Method and apparatus for connecting lines in line concentrator of switching system

A method of connecting a line concentrator to other line concentrators in a switching system, wherein a switching unit accommodates the line concentrator is provided. The method includes the steps of analyzing a call received by the switching unit through the line concentrator, and connecting lines that are accommodated by the line concentrator and are connected to the other line concentrators without connecting through the switching unit when the call is detected to be made between the line concentrator and the other line concentrators. Thus, a call can be made to a specific dial number without connecting through the switching unit provided in the switching system. Additionally, a call can also be made by use of another switching unit when a communication path between the switching unit and the line concentrator is overcrowded.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a switching system and a method of controlling a line concentrator provided in the switching system to connect lines, and more particularly relates to a switching system and a method of controlling a line concentrator provided in the switching system to connect to other line concentrators.

2. Description of the Related Art

Recently, with the rapid spread of Internet usage, the number of Internet service providers (ISP) that provide services to connect to the Internet, as well as the number of subscribers who use the Internet through dial-up ISP connections, has increased sharply. Additionally, introduction of a flat rate to Internet services has been considered, and thus it is predictable that a percentage of occupation of lines for a long period by ISP-connection subscribers will increase further in the future. Each corporation has been researching and developing a new network that allows Internet users to connect to the Internet constantly. However, it is predicted that it takes time to replace the existing network with the new network completely, and the investment in equipment for each ISP with introduction of the new network is enormous. Under such circumstances, each ISP needs to cope with the rapid spread of the Internet use by using the existing network effectively as well as constructing the new network.

A description will now be given of a network using a conventional switching system with reference toFIG. 1. InFIG. 1, a line-concentrating unit12is connected to a switching unit10. Additionally, a remote line-concentrating unit16is connected to the switching unit10through trunks13and14. Subscribers A and B are connected to the line-concentrating unit12. An ISP network20is also connected to the line-concentrating unit12through a PRI (Primary Rate Interface)15of an ISDN (Integrated Services Digital Network) line. Subscribers C and D are connected to the remote line-concentrating unit16. A subscriber E is connected to the switching unit10. A switching unit30is connected to the switching unit10through trunks17and18. A line-concentrating unit32is connected to the switching unit30. In addition, a remote line-concentrating unit36is connected to the switching unit30through trunks33and34. Subscribers F and G are connected to the line-concentrating unit32. Subscribers H and I are connected to the remote line-concentrating unit36. Additionally, a subscriber J is connected to the switching unit30.

In a case that the subscribers A and C located under the switching unit10attempt to connect to the ISP network20by a dial-up connection, the subscribers A and C are connected through the PRI15to the ISP network20. To be concrete, in a case that an ISP-connection subscriber such as the subscriber A and the PRI15that is connected to the ISP network20are located under the line-concentrating unit12, the subscriber A connects to the PRI15via only the line-concentrating unit12not through the switching unit10when the subscriber A has captured the PRI15under the line-concentrating unit12. Such a method to connect to the PRI15simply through the line-concentrating unit12by the subscriber A is called an off-road function of a line-concentrating unit.

However, other ISP-connection subscribers such as the subscribers F and H located under the switching unit30also connect to the ISP network20, and they call the switching unit10through trunks17and18, and are connected to the ISP network20via the PRI15located under the switching unit10. If a large number of the ISP-connection subscribers occupy the lines by use of dial-up connections for a long period as described above, the lines are overcrowded at overcrowded points A through F, and thus general subscribers such as subscribers B, D, E, G, I and J cannot capture the lines for regular calls. The first method to solve the above-described problem is to increase the number of switches in the switching unit10and the number of trunks located between the switching units10and30by a common carrier considering the occupation time of the lines by the dial-up ISP connections. The second method is to provide a new ISP access point37indicated by a broken line by an Internet service provider and to provide a PRI38that is indicated by a broken line and is connected to the ISP access point37under the line-concentrating unit32by the common carrier.

