Abstract:
An apparatus for transmission between subscriber terminals and digital switches that employ a 4:1 time-division-multiplexing scheme includes a main path for carrying signals between the subscriber terminals and digital switches, a control path which is separate from said main path and carries control information for maintenance purpose, and an interface-&amp;-control unit which converts the control information so as to conform to specifications of the said digital switches wherein the conversion of the control information is based on switch settings made to said interface-&amp;-control unit.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention generally relates to subscriber-system transmission apparatuses, and particularly relates to a subscriber-system transmission apparatus which is equipped with interface connectable to any types of switches of the IDLC type or the UDLC type and connectable to various switches of different vendors. 
   2. Description of the Related Art 
     FIG. 15  is an illustrative drawing showing a remote-station switch system having a UDLC (universal digital loop carrier) network configuration.  FIG. 16  is an illustrative drawing showing a remote-station switch system having an IDLC (integrated digital loop carrier) network configuration. 
   The remote-station switch system of  FIG. 15  having a UDLC network configuration is of a type that is used together with an analog switch. A subscriber-system transmission apparatus  1  shown in  FIG. 15 , which serves as a remote terminal RT, covers subscriber terminals that are scattered around in remote areas (CSA: carrier service areas) far away from an analog switch  4 . The subscriber-system transmission apparatus  1  connects these subscriber terminals to the analog switch  4  via optical-fiber transmission lines. In one example of the UDLC network configuration, the subscriber-system transmission apparatus  1  is connected to the analog switch  4  via an optical-fiber transmission line, an optical transmission apparatus  9  having optical-signal multiplexing/demultiplexing functions, and a transmission apparatus  8  having electrical-signal multiplexing/demultiplexing functions. In another example of the UDLC network configuration, the subscriber-system transmission apparatus  1  is connected to the analog switch  4  via an optical-fiber transmission line and a center-station-system (COT) transmission apparatus  5  having optical-signal multiplexing/demultiplexing functions. 
   The remote-station switch system of  FIG. 16  having an IDLC network configuration is of a type that is used together with a digital switch. In one example of this configuration, the subscriber-system transmission apparatus  1  is connected to a digital switch  3  of a TR08 type via an optical-fiber transmission line and the optical transmission apparatus  9  having optical-signal multiplexing/demultiplexing functions. In another example of this configuration, the subscriber-system transmission apparatus  1  is connected to a digital switch of a TR303 type via an optical-fiber transmission line and the optical transmission apparatus  9 . The former example corresponds to a scheme in which signaling is incorporated into subscriber lines when it is transmitted, and the latter example corresponds to a scheme in which signaling is transmitted via another path. In either example of the IDLC network configuration, the transmission apparatus  8  having electrical-signal multiplexing/demultiplexing functions is made obsolete while it is of necessity in the UDLC network configuration. 
   The subscriber-system transmission apparatus  1  that is connected to the analog switch  4  or the digital switch  2  or  3  covers ISDN subscriber terminals. In what follows, a description will be given of an ISDN multiplexing scheme. 
   ISDN Multiplexing Scheme 
   At a U point on a digital subscriber line, signals corresponding to the following items are provided.
         64 kbps B1 and B2 channels (data channel)   16 kbps D channel (data channel)   synchronization word   M channel (for maintenance purpose)
           1. crc (cyclic redundancy check) bit   2. febe (far end block error) bit   3. eoc (embedded operation channel) bit   4. I (indicator) bit   
               

     FIG. 17  shows a related-art UDLC system. 
   In the related-art UDLC system as shown in  FIG. 17 , 2B+D signals and M-channel signal are multiplexed as they are received through a U point (i.e., an interface between a switch and a COT), followed by transmitting the multiplexed signals to the subscriber-system transmission apparatus  1  situated on the remote station side. The subscriber-system transmission apparatus  1  demultiplexes the received signals, and transmits the demultiplexed signals to a U point (i.e., an interface between a remote station RT and an NT 1  apparatus  6 ). The same operation is performed when signals are transmitted in the opposite direction. 
   The signals described above are multiplexed by the unit of DS0 (64 kbps) on digital facility. Multiplexing schemes used in this operation include:
         1) 3DS0 time division multiplexing; and   2) 4:1 time division multiplexing.       

