RU2127489C1 - Method for building multiple-ring hierarchical network around fiber-optic communication lines and base network for implementing it - Google Patents

Abstract

FIELD: communications engineering, ring-type channel-multiplexing digital data transmission networks. SUBSTANCE: method involves routing multiconductor main cable made of optical fibers; organizing ring of first hierarchy level from multiconductor cable; organizing at least one ring of second hierarchy level from multiconductor cable; connecting ring of first hierarchy level to base station which is coupled with communications center; connecting ring of first hierarchy level to that of second level through first-level switching station. In addition, second ring of first hierarchy level is organized around other spatial area and connected to second base station coupled with communications center. Both base stations are arranged in vicinity of geometric center of network. In addition, at least one ring of second hierarchy level is organized and connected to second ring of first hierarchy level through its respective first-level switching station. In organizing ring of second hierarchy level, main cable is built up of greater number of optical fibers than that required for connecting switching station by at least two optical fibers. Network extension is made by organizing at least one additional ring of first hierarchy level connected to its respective third base station coupled with communications center. In organizing additional ring of first hierarchy level, use is made of optical fibers of main cable sections routed earlier for organizing ring of second hierarchy level. EFFECT: reduced capital outlays for organizing new service areas and provision for gradual extension of entire network without raising data transmission speed in rings of first hierarchy level. 9 cl, 4 dwg

Description

 The invention relates to communication technology and can be used in ring-type networks for transmitting digital information with channel multiplexing.

 A known system for transmitting blocks of information along interconnected closed routes, containing a central station connected to a ring of the first hierarchy level, designed to transmit information at a higher speed, rings of the second hierarchy level, designed to transmit information at a lower speed than in the ring of the first hierarchy level, connected with a ring of the first level of the hierarchy through switching stations of the first level, rings of the third level of the hierarchy, designed to transmit information with a lower speed than in the rings of the second level of the hierarchy connected to the corresponding ring of the second level of the hierarchy through switching stations of the second level, the subscriber equipment connected to the ring of the third level of the hierarchy through the equipment of subscriber connection [1].

 In this technical solution, several stations have access to each ring for inputting and outputting messages from information blocks of standard sizes transmitted along the ring route. Various ring routes are interconnected by switching stations, which, depending on the address information at the beginning of each information block, selectively switch this block to an interconnected ring.

 The advantage of such multi-ring systems is the ability to provide bypass and backup routes, due to which high reliability of information transmission is achieved.

 A limitation of such a system is the complexity of its reconfiguration. So, the connection of new subscribers is carried out due to the additional build-up of inter-office connections of rings of the lower hierarchy and the installation of additional switching stations. If it is necessary to increase the speed of information transfer in any of the hierarchical rings, it is necessary to replace the station equipment with more productive one. If it is necessary to increase the speed of information transfer in general, the system requires replacement of equipment in each of the hierarchical rings. In addition, the equipment of this system is not designed to transmit information via fiber-optic communication lines, which dramatically reduces the speed and quality of information transfer.

 There is also a standard called the optical interface to distributed data (OIRD), which provides for a combination of two rings made on the basis of fiber-optic channels [2].

 This standard describes a system designed for combining smaller networks within enterprises, connecting workstations, graphic devices, personal computer networks, laser printers, etc., and the secondary ring is used to reconfigure the system in case of failure of the primary ring elements. The secondary ring is laid along the same route and serves to double the overall system performance and increase the reliability of its operation.

 When reconfiguring the system or increasing the speed of its functioning, as well as in the previous technical solution, a complete replacement of station equipment is required.

 The closest is a method of constructing a multi-ring hierarchical network on fiber-optic communication lines (FOCL), including laying a trunk cable (MK) made of multicore optical fibers, forming from the trunk cable a ring of the first hierarchy level with a higher information transfer rate and covering the spatial domain, forming from the trunk cable at least one ring of the second level of the hierarchy with a lower information transfer rate than in the ring of the first level archaea, connecting the ring of the first level to the central station (CA), which is connected with the central communication center (CSC), connecting the ring of the second level of the hierarchy to the ring of the first level of the hierarchy through the switching station (CS) of the first level, building up the network [3].

 In this method, the network is also expanded by forming additional rings of the lower hierarchy, i.e. with a predetermined information transfer rate and coverage by such rings of new areas of subscription service, therefore, the above limitations are inherent in it.

