BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to digital subscriber line (xDSL) network systems permitting transmission of digital data signals over conventional telephone lines used for plain old telephone service (POTS), and in particular to a subscriber circuit in a telephone central office, connected to an interface unit at a subscriber location through a conventional telephone line. 
     2. Description of the Related Art 
     The existing public telephone network has been increasingly used to transmit digital data although it was originally designed for transmitting analog voice signals. 
     Digital Subscriber Line (xDSL) technology allows the telephone service and high-speed data transmission service over POTS lines. Especially, data transmission of Asymmetric DSL (ADSL) permits transmission of digital data over the conventional twisted wire pairs that are used for POTS, providing for an upstream channel at a maximum rate of 1 megabits per second (Mbps) and a downstream channel at a maximum rate of 9 Mbps. 
     The basic ADSL architecture has an ADSL interface unit at the telephone central office and an ADSL interface unit at the subscriber location. Each ADSL interface unit includes a POTS splitter or filter and ADSL modem. 
     In Japanese Patent Application Unexamined Publication No. 11-308352, an ADSL communication system has been disclosed, which is provided with a path switch at each of the central office and the subscriber location. When the telephone service is stopped, a controller deactivates the splitter and activates the path switch to connect the subscriber line directly to the ADSL mode. As a result, the frequency bandwidth assigned to the telephone service can be efficiently used. 
     However, the conventional architecture as described above is provided with the splitter between the subscriber circuit and the subscriber terminal. In general, a splitter has a filter composed of a coil and a capacitor to split multiplexed signals on the subscriber line into an analog POTS signal and a high-speed digital data signal. Accordingly, it is not possible to properly perform an accurate subscriber line test through such a splitter. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a subscriber circuit allowing a reliable and proper subscriber line test. 
     According to the present invention, a subscriber circuit connected to subscriber equipment through a subscriber line, includes: a splitter for splitting a signal on the subscriber line into a lower-frequency band signal and a higher-frequency band signal; a feeder for feeding a current to the subscriber equipment through the subscriber line; a path selector for selecting one of a first path and a second path between the subscriber line and the feeder, wherein the first path has the splitter inserted therein and the second path has no splitter inserted therein; and a controller controlling path selection of the path selector depending on whether a subscriber line test instruction is received from a host. 
     The path selector normally selects the first path. When the subscriber line test instruction is received, the path selector selects the second path. 
     According to an aspect of the present invention, a subscriber circuit connected to subscriber equipment through a subscriber line, includes: a splitter for splitting a signal on the subscriber line into a lower-frequency band signal and a higher-frequency band signal; a feeder for feeding a current to the subscriber equipment through the subscriber line; a switch connected to the subscriber line, for selectively connecting and disconnecting the feeder to the subscriber line; a path selector connected between the switch and the feeder, for selecting one of a first path and a second path, wherein the first path has the splitter inserted therein and the second path directly connects the feeder and the switch; and a controller controlling the switch and the path selector depending on whether a subscriber line test instruction is received from a host. 
     The path selector may include: a first selector having a first common port and first and second selection ports, wherein the first common port is connected to the switch and the first selection port is connected to the splitter; and a second selector having a second common port and third and fourth selection ports, wherein the second common port is connected to the feeder, the third selection port is connected to the first selection port of the first selector through the splitter to form the first path, and the fourth selection port is connected directly to the second selection port of the first selector to form the second path. 
     According to another aspect of the present invention, a subscriber circuit connected to subscriber equipment through a subscriber line, includes: a splitter for splitting a signal on the subscriber line into a lower-frequency band signal and a higher-frequency band signal; a feeder for feeding a current to the subscriber equipment through the subscriber line; a selector connected to the subscriber line, for selecting one of a first path and a second path, wherein the first path has the splitter inserted therein and the second path directly connects the feeder and the subscriber line; a switch for selecting one of the first path and the second path synchronously with the selector, wherein the switch selectively connects and disconnects the feeder to the subscriber line; and a controller controlling the switch and the selector depending on whether a subscriber line test instruction is received from a host. 
     Preferably, the selector has a first common port and first and second selection ports, wherein the first common port is connected to the subscriber line and the first selection port is connected to the splitter. The switch has a second common port and third and fourth selection ports, wherein the second common port is connected to the feeder, the third selection port is connected to the first selection port of the first selector through the splitter to form the first path, and the fourth selection port is connected directly to the second selection port of the first selector to form the second path. 
     As described above, according to the present invention, the splitter is incorporated within the subscriber circuit in such a manner that the splitter can be isolated from a current feeding system. Accordingly, a reliable and proper subscriber line test can be achieved. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram showing a subscriber circuit according to a first embodiment of the present invention; and 
     FIG. 2 is a block diagram showing a subscriber circuit according to a second embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, a telephone company central office is provided with a plurality of subscriber circuits according to a first embodiment of the present invention, each of which accommodates corresponding subscriber equipment. A subscriber circuit  10  is connected to a corresponding subscriber terminal  11  through a subscriber line  12  that is an ordinary telephone line composed of a twisted wire pair. 
     The subscriber circuit  10  includes a controller  101 , a feeding circuit  102 , a selector  103 , a splitter  104 , a selector  105 , a switch  106 , and other necessary circuits (not shown). The controller  101  controls the feeding circuit  102 , the selector  103 , the selector  105 , and the switch  106  depending on control information received from a host. 
