the optimal rerouting can be accomplished without having to grasp the overall situation of the communication network including the identification of the source of redundancy as the shift of the own terminal or that of the other party to the terminal or any other factor. 
     The number of hops of the currently received data at the reception terminal and the number of hops of the immediately preceding received data at the reception terminal are compared at the comparative step, and routing is altered at the optimizing step so as to reduce the number of hops if the result of comparison at the comparative step reveals that the number of hops of the currently received data is greater than the number of hops of the immediately preceding received data. 
     By monitoring the number of hops every time data are received, the optimal rerouting can be accomplished on a substantially real time basis while the mobile terminal is engaged in communication. 
     The number of hops is acquired by acquiring on the basis of parameters in data, the parameters being modified by routers which are passed between the transmission terminal and the reception terminal during the time they communicate, the variances between the values of the parameters at the starting point of counting and the values of the parameters at the ending point of counting, and identifying the number of routers corresponding to those variances. 
     This makes it possible to acquire the number of hops in data reception. Further by using parameters contained in the data, it is made possible to acquire the number of hops on a substantially real time basis along with the transmission and reception of data and without having to perform any special control over the communication of parameters. 
     The parameters are initialized at the starting point, and the variances can be acquired on the basis of the resultant initial values. 
     This makes it possible to acquire the number of hops accurately. 
     A terminal according to the invention includes an acquiring unit which acquires the number of hops and an issuing unit which issues, if the number of hops currently acquired by the acquiring unit is greater than the number of hops acquired in the past, an optimizing instruction to alter routing to the other terminal to reduce the number of hops and optimize it, it being so arranged that an external apparatus having received the instruction optimizes the inter-terminal routing. 
     A router according to the invention includes an acquiring unit which acquires the number of hops and an issuing unit which issues, if the number of hops currently acquired by the acquiring unit is greater than the number of hops acquired in the past, an optimizing instruction to alter the inter-terminal routing to reduce the number of hops and optimize it, it being so arranged that an external apparatus having received the instruction optimizes the inter-terminal routing. 
     The router may further include a receiver unit which receives the number of hops acquired by another apparatus in the past, wherein the issuing unit issues, if the number of hops currently acquired by the acquiring unit is greater than the past number of hops acquired by the receiver unit, an optimizing instruction to alter the inter-terminal routing to reduce and optimize it, and an external apparatus having received the instruction optimizes the inter-terminal routing. 
     The router may further include a transmitter unit which transmits the number of hops acquired by the acquiring unit to an external comparator which detects any increase in the number of hops in the inter-terminal communication on the basis of the acquired number of hops. 
     An embodiment of a router of the invention includes an initializing unit which initializes parameters in data modified by a router installed between the terminals for the communication of data to identify the number of hops, wherein the variances of said parameters matching said number of hops can be acquired on the basis of the resultant initialized values. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a flow chart of a method of detecting a trigger to rerouting according to the present invention; 
         FIG. 2  illustrates how routing takes place if the method of detecting a trigger to rerouting according to the invention is applied to a case in which a correspondent node (CN) is in another network; 
         FIG. 3  illustrates how routing takes place if the method of detecting a trigger to rerouting according to the invention is applied to a case in which a CN is in the own network; 
         FIG. 4  is a block diagram of the configuration of a terminal for use in an implementation of this method; 
         FIG. 5A  is a block diagram of a configuration of an access router for use in the implementation of this method;  FIG. 5B , a block diagram of a configuration of an access router for transmitting the number of hops to an access router at the destination of shifting; and  FIG. 5C , a block diagram of a configuration of an access router for receiving the number of hops from an access router in the position before the shift; 
         FIG. 6  is a block diagram of a configuration of a router for use in the implementation of this method; and 
         FIG. 7A  illustrates how routing takes place if a radio channel between a mobile communication terminal and a base transceiver station is a common channel in a conventional GPRS system; and  FIG. 7B , how routing takes place if the radio channel between the mobile communication terminal and the base transceiver station is a dedicated channel in the conventional GPRS system. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will be described below with reference to the accompanying drawings. In the following description, equivalent elements in different drawings are denoted by respectively the same reference signs. 
     Embodiment 1  
       FIG. 1  is a flow chart of a method of detecting a trigger to rerouting according to the present invention. 
     At step S 101  in  FIG. 1 , transmission and reception of data between specific terminals is monitored. 
     At step S 102 , if transmission and reception of any data was detected at step S 101 , the number of routers the data have passed from the time they were transmitted from one terminal by the time they are received by another terminal, which is the communication partner, i.e. the number of hops, is acquired. 