 The main building of the network in this method is carried out by forming rings of the third level of the hierarchy with a lower information transfer rate than for rings of the second level of the hierarchy, while the rings of the third level are connected to the rings of the second level through switching stations of the second level.

 If it is necessary to increase the speed of information transfer in certain areas of the functioning of the system or in general, it is necessary to replace station equipment with the corresponding ring hierarchy in terms of information transfer speed.

 The closest device to implement the method is a core network containing a central communication center (CSC), a central station (CA), at least one switching station (CS) of the first level, a ring of the first level of the hierarchy, intended for transmission of information with a higher speed, made of a trunk cable (MK), multi-core of optical fibers, and connected to a central station (CS) and a switching station (CS) of the first level, at least one ring of the second level I hierarchy, designed to transmit information at a lower speed than for the ring of the first level of the hierarchy, made of a trunk cable (MK), multi-strand of optical fibers, and connected to the ring of the first level of the hierarchy through a switching station (CS) of the first level [3].

 In this technical solution, only one ring of the first level of the hierarchy is formed. The network is configured by connecting through the CS to the ring of the first level of the hierarchy of rings of the lower level of the hierarchy, i.e. with a lower information transfer rate, and multi-ring hierarchical structure cover the area of subscription service. If it is necessary to commission a new area of subscriber services, the corresponding ring of the lower hierarchy is connected through the SC.

 The limitations of such a system are the complexity and high costs of reconfiguring it if it is necessary to introduce large areas of subscription service into the system, as well as if it is necessary to increase the speed of information transfer in certain areas of subscription service or in the system as a whole. A large expansion of subscriber service areas necessitates a sharp increase in the speed of information transfer along hierarchical rings, where CSs of the first hierarchy level should provide the maximum information transfer speed in the system.

 The problem solved by the invention is to increase the efficiency and reliability of the system, as well as expanding the arsenal of funds and reducing the cost of its reconfiguration.

 The technical result that can be obtained by implementing the method is to reduce capital costs for connecting new areas of subscription service and to ensure smooth growth of the network as a whole without increasing the speed of information transfer in the rings of the first hierarchy level.

 The technical result that can be obtained by performing the basic system for implementing the method is to increase the efficiency of using network resources.

 To solve the problem with achieving a technical result in the known method of constructing a multi-ring hierarchical network at a fiber optic link, including laying an MK made of multicore optical fibers, forming from the MK a ring of the first level of the hierarchy with a higher information transfer rate and covering the spatial domain, forming from MK at least at least one ring of the second level of the hierarchy with a lower information transfer rate than in the ring of the first level of the hierarchy, connecting the ring of the first level I am connected to the central office, which is connected with the central control center, connecting the ring of the second hierarchy level to the ring of the first hierarchy level through the first level CS, building up the network, according to the invention, when laying MK, additionally form the second ring of the first hierarchy level, which covers another spatial area and which is connected to the second CA connected with the central control center, both central centers are located in the vicinity of the geometric center of the network, additionally form at least one second ring of the second hierarchy level, which is connected to the second ring At the first level of the hierarchy, through its first level CS, when forming rings of the second hierarchy level, the number of optical fibers in the MC is selected to be larger than necessary for connecting the first level CS to at least two optical fibers, and the network is built up by forming at least one an additional ring of the first level of the hierarchy, which is connected to its third CA associated with the NCC, while the formation of the additional ring of the first level of the hierarchy uses the optical fibers of the sections paved MK, used to form the rings of the second level of the hierarchy.

There are additional options for implementing the method, in which it is advisable that:
- the third CA of the additional ring of the first level of the hierarchy was located in the vicinity of the geometric center of the network;
- additional network was built up by forming rings of the third level of the hierarchy with a lower information transfer rate than for rings of the second level of the hierarchy, while the rings of the third level were connected to the rings of the second level through the CS of the second level;
- after building up the network with an additional ring of the first level of the hierarchy, when the rings of the third level of the hierarchy are formed, the released sections of the MK ring of the second level of the hierarchy are used;
- the ring of the second level of the hierarchy was connected to the additional ring of the first level of the hierarchy using the first level CS, which was released after the network was built up;
- when forming rings of the first level of the hierarchy from MK, the number of optical fibers in the trunk cable was selected to be at least two optical fibers greater than necessary for connecting the first level KS, and when forming an additional ring of the first level of the hierarchy, optical fibers of sections of previously laid MKs were used, used for previously formed rings of the first level of the hierarchy;
- the building was additionally carried out by expanding one of the rings of the first hierarchy level to cover a larger spatial area, for which optical fibers of sections of previously laid MK were used to form rings of the first and second hierarchy levels;
- after the expansion of one of the rings of the first level of the hierarchy, the released sections of the MK of the previously formed ring of the second level of the hierarchy were used to form the rings of the second level of the hierarchy.