However, in the first method, a large number of inefficiently used extension equipment such as the trunks and the switches must be added to the switching units10and30so as to correspond to long-period calls concentrated in a specific time of a day. Such a method does not follow a design concept of a conventional switching system that minimizes the investment in equipment such as the trunks and the switches by sharing the equipment of the switching system among calls on the assumption that most of the calls are short-period calls. Additionally, the first method includes a problem that it is not simple to maintain and manage the entire network shown inFIG. 1for providing sufficient services to each of the ISP-connection subscribers and the general subscribers since the equipment such as the trunks and the switches of the switching units10and30are shared among the long-period calls that are not considered in the conventional switching system and the short-period calls. In the second method, the Internet service provider must extend the ISP network20to a remote location for preparing the new ISP access point37, and thus additional investment in equipment used for extending the ISP network20is necessary. In addition, a dial number for accessing each line-concentrating unit must be changed for a subscriber located under a line-concentrating unit to capture a PRI under the line-concentrating unit when accessing the ISP network20.

SUMMARY OF THE INVENTION

Accordingly, it is a general object of the present invention to provide a method and an apparatus for controlling a switching system to generate alternative communication paths for a call. A more particular object of the present invention is to provide a switching system and a method of connecting a call to a specific dial number without connecting through a switching unit provided in the switching system, and connecting a line concentrator provided in the switching system to another switching unit when a communication path between the switching unit and the line concentrator is overcrowded.

The above-described object of the present invention is achieved by a method of connecting a line concentrator to other line concentrators in a switching system, wherein a switching unit accommodates the line concentrator, the method including the steps of analyzing a call received by the switching unit through the line concentrator, and connecting lines that are accommodated by the line concentrator and are connected to the other line concentrators without connecting through the switching unit when the call is detected to be made between the line concentrator and the other line concentrators.

Thus, a call can be made to a specific dial number without connecting through the switching unit provided in the switching system. Additionally, a call can also be made by use of another switching unit when a communication path between the switching unit and the line concentrator is overcrowded.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A description will now be given of preferred embodiments of the present invention, with reference to the accompanying drawings.

FIG. 2is a block diagram showing a network to which a switching system of the present invention is applied, according to a first embodiment of the present invention. A line-concentrating unit42is connected to a switching unit40. Additionally, a remote line-concentrating unit46is connected to the switching unit40through trunks43and44. A subscriber A, a subscriber B and a trunk47are connected to the line-concentrating unit42. In addition, an ISP network50is connected to the line-concentrating unit42through a PRI45. A subscriber C, a subscriber D and a trunk48are connected to the remote line-concentrating unit46. The trunk48is connected to the line-concentrating unit42through the trunk47. A subscriber E is connected to the switching unit40.

A switching unit60is connected to the switching unit40through trunks52and53. A line-concentrating unit62is connected to the switching unit60. Additionally, a remote line-concentrating unit66is connected to the switching unit60through trunks63and64. A subscriber F, a subscriber G and a trunk68are connected to the line-concentrating unit62. The trunk68is connected to the line-concentrating unit42through the trunk47. A subscriber H, a subscriber I and a trunk69are connected to the remote line-concentrating unit66. The trunk69is connected to the line-concentrating unit42through the trunk47. In addition, a subscriber J is connected to the switching unit60.

In a case that the subscribers C and D both access the ISP network50, the remote line-concentrating unit46connects each of the subscribers C and D to the trunk48, and the line-concentrating unit42connects the trunk47and the PRI45. Accordingly, each of the subscribers C and D is connected to the ISP network50through the remote line-concentrating unit46, the trunk48, the trunk47, the line-concentrating unit42and the PRI45, but not through the switching unit40. Additionally, in a case that the subscribers F and G both access the ISP network50, the line-concentrating unit62connects each of the subscribers F and G to the trunk68, and the line-concentrating unit42connects the trunk47and the PRI45. Accordingly, each of the subscribers F and G is connected to the ISP network50through the line-concentrating unit62, the trunk68, the trunk47, the line-concentrating unit42and the PRI45, but not through the switching units40and60. Similarly, each of the subscribers H and I is connected to the ISP network50through the remote line-concentrating unit66, the trunk69, the trunk47, the line-concentrating unit42and the PRI45, but not through the switching units40and60. On the other hand, regular calls among the subscribers B, D, E, G, I and J are connected via the switching units40and60.

