Communication network design method and communication system

Abstract: In order to establish VC networks on an ATM network, a different VCH interconnection network for each media or service consisting of information to be transferred is established on a VP network; and VC networks corresponding to these media or services are formed by means of these respective VCH interconnection networks, whereby a plurality of independent VC networks for different media or services are established simultaneously on a single physical network.

Description: This invention is utilized in asynchronous transfer mode (ATM) communication networks, and relates in particular to techniques for actualizing a different network topology for each media or service on a single physical network. FIG.1 and FIG.2 show the configurations of two prior art communication networks. The network shown in FIG. 1 may be utilized for telephony, and comprises: local switches 11 and 13 accommodating subscriber lines; transit switch 12 interconnecting these local switches 11 and 13; transmission links 14 interconnecting local switches 11 and 13 and transit switch 12; and cross-connect switches 15. As opposed to this, as shown in FIG.2, in data communications as typified by Ethernet, cross-connect switches 21 and transmission links 22 are connected in a single loop or a single bus, and add/drop multiplexers 23 are provided for inserting and branching off information. With prior art network configurations, two communication networks have been necessary as the different topologies for the two services mentioned above. This is because a service such as telephony occupies considerable resources since each call is at a constant bit rate (CBR) and has a comparatively long connection time. Also, traffic is dispersed over a wide area, and so transit switches are required due to the necessity of concentrating circuits for long-distance communication. As opposed to this, data is generally transmitted at variable bit rate (VBR) with the traffic itself in the form of bursts, and although the peak rate is high, the average rate is low and some degree of delay is allowed. A network utilized for data transmission therefore has a topology whereby a bus or a loop is shared as a single resource. Because services with different traffic characteristics have thus used physical networks with separate topologies, there is the problem that there will be a different optimum topology every time a new service or media appears in this multimedia age, with the result that many networks will be required. To put it another way, each service will require its own coaxial or optical fibre cable, and this arrangement is lacking in flexibility. Known from Arvidsson A.: "Management of reconfigurable virtual path networks", Fundamental Role of teletraffic in the evolution of telecommunication networks, Proceedings of the 14 th international Teletraffic Congress - ITC 14 Juan-Lees-Pins, June 6-10, 1994, No. 1B, 6 June 1994, Labetoulle J., Roberts J.W. (Eds.), is an ATM communication method wherein different VP networks are established for different service classes. The known design method provides integer solutions, handles non-linear equivalent bandwidths, finds its own paths and allows for a design optimisation criterion. It is an object of the present invention to provide an efficient and flexible network design method and communication equipment which solve such problems. According to a first aspect, this invention provides a communication network design method which: physically connects, by means of a transmission link network, a plurality of virtual channel handlers (VCHs) which perform switching in units of virtual channels (VCs), and a virtual path handler (VPH) which changes over information transfer routes in units of virtual paths (VPs) that accommodate a plurality of VCs; establishes a VP network on this transmission link network, said VP network being formed from VPs which logically connect a plurality of VCHs; and establishes a VC network on this VP network, said VC network being formed from VCs which logically connect subscribers; said communication network design method being characterised in that: a different VCH interconnection network for each media or service is established on the VP network, and a VC network corresponding to a given media or service is formed by means of each VCH interconnection network, whereby a plurality of independent VC networks for different media or services are established simultaneously on a single physical network. In other words, in order to actualize an ideal topology for each media or service on a single physical network, logical connections between VCHs are established for each media or service in an ATM network comprising a VC network and a VP network. The VCs of these media or services are established on the VCH interconnection networks thereby obtained. In this specification, "media" signifies a communication system with a distinctive set of communication standards, quality, mode, capacity, and so forth. For example, there is voice communication and image communication, while the latter is subdivided into still and moving images. Still images are further subdivided into recorded images (FAX) and display images (LCD), while moving images are subdivided into standard TV images and simplified TV images. A plurality of media generally coexist in a single communication system (this is termed "multimedia") and each media will deal with a different kind of signal and have a different signal quality requirement. The quality that users require for each media includes specified values for call blocking probability, cell loss probability, and delay. The VC and VP networks of an ATM network correspond respectively to the circuit and path networks in a conventional network. The VP network is a network which is established logically on a physical network, and it can logically connect all VCHs in a mesh. The VC network is a network which logically connects subscribers on the VP network by means of VCs. A VCH interconnection network as used in this invention is formed by means of a logical network which connects subscriber VCHs used by a given media or service and a plurality of transit VCHs; and VCs for this media or service are established by selection on this VCH interconnection network only. A VCH interconnection network for a given media or service corresponds to the VC network of that media or service. However, since a VC network is a network with subscribers as the terminal points, it establishes connections between subscribers and VCHs as well as between VCHs; whereas a VCH interconnection network is a network which establishes logical connections between VCHs. Although a VCH interconnection network does not take connections to subscribers into consideration, designing VCH interconnection networks suited to each media or service is equivalent to designing VC networks suited to the same media or services. All VCHs can be connected in a mesh by ensuring that a VP which does not pass through any other VCH can be established between any two given VCHs. The establishing of a VCH interconnection network may be performed by assigning different VPs to each media or service consisting of information to be transferred. Alternatively, it is also feasible to establish a VCH interconnection network by allowing a plurality of media or services with the same destination VCH to be assigned to the same VPs. In this case, priority control should be performed between VPs with the same destination in accordance with a priority ranking of the media or services. The network topologies of a plurality of VCH interconnection networks for different kinds of media or service may also be designed simultaneously after taking their overall optimization into consideration. Alternatively, they may be designed in a particular order. When designing in a particular order, an optimized network topology may be obtained by calculating the network resource efficiency from the working VP ratio, which is defined as the ratio of working VP capacity to total VP capacity (i.e., both working and back-up VPs), and from the VP use efficiency, which has been calculated from the traffic type, traffic demand and required cell loss probability in VCs established between subscribers utilizing this VCH interconnection network. In this case, the number of disjoint routes D (i.e., the number of paths between origin and destination VCHs which do not pass through the same VPHs or VP links) may be set as a parameter of trial network topologies, and satisfying the