     At step S 103 , the number of hops acquired at step S 102  is compared with the number of hops acquired by earlier transmission and reception of data than at step S 101 . If this comparison reveals a greater number of hops at step S 102 , i.e. that the shift of the communicating terminal in the meantime has made the routing redundant, the process goes ahead to step S 104 . 
     At step S 104 , rerouting is carried out so as to reduce the number of hops. 
       FIG. 2  illustrates how routing takes place if the method of detecting a trigger to rerouting according to the invention is applied to a case in which a correspondent node (CN), with which a mobile terminal communicates, is in another network (hereinafter referred to as the other network) than the network to which the mobile terminal belongs (hereinafter referred to as the own network). 
     A CN  8  in this case may be a mobile terminal connected to the other network or any other terminal or a server. AncRs  51  and  52  are routers present on the route between the mobile communication terminal M and the CN  8 , and communication takes place via them. The AncR  51  in Embodiment 1 usually is likely to be a router present on the boundary between the own network and the other network. The AncR  51  further has a function to capsulate a data packet destined from the CN  8  for the mobile communication terminal M when it passes and to transmit it to the mobile communication terminal M and a function to set in that data packet the initial number of hops in the form of combining consciousness with an access router (AR). RTs  61  and  62  are usual routers (RTs) present in the network. ARs  71  through  74  are routers present at one end or another of the network, and the mobile communication terminal M connects with the AR  72  among them and engages in wireless communication with the AR  72 . 
       FIG. 2  shows a situation in which the mobile communication terminal M is already connected to the AR  72  before a handover. The routing of a data packet from the CN  8  to the mobile communication terminal M then consists of a route R 5  via the AncR  52 . The data packet from the CN  8  is capsulated by the AncR  52 , and the number of hops is initialized and transmitted to the mobile communication terminal M. The AR  72  detects from the received data packet (destined for the mobile terminal) the number of hops required between the CN  8  and the AR  72  on the basis of the initial number of hops set by the AncR  52 , and stores that number. 
     When the mobile communication terminal M shifts to under the command of the AR  73  as indicated by an arrow Y 1  and a handover takes place, immediately after the handover it is routed via a route R 6  because it is still communicating via the AncR  52 . At the time of this handover, information regarding the number of hops of the reception data of the mobile communication terminal M, acquired and stored by the AR  72 , is succeeded and stored by the AR  73 . The optimal (shortest) path then is a route R 7 , and the route R 6  would be a redundant path for routing from the CN  8  to the AncR  51  via the AncR  52 . Then the AR  73  compares the number of hops between the CN  8  and the AR  72  at the AR  72  via the route R 5 , succeeded from the AR  72 , with the number of hops between the CN  8  and the AR  73  detected when on the route R 6 , and detects an increase in the number of hops. 
     Triggered by this detection, the AR  73  invokes control to optimize the route, i.e. to reselect an AncR. In the state shown in  FIG. 2 , as the route using the AncR  51  as the relay node is the shortest cut, control to select the AncR  51  as the relay node is invoked. A number of ways of AncR reselection control are conceivable, including direct notification by the AR  73  to the CN  8  that the subsequent communication will take place via the AncR  51 , and a request by the AR  73  to the AncR, another router or mobile terminal to notify the CN  8  of the reselection. After the AncR reselection, the routing is changed to the route R 7  having the AncR  51  as its relay node, resulting in a switch to the optimal route. Various specific means are conceivable for handing over the data of the number of hops from the AR  72  to the AR  73  in this embodiment, including direct transmission of the data from the AR  72  to the AR  73 , transmission of the data via another node (router) in the network, and transmission of the data via the pertinent mobile terminal. 
     Embodiment 2  
       FIG. 3  illustrates how routing takes place if the present invention is applied to a case in which a CN is in the own network. 
     There is shown a situation in which, in the initial state, the mobile communication terminal M is connected to the AR  77  and the CN  8 , to the AR  76 , and the data routing from the CN  8  to mobile communication terminal M uses an RT  63  as the AncR. Therefore, the data routing from the CN  8  to the mobile communication terminal M in the initial state uses a route R 8 . In this case, too, as in Embodiment 1, the data destined from the CN  8  to the mobile communication terminal M are capsulated and the initial number of hops is set at the RT  63 , which is the AncR, and the AR  77  detects from the reception data the number of hops between the CN  8  and the AR  77  and stores it. 