FIG. 3is a block diagram showing a structure of the switching system according to a second embodiment of the present invention. The switching system according to the present invention includes a switching unit80and a line-concentrating unit90. The switching unit80shown inFIG. 3includes a number-analyzing unit81, a call-processing unit82, a line-control unit83, a line-selecting unit84, a signal-receiving unit85, an overcrowding-control unit86, an interface-selecting data table87and a line-selecting data table88. The line-concentrating unit90includes a line-connecting unit91, a call-processing unit92and a signal-transmitting unit93.

The number-analyzing unit81analyzes a dial number, and selects its destination line for a regular call switching. The line-selecting unit84selects lines located under the line-concentrating unit90, and then directs the line-concentrating unit90to connect the lines therein. The call-processing unit82detects a call out and a call in from a line under the switching unit80as well as analyzes a call-out signal and a call-in signal supplied from the line-concentrating unit90, and then executes a call-out process. The line-control unit83starts up trunks and executes a call-in process to a subscriber. Additionally, the line-control unit83sets a communication path in the switching unit80, and sets a communication path in the line-concentrating unit90when lines connecting a subscriber to another subscriber, a PRI or an ISP network are located under the line-concentrating unit90. The signal-receiving unit85receives an overcrowding-process signal from the signal-transmitting unit93of the line-concentrating unit90, and then starts up the overcrowding-control unit86. The overcrowding-control unit86controls the line-concentrating unit90to connect a subscriber located under the line-concentrating unit90and a predetermined line located under the line-concentrating unit90. It should be noted that the line-selecting unit84has been added to the number-analyzing unit81of a conventional switching unit. The signal-receiving unit85and the overcrowding-control unit86have been added to the line-control unit83of the conventional switching unit. Additionally, the signal-transmitting unit93has been added to the call-processing unit92of a conventional line-concentrating unit. A description will be given of the interface-selecting data table87and the line-selecting data table88later.

The line-connecting unit91provides a communication path by connecting lines that are located under the line-concentrating unit90by following an instruction from the line-selecting unit84of the switching unit80. The call-processing unit92detects a call out and a call in from a line located under the line-concentrating unit90, and secures a communication path between the line-concentrating unit90and the switching unit80as well as transmits the call-out signal and the call-in signal to the call-processing unit82of the switching unit80. The signal-transmitting unit93detects overcrowding on the communication path between the line-concentrating unit90and the switching unit80, and transmits the overcrowding-process signal to the signal-receiving unit85of the switching unit80.

The above-described line-selecting unit84can detect a call that is to be made between different line-concentrating units in the switching system from a single dial number received from a subscriber located under any line-concentrating unit90that is located under the switching unit80, and can certainly select a line, an interface connecting the different line-concentrating units and an interface connected to a final destination of the call that are located under the same line-concentrating unit90as the subscriber. For instance, when the subscriber F shown inFIG. 2dials a number to access the ISP network50, the switching unit60captures the trunk68certainly, and connects the subscriber F to the ISP network50through the line-concentrating unit62, the trunk68, the trunk47, the line-concentrating unit42and the PRI45.

When overcrowding has occurred on the communication path between the switching unit80and the line-concentrating unit90, the signal-transmitting unit93provided in the line-concentrating unit90detects the overcrowding, and transmits the overcrowding-process signal to the signal-receiving unit85of the switching unit80. Subsequently, the signal-receiving unit85receives the overcrowding-process signal from the signal-transmitting unit93, and starts up the overcrowding-control unit86. The overcrowding-control unit86enables another line-concentrating unit to execute a call-out process by controlling the line-concentrating unit90to connect a subscriber as a calling end and a predetermined line, both being located under the line-concentrating unit90. Accordingly, even if the communication path between the switching unit80and the line-concentrating unit90is overcrowded when a subscriber makes a call through the communication path, another line-concentrating unit90can execute the call-out process for the subscriber, thereby enabling a communication by a call being made between different line-concentrating units.