   3DS0 Time-division-multiplexing Scheme 
   The 3DS0 time-division-multiplexing scheme uses 3 DS0s for the purpose of multiplexing 2B+D on a digital subscriber line into one set of signals. Allocation of DS0s is as follows.
         B1 channel—DS0 (B1)   B2 channel—DS0 (B2)   D channel, M channel—DS0 (D+) DS0 (B1), DS0(B2), and DS0(D+) as described above together constitute 3DS0.       

   DS0 that is allocated to the D channel and the M channel is called a D+ byte. The related-art ISDN switches employ 3DS0 time-division-multiplexing scheme, and three DS0 time slots are allocated on the digital facility at all times. 
   4:1 Time-division-multiplexing Scheme 
   In the 4:1 time-division-multiplexing scheme, the B1 and B2 channels are allocated to 2 DS0s, and four D channels are allocated to one DS0 for the purpose of multiplexing 2B+D on the digital subscriber line into one set of signals. Allocation of DS0s is as follows. 
   
     
       
             
             
             
           
             
           
             
             
             
           
             
           
             
             
             
           
             
           
             
             
             
           
             
           
             
             
             
           
             
           
         
             
                 
                 
             
           
           
             
                 
                B1 channel 
               - - - DSO 
             
             
                 
               B2 channel 
               - - - DSO 
             
             
                 
               D channel 
               - - - DSO 
             
           
        
         
             
                (own DSL: 2 bits such as bits 0 and 1) 
             
           
        
         
             
                 
                D channel 
               - - - same as above 
             
           
        
         
             
                (other DSL: 2 bits such as bits 2 and 3) 
             
           
        
         
             
                 
                D channel 
               - - - same as above 
             
           
        
         
             
                (other DSL: 2 bits such as bits 4 and 5) 
             
           
        
         
             
                 
                D channel 
               - - - same as above 
             
           
        
         
             
                (other DSL: 2 bits such as bits 6 and 7) 
             
           
        
         
             
                 
                M channel (eoc + I bit) 
               - - -  EOC path 
             
           
        
         
             
                 (eoc: embedded operation channel, I: indicator) 
             
             
                 
             
           
        
       
     
   