given number of disjoint routes D may be set as a constraint. A trial network topology such that the total length of all the VP links is minimized is then formed, and the number of disjoint routes D is changed when the trial network topology is changed. The network resource efficiency may then be calculated for the trial network topology obtained. Network topologies can also be designed simultaneously so that overall network cost is minimized. In this case, connection or disconnection of VP connections between VCHs for establishing a VCH interconnection network may be performed in parallel with establishing VCH interconnection networks for other media or services When a plurality of network topologies are designed in a particular order, the design priority order should be determined in the order of decreasing network cost of the media or services supported by the VCH interconnection networks in question. After network topologies have been designed in this manner, the proportion of overall network cost accounted for by the network cost of the VCH interconnection network for each media or service can be calculated and the design priority order can be obtained again in accordance with the results - of this calculation. If this design priority order is different from the previous time, the design of network topologies can be repeated in accordance with the new design priority order. A limit may be set for the number of such repetitions. It is also feasible to classify media or services into a priority group and a non-priority group according to their network cost. The design of VCH interconnection network topologies for the priority group is performed in the order of decreasing network cost, while the design of VCH interconnection network topologies for the non-priority group is performed simultaneously so that overall network cost is minimized. When designing network topology, it is desirable to set an upper limit for the number of VPs passed through from an origin VCH to a destination VCH. According to a second aspect, this invention provides communication system which has: a plurality of VCHs which perform switching in units of VCs; a VPH which changes over routes in units of VPs which accommodate a plurality of VCs; a transmission link network which physically connects the plurality ofVCHs and the VPH; and a higher order means which establishes a VP network on this transmission link network, said VP network being formed from VPs which logically connect the plurality of VCHs, and which also establishes a VC network on this VP network, said VC network being formed from VCs which logically connect subscribers; said communication equipment being characterised in that the higher order means includes: a VCH interconnection network establishing means which establishes, on the VP network, a different VCH interconnection network for each media or service; and a means which forms a VC network corresponding to a given media or service on the basis of each VCH interconnection network, whereby a plurality of VC networks for different media or services are established simultaneously on a single physical network. FIG. 1 shows a prior art communication network configuration. FIG.2 shows another prior art communication network configuration. FIG. is a block diagram showing communication equipment according to a first embodiment of this invention. FIG.4(a)-(c) shows examples of logical network topology for various services. FIG.5(a)-(d) shows configurations for actualizing a plurality of topologies on a single physical network. FIG.6 shows an example of a VCH configuration. FIG.7 shows another example of a VCH configuration. FIG.8 serves to explain the manner in which VPs are disposed on a transmission link. FIG. 9 shows another example of a VCH configuration. FIG.10 is a flowchart showing a conventional method for establishing a VC network in an ATM network. FIG.11 is a flowchart showing a method for obtaining a different optimum VC network for each media. FIG. 12 is a flowchart showing an example of a method for forming a trial VCH interconnection network topology. FIG.13 is a flowchart showing an example of a method for changing the trial VCH interconnection network topology. FIG.14 shows trial network topologies when the number of disjoint routes D has been set as a parameter. FIG. 15 shows an example of results from an optimization of VCH interconnection network topology. FIG.16 shows another example of results from an optimization of VCH interconnection network topology. FIG. 17 shows the overall flow of a method for establishing a plurality of VCH interconnection networks that minimize cost. FIG. 18 shows the design flow of each VCH interconnection network topology. FIG. 19 shows the design flow of a VCH interconnection network topology for one given media. FIG.20 is a flowchart showing a method for designing a VCH interconnection network topology for each media after setting the design priority order of the VCH interconnection networks in the order of decreasing network cost. FIG.21 is a flowchart showing another method for designing a VCH interconnection network topology for each media. FIG.22 shows another example of a method for designing a VCH interconnection network topology for media m. FIG.23 shows still another method for designing VCH interconnection network topologies. FIG.24 shows a method for designing VCH interconnection network topologies simultaneously for a plurality of media. FIG.25 shows an example of a method for designing VCH interconnection network topologies simultaneously for each of M media. FIG.26 shows the flow of a method for determining the order of pairs of VCHs between which VP connections will be added or deleted. FIG.27 serves to explain this method. FIG.28 shows the processing flow for establishing VCH interconnection networks by combining the method for designing a VCH interconnection network topology for each media in a particular order with the method for designing simultaneously. FIG.29 serves to explain this design method. FIG.30 shows another method for establishing VCH interconnection networks by combining the method for designing a VCH interconnection network topology for each media in a particular order with the method for simultaneously designing a plurality of VCH interconnection network topologies. FIG.31 shows still another method. FIG.3 is a block diagram showing communication equipment according to a first embodiment of this invention. This embodying equipment has: a plurality of virtual channel handlers VCH1-VCH5 which perform switching in units of VCs that logically connect subscribers; VPH1-VPH3 which switch the routes of these VCs in units of VPs which accommodate a plurality of VCs; a transmission link network formed from transmission links 31-37 which physically connect VCH1-VCH5 and VPH1-VPH3; and an operation system OpS which establishes, on this transmission link network, a VP network formed from VPs that logically connect VCH1-VCH5, and also establishes, on this VP network, a VC network formed from VCs. OpS can establish a VP between any two given VCHs that does not pass through any other VCH. It can also simultaneously establish a plurality of VC networks for different media or services on a single physical network by establishing, on the VP network, a different VCH interconnection network for each media or service consisting of information to be transferred, and by using each VCH interconnection network to form a VC network corresponding to that media or service. VCH1-VCH5 are respectively arranged in various regions, and any given VCHs can be connected directly via the VP network. Although there are circumstances in which a connection between VCHs will involve passing through one or more of VPH1-VPH3, VPH1-VPH3 do not need to be switched in VC units and generally operate faster and with simpler hardware than VCH1-VCH5. Accordingly, a VP network in which the routes of the paths are switched by VPH1-VPH3 is a fast high-quality network with little cell loss. FIG.4(a)-(c) shows examples, realizable by means of this invention, of logical VCH interconnection network topologies for various services. In the examples shown here, the VCH interconnection networks are all formed from VPs. Category 1 shown in FIG.4(a) is a star topology as typified by telephony; Category 2 shown in FIG.4(b) is a loop topology as typified by electronic mail and data; while Category 3 shown in FIG.4(c) is a mesh topology as utilized for on-line banking, etc., where especially high reliability is required. FIG.5(a)-(d) shows the configurations which