FIG. 4is a block diagram showing a data structure of the interface-selecting data table87according to a third embodiment of the present invention. The interface-selecting data table87includes a number-analysis table87A and a line-selection table87B. Line-concentrator-trunk-group (LCTG) numbers are initially registered in the number-analysis table87A as a new trunk group class. The LCTG number is a group number for a group of trunks located under certain line-concentrating units. By searching through the number-analysis table87by using a dial number dialed by a subscriber for a call between different line-concentrating units, a LCTG number corresponding to the dial number is obtained therefrom. Additionally, the number-analysis table87A stores identification numbers of regular trunks, terminals, and primary rate interfaces (PRI) for a dial number that is not used for the call between the different line-concentrating units.

The line-selection table87B stores a chain of idle trunks, that is, an idle chain, for each line-concentrating unit. The idle chain is created by connecting a number of trunks that are located under a specific line-concentrating unit and are currently not used by any connection lines. An idle trunk is selected from an idle chain that corresponds to a line-concentrating-unit number of a subscriber as a calling end by searching through the line-selection table87B by use of the LCTG number obtained from the number-analysis table87A and the line-concentrating-unit number of the subscriber. Accordingly, an interface (a trunk) connecting different line-concentrating units or an interface (a PRI) connected to a final destination can be selected accurately for each of subscribers located under the same switching unit by using the same dial number for each of the subscribers.

FIG. 5is a block diagram showing a switching system used for describing a first method to control a line-concentrating unit to connect lines. In the first method, a description will be given of a case that the subscriber F connects to the ISP network50by a dial-up connection. Each unit with a number shown inFIG. 5corresponds to a unit with the same number shown inFIG. 2. As shown inFIG. 5, the line-concentrating unit42(a line-concentrating unit #3) is connected to the switching unit40(a switching unit #2). The ISP network50is connected to the line-concentrating unit42through the PRI45. Additionally, the trunk47is connected to the line-concentrating unit42. The line-concentrating unit62(a line-concentrating unit #1) and the line-concentrating unit66(a line-concentrating unit #2) are connected to the switching unit60(a switching unit #1). The subscriber F and the trunk68are connected to the line-concentrating unit62. The subscriber H and the trunk69are connected to the line-concentrating unit66.

The switching unit60includes the above-described number-analysis table87A and the line-selection table87B shown inFIG. 6. A dial-up access number of the ISP network50“ABC-1234” is registered as an entry in the number-analysis table87A as shown inFIG. 6. An LCTG #1that includes a group of line-connection interfaces (trunks) #a through #m is registered as data to be selected as a result of a number analysis executed with the dial-up access number “ABC-1234” in the number-analysis table87A. Additionally, the line-selection table87B initially stores contents of the LCTG #1, which are the line-connection interfaces #a through #f located under the line-concentrating unit62(the line-concentrating unit #1) for a line-concentrating-unit number #1, and the line-connection interfaces #g through #m located under the line-concentrating unit66(the line-concentrating unit #2) for a line-concentrating-unit number #2. On the other hand, the switching unit40includes the number-analysis table87A shown inFIG. 7. In the number-analysis table87A shown inFIG. 7, the PRI45that is an interface connecting to the ISP network50is registered corresponding to the dial-up access number of the ISP network50“ABC-1234”.