   In the 4:1 time-division-multiplexing scheme, four D channels exclusively occupies the whole DS0. Because of this, it is necessary to attend to message conversion between EOC/eoc and EOC/Ibit on the ISDN-channel card with respect to M-channel information for maintenance purposes, so that the information is transmitted to the switch through an EOC path serving as a maintenance channel for the transmission device, i.e., through a control path between the switch and the subscriber-system transmission apparatus. 
   In the 3DS0 time-division-multiplexing scheme that is typically used in the analog-switch system or in the TR08-type digital-switch system, one DS0 is allocated to one D channel of the ISDN. For the purpose of efficient use of transmission lines, however, it has been required in an increasing number of cases to use the 4:1 time-division-multiplexing scheme in which one DS0 is allocated to four D channels. In the TR303-type digital switch system that has been recently chosen more often than not, the 4:1 time-division-multiplexing scheme is in prevalent use. 
   The 4:1 time-division-multiplexing scheme multiplexes four D channels, and transmits them as one DS0. While the conventional 3DS0 time-division-multiplexing scheme transmits the M channel as the D+ byte, the 4:1 time-division-multiplexing scheme cannot use the DS0 space for this purpose. As a result, it becomes necessary to convert the eoc bit and the I (indicator) bit of the M channel into EOC (i.e., the control path between the switch and the subscriber-system transmission apparatus) so as to attend to transmission between the switch and the subscriber-system transmission apparatus by using the EOC path that is separate from DS0. 
   If a new interface is newly designed for the EOC path, compatibility with conventional channel cards would be lost. A further problem is that there is a need to reconstruct the entire configuration of apparatus in response to the creation of new interface. 
   Switches that support the TR303-type digital switches are currently manufactured by various manufacturers. In order to connect with the TR303-type digital switches of various manufactures, alarm information on the ISDN subscriber lines needs to be transmitted by using a method of notification that complies to the different requirements of switches of different manufactures. Because of this, the subscriber-system transmission apparatus needs to manage and control alarm information on the subscriber line and to transmit the information through the control path (i.e., EOC) between the switch and the subscriber-system transmission apparatus. 
   The TR303-digital-switch system as described above employs the 4:1 time-division-multiplexing mode function for the purpose of connecting with IDTs (integrated digital terminals). In the 4:1 time-division-multiplexing mode, the EOC (embedded operation channel) is used to exchange control information between the TR303-type switch and the subscriber-system transmission apparatus  1  of the remote station. The standard TR-397 concerning the ISDN does not set forth details of the EOC (i.e., standards of alarm notification for the ISDN subscribers) in the 4:1 time-division-multiplexing mode. Manufactures that make switches supporting the TR303-type switches design and produce switches based on their own IDSN alarm-detection specifications. The subscriber-system transmission apparatuses also need to be changed in terms of their design specifications in compliance to the requirements of manufacturers. This gives rise to a problem of lack of universal applicability. 
   Accordingly, there is a need for a subscriber-system transmission apparatus which is adaptable to either type of ISDN switches that is based on the 3DS0 time-division-multiplexing scheme or the 4:1 time-division-multiplexing scheme while keeping compatibility with the prior-art multiplexing/demultiplexing interface. 
   SUMMARY OF THE INVENTION 
   It is a general object of the present invention to provide an apparatus that substantially obviates one or more of the problems caused by the limitations and disadvantages of the related art. 
   Features and advantages of the present invention will be set forth in the description which follows, and in part will become apparent from the description and the accompanying drawings, or may be learned by practice of the invention according to the teachings provided in the description. Objects as well as other features and advantages of the present invention will be realized and attained by the apparatus particularly pointed out in the specification in such full, clear, concise, and exact terms as to enable a person having ordinary skill in the art to practice the invention. 
   To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides an apparatus for transmission between subscriber terminals and digital switches that employ a 4:1 time-division-multiplexing scheme, the apparatus including a main path for carrying signals between the subscriber terminals and digital switches, a control path which is separate from said main path and carries control information for maintenance purpose, and an interface-&amp;-control unit which converts the control information so as to conform to specifications of the said digital switches wherein the conversion of the control information is based on switch settings made to said interface-&amp;-control unit. 
   In the above apparatus, the switch settings are selected in accordance with the types of the digital switches, and are used to conform to the specifications of the digital switches. This ensures that the apparatus can cope with various types of digital switches, providing universal applicability in terms of connections with digital switches. 
   Further, according to another aspect of the present invention, an apparatus for transmission between subscriber terminals and digital switches that employ a 4:1 time-division-multiplexing scheme includes a main path for carrying signals between the subscriber terminals and digital switches, a control path which is separate from said main path and carries control information for maintenance purpose, and a subscriber-side interface unit which transmits an alarm notification to the digital switches via said control path wherein the alarm notification conforms to specifications of said digital switches based on switch settings made to said subscriber-side interface unit. 
   In the apparatus as described above, the switch settings are selected in accordance with the types of the digital switches used in the network to which subscriber terminals belong, and are used to conform to the specifications of the digital switches. This ensures that the apparatus can cope with the various types of digital switches used in the network regardless of the network to which the subscribers belong. In other words, any user using any network can be covered by the apparatus of the present invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of an IDLC system including a subscriber-system transmission apparatus according to an embodiment of the present invention; 
       FIG. 2  is a block diagram of a relevant portion of the subscriber-system transmission apparatus for the purpose of explaining an EOC path during a 4:1 time-division-multiplexing mode; 
       FIG. 3  is a block diagram showing details of a portion relevant to settings of switch options in the subscriber-system transmission apparatus; 
       FIG. 4  is a block diagram showing a configuration of a U-point-termination unit; 
       FIG. 5  is a table showing ISDN-alarm-notification commands and values thereof that are transmitted in response to the switch option settings corresponding to each switch vendor; 
       FIG. 6  is a table showing ISDN-alarm-notification commands and values thereof that are transmitted in response to the switch option settings corresponding to each switch vendor; 
       FIG. 7  is a table showing ISDN-alarm-notification commands and values thereof that are transmitted in response to the switch option settings corresponding to each switch vendor; 