realize the plurality of VCH interconnection network topologies shown in FIG.4(a)-(c) on a single physical network. FIG.5(a) shows the configuration of the physical network, while FIG.5(b)-(d) shows the respective VCH interconnection network topologies. In Category 1, VCH5 is taken as a transit VCH and VCH1-VCH4 are taken as local VCHs. VCH5 is connected to VCH1-VCH4 by virtual paths VP1-1 to VP1-4, and all traffic amongst VCH1-VCH4 can be connected via VCH5. In Category 2, which is completely independent from the VPs of Category 1, the loop topology is realized by means of virtual paths VP2-1 to VP2-5. In the case of Category 3, the mesh topology is realized by means of virtual paths VP3-1 to VP3-10, independently of Categories 1 and 2. VCHs can thus be connected using VPs in such manner that the desired topologies are obtained. In these topologies, VCH5 (by way of example) functions as a transit VCH in Category 1; as an add-drop VCH in Category 2; and as a local VCH in Category 3. FIG.6 shows an example of the configuration of a VCH. This VCH has service distribution part 61 which distributes cells with the same destination. It also has, for each category, a shaper 62 which temporarily stores distributed cells and then sends them to a respective separate VP. Each VCH assigns a VP to each service and performs shaping at the speed required for each service category. A VP is established for each destination: i.e., for each destination VCH, and for each service. FIG.7 shows another example of the configuration of a VCH. This VCH has, in addition to the configuration shown in FIG.6, read control part 63 which controls the reading of shapers 62. This read control part 63 performs read control in accordance with the priority or non-priority of each service category with the same destination. For example, when telephone traffic, which is strongly real-time in nature, has increased, read control part 63 increases the bandwidth of the corresponding VP and makes traffic of other categories wait, said other categories being such that waiting is permissible. The manner in which VPs are disposed on a transmission link is shown in FIG.8. Priority control of this sort is priority control among VPs with the same destination, and does not affect resource management in the VP network. It enables the quality of service (delay and cell loss) of a plurality of media or services to be controlled efficiently at a switch. In the foregoing embodiment, an explanation was given of the case where an individual VP is assigned to each media, i.e., to each service category. However, a plurality of media with the same destination can also be assigned to the same VP. VCH configuration in such a case is shown in FIG. 9. The VCH shown in FIG.9 has: a shaper 62-1 for each category; VP shaper 62-2 for each destination; and read control part 63 which controls the reading of shapers 62-1. For each category, a shaper 62-1 controls the quality of service for that category. VP shaper 62-2 sends cells that have been read from a shaper 62-1 for each category to a common VP. In this case, Category 1 may for example be assigned to virtual channel identifiers (VCIs) "1"-"100", while Category 2 may be assigned to VCIs "101"-"200". This results in one VP per destination at the transmission link level, which means that the number of VPs can be decreased and the communication network can be realized with smaller VPHs. In the embodiments explained above, an ideal network topology is realized for each service or media in a single physical network. Because quality of service can be controlled as occasion demands for each service, multimedia can be handled economically and flexibly. Next, explanations will be given of procedures for establishing the VCH interconnection networks described above. FIG.10 shows, as an example of a method for establishing a VCH interconnection network, the utilization of a known method for establishing a VC network. In order to establish a VCH interconnection network, the initial values for a trial VCH interconnection network topology are first of all input. The quality required by users includes reliability. As an example, reliability will be considered as the recovery ratio of working VPs when a fault has occurred. In an ATM network, the reliability required for each media is different. The back-up VP capacity is assigned so as to guarantee the highest required reliability, i.e., the recovery ratio of working VPs when a fault has occurred. The back-up VP capacity is set within the range of the given trial VCH interconnection network topology. The working VP ratio is defined as: total working VP capacity ( total working VP capacity + total back-up VP capacity ) The working VP ratio is thus obtained from the working VP capacity and the back-up VP capacity, which have been set so as to guarantee the required reliability. It is then decided whether the working VP ratio is maximum. If it is not maximum, the trial VCH interconnection network topology is changed and the working VP ratio is obtained again in the same manner. If the working VP ratio is maximum, the VCH interconnection network topology in question will be an optimum network topology. However, in an ATM network in which a plurality of media coexist, the method illustrated in FIG.10 ends up establishing a VCH interconnection network suited to the media which requires the highest reliability, which means that excessive network resources are given to media which do not require such reliability. In addition, VCs that handle burst type traffic and VCs that handle non-burst traffic are multiplexed onto the same VPs, and if the traffic type (i.e., whether in the form of bursts or not), VC number, required cell loss probability, etc., of VCs established between subscribers using this VCH interconnection network are not reflected in the network resource efficiency, it will be impossible to make an accurate evaluation of the network resource efficiency. FIG.11 shows a method for obtaining a different optimum VCH interconnection network for each media, thereby overcoming these defects. In this method, a different VCH interconnection network is configured in accordance with the required quality, traffic type and traffic demand of each media, so that network resource efficiency is maximized. In establishing a VCH interconnection network for each media, network resource efficiency is calculated not just from the working VP ratio, but also from the VP use efficiency, which has been calculated from the traffic type (whether in the form of bursts or not), traffic demand (VC number) and required cell loss probability (required quality) of VCs established between subscribers on the basis of the VCH interconnection network. Network resource efficiency thus calculated is used to evaluate the optimization, and the trial VCH interconnection network topology is changed on this basis. This method will now be explained in greater detail. In the optimization of a VCH interconnection network for a given media, the initial values for a trial VCH interconnection network topology are first of all given (S1), and VC routes are established for this trial VCH interconnection network topology. VC routes may be considered as taking the shortest path between origin and destination VCHs. VC routes pertaining to the same media and passing between two VCHs may be multiplexed onto the same VP. The VP capacity required for this is calculated from the traffic type (whether in the form of bursts or not), VC number and cell loss probability of the VCs; and the VP use efficiency is found at the same time. The VC number is equivalent to the traffic demand of the media, while cell loss probability is part of the quality required of the media. The use efficiency of any given VP is defined as: sum of mean rates of VCs accommodated in given VP required VP capacity When considered in terms of the whole network, the overall VP use efficiency will be: Σ ( sum of mean rates of VCs accommodated in a VP ) Σ required VP capacity where E indicates the sum total for all the VPs. When the reliability required of a media is considered as the recovery ratio in the event of a fault in a VP, the back-up VP capacity is set so as to be sufficient to guarantee the required recovery ratio in an alternative path, in order to achieve recovery between faulty VP terminals. An alternative path may for example be considered as being the shortest path within the limits of the given VCH interconnection network topology. The working VP ratio is defined as: total working VP capacity total working VP capacity + total back - up VP capacity The overall network resource efficiency is defined as: working VP ratio × VP use efficiency This expresses the state of utilization of network resources. It is decided whether this network resource efficiency is maximum, and if it is not maximum, the trial VCH interconnection network topology is changed and the same process is repeated. If the network resource efficiency is maximum, the given VCH interconnection network topology becomes the optimum network topology for that media. Optimization of a VCH interconnection network for a new media is begun and the same process is performed until optimization of VCH interconnection networks is completed for all media. Thus, because a different VCH interconnection network for each media is optimized, the VCH interconnection networks are not all designed to suit the media that requires the highest quality (reliability and cell loss probability), and excess network resources are not given to media that do not require such high quality. Moreover, network resource efficiency is calculated not just from the working VP ratio, but also from the VP use efficiency as calculated from the traffic type (whether in the form of bursts or not), traffic demand (VC number) and required cell loss probability of VCs established between subscribers utilizing these VCH interconnection networks. As a result, VCH interconnection networks can be optimized in correspondence with required quality, traffic type and traffic demand, which differ for different media. FIG. 12 and FIG.13 show examples of how a trial VCH interconnection network topology is formed (S1) so that network resource efficiency can be calculated. FIG.14(a)-(d) shows trial network topologies when the number of disjoint routes D has been set as a parameter. The number of disjoint routes D between VCHs is set as a parameter when the initial values for a trial VCH interconnection network topology are given or when a trial VCH interconnection network topology is changed. The number of disjoint routes between VCHs is the number of paths between origin and destination VCHs which do not pass through the same VCH or VP link. In the present case, the number of disjoint routes will be considered to be the same between all VCHs. In the method shown in FIG.12, a given number of disjoint routes D is set as a parameter of trial VCH interconnection network topology, and satisfying this number of disjoint routes D will be taken as a constraint. Initial values for a trial VCH interconnection network topology are formed so that the total length of all the VP links is minimized. The network resource efficiency of the trial VCH interconnection network topology obtained is calculated using the method described above. When the trial VCH interconnection network topology is changed, the number of disjoint routes D is also changed, as shown in FIG.13. By thus setting the number of disjoint routes D as a constraint, the number of possible trial VCH interconnection network topologies is reduced, so that optimization of VCH interconnection networks can be speeded up. FIG.15 and FIG.16 show results of optimizations of VCH interconnection network topology in which the number of disjoint routes D is set as a parameter of VCH interconnection network topology, and network resource efficiency is maximized. FIG.15 shows the optimum number of disjoint routes for guaranteeing a 100% recovery ratio, while FIG.16 shows the optimum number of disjoint routes for guaranteeing a 30% recovery ratio. It was assumed in both cases that VC mean rate is 150 kbps, burst length is 4 kbyte, and cell loss probability is 10 -6 . The horizontal axis represents traffic demand T, while the vertical axis represents peak ratio = VC peak rate/VC mean rate. The number and arrangement of VCHs was as in the network models illustrated in FIG.14(a)-(d). Traffic demand T was defined as the number of VCs required between origin and destination VCHs, and it was assumed that the number of VCs required was the same between all origin and destination VCHs. As has been explained above, by optimizing a different VCH interconnection network for each media, the VCH interconnection networks are not all designed to suit the media that requires the highest quality (reliability and cell loss probability), and excess network resources are not given to media that do not require such high quality. Moreover, network resource efficiency is calculated not just from the working VP ratio, but also from the VP use efficiency as calculated from the traffic type, traffic demand and required cell loss probability of VCs established between subscribers utilizing these VCH interconnection networks. As a result, VCH interconnection networks can be optimized in correspondence with required quality, traffic type and traffic demand, which differ for different media. In addition, by setting the number of disjoint routes D as a constraint, the number of possible trial VCH interconnection network topologies is reduced, so that optimization of VCH interconnection networks can be speeded up. Thus far, explanations have been given of methods for maximizing network resource efficiency in terms of VPs. As opposed to this, it is also feasible to design VCH interconnection networks so as to minimize the network cost of all elements that are required for establishing VCH interconnection networks, i.e., of all elements, including VCHs and VCH interconnections, that are used in establishing individual VCs. Methods for accomplishing this will now be explained. FIG. 17 shows the overall flow of a method for establishing a plurality of VCH interconnection networks that minimize network cost. FIG.18 shows the design flow of each VCH interconnection network topology. FIG.19 shows the design flow of a VCH interconnection network topology for one given media. A method for designing VCH interconnection network topologies for a plurality of media will be explained with reference to FIG.17. First of all, the capacity C (a vector quantity) of the ATM physical network is input to operation system OpS. This capacity C includes information regarding the capacity of each VCH and each VPH, the capacity of each transmission link, and so forth. When there are M media, the characteristics of each of these M media are input to the OpS. Specifically, items thus input include the required quality (call blocking probability, cell loss probability, delay), traffic characteristics (VBR or CBR), traffic demand (geographical distribution of traffic), and so forth. Next, the order of the media for which VCH interconnection network topologies are to be designed is determined. This order can be determined randomly. Alternatively, it can be determined in the probable order of decreasing proportion of overall network cost accounted for by the media in question. A media which comes m th in this order will hereinafter be termed "media m". A VCH interconnection network topology is then designed for each media in accordance with the determined order (S2). This design procedure will be explained with reference to FIG.18. First of all, m is initialized to "1". Next, setting capacity C of the physical network and the quality required for media m as constraints, a VCH interconnection network topology for media m is designed so that network cost of media m is minimized (S3). When a VCH interconnection network topology for media m is designed, capacity C m (a vector quantity) exhibited by media m in the ATM physical network can be obtained. This capacity C m includes the capacity of transmission links, VCHs and VPHs, etc. occupied by media m. Next, C m is subtracted from capacity C of the physical network and this is taken as the new C. This altered C becomes a constraint of the physical network when a VCH interconnection network topology is designed for the next media. After m has been increased by 1, if m is less than or equal to the number of media M, that VCH interconnection network topology is designed. The remaining capacity C of the physical network (i.e., excluding use of previously established media) is set as a constraint, and a VCH interconnection network topology is designed for the next media m in the order. If this processing is repeated until m becomes larger than M, the design of all the VCH interconnection network topologies will be finished. This means that VCH interconnection network topologies have been respectively obtained for all the M media, as shown in FIG.17. VCH interconnection networks are then established on the VP network in accordance with these topologies. An example of a