FIG. 8is a diagram showing processes taken by the line-concentrating unit62(the line-concentrating unit #1) and the switching unit60(the switching unit #1) on a calling end in the first method to control a line-concentrating unit to connect lines. At a step S1shown inFIG. 8, a subscriber F located under the line-concentrating unit #1calls for accessing the Internet. The call-processing unit92provided in the line-concentrating unit #1detects the call made by the subscriber F, and sets a communication path to the switching unit #1, as well as notifies the switching unit #1of the call made by the subscriber F and information about the subscriber F including such as its device number and line-concentrating-unit number through a signal path. At a step S2, the call-processing unit82provided in the switching unit #1prepares for receiving a dial number from the line-concentrating unit #1after being notified about the call made by the subscriber F by the line-concentrating unit #1. At a step S3, the subscriber F dials a dial-up access number (a dial number) of the ISP network50“ABC-1234”. This dial number “ABC-1234” is then transmitted from the line-concentrating unit #1through the communication path to the switching unit #1. Subsequently, at a step S4, the number-analyzing unit81of the switching unit #1analyzes the dial number “ABC-1234” received from the line-concentrating unit #1and selects the LCTG #1including a group of the line-connection interfaces by use of the number-analysis table87A shown inFIG. 6. At a step S5, the line-selecting unit84provided in the number-analyzing unit81of the switching unit #1searches through the line-selection table87B shown inFIG. 6by use of the line-concentrating-unit number #1of the subscriber F and the LCTG #1obtained from the number-analysis table87A, thereby locating an idle chain of line-connection interfaces #a through #f that correspond to the line-concentrating-unit number #1. Subsequently, the line-selecting unit84selects the trunk #e, a header of the idle chain.

At a step S6, the line-control unit83provided in the switching unit #1starts up the trunk #e selected by the line-selecting unit84. At a step S7, the trunk #e located under the line-concentrating unit #1transmits a startup signal and the dial number “ABC-1234” dialed by the subscriber F to a line-connection interface connected to the line-concentrating unit #3located under the switching unit #2. It should be noted that a description about processes executed by the line-concentrating unit #3and the switching unit #2will be given later. At a step S8, after receiving a response signal from the switching unit #2, the trunk #e transmits the response signal via the line-concentrating unit #1through the signal path to the line-control unit83provided in the switching unit #1. At a step S9, after receiving the response signal from the trunk #e, the line-control unit83directs the line-connecting unit91provided in the line-concentrating unit #1to release the communication path that has been secured for the subscriber F between the line-concentrating unit #1and the switching unit #1and to connect a communication path between the subscriber F and the trunk #e in the line-concentrating unit #1. At a step S10, the line-connecting unit91, being directed by the line-control unit83of the switching unit #1connects the communication path between the subscriber F and the trunk #e in the line-concentrating unit #1and releases the communication path that has been secured for the subscriber F between the line-concentrating unit #1and the switching unit #1. As a result of taking the above-described steps, the communication path between the subscriber F and the trunk #e is established without connecting through the switching unit #1.

FIG. 9is a diagram showing processes taken by the line-concentrating unit42(the line-concentrating unit #3) and the switching unit40(the switching unit #2) on a receiving end in the first method to control a line-concentrating unit to connect lines. At a step S11, after receiving the startup signal transmitted from the trunk #e at the step S7, the call-processing unit92provided in the line-concentrating unit #3secures a communication path between the line-concentrating unit #3and the switching unit #2, and notifies the switching unit #2about the reception of the startup signal from the trunk #e. At a step S12, the call-processing unit82provided in the switching unit #2prepares for receiving the dial number after being notified by the call-processing unit92of the line-concentrating unit #3about the reception of the startup signal. At a step S13, the call-processing unit92of the line-concentrating unit #3receives the dial number “ABC-1234” from the trunk #e, and transmits the dial number “ABC-1234” to the switching unit #2. At a step S14, after receiving the dial number “ABC-1234” from the line-concentrating unit #3, the number-analyzing unit81provided in the switching unit #2analyzes the dial number, and then selects the PRI45that is a line-connection interface connected to the ISP network50as shown inFIG. 7. Subsequently, at a step S15, the line-control unit83provided in the switching unit #2executes a calling process to the ISP network50by use of the PRI45. At a step S16, the PRI45transmits a setup signal to the ISP network50. After receiving a response signal transmitted from the ISP network50, the PRI45notifies the switching unit #2about the reception of the response signal at a step S17. At a step S18, the line-control unit83executes a response signal transmitting process after being notified about the response signal transmitted from the ISP network50. At a step S19, the trunk #e transmits another response signal to the switching unit #1. Subsequently, at a step S20, the line-control unit83provided in the switching unit #2directs the line-connecting unit91provided in the line-concentrating unit #3to release the communication path that has been secured for the trunk #e between the line-concentrating unit #3and the switching unit #2, and to connect a communication path between the trunk #e and the PRI45in the line-concentrating unit #3. At a step S21, the line-connecting unit91, being directed by the line-control unit83of the switching unit #2, connects the communication path between the trunk #e and the PRI45in the line-concentrating unit #3, and releases the communication path that has been secured for the trunk #e between the line-concentrating unit #3and the switching unit #2. As a result of taking the above-described steps, the communication path between the trunk #e and the PRI45is established without connecting through the switching unit #2. Consequently, after taking the steps S1through S21, a communication path is established between the subscriber F and the ISP network50.