       FIG. 8  is a table showing ISDN alarm notification commands and values thereof transmitted in response to switch settings of an EOC-control unit of an EOC control card; 
       FIG. 9  is a table showing ISDN alarm notification commands and values thereof transmitted in response to switch settings of the EOC-control unit of the EOC control card; 
       FIG. 10  is a table showing ISDN alarm notification commands and values thereof transmitted in response to switch settings of the EOC-control unit of the EOC control card; 
       FIG. 11  is a table showing ISDN alarm notification commands and values thereof transmitted in response to switch settings of the EOC-control unit of the EOC control card; 
       FIG. 12  is a table showing standards of ISDN alarm messages that are transmitted in response to switch settings of the SW-interface unit of the SW-interface unit; 
       FIG. 13  is a table showing standards of ISDN alarm messages that are transmitted in response to switch settings of the SW-interface unit of the SW-interface unit; 
       FIG. 14  is a table showing standards of ISDN alarm messages that are transmitted in response to switch settings of the SW-interface unit of the SW-interface unit; 
       FIG. 15  is an illustrative drawing showing a remote-station switch system having a UDLC network configuration; 
       FIG. 16  is an illustrative drawing showing a remote-station switch system having an IDLC network configuration; and 
       FIG. 17  shows a related-art UDLC system. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In the following, embodiments of the present invention will be described with reference to the accompanying drawings. 
     FIG. 1  is a block diagram of an IDLC system including a subscriber-system transmission apparatus according to an embodiment of the present invention. 
   In  FIG. 1 , the subscriber-system transmission apparatus  1  is connected to TR303-type digital switches  2 A,  2 B, and  2 C corresponding to respective vendors A, B, and C. This connection is established via optical-fiber transmission lines based on DS1 interface. Further, the subscriber-system transmission apparatus  1  is connected to analog switches  4   a ,  4   b , and  4   c  corresponding to respective vendors a, b, and c via optical-fiber transmission lines and a center-station-system transmission apparatus  5 . 
   The subscriber-system transmission apparatus  1  covers a plurality of ISDN subscriber terminals  7  via NT 1  (network-terminal-apparatus- 1 ) apparatuses  6 . Each user of the subscriber terminals  7  belongs to his/her choice of a service-provider network where service-provider networks adopt switches manufactured by vendors of their own choice. When a user attempts to transmit or receive a call, he/she establishes connection with the service-provider network to which he/she belongs. 
   A control console  10  connected to the subscriber-system transmission apparatus  1  is comprised of a personal computer or the like, which is used for making various settings to the subscriber-system transmission apparatus  1  in terms of switch options and the like. 
   In this manner, the subscriber-system transmission apparatus  1  is provided with the DS1 interface for the IDTs (integrated digital terminals), and includes an EOC interface unit and a cross-connect unit therein for switching between the 3DS0 time-division-multiplexing scheme and the 4:1 time-division-multiplexing scheme. This makes it possible to assume an IDLC configuration based on the 4:1 time-division-multiplexing scheme. Further, this system can be regarded as the 3DS0-scheme system having an additional configuration based on the 4:1 time-division-multiplexing scheme. A mode-switch unit is provided to operate under control of the operation system to switch between the 3DS0 time-division-multiplexing scheme and the 4:1 time-division-multiplexing scheme. Because of such a configuration, this system can be connected to both the switches of 3DS0 time-division-multiplexing scheme and the switches of the 4:1 time-division-multiplexing scheme. 
     FIG. 2  is a block diagram of a relevant portion of the subscriber-system transmission apparatus  1  according to this embodiment for the purpose of explaining an EOC path during the 4:1 time-division-multiplexing mode. 
   As show in  FIG. 2 , the subscriber-system transmission apparatus  1  includes a common shell  11  and narrow band shells  12 . The common shell  11  includes a multiplexing/demultiplexing card  13 , a cross-connect card  14 , a SW-interface unit  15 , and an EOC control card  16 . The multiplexing/demultiplexing card  13  is comprised of a HC1A/HO10 unit, a TS1A unit, MC1O unit, etc. The cross-connect card  14  is comprised of a TS1C unit or the like, and the SW-interface unit  15  includes an EP1C unit or the like. The EOC control card  16  includes an EM1C unit or the like. 
   The narrow band shells  12  are provided as many as 10 units in parallel, and are connected to the cross-connect card  14  of the common shell  11 . Each narrow band shell  12  includes a multiplexing/demultiplexing card  17  and 48 ISDN channel cards  18 . 
   The multiplexing/demultiplexing card  13  of the common shell  11  is connected to the optical-fiber transmission lines via various interfaces such as OC3/OC12, OC3/D3/STS1, or DS1. Main signals 2B+D demultiplexed by the multiplexing/demultiplexing card  13  are supplied to the cross-connect card  14  during a period when the 4:1 time-division-multiplexing scheme is selected. In an opposite direction, the main signal 2B+D multiplexed by the cross-connect card  14  are supplied to the multiplexing/demultiplexing card  13 . 
   An EOC signal demultiplexed by the multiplexing/demultiplexing card  13  is supplied to the cross-connect card  14  as an internal EOC after passing through the SW-interface unit  15  and the EOC control card  16 . In the opposite direction, the internal EOC multiplexed by the cross-connect card  14  is supplied to the multiplexing/demultiplexing card  13  via the EOC control card  16  and the SW-interface unit  15 . 
   The narrow band shells  12  attend to EOC/eoc conversion processing and EOC/I-bit conversion processing, and supply or receive the 2B+D and M channels at the U-point interface. 
   In  FIG. 2  showing the EOC path during the 4:1 time-division-multiplexing mode, signals transmitted to or received from OC3/OC12 are exchanged with the cross-connect card  14  via the multiplexing/demultiplexing card  13 . On the other hand, the EOC (e.g., eoc bit+I bit) during the 4:1 time-division-multiplexing mode is exchanged between the multiplexing/demultiplexing card  13  and the narrow band shells  12  via the SW-interface unit  15  and the EOC control card  16  where SW-interface unit  15  provides necessary interface, and the EOC control card  16  attends to EOC control. 
   As shown in  FIG. 1 , the switch system employing the subscriber-system transmission apparatus  1  does not require the related-art center-station (COT) transmission device  5  when it is connected to the TR303-type digital switches  2 A through  2 C. That is, the subscriber-system transmission apparatus  1  is connected to the TR303-type digital switches  2 A through  2 C via interface such as DS1. Since interface specifications (e.g., ISDN alarm detection) differ from vendor to vendor who manufacture the TR303-type digital switches  2 A through  2 C, a method of notifying an ISDN alarm is changed depending on the types of the TR303-type digital switches  2 A through  2 C. 