method for designing a VCH interconnection network topology for media m (S3) will be explained with reference to FIG.19. First of all, the initial values for the trial VCH interconnection network topology A m for media m are input. A VCH interconnection network topology can be expressed, for example, by an adjacency matrix A. A is an N×N matrix, where N is the number of VCHs. a ij , the (i,j) element of A, is defined as a ij =1 if there is a VP connection from VCHi to VCHj, and as a ij =0 if there is no VP connection from VCHi to VCHj. Adjacency matrix A is generally taken as a variable when a VCH interconnection network is established, and the VCH interconnection network topology is determined on this basis. Z min is initialized as infinity. Next, VP routes and capacities and VC routes that will guarantee the quality required of media m (e.g., call blocking probability, cell loss probability, and delay) and result in lower network cost are obtained, and network cost Z for media m is calculated. It will be assumed that a VP is established only when a VP connection between VCHs has been established in the aforementioned VCH interconnection network topology A m (i.e., only when a ij =1). It is also assumed that the system in use is the one where VCs belonging to different media are accommodated in separate VPs. Accordingly, when considering the accommodation of VCs pertaining to a given media in a VP, no account has to be taken of VCs of other media accommodated in the same VP. There may be one or more than one VP between VCHs. The VC routes are selected within the limits of the VP connections represented by the given VCH interconnection network topology. The network cost Z for media m may comprise the sum of value Z 1 , which is calculated from the quantity of facilities used in the ATM physical network for media m, and a penalty function Z 2 . In other words, Z=Z 1 +Z 2 . Specifically, the "quantity of facilities used in the ATM physical network for media m" means the quantity of facilities used for transmission links, VCHs and VPHs. The penalty function is formulated so that it is "0" when the constraints on physical network capacity and required quality are met, and so that its value Z 2 increases in correspondence with the degree to which these constraints are not met. Although the value of Z 2 may become larger than "0" during the process whereby the design of VCH interconnection network topology is optimized, Z 2 has to become "0" at the point in time when the optimization process is finished. Next, it is decided whether the value of Z obtained in the manner described above is SMALLER than Z min . If it is, the value of Z is substituted for Z min , and A m is substituted for A mopt . Processing then proceeds to the next step. If Z is not smaller than Z min , processing proceeds to the next step without changing the values of Z min and A mopt . In said next processing step, it is decided whether all possible VCH interconnection network topologies A m have been tried. If this is so, the DESIGN OF A VCH INTERCONNECTION NETWORK TOPOLOGY for media m is terminated and processing returns to the steps shown in FIG.18. However, if this is not so, VCH interconnection network topology A m is changed and the processing described above is repeated. By thus designing VCH interconnection network topologies after determining the order of the media, VCH interconnection networks that are suited to the characteristics of the media can be established and efficient use can be made of network resources. FIG.20 is a flowchart showing a method for designing a VCH interconnection network topology for each media after setting a design priority order for the VCH interconnection networks in the order of decreasing network cost. The processing from input of ATM physical network capacity C to obtaining a VCH interconnection network topology for each media is the same as in the method illustrated in FIG.17. At this point in time, the network cost of each VCH interconnection network and the overall network cost are known. The media are therefore arranged in the order of decreasing proportion of overall network cost accounted for by the VCH interconnection network cost of each media. It is then decided whether the order has changed as a result of this rearrangement. If it has, the design of a VCH interconnection network topology for each media is repeated. If the order has not changed, this means that the order of the media has converged and that the desired VCH interconnection network topology for each media has been obtained. Because the method illustrated in FIG.20 thus arranges the design order in the order of decreasing proportion of overall network cost accounted for by the VCH interconnection network cost of each media, and designs a VCH interconnection network topology for each media in accordance with this order, it can give priority to establishing VCH interconnection networks for media that have a greater effect on overall network cost. This method can therefore reduce overall network cost and establish VCH interconnection networks that are suited to media characteristics, thereby making efficient use of network resources. FIG.21 is a flowchart showing another method for designing VCH interconnection network topologies for each media. This method differs from the method shown in FIG.20 in that a limit is imposed on the number of times the order can be changed. Namely, the number of times that VCH interconnection network topologies are designed in a particular order is restricted beforehand to K. After an initial order has been set, a count is initialized to k=1, whereupon the value of k is increased by 1 each time the order is rearranged. It is then decided whether K≥k. If this is so, the processing shifts to the decision regarding whether the order has changed. If this is not so, it is decided that the design of a VCH interconnection network topology for each media is finished and processing is terminated. The number of times that the design of a VCH interconnection network topology for each media is repeated is thus predetermined, and even if the order has changed, if this number of times is exceeded, it is decided that the design of a VCH interconnection network topology for each media is finished, whereupon processing is terminated. Because a restriction is thus imposed on the calculation time spent on designing the VCH interconnection network topologies, the communication network can be established more efficiently. FIG.22 shows a method for designing a VCH interconnection network topology for media m which differs from the design method illustrated in FIG.19. According to this method, an optimal solution for a VCH interconnection network is found using I trial VCH interconnection network topologies. In the subsequent explanation, the subscript will be omitted from the symbol A m which indicates the VCH interconnection network topology for the m th media. First of all, the number of repetitions i of the trial VCH interconnection network topology is set to "1" and Z opt is set to infinity. Initial values for the trial VCH interconnection network topology A for media m are input as the initial topology. VP routes and capacities and VC routes that will guarantee the quality required of media m (e.g., call blocking probability, cell loss probability, and delay) and result in lower network cost are obtained, and network cost Z for media m is calculated. The methods used for establishing VP routes and capacities and VC routes, and for calculating network cost Z, are the same as those explained with reference to FIG.19. The value of Z obtained is substituted for Z min . A VCH interconnection network topology can be expressed, for example, by an adjacency matrix A. A is an N×N matrix, where N is the number of VCHs. a ij , the (i,j) element of A, is defined as a ij =1 if there is a VP connection from VCHi to VCHj, and as a ij =0 if there is no VP connection from VCHi to VCHj. Adjacency matrix A is generally taken as a variable when a VCH interconnection network is established, and the VCH interconnection network topology is determined on this basis. By repeatedly adding and deleting VP connections between VCHs, the method illustrated in FIG.22 optimizes VCH interconnection network topology so that network cost for media m is minimized. To achieve this, an order is first of all determined for pairs of VCHs between which VP connections are to be added or deleted. This order can be determined for example by arranging the VCH pairs randomly. VCH pairs (i,j) are then selected in accordance with