Accordingly, a call to a specific dial number can be connected at a line-concentrating unit without going through a switching unit, since a trunk that connects lines located under a single line-concentrating unit is selected by searching through the line-selection table87B by use of a line-concentrating-unit number of the line-concentrating unit accessed by a subscriber making the call and a LCTG number obtained from the number-analysis table87A.

In a case that all of communication paths between the line-concentrating unit42(the line-concentrating unit #3) and the switching unit40(the switching unit #2) are being used and overcrowded as shown inFIG. 10, the signal-transmitting unit93provided in the line-concentrating unit42transmits an overcrowding-process message included in an overcrowding-process signal through the signal path to the switching unit40after the call-processing unit92provided in the line-concentrating unit42has detected overcrowding of the communication paths. The overcrowding-process message includes a message number, location information of a subscriber and a line-concentrating-unit number as shown inFIG. 11. After receiving the overcrowding-process message from the signal-transmitting unit93of the line-concentrating unit42, the signal-receiving unit85included in the line-control unit83of the switching unit40starts up the overcrowding-control unit86. Subsequently, the overcrowding-control unit86obtains a line-concentrator-trunk-group number (LCTG) #i with reference to the line-selecting data table88shown inFIG. 12that is used for selecting a line when communication paths are overcrowded, and selects a line-connection interface located under the same line-concentrating unit as a subscriber who has made a call to a specific dial number from line-connection interfaces registered on the line-selection table87B. Additionally, the line-control unit83starts up the selected line-connection interface as well as directs the line-connecting unit91of the line-concentrating unit42to establish a communication path between the subscriber and the selected line-connection interface in the line-concentrating unit42.

FIG. 13is a block diagram showing a second method to control a line-concentrating unit to connect lines according to the present invention. InFIG. 13, each unit with a number corresponds to a unit with the same number inFIG. 2. A case in which a subscriber H connects to the ISP network50by a dial-up connection will be described in the second method. As shown inFIG. 13, the line-concentrating unit42(the line-concentrating unit #3) is connected to the switching unit40(the switching unit #2). The ISP network50is connected to the line-concentrating unit42through the PRI45. Additionally, the trunk47is connected to the line-concentrating unit42. The line-concentrating unit62(the line-concentrating unit #1) and the line-concentrating unit66(the line-concentrating unit #2) are connected to the switching unit60(the switching unit #1). The subscriber F and the trunk68are connected to the line-concentrating unit62. The subscriber H and the trunk69are connected to the line-concentrating unit66. The trunks68and69are connected to the line-concentrating unit42through the trunk47. In addition, it is assumed that the line-concentrating unit66is overcrowded.

The switching unit60includes the line-selection table87B and the line-selecting data table88as shown inFIG. 14. The LCTG #1that includes a group of line-connection interfaces (trunks) #a through #m is registered as data to be selected in the line-selecting data table88. Additionally, the line-selection table87B initially stores contents of the LCTG #1which are the line-connection interfaces #a through #f located under the line-concentrating unit62for the line-concentrating-unit number #1and the line-connection interfaces #g through #m located under the line-concentrating unit66for the line-concentrating-unit number #2. On the other hand, the switching unit40includes the number-analysis table87A shown inFIG. 15. In the number-analysis table87A shown inFIG. 15, the PRI45that is an interface connecting to the ISP network50is registered corresponding to the dial-up access number of the ISP network50“ABC-1234”.