     The feeding circuit  102  supplies a necessary current to the subscriber terminal  11  through the subscriber line  12  under control of the controller  101 . 
     The common port of the selector  103  is connected to the feeding circuit  102  and the common port of the selector  105  is connected to the switch  106 . Selection ports of the selectors  103  and  105  are connected to the splitter  104  to form a path and the other selection ports thereof are directly connected to form a bypass  107 . The selectors  103  and  105  are synchronized to select one of the path through the splitter  104  and the bypass  107  under control of the controller  101 . 
     The splitter  104  connected between the selectors  103  and  105  is composed of a low pass filter to isolate a POTS frequency band from an ADSL frequency band which is higher than the POTS frequency band. 
     The switch  106  connects the subscriber line  12  to the common port of the selector  105  and performs connection and disconnection of the subscriber line  12  to the feeding circuit  102  under control of the controller  101 . 
     The controller  101  controls the feeding circuit  102 , the selector  103 , the selector  105 , and the switch  106  depending on whether the subscriber line test is performed. The details will be described hereafter. 
     The path of the splitter  104  is normally selected to connect the feeding circuit  102  to the subscriber line  12  through the splitter  104 . The bypass  107  is selected to connect the feeding circuit  102  directly to the switch  106  when the subscriber line test is performed. 
     More specifically, when normally, that is, the subscriber line test is not performed, the controller  101  receives control information instructing not to perform the subscriber line test and therefore instructs the selectors  103  and  105  to select the path of the splitter  104 . Accordingly, the splitter  104  is connected between the switch  106  and the feeding circuit  102  and thereby a predetermined current required for telephone communication is supplied to the subscriber terminal  11  and the subscriber line  12  through the switch  106 . 
     On the other hand, when the subscriber line test is performed, the controller  101  receives control information instructing to perform the subscriber line test and detects test conditions from it. Following the test conditions, the controller  101  instructs the selectors  103  and  105  to select the bypass  107  to connect the feeding circuit  102  directly to the switch  106 . Thereafter, the controller  101  controls the feeding circuit  102  and the switch  106  so as to perform the designated subscriber line test. For example, a test current is supplied from the feeding circuit  102  to the subscriber line  12  through the switch  106  without the intervention of the splitter  104 . In other words, the splitter  104  is isolated from the subscriber line test system. Accordingly, the subscriber line test can be accurately and reliably performed without influence of the low pass filter of the splitter  104 . 
     Referring to FIG. 2, a subscriber circuit according to a second embodiment of the present invention differs from the first embodiment of FIG. 1 in that a switch  201  is provided in replace of the selector  103  of FIG.  1 . In FIG. 2, other blocks similar to those previously described with reference to FIG. 1 are denoted by the same reference numerals and the descriptions will be omitted. 
     The switch  201  has a common port and two selection ports and further a function of connection and disconnection of the feeding circuit  102  under control of the controller  101 . 
     The common port of the switch  201  is connected to the feeding circuit  102  and the common port of the selector  105  is connected to the subscriber line  12 . Selection ports of the switch  201  and the selector  105  are connected to the splitter  104  to form a path and the other selection ports thereof are directly connected to form a bypass  107 . The switch  201  and the selector  105  are synchronized to select one of the path through the splitter  104  and the bypass  107  under control of the controller  101 . 
     The controller  101  controls the feeding circuit  102 , the switch  201 , and the selector  105  depending on whether the subscriber line test is performed. The details will be described hereafter. 
     As in the case of FIG. 1, the path of the splitter  104  is normally selected to connect the feeding circuit  102  to the subscriber line  12  through the splitter  104 . When the subscriber line test is performed, the bypass  107  is selected to connect the feeding circuit  102  directly to the subscriber line  12 . 
     More specifically, when the subscriber line test is not performed, the controller  101  instructs the switch  201  and the selector  105  to select the path of the splitter  104 . Accordingly, the splitter  104  is connected between the subscriber line  12  and the feeding circuit  102  and thereby a predetermined current required for telephone communication is supplied to the subscriber terminal  11  and the subscriber line  12 . 
     When the subscriber line test is performed, the controller  101  receives control information instructing to perform the subscriber line test and detects test conditions from it. Following the test conditions, the controller  101  instructs the switch  201  and the selector  105  to select the bypass  107  to connect the feeding circuit  102  directly to the subscriber line  12 . Thereafter, the controller  101  controls the feeding circuit  102  so as to perform the designated subscriber line test. For example, a test current is supplied from the feeding circuit  102  directly to the subscriber line  12  without the intervention of the splitter  104 . Accordingly, as in the case of the first embodiment, the subscriber line test can be accurately and reliably performed without influence of the low pass filter of the splitter  104 . 
     According to the first and second embodiments as described above, the splitter  104  is incorporated within the subscriber circuit  10  in such a manner that the splitter  104  can be isolated from the current feeding system. Accordingly, a reliable and proper subscriber line test can be achieved and further the work of splitter connection to the subscriber line can be eliminated.
A subscriber circuit allowing a reliable and proper subscriber line test is disclosed. The subscriber circuit includes a splitter and a feeder for feeding a current to subscriber equipment through a subscriber line. A path selector is provided to select one of a first path and a second path between the subscriber line and the feeder. The first path has the splitter inserted therein and the second path has no splitter inserted therein. The path selector normally selects the first path. When the subscriber line test instruction is received, the path selector selects the second path.