 To solve this problem in a well-known core network, containing a central control center, a central nervous system, at least one first-level CS connected to it, a ring of the first hierarchy level, designed to transmit information at a higher speed, made of MK, multi-strand of optical fibers, and connected with a first level CS and CS, at least one ring of the second level of the hierarchy, designed to transmit information at a lower speed than for the ring of the first level of the hierarchy, made of MK, multi-strand of optical fibers, and connected e with a ring of the first hierarchy level through the first level CS, according to the invention, a second ring of the first hierarchy level with a higher information transfer rate in the network is introduced, a second CA connected to the second ring of the first hierarchy level and connected to the central control center, at least one ring of the second hierarchy level , A first level CS, while the ring of the second hierarchy level is connected to the second ring of the first hierarchy level through the entered first level CS, the ring of the first hierarchy level and the second ring of the first hierarchy level s covering different spatial regions, both of these CAs are installed in the vicinity of the geometric center of the network and the number of optical fibers in the MC to the second level of the hierarchy of rings is chosen larger than necessary for connecting the COP of the first level, not less than two optical fibers.

 Due to the formation of an additional ring of the first level of the hierarchy with its own CA, placement of both CAs in the vicinity of the geometric center of the network, it was possible to solve the problem with achieving a technical result.

 These advantages, as well as features of the present invention are illustrated by the best option for its implementation with reference to the accompanying drawings.

FIG. 1 depicts a core network of the claimed multi-ring hierarchical network at a fiber optic link intended for reconfiguration;
FIG. 2 is the same as FIG. 1, after reconfiguration;
FIG. 3 is the same as FIG. 2, when using sections of the trunk cable of the rings of the first hierarchy level to form an additional ring of the first hierarchy level;
FIG. 4 is the same as FIG. 2, with additional growth and expansion of the spatial region of the ring of the first level of the hierarchy.

 Since the claimed method is implemented when configuring and modifying the core network (Fig. 1), a description of the method is given as the description of changes in the topology of the circuit.

 The multi-ring hierarchical network at the FOCL, selected as the core network for configuration (Fig. 1), contains the DSP 1 and the DSP 2 connected to the DSP 1. The system has at least one first-level DSP 3. Ring 4 of the first level of the hierarchy (Fig. 1, 2, 3, 4 is schematically shown as an ellipse) is designed to transmit information at a higher speed, made of MK, multi-strand of optical fibers, and connected to the DS 2 and CS 3 of the first level. The network has at least one ring 5 of the second hierarchy level (shown in Fig. 1, 2, 3, 4 in a rectangle), which is designed to transmit information at a lower speed than for ring 4 of the first hierarchy level. Ring 5 is made of MK, a multicore optical fiber, and is connected to ring 4 through KS 3 of the first level.

 Introduced: the second ring 6 of the first level of the hierarchy, the second CA 7 connected to the second ring 6 and connected to the central control center 1, at least one ring 5 of the second level of the hierarchy, COP 3 of the first level. Ring 5 of the second level of the hierarchy is connected to the second ring 6 of the first level of the hierarchy through the introduced COP 3 of the first level. Ring 4 of the first level of the hierarchy and second ring 6 of the first level of the hierarchy are made covering various spatial areas. Both mentioned DS 2 and DS 7 are installed in the vicinity of the geometric center of the network. The number of optical fibers in MK for rings 5 of the second level of the hierarchy is chosen to be larger than necessary for connecting KS 3 of the first level to at least two optical fibers.

 In FIG. 1, 2, 3, 4 also schematically in the form of circles shows rings 8 of the third hierarchy level for transmitting information at a lower speed than in rings 5, KS 9 of the second hierarchy level for connecting rings 8 to rings 5, subscriber connection equipment 10 and user equipment 11. Connection of subscriber equipment 11 for the claimed method is not fundamental and is carried out in a known manner.