the order thereby determined, and the value of a ij is inverted. In other words, if a ij =1 it is changed to "0", while if a ij =0 it is changed to "1". VP routes and capacities and VC routes that will guarantee the quality required of media m (e.g., call blocking probability, cell loss probability, and delay) and result in lower network cost are obtained for the altered VCH interconnection network topology, and network cost Z for media m is calculated. It is then decided whether Z min >Z. If this is so, the value of Z is substituted for Z min . This signifies that it is possible that the VCH interconnection network topology at that point in time will become an optimal solution. If Z min is not greater than Z, the value of a ij is restored to its previous value. It is then decided whether the foregoing processing has been performed for all VCH pairs. If it has, processing proceeds to the next step. If it has not, the processing returns to the calculation of network cost Z, and the same processing is performed on the next VCH pair (i,j) in the order. When the foregoing processing has been performed on all VCH pairs, it is decided whether the VCH interconnection network topology has converged. Convergence of a VCH interconnection network topology means that there is no change, at the point in time of this convergence decision, in the previous VCH interconnection network topology for which the order of the VCH pairs is determined. If it is decided that the VCH interconnection network topology has not converged, processing returns to determining the order of VCH pairs between which VP connections will be added or deleted. If it is decided that the VCH interconnection network topology has converged, it is then decided whether Z opt >Z min . If this is so, Z opt and A opt are updated with the values of Z min and A, respectively. The value of i is increased by 1 and it is decided whether I≥i. If this is so, processing returns to CALCULATING NETWORK COST. If it is not so, this processing (S3) is terminated. By thus adding or deleting VP connections for I trial VCH interconnection network topologies, the method illustrated in FIG.22 enables an optimal solution for VCH interconnection network topology to be obtained efficiently. FIG.23 shows still another method for designing VCH interconnection network topologies. This method differs from the method illustrated in FIG.22 in that it performs a decision and related processing regarding a delay condition. Namely, after network cost Z has been calculated, it is decided whether a delay condition is satisfied. In the method shown in FIG.19 or the method shown in FIG. 22, failure to meet the delay conditions was reflected in the value of a penalty function Z 2 . As opposed to this, in the method shown in FIG 23, in addition to this, delay is reduced by establishing a VP between VCHs which are not directly connected by a VP. In other words, when the delay condition is not satisfied, it is decided whether a ij =1. If a ij =1, the value Z min is updated to Z, irrespective of the value of the network cost. If a ij is not equal to 1, the value of a ij is restored and a ij is set to 1. a ij =1 signifies that a VP has been established between VCHi and VCHj, and because there is no intermediary VCH, delay can be reduced. By thus incorporating a decision and related processing regarding a delay condition into the algorithm for optimizing VCH interconnection network topology, an optimal solution for VCH interconnection network topology can be obtained efficiently. Explanations have been given, with reference to FIG.17-FIG.23, of methods for designing a plurality of VCH interconnection network topologies in an order that will minimize network cost. As opposed to this, it is also feasible to design a plurality of VCH interconnection network topologies simultaneously. Such methods will be explained below. FIG.24 shows a method for designing VCH interconnection network topologies simultaneously for a plurality of media. In this method, capacity C (a vector quantity) of the ATM physical network is first of all input to operation system OpS. This capacity C includes information regarding the capacity of each VCH, the capacity of each transmission link, and so forth. When there are M media, the characteristics of each of these M media are input to the OpS. Specifically, items thus input include the required quality (call blocking probability, cell loss probability, delay), traffic characteristics (VBR or CBR), traffic demand (geographical distribution of traffic), and so forth. Next, rather than designing the VCH interconnection network topologies for each of the M media in a particular order, they are designed simultaneously. An example of this design method will be explained with reference to FIG.25. First of all, the number of repetitions i of the trial VCH interconnection network topology is set to "1", and Z opt is set to infinity. For all m (m=1, 2, ..., M), initial values for the trial VCH interconnection network topology A for media m are input as the initial topology. Next, for all m, VP routes and capacities and VC routes are obtained that will guarantee the quality required of media m (e.g., call blocking probability, cell loss probability, and delay) and result in a smaller sum total of the VCH interconnection network costs arising from each media, i.e., in lower overall network cost. Overall network cost Z is then calculated. VP routes and capacities and VC routes, and overall network cost Z, are obtained by applying the methods explained with reference to FIG.19 to all media in succession. THE VALUE OF Z OBTAINED IS SUBSTITUTED FOR Z min . A VCH interconnection network topology can be expressed by an N×N matrix (where N is the number of VCHs). In the present case, a VCH interconnection network topology for media m will be represented by adjacency matrix Am. am ij , the (i,j) element of Am, is defined as am ij =1 if there is a VP connection from VCHi to VCHj, and as am ij =0 if there is no VP connection from VCHi to VCHj. By repeatedly adding and deleting VP connections between VCHs in VC networks for all media, the method illustrated in FIG.25 optimizes VCH interconnection network topologies and establishes all VCH interconnection networks so that the overall network cost resulting from all media is minimized. After the calculation of network cost Z, the order of the VCH pairs between which VP connections will be added and deleted is determined for all media. An example of the method for determining this order will be explained with reference to FIG.26 and FIG.27. First of all, as Procedure 1, the order of VCH pairs is determined for each media. An example for the case where there are three media is shown in FIG.27. Next, as Procedure 2, the order in which the media are distributed is determined. In the example given in FIG. 27, this order is: media 2, media 1, media 3, media 2, ... . WHEN THERE ARE THREE MEDIA, THE ORDER OF THE FIRST THREE VCH PAIRS MAY BE DETERMINED BY SELECTING FROM THE MEDIA IN RANDOM ORDER. THIS RANDOM ORDER IS THEN REPEATED EVERY THREE PAIRS. Next, as Procedure 3, VCH pairs am ij are assigned in accordance with the order determined by Procedures 1 and 2. An example of this is shown in FIG.27. VCH pairs am ij are selected in turn in accordance with the order determined in Procedure 3, and the value of am ij is inverted. In other words, if am ij =1, it is changed to "0", and if am ij =0, it is changed to "1". VP routes and capacities and VC routes that will guarantee the quality required of media m (e.g., call blocking probability, cell loss probability, and delay) and result in lower network cost are obtained for the altered VCH interconnection network topology for media m; a VCH interconnection network configuration is obtained for media m; and overall network cost Z is calculated. At this point, VCH interconnection networks for media other than media m do not change. It is then decided whether Z min >Z. If this is so, the value of Z is substituted for Z min . This signifies that it is possible that the VCH interconnection network topology at that point in time will become an optimal solution. If Z min is not greater than Z, the value of am ij is restored to its value prior to the change. It is then decided whether the foregoing processing has been performed for VCH pairs of all the media. If it has, processing proceeds to the next step. If it has not, the same processing is performed on the next VCH pair (i,j) of media m in the sequence. Next, for all m, it is decided whether the VCH interconnection network topology