   To this end, the control console  10  connected to the subscriber-system transmission apparatus  1  is used for making settings to switch options SWOPT. These settings make it possible to perform ISDN-alarm notification that complies to each of the switches. In the subscriber-system transmission apparatus  1 , the switch options SWOPT may define that the companies A and C correspond to SWOPT 1  and that the company B correspond to SWOPT 2 . The control console  10  serving as a switch-option setting unit is used for setting appropriate data to the SW-interface unit  15 , the EOC control card  16 , and the ISDN channel cards  18 , thereby complying to the requirements of the switch to be connected. Switch option SWOPT 3  is not in use because there is no vendor corresponding to this option. However, operation is guaranteed based on software specifications prior to correction of problems, so that backward compatibility is secured so as not to cause malfunction in apparatuses having only the EM1C unit thereof updated. 
     FIG. 3  is a block diagram showing details of a portion relevant to settings of switch options in the subscriber-system transmission apparatus  1 .  FIG. 3  is used for explaining a method of controlling alarm when the subscriber-system transmission apparatus  1  is connected to all the switches. 
   In  FIG. 3 , the TR303-type digital switch  2 A is made by the manufacturer A, and is connected to the subscriber-system transmission apparatus  1  via DS1. The switch setting is SWOPT 1 . The TR303-type digital switch  2 B is made by the manufacturer B, and is connected to the subscriber-system transmission apparatus  1  via OC 3 . The switch setting is SWOPT 2 . The TR303-type digital switch  2 C is a product of the manufacturer C, and is connected to the subscriber-system transmission apparatus  1  via OC12. The switch setting is SWOPT 1 . 
   The SW-interface unit  15  of the subscriber-system transmission apparatus  1  includes three SW-interface units  151  through  153  corresponding to the respective TR303-type digital switches. The SW-interface units  151  through  153  can be set to the switch option SWOPT 1 , SWOPT 2 , or SWOPT 3  by using the control console (SWOPT setting unit)  10 . In this example, the SW-interface units  151  and  153  respectively connected to the TR303-type digital switches  2 A and  2 C are set to the switch setting (switch option) SWOPT 1 , and the SW-interface unit  152  connected to the TR303-type digital switch  2 B is set to the switch option SWOPT 2 . 
   The SW-interface units  151  through  153  attend to conversion resembling language conversion, as will be described later, which corresponds to specifications of each of the TR303-type digital switches  2 A through  2 C. 
   The EOC control card  16  includes three EOC-control units  161  through  163 , which correspond to the SW-interface units  151  through  153  of the SW-interface unit  15 , respectively. The EOC-control units  161  through  163  are set to the switch option SWOPT 1 , SWOPT 2 , or SWOPT 3  to conform to the switch-option settings of the SW-interface units  151  through  153  set by the control console  10 . In the example shown in  FIG. 3 , the EOC-control unit  161  has the switch option thereof set to SWOPT 1  in conformity with the switch option setting of the SW-interface unit  151 , and controls the EOC supplied from the SW-interface unit  151  via an EOC-termination unit  164 . Further, the EOC-control unit  161  exchanges the internal EOC with the ISDN channel cards  18  via an internal-EOC-exchange unit  165 . 
   The EOC-control units  161  through  163  convert alarm formats to conform to the specifications of the TR303-type digital switches  2 A through  2 C, which will be described later. 
   The cross-connect card  14  is provided between the EOC control card  16  and the ISDN channel cards  18 , and attends to cross-connect processing. Under the remote control of the control console  10 , the cross-connect card  14  can switch between the 4:1 time-division-multiplexing scheme and the 3DS0 time-division-multiplexing scheme with respect to each subscriber, and can establish a cross-connect between a given subscriber and any given subscriber number of any given switch. Further, the cross-connect card  14  has service statuses (e.g., used/unused statuses of the B1 and B2 channels) and provisioning (e.g., settings of 4:1 TDM and 3DS0 TDM) thereof set through remote controlling. 
   Each of the ISDN channel cards  18  has 2 channels, each of which is connected to a corresponding one of the subscriber terminals (TE)  7  via the NT 1  apparatus  6 . Each channel of each ISDN channel card  18  includes three alarm-control units  181  through  183 , an internal-EOC-exchange unit  184 , and a U-point-termination unit  185 . The alarm-control units  181  through  183  are set to the switch option SWOPT 1 , SWOPT 2 , or SWOPT 3 . The internal-EOC-exchange unit  184  is equipped with the function of EOC/eoc conversion and the function of EOC/I-bit conversion. 
     FIG. 4  is a block diagram showing a configuration of the U-point-termination unit  185 . 
   The U-point-termination unit  185  includes a U-point-synchronization-deviation-detection unit  1851 , a NT 1 -power-supply-disconnection-detection unit  1852 , a T-point-synchronization-deviation-detection unit  1853 , alarm-control-switching unit  1854 . The U-point-synchronization-deviation-detection unit  1851  detects loss of synchronization when synchronization is lost at the U point serving as connection interface with the NT 1  apparatus  6 . The NT 1 -power-supply-disconnection-detection unit  1852  detects disconnection of power supply to the NT 1  apparatus  6 . The T-point-synchronization-deviation-detection unit  1853  detects loss of synchronization when synchronization is lost at the T point. The alarm-control-switching unit  1854  selectively connects to one of the alarm-control units  181  through  183 . 
   The alarm-control-switching unit  1854  is almost permanently connected to one of the alarm-control units  181  through  183 . This connected one of the alarm-control units  181  through  183  has the switch option SWOPT thereof set to conform to a TR303-type digital switch that is employed by the network to which the user of the channel belongs. 
   In the example of  FIG. 3 , the channel CH 1  of the ISDN channel card  18  is used by a user who belongs to a service-provider network that employs the TR303-type digital switch  2 A manufactured by the manufacturer A. In order to connect the channel CH 1  to the TR303-type digital switch  2 A, the control console  10  is used to make connection settings to the cross-connect card  14 . After this connection is established, the channel CH 1  is set by the EOC control card  16  such as to carry out alarm control of the switch option SWOPT 1 . 
   FIG.  5  through  FIG. 7  are tables showing ISDN-alarm-notification commands and values thereof that are transmitted in response to the switch option settings corresponding to each switch vendor.  FIG. 5  shows a case in which the alarm condition indicates loss of synchronization at the U point.  FIG. 6  shows a case where the power supply to the NT 1  is disconnected.  FIG. 7  shows a case in which synchronization is lost at the T point. 
   With respect to the switch venders A and C, a notification command that corresponds to the switch option SWOPT of 1 is used. With respect to the switch vendor B, a notification command that corresponds to the switch option SWOPT of 2 is used. Since the switch option SWOPT of 3 does not correspond to any vendor, it is not currently used. However, operation is guaranteed based on software specifications prior to correction of problems, so that backward compatibility is secured so as not to cause malfunction in apparatuses having only the EM1C unit thereof updated. What is shown in the figure is an EOP report that the subscriber-system transmission apparatus  1  transmits to the switch  2 . LTOH represents m bits directed downstream at the U point, and NTOH represents m bits directed upstream at the U point. 
   In the case of loss of synchronization at the point U as shown in  FIG. 5 , the Alarm Status Change Report is transmitted if the switch setting is SWOPT 1  corresponding to the vendor A. In this case,
         LTOH=‘0111 1111 111’ b, and   NTOH=‘0000 0000 000’ b.
 