FIG. 16is a diagram showing processes taken by the line-concentrating unit66(the line-concentrating unit #1) and the switching unit60(the switching unit #1) on a calling end in the second method to control a line-concentrating unit to connect lines. At a step S101shown inFIG. 16, a subscriber H located under the line-concentrating unit #2calls for accessing the Internet. At a step S102, the call-processing unit92provided in the line-concentrating unit #2starts up the signal-transmitting unit93provided therein since the line-concentrating unit2is overcrowded. Subsequently, at a step S103, the signal-transmitting unit93transmits an overcrowding-process signal including an overcrowding-process message to the switching unit #1through a signal path located between the switching unit #1and the line-concentrating unit #2. At a step S104, the signal-receiving unit85provided in the switching unit #1starts up the overcrowding-control unit86after receiving the overcrowding-process signal from the line-concentrating unit #2. At a step S105, the overcrowding-control unit86selects the LCTG #1with reference to the line-selecting data table88. At a step S106, the overcrowding-control unit86searches through the line-selection table87B by use of the line-concentrating-unit number #2of the subscriber H and the LCTG #1thereby locating an idle chain of the line-connection interfaces #g through #m that correspond to the line-concentrating-unit number #2. Subsequently, the line-selecting unit84selects the trunk #i, a header of the idle chain. At a step S107, the line-control unit83provided in the switching unit #1starts up the trunk #i. At a step S108, the trunk #i located under the line-concentrating unit #2transmits a startup signal to a line-connection interface connected to the line-concentrating unit #3located under the switching unit #2by a startup process taken by the switching unit #1. It should be noted that a description about processes executed by the line-concentrating unit #3and the switching unit #2will be given later. At a step S109, the line-control unit83of the switching unit #1directs line-connecting unit91of the line-concentrating unit #2to establish a communication path between the subscriber H and the trunk #i therein. Subsequently, at a step S110, the line-connecting unit91provided in the line-concentrating unit #2, being directed by the line-control unit83of the switching unit #1, establishes the communication path between the subscriber H and the trunk #i therein. At a step Sill, the subscriber H dials the number “ABC-1234”. Then the number “ABC-1234” is transmitted to the switching unit #2through the line-concentrating unit #2and the trunk #i at a step S112. By taking the above-described steps, the communication path is established between the subscriber H and the trunk #i in the line-concentrating unit #2and not through the switching unit #1.

FIG. 17is a diagram showing processes taken by the line-concentrating unit42(the line-concentrating unit #3) and the switching unit40(the switching unit #2) on a receiving end in the second method to control a line-concentrating unit to connect lines. At a step S113, after receiving the startup signal that has been transmitted from the trunk #i at the step S108, the call-processing unit92provided in the line-concentrating unit #3secures a communication path between the line-concentrating unit #3and the switching unit #2as well as notifies the switching unit #2about reception of the startup signal that has been transmitted from the trunk #i. At a step S114, the call-processing unit82provided in the switching unit #2prepares for receiving a dial number after being notified about the reception of the startup signal. At a step S115, the line-concentrating unit #3receives the dial number “ABC-1234” from the trunk #i, and transmits the dial number “ABC-1234” to the switching unit #2. After receiving the dial number “ABC-1234” from the line-concentrating unit #3, the number-analyzing unit81of the switching unit #2analyzes the dial number “ABC-1234,” thereby selecting the PRI45that is an interface connected to the ISP network50with reference to the number-analysis table87A at a step S116. At a step S117, the line-control unit83of the switching unit #2executes a call process to the ISP network50by use of the PRI45. Following the call process that has been executed by the line-control unit83, the PRI45transmits a setup signal to the ISP network50at a step S118. At a step S119, the PRI45receives a response signal transmitted from the ISP network50, and notifies the switching unit #2about reception of the response signal. At a step S120, the line-control unit83of the switching unit #2directs the line-connecting unit91of the line-concentrating unit #3to release the communication path that has been secured for the trunk #i, and to establish a communication path between the trunk #i and the PRI45in the line-concentrating unit #3. Then, at a step S121, the line-connecting unit91establishes the communication path between the trunk #i and the PRI45therein, and releases the communication path that has been established for the trunk #i between the line-concentrating unit #3and the switching unit #2. Accordingly, by taking the above-described steps, the communication path is established between the trunk #i and the PRI45in the line-concentrating unit #3without going through the switching units #1and #2.