 A feature designed to configure the system is that the number of MK optical fibers for forming rings 5 of the second level of the hierarchy is chosen to be larger than necessary for connecting existing equipment. The number k of additional optical fibers in the MK required for connecting new equipment can be defined as k ≥2 • n, where n is the number of hierarchy levels built on this ring. Thus, when constructing the network of the claimed method with n = 1, at least two additional optical fibers are required.

 If, in the known system, CA 2 is installed, for example, in the central part of the city, while ring 4, as it were, covers the central region, then in the system proposed for configuration, CA 2 and CA 7 are jointly installed in the vicinity of the geometric center of the network, while rings 4 and 6 also jointly cover the central region, but its various spatial areas.

 Changing the basic network model allows us to further increase the efficiency of using basic network resources, despite the initial increase in the cost of installing additional equipment, which is not obvious and unknown from the prior art.

 Indeed, the system (Fig. 1) can be reconfigured with a reduction in capital costs for connecting new customer service areas and with a smooth expansion of the network as a whole without increasing the level of the hierarchy of CS rings 4 and 6 of the first level of the hierarchy. To do this, when laying MK to form rings 5 of the second level of the hierarchy, the number of optical fibers in them is chosen larger than necessary for connecting the CS 3. The network is expanded (Fig. 2) by forming at least one additional ring 12 of the first level of the hierarchy, which is connected to its CA. 13. CA 13, as well as CA 2 and CA 7, are independently connected to the CCC 1, for example, fiber-optic communication lines or other means. When forming an additional ring 12, optical fibers of sections 14 of previously laid MKs used to form rings 5 of the second hierarchy level are used (in Figs. 1, 2, such sections 14 are marked in bold lines).

 Thus, the expansion of the network coverage area is carried out by increasing the number of rings of the highest hierarchy and creating new rings through the use of free fibers of existing MKs and extending, if necessary, the radial MKs to the dimensions of the new coverage zone with their closure by new segments (Fig. 2 is shown by a dashed line ) trunk cable.

 The sizes and distances between rings 4 and 6 can be selected based on the existing geometry of the subscription service area, for example, based on a city plan. A particularly large reduction in capital costs is achieved when the system is reconfigured in accordance with the claimed method in cities of a radial-ring structure, while to further reduce costs, it is advisable that when building rings 4 and 6 territorially cover the entire maximum network coverage area, and COP 3 and COP 9 included in rings 5 and 8 of the lower hierarchy, respectively, were located inside and between the rings of a higher hierarchy level.

 Also, in order to further reduce the capital costs of communication with the central control center 1, the CA 13 of the additional ring 12 of the first hierarchy level should be located in the vicinity of the geometric center of the network (Fig. 2). However, in the general case, with a specific city plan, for example, with an elongated structure of districts, CA 13 can be located independently of CA 2 and 6, since each of CA 2,6, 13 is independently connected to the center 1, which further extends the functional features when configuring the system.

 The proposed method does not interfere with the growth of the network by known means. As in the known method, additional network expansion can be done by additionally forming rings 8 of the third hierarchy level (Fig. 1, 2) with a lower information transfer rate than for rings 5 of the second hierarchy level. Rings 8 are connected to rings 5 of the second level through KS 9 of the second level.

 However, additional cost reduction and expanding the arsenal of funds for reconfiguring the system in the claimed method can be achieved by forming rings 8 of the third hierarchy level, in which the freed sections (in Fig. 2 are shown as a dashed line) of MK ring 5 of the second hierarchy level, which reconfigured.

 In addition, the claimed method allows the ring 5 of the second level of the hierarchy to be connected to the additional ring 12 of the first level of the hierarchy using a switching station KS 3 (shown in Fig. 2 with a dashed outline), released after building and reconfiguring the network. In this case, the claimed method additionally allows to reduce the cost of installing new equipment and use the existing one.

 Additionally (Fig. 3), when forming rings 4 and 6 of the first level of the hierarchy from the MK, the number of optical fibers in the MK is chosen to be larger than necessary for connecting the KS 3 of the first level to at least two optical fibers. When forming an additional ring 12 of the first level of the hierarchy, optical fibers of sections 15 (shown in bold lines in FIG. 3) of previously laid MK are used, which are used for the previously formed rings 4 and 6 of the first level of the hierarchy. In this case, two rings 4 and 12, as well as 6 and 12, are formed in sections 15 of the MK. Thus, sections 15 use the same previously laid trunk cable to form different rings, which further expands the arsenal of funds and reduces the cost of building network.