     When the CN  8  hands over here the command to the AR  75  as indicated by an arrow Y 2 , immediately after the handover the data routing from CN  8  to the mobile communication terminal M runs via a route R 9  because the AncR still is the RT  63 . As the optimal route then is the route R 10 , the path from the RT  62  to the RT  63  is redundant for the route R 9 . Then, the AR  77  detects the redundancy of routing by comparing the number of hops obtained from the reception data from the CN  8 , and invokes control to optimize the route (reselect an AncR). Since the optimal AncR then is the RT  62 , the control to change the AncR to the RT  62  is invoked. For reselection control then, a number of ways are conceivable as described above with reference to Embodiment 1. After AncR reselection to use the RT  62 , the routing runs via the route R 10 , resulting in a switch to the optimal route. 
     Specific parameters to be used in calculating the number of hops according to the invention include, for instance, a Time To Live (TTL) in IPv 4  and a hop limit parameter in IPv 6 . 
     Further, regarding the functions and operations of an AR according to the invention, a mobile communication terminal can also have the same functions and operations as the AR except the AR before the mobile communication terminal shifts to hand over the received number of hops that has been acquired to the AR to which the shift was desired for and the AR to which after the mobile communication terminal shifts to receive and store the number of hops received before the shift. Thus, the object of the invention is also attained when the mobile communication terminal acquires the number of hops, compares the currently acquired number of hops with the number of hops acquired in the past and issues an instruction to change the routing to another terminal is changed when the comparison reveals the current number of hops to be greater than the past number of hops. 
     In order to realize the method described above, the terminal can be configured as shown in  FIG. 4 , or the access router configured as shown in  FIG. 5  (A to C), and the router configured as shown in  FIG. 6 . 
     Thus, as illustrated in  FIG. 4 , the terminal M comprises a number of hops acquiring unit M 1  and a command issuing unit M 2 . The number of hops acquiring unit M 1  acquires, every time it receives data as indicated by an arrow Y 6 , the number of hops needed for the reception of the data. The command issuing unit M 2  issues an instruction to change the routing to another terminal is changed when the number of hops received by the number of hops acquiring unit M 1  proves greater than the number of hops acquired in the past. 
     Alternatively, an access router  7  may comprise a number of hops acquiring unit  7 A, a transmitter unit  7 B, a command issuing unit  7 C and a receiver unit  7 D as shown in  FIG. 5A . The number of hops acquiring unit  7 A has similar functions to those of the number of hops acquiring unit provided on the terminal. The receiver unit  7 D receives the number of hops transmitted from another access router  7 . Thus, when the terminal has shifted as described above, the access router  7  to be connected to that terminal will change. In this case, by receiving the past number of hops to be compared with from the access router  7  connected before the shift and making comparison, any redundancy in the routing after the shift can be detected. The transmitter unit  7 B transmits, when the terminal under the command of the access router shifts as described above, the number of hops acquired in data communication immediately before the shift, to the other access router  7  which is to compare the number of hops. The command issuing unit  7 C compares the number of hops acquired by the number of hops acquiring unit  7 A or received by the receiver unit  7 D, and issues an instruction to on the basis of the result of comparison as described above.  FIG. 5B  illustrates the minimum configuration the access router  7  requires when a terminal under its command has shift, and  FIG. 5C , that the access router  7  requires when a terminal has been newly connected under its command. Thus, the access router  7  a terminal under whose command has moved out transmits the number of hops acquired by the number of hops acquiring unit  7 A through the transmitter unit  7 B. The access router  7  under whose command a terminal has been newly connected receives through the receiver unit  7 D the number of hops before the shift, transmitted by that access router  7 , i.e. the past number of hop, acquires the current number of hops through the number of hops acquiring unit  7 A, and compares the current and past numbers of hops using the command issuing unit  7 C. Whereas any redundancy in routing can be detected in this manner, in practice an access router  7  of a configuration shown in  FIG. 5A  is installed in the network. 
     As shown in  FIG. 6 , a router  9  includes an initializing unit  9 A for setting initial values for parameters in data. The parameters are initialized first and then undated by a router installed between terminals engaged in communication of data to identify the number of hops. These parameters are the hop limit parameter and others as mentioned above. By initializing the number of hops at the router  9 , especially the anchor router where the counting starts, having the access router or the like to acquire that parameter perceive the initial value, and acquiring the number of hops on the basis of the variance from the initial value, the number of hops can be acquired accurately. 
     As hither to described, because an AR or a mobile terminal detects the number of hops from data received in communication and stores it, compares it with the number of hops in new received data and thereby provides a trigger to reselection according to the present invention, eventually it can switch to the optimal route on a substantially real time basis while the mobile terminal is engaged in communication. Furthermore, as the invention can be applied irrespective of whether the communication partner is in another network or in the own network, and as it allows the detection of any redundancy in routing irrespective of the cause of redundancy, whether it is due to the shift of the own terminal, that of the communication partner or to any other factor, the resources in the network can be utilized more effectively by eliminating redundant routing.