Accordingly, when a communication path between a line-concentrating unit and a switching unit is overcrowded, a call made by a subscriber located under the switching unit can be connected to a destination address or dial number through another switching unit by connecting lines located under the line-concentrating unit since a trunk that connects lines located under the line-concentrating unit is selected by searching through the line-selection table87B by use of a line-concentrating-unit number of the line-concentrating unit accessed by a subscriber making the call and a LCTG number obtained from the line-selecting data table88.

As described above, the switching system according to the present invention can prevent overcrowding of a communication path by separating communication paths used by ISP-connection subscribers and general subscribers. Additionally, the switching system does not need to add extra trunks to each of the switching units40and60for connecting the switching units40and60. Furthermore, a common carrier can easily maintain and manage the switching system, and can keep maintaining a call to connect to an ISP network and a general call as before since communication paths used by the ISP-connection subscribers and the general subscribers can be separated.

A description will now be given of a network system where the switching unit of the present invention is adopted according to a fourth embodiment of the present invention with reference toFIG. 18.FIG. 18shows a network system that can secure a line for an emergency call. As shown inFIG. 18, a line-concentrating unit142is connected to a switching unit140. Additionally, a remote line-concentrating unit146is connected to the switching unit140through trunks143and144. A subscriber A, a subscriber B and a trunk147are connected to the line-concentrating unit142. A police/fire station150is connected to the line-concentrating unit142through a FPT (Fire Police Trunk)145. A subscriber C, a subscriber D and a trunk148are connected to the remote line-concentrating unit146, wherein the trunk148is connected to the line-concentrating unit142through the trunk147. In addition, a subscriber E is connected to the switching unit140. Furthermore, a switching unit160is connected to the switching unit140through trunks152and153. A line-concentrating unit162, a subscriber J and a police/fire station170are connected to the switching unit160. In addition, a remote line-concentrating unit166is connected to the switching unit160through trunks163and164. A subscriber F, a subscriber G and a trunk168are connected to the line-concentrating unit162, wherein the trunk168is connected to the line-concentrating unit142through the trunk147. A subscriber H, a subscriber I and a trunk169are connected to the remote line-concentrating unit166, wherein the trunk169is connected to the line-concentrating unit142through the trunk147.

For instance, in a case that overcrowding occurs at overcrowded points A through E because of increase in communication traffic caused by unusual conditions such as a disaster at the switching unit160, the subscriber G cannot normally reach either of police/fire stations150and170even if the subscriber G tries to make an emergency call because of the overcrowding. However, with the present invention, the subscriber G can reach the police/fire station150by using the FPT145provided under the line-concentrating unit142. When the subscribers C and D access the police/fire station150, they are connected to the FPT145through the remote line-concentrating unit146, the trunk148, the trunk147and the line-concentrating unit142, but not though the switching unit140. When the subscribers F and G access the police/fire station150, they are connected to the FPT145through the line-concentrating unit162, the trunk168, the trunk147and the line-concentrating unit142, but not through the switching units140and160. Similarly, when the subscribers H and I access the police/fire station150, they are connected to the FPT145through the remote line-concentrating unit166, the trunk169, the trunk147and the line-concentrating unit142. On the other hand, calls among the subscribers D, E, I and J are made through the switching units140and160as before.

The above description is provided in order to enable any person skilled in the art to make and use the invention and sets forth the best mode contemplated by the inventors of carrying out the invention.

The present invention is not limited to the specially disclosed embodiments, and variations and modifications may be made without departing from the scope and spirit of the invention.

The present application is based on Japanese Priority Application No. 2000-099379, filed on Mar. 31, 2000, the entire contents of which are hereby incorporated by reference.