 In addition, the network is expanded (Fig. 4) by expanding one of the rings, for example, 6 of the first hierarchy level to cover a larger spatial area. To do this, use the optical fibers of sections 16 (shown in Fig. 4 by a thick line) of previously laid MKs to form rings b and 5 of the first and second levels of the hierarchy. In this case, the number of MK fibers during the formation of the ring 6 can be selected and not exceeding the value necessary for the normal functioning of KS 3, but the number of MK fibers for the formation of rings 5 must be selected to satisfy the above condition.

 After the expansion of ring 6, for the formation of new rings 5 of the second level of the hierarchy, the freed sections of the previously formed ring 5 of the second level of the hierarchy after its reconfiguration are used (Fig. 4 is shown as a dashed line). In addition, the released MK sections for ring 6 can be used.

 Rings 4, 6, 12 of the first hierarchy level, rings 5 of the second hierarchy level and rings 8 of the third hierarchy level are shown respectively in FIG. 1, 2, 3, 4 in the form of ellipses (parts of ellipses, Fig. 4), rectangles and circles only for a more understandable illustration of the invention. In practice, they have a different shape associated with the selected technological routes of the laid MK. When comparing the circuits (Fig. 1, 2, 3, 4), you can notice that the claimed method and the basic circuit for its implementation allows you to use the already laid sections of the trunk cable to create new rings of a higher hierarchy, as well as use the freed sections of the cable to create rings the same or lower hierarchy. The same principle is used, for example, to increase the third level of the hierarchy of rings 8 to the second.

 An example implementation of the method.

 The network is based on the formation of two rings 4 and 6 of the first hierarchy level with the coverage of the territory conditionally delimited, for example, north and south with switching stations 3, made, for example, on the basis of the synchronous transport module STM-4 (multiplexer). Central stations 2 and 7 are located in the vicinity of the geometric center in the future proposed for network expansion and are information exchange nodes. To reduce the cost of connecting and connecting to the central control center 1, the central control center 2 and central control center 7 should be located in the same building, for example, in the same place as the central control center 1. The central control center 1 represents a conglomerate of multiplexing and routing equipment that provides the formation and direction of information flows of various nature, for example, IP service, X.25, Frame Relay, satellite channels, telephone traffic, etc.

 The formation of two rings 4 and 6 doubles the network’s ability to transfer service flows and increases the reliability and survivability of the network by constructing its topology along independent routes. As the switching stations 9 of the second level, when used as the switching stations 3 of the first level of STM-4, the synchronous transport module STM-1 is selected.

 Taking into account future network expansion, it is advisable to use MK with an increased number of cores, for example, containing 48 optical fibers, the main part of which is designed to transmit service information flows, and at least two optical fibers are left in reserve when connecting existing equipment.

 If it is necessary to cover new areas with developing infrastructure in accordance with the claimed method, construction should be carried out on the basis of fiber optic links operated by means of technological joints on the routes by means of branching couplings and the inclusion of new KS 3 (STM-4) nodes in the ring structure. Thus, geographical expansion of the existing network topology. The construction of rings 5 of the second level of the hierarchy with KS 9 is carried out according to the traditional scheme, but using the freed sections of the MK.

 The effectiveness of the proposed method consists in saving money on the purchase of node equipment, a higher data transfer rate and reducing the involvement of labor resources for network reconfiguration, in saving money on the construction of new FOCLs, since in the future it is planned to complete the FOCL on the already built MK.

 The most successfully claimed method of constructing a multi-ring hierarchical network on fiber-optic communication lines and a core network for its implementation can be used in communication technology, mainly in synchronous and asynchronous ring-type networks for transmitting digital information with channel multiplexing. The method allows to efficiently build a network on the basis of equipment that implements Synchronous Digital Hierarchy (SDH), Asynchronous Transfer Mode (ATM), Wave Length Division Multiplexing (WDM) technology, both from the point of view of optimal organization of high-speed information transfer streams and from the point of view of economy future funds to finance the entire project.

Sources of information
1. US patent N 3731002, H 04 J 3/06, 1973.