If the switch setting is SWOPT 2  corresponding to the vendor B, the Alarm Status Change Report and the NT 1  Overhead Change Report are transmitted. In this case,
   LTOH=‘0111 1111 111’ b, and   NTOH=‘000x x101 111’ b,
 
where “x” is the same value as one observed before loss of synchronization at the U point. If the switch setting is a default setting SWOPT 3  corresponding to no vendor, the Alarm Status Change Report is transmitted. In this case,
   LTOH=‘1111 1111 111’ b, and   NTOH=value received from the NT 1  apparatus.       

   In this manner, the U-point-synchronization-deviation-detection unit detects loss of synchronization at the U point, and notifies the switch of the alarm status. In so doing, only the Alarm Status Change Report should be sent to the switch if the switch is a product of the vendor A or C corresponding to the switch option SWOPT 1 . Further, the act bit of LTOH should be set to 0, and all bits of NTOH should be set to 0. If the switch is a product of the vendor C corresponding to the switch option SWOPT 2 , both the Alarm Status Change Report and the NT 1  Overhead Change Report should be sent. In this case, the act bit of LTOH should be set to 0. Further, the act bit, the ps 1  bit, the ps 2  bit, and the sai bit of NTOH should be set to 0. 
   In the case of disconnection of power supply to the NT  1  as shown in  FIG. 6 , the Alarm Status Change Report is transmitted if the switch setting is SWOPT 1  corresponding to the vendor A. In this case,
         LTOH=‘0111 1111 111’ b, and   NTOH=‘0000 0000 000’ b.
 