Am for media m has converged. Convergence of a VCH interconnection network topology means that there is no change, at the point in time of this decision, in VCH interconnection network topologies Am (for all m, where m=1, 2, ..., M) prior to starting processing step S5 for determining the order of the VCH pairs. If it is decided that there has been convergence, the processing proceeds to the next step. If it is decided that convergence has not occurred, the processing returns to determining the order of VCH pairs between which VP connections will be added or deleted, and the same processing as described above is repeated. In the subsequent processing step, it is decided whether Z opt >Z min . If this is so, Z opt and Am opt are updated with the values of Z min and Am, respectively. The value of i is increased by 1 and it is decided whether I≥i for a predetermined I. If this is so, processing returns to the calculation of network cost Z and the operations described above are repeated. In the present case, I is the number of times that a trial VCH interconnection network topology is formed. If it is decided that I is not greater than or equal to i, this processing (S4) is terminated. As a result of this processing, a VCH interconnection network for each media can be established for all the media. Because a VCH interconnection network topology is designed simultaneously for each media, VCH interconnection networks suited to the characteristics of the respective media can be established while taking into consideration the effects which the respective VCH interconnection networks have on each other. As a result, efficient use can be made of network resources. FIG.28 and FIG.29 show a method for establishing VCH interconnection networks by combining the method for designing VCH interconnection network topologies for respective media in a particular order with the method for designing them simultaneously. FIG.28 shows the processing flow, while FIG.29 explains the design method. In this method, the capacity C (a vector quantity) of the ATM physical network is first of all input to operation system OpS. This capacity C includes information regarding the capacity of each VCH and each VPH, the capacity of each transmission link, and so forth. When there are M media, the characteristics of each of these M media are input to the OpS. Specifically, items thus input include the required quality (call blocking probability, cell loss probability, delay), traffic characteristics (VBR or CBR), traffic demand (geographical distribution of traffic), and so forth. Next, M' media are selected from the M media. Because priority is given to the design of VCH interconnection network topologies for the M' media, the selection of the M' media may involve, by way of example, selecting media which may have a large influence on network cost. For the M' media that are selected, the media order in which networks are to be established is determined, a VCH interconnection network topology is designed for each media in this order, and VCH interconnection networks are established. After this, respective VCH interconnection network topologies for the remaining M-M' media are designed. At this point, the capacity of the given physical network is the remaining capacity of the part occupied by the VCH interconnection networks of the M media which have been established as described above. When this processing is completed, a VCH interconnection network for each media is established for the remaining M-M' media. By thus establishing VCH interconnection networks by combining the method for designing VCH interconnection network topologies for respective media in a particular order with the method for designing a plurality of VCH interconnection network topologies simultaneously, media with a high priority ranking can obtain preferential assignment of network resources, while the remaining media can obtain efficient assignment of network resources that involves taking into consideration the mutual influence of each VCH interconnection network. FIG.30 shows another method for establishing VCH interconnection networks by combining the method for designing VCH interconnection network topologies for respective media in a particular order with the method for designing a plurality of VCH interconnection network topologies simultaneously. This method differs from the method illustrated in FIG.28 and FIG. 29 in that M' media for preferential network establishment are selected from the M media, and VCH interconnection network topologies are designed for these M' media in the order of decreasing proportion of overall network cost accounted for by the VCH interconnection network cost of a given media. In other words, this method differs from the method illustrated in FIG.28 and FIG.29 in that the method illustrated in FIG.20 is utilized. In this method, media with a high priority ranking can obtain preferential assignment of network resources, while the remaining media can obtain efficient assignment of network resources that involves taking into consideration the mutual influence of each VCH interconnection network. In particular, because the media are arranged in the order of decreasing proportion of overall network cost accounted for by their respective VCH interconnection networks, and because VCH interconnection network topologies for the respective media are designed in accordance with this order, media which have a large influence on overall network cost are established first. Accordingly, overall network cost can be reduced; VCH interconnection networks suited to the characteristics of the respective media can be established; and efficient use can be made of network resources. In FIG.31, in similar manner to the method illustrated in FIG.28 and FIG.29 and to the method illustrated in FIG.30, VCH interconnection networks are established by combining the method for designing VCH interconnection network topologies for respective media in a particular order with the method for designing a plurality of VCH interconnection network topologies simultaneously. However, the method shown in FIG.31 differs in that the number of times a VCH interconnection network topology is designed for each media is set in advance: in other words, it differs in that the method illustrated in FIG.21 is utilized. Namely, when preferentially establishing networks for M' media, the number of times a VCH interconnection network topology is designed for each media is set in advance, and even though the order has changed, if it seems that this number of times will be exceeded, it is decided that a VCH interconnection network topology has been designed for each of the M' media, and the processing shifts to the method for designing the remaining M-M' VCH interconnection network topologies simultaneously. Accordingly, because the time spent in calculating and thereby establishing a VCH interconnection network is restricted, communication networks can be established efficiently. As has now been explained, this invention establishes a different VCH interconnection network for each media and establishes VC networks on the basis of these VCH interconnection networks. Consequently, the VC network is not designed to guarantee the highest quality (call blocking probability, cell loss probability and delay) required, and excessive network resources are not given to media which do not require such high quality. In addition, network cost can be reduced by setting the capacity of the physical network (i.e., the capacity of transmission links, VCHs and VPHs) and the qualities required for a plurality of media as constraints, and by establishing a respective VCH interconnection network for each media on the basis of the media characteristics (traffic type, traffic demand) that have been input, so that overall network cost is minimized when the VCH interconnection network topologies constituted by the plurality of media are combined. As has been explained above, this invention establishes a different VCH interconnection network for each media in correspondence with the quality required by the media or the media traffic characteristics. It can therefore utilize this to establish VC networks that connect subscribers. Moreover, when a plurality of media coexist in a communication network, it can design the network on the basis of minimum network cost. The constituent elements of the communication network can therefore be utilized effectively. Furthermore, communication networks in which a plurality of media coexist can be designed quickly by means of this invention.