 2. Local digital communication lines Express information. Collection "Information Transfer". USSR State Committee for Science and Technology, USSR Academy of Sciences, All-Union Institute of Scientific and Technical Information. - M., 1986, N 40, p. 5 - 8.

 3. Synchronous Transmission Systems. Nortel Limited, 1995, London, 1995, p. 40-44.

Claims (9)

1. A method of constructing a multi-ring hierarchical network on fiber-optic communication lines, including laying a trunk cable made of multicore optical fibers, forming from the trunk cable a ring of the first level of the hierarchy with a higher transmission rate of information covering the spatial domain, forming at least one ring of the second level of the hierarchy with a lower information transfer rate than in the ring of the first level of the hierarchy, connecting the ring of the first level to the central station, which is connected with the central communication node, connecting the ring of the second hierarchy level to the ring of the first hierarchy level through the switching station of the first level, network extension, characterized in that when laying the trunk cable, a second ring of the first hierarchy level is additionally formed, which covers another spatial area and which is connected to the second central station connected to the central communication center, both central stations have a geometer in the vicinity Cesky network center further forming at least one second ring of the second hierarchical level, which is connected to the second ring of the first hierarchy level via a commutation station of the first level,
when forming rings of the second level of the hierarchy, the number of optical fibers in the trunk cables is selected to be at least two optical fibers greater than necessary for connecting the switching stations of the first level, and the network is built up by forming at least one additional ring of the first level of the hierarchy, which is connected to their third central station, connected to the central communication center, while using optical Loknya of extended portions previously trunk cables, used for the formation of rings of the second hierarchy level.  2. The method according to claim 1, characterized in that the third central station of the additional ring of the first level of the hierarchy is located in the vicinity of the geometric center of the network.  3. The method according to claim 1, characterized in that the network is further expanded by forming rings of the third hierarchy level with a lower information transfer rate than for rings of the second hierarchy level, while the rings of the third level are connected to the rings of the second level through switching stations of the second level .  4. The method according to claim 3, characterized in that after building up the network with an additional ring of the first level of the hierarchy, when the rings of the third level of the hierarchy are formed, the released sections of the trunk cable of the ring of the second level of the hierarchy are used.  5. The method according to claim 1, characterized in that the ring of the second level of the hierarchy is connected to the additional ring of the first level of the hierarchy using a switching station of the first level, freed up after the network is built up.  6. The method according to claim 1, characterized in that the formation of the rings of the first level of the hierarchy from the trunk cable, the number of optical fibers in the trunk cable is selected to be larger than necessary for connecting the first level switching stations, at least two optical fibers, and when forming additional rings of the first hierarchy level use optical fibers of the sections of previously laid trunk cables, which are used for previously formed rings of the first hierarchy level.  7. The method according to claim 1, characterized in that the extension is carried out by expanding one of the rings of the first hierarchy level to cover a larger spatial area, for which optical fibers of sections of previously laid trunk cables are used to form rings of the first and second levels of the hierarchy.  8. The method according to claim 7, characterized in that after the expansion of one of the rings of the first hierarchy level, the freed sections of the trunk cable of the previously formed ring of the second hierarchy level are used to form the rings of the second hierarchy level.  9. A core network comprising a central communication node, a central station, at least one first-level switching station, a first-level ring of a hierarchy, designed to transmit information at a higher speed, made of a trunk cable, multi-core, of optical fibers and connected to the central station and the switching station of the first level, at least one ring of the second level of the hierarchy, designed to transmit information at a lower speed than for the ring of the first level archives made of a trunk cable, multicore, of optical fibers and connected to a ring of the first hierarchy level through a switching station of the first level, characterized in that a second ring of the first hierarchy level with a higher information transfer rate is introduced, a second central station connected to the second ring of the first hierarchy level and associated with the central communication node, at least one ring of the second hierarchy level, the switching station of the first level, while the ring of the second hierarchy level in connected to the second ring of the first level of the hierarchy through the introduced switching station of the first level, the ring of the first level of the hierarchy and the second ring of the first level of the hierarchy are made covering various spatial areas, both of these central stations are installed in the vicinity of the geometric center of the network, and the number of optical fibers in the trunk cable for the rings the second level of the hierarchy is chosen larger than necessary for connecting the switching station of the first level, at least two optical fibers on.

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