Claims: Communication network design method for use with an asynchronous transfer mode (ATM) communication network, which: physically connects, by means of a transmission link network, a plurality of virtual channel handlers (VCHs) (VCH1...VCH5) which perform switching in units of virtual channels (VCs), and a virtual path handler (VPH) (VPH1..VPH3) which changes over information transfer routes in units of virtual paths (VPs) (VP1-1...VP1-4,...,VP3-1...VP3-10) that accommodate a plurality of VCs; and establishes a VP network on this transmission link network, said VP network being formed from VPs which logically connect the aforementioned plurality of VCHs (VCH1...VCH5); and also establishes a VC network on this VP network, said VC network being formed from VCs which logically connect subscribers; said communication network design method being characterised in that : a different VCH interconnection network for each media or service consisting of information to be transferred is established on the VP network; and a VC network corresponding to a given media or service is formed by means of each VCH interconnection network, whereby a plurality of independent VC networks for different media or services are established simultaneously on a single physical network; wherein the VCH interconnection networks are formed on the ATM communication network. Communication network design method as set forth in Claim 1, wherein it is ensured that, for the aforementioned plurality of VCHs (VCH1...VCH5), a VP (VP3-1...VP3-10) which does not pass through any other VCH (VCH1...VCH5) can be established between any two given VCHs (VCH1...VCH5). Communication network design method as set forth in Claims 1 or 2, wherein a VCH interconnection network is established by assigning different VPs (VP1-1...VP1-4,...,VP3-1...VP3-10) to each media or service consisting of information to be transferred. Communication network design method as set forth in Claims 1 or 2, wherein a VCH interconnection network is established by allowing a plurality of media or services with the same destination VCH to be assigned a single VP (VP1-1...VP1-4,...,VP3-1...VP3-10). Communication network design method as set forth in Claim 4, wherein priority control is performed between VCs with the same destination in accordance with a priority ranking of the media or service consisting of information to be transferred. Communication network design method as set forth in any of Claims 1 to 5, wherein the network topologies of a plurality of VCH interconnection networks for different kinds of media or service are designed simultaneously after taking their overall optimization into consideration. Communication network design method as set forth in Claim 6, wherein an optimized network topology is obtained by calculating the network resource efficiency from the working VP ratio, which is defined as the ratio of working VP capacity to total capacity of working and back-up VPs (VP1-1...VP1-4,...,VP3-1...VP3-10), and from the VP use efficiency, which has been calculated from the traffic type, traffic demand and required cell loss probability of VCs. Communication network design method as set forth in Claim 7, wherein the number of disjoint routes D (i.e., the number of paths between origin and destination VCHs which do not pass through the same VCHs or VP links) is set as a parameter of trial network topology; and satisfying the given number of disjoint routes D is set as a constraint; and a trial network topology such that the total length of all the VP links is minimized is formed; and the aforementioned number of disjoint routes D is changed when the trial network topology is changed; and network resource efficiency is calculated for the trial network topology obtained. Communication network design method as set forth in Claim 7, wherein network topology is designed so that the overall communication network cost is minimized. Communication network design method as set forth in Claim 9, wherein connection or disconnection of VP connections between VCHs (VCH1...VCH5) for establishing VCH interconnection network is performed in parallel with establishing VCH interconnection networks for other media or services. Communication network design method as set forth in any of Claims 1 to 5, wherein the network topologies of a plurality of VCH interconnection networks for different kinds of media or service are designed in a particular order. Communication network design method as set forth in Claim 11, wherein the design priority order of the topologies of a network is set in the order of decreasing network cost of the media or services supported by its VCH interconnection networks. Communication network design method characterised in that : network topologies are designed in accordance with the communication network design method set force in Claims 11 or 12, the proportion of overall network cost accounted for by the network cost of the VCH interconnection network for each media or service is calculated; the design priority order is set again in accordance with the results of this calculation; and if this design priority order is different from the previous time, the design of network topologies is repeated in accordance with the new design priority order. Communication network design method as set forth in Claim 13, wherein a limit is set for the number of repetitions. Communication network design method as set forth in any of Claims 9 to 14, wherein network topologies are designed after setting an upper limit for the number of VPs (VP1-1...VP1-4,...,VP3-1...VP3-10) passed through from an origin VCH (VCH1...VCH5) to a destination VCH (VCH1...VCH5). Communication network design method as set forth in any of Claims 1 to 5, wherein: media or services are classified into a priority group and a non-priority group according to their network cost; the design of VCH interconnection network topologies for the priority group is performed in the order of decreasing network cost; the design of VCH interconnection network topologies for the non-priority group is performed simultaneously in such manner that overall network cost is minimized. Communication system for an asynchronous transfer mode (ATM) communication network, which has: a plurality of virtual channel handlers (VCHs) (VCH1...VCH5) which perform switching in units of virtual channels (VCs); a virtual path handler (VPH) (VPH1...VPH3) which changes over routes in units of virtual paths (VPs) (VP1-1...VP1-4,...,VP3-1...VP3-10) which accommodate a plurality of VCs; a transmission link network which physically connects the aforementioned plurality of VCHs (VCH1...VCH5) and the VPH (VPH1...VPH3); and a higher order means which establishes a VP network on this transmission link network said VP network being formed from VPs (VP1-1...VP1-4,...,VP3-1...VP3-10) which logically connect the aforementioned plurality of VCHs (VCH1...VCH5); and which also establishes a VC network on this VP network, said VC network being formed from VCs which logically connect subscribers; said communication system being characterised in that the aforementioned higher order means includes: a VCH interconnection network establishing means which establishes, on the VP network, a different VCH interconnection network for each media or service consisting of information to be transferred; and a means which forms VC networks corresponding to these media or services by means of these respective VCH interconnection networks, whereby a plurality of VC networks for different media or services are established simultaneously on a single physical network, wherein the VCH interconnection networks are formed on the ATM communication network.

Inventors: OKI EIJI, YAMANAKA NAOAKI, OKI, EIJI, YAMANAKA, NAOAKI
Applicants: NIPPON TELEGRAPH & TELEPHONE, NIPPON TELEGRAPH AND TELEPHONE CORPORATION
Application year: 1995
Main classification: H04Q11
IPC codes: H04L12, H04Q11