If the switch setting is SWOPT 2  corresponding to the vendor B, the Alarm Status Change Report and the NT 1  Overhead Change Report are transmitted. In this case,
   LTOH=‘0111 1111 111’ b, and   NTOH=‘000x x101 111’ b,
 
where “x” is the same value as one observed before disconnection of power supply to the NT 1 . If the switch setting is a default setting SWOPT 3  corresponding to no vendor, the Alarm Status Change Report is transmitted. In this case,
   LTOH=‘1111 1111 111’ b, and   NTOH=value received from the NT 1  apparatus.       

   In this manner, the status of disconnection of power supply to the NT 1  is the same as the status of loss of synchronization at the U point, resulting in transmission of the same messages. 
   In the case of loss of synchronization at the T point as shown in  FIG. 7 , the NT 1  Overhead Change Report is transmitted if the switch setting is SWOPT 1  corresponding to the vendor A. In this case,
         LTOH=‘1111 1111 111’ b, and   NTOH=‘0xxx xxxx xxx’ b,
 
where “x” is the same value as one observed before loss of synchronization at the T point. If the switch setting is SWOPT 2  corresponding to the vendor B, the NT 1  Overhead Change Report are transmitted. In this case,
   LTOH=‘1111 1111 111’ b, and   NTOH=‘0xxx xx0x xxx’ b,
 
where “x” is the same value as one observed before loss of synchronization at the T point. If the switch setting is a default setting SWOPT 3  corresponding to no vendor, the NT 1  Overhead Change Report is transmitted. In this case,
   LTOH=‘1111 1111 111’ b, and   NTOH=value received from the NT 1  apparatus.       

   In this manner, the T-point-synchronization-deviation-detection unit detects loss of synchronization at the T point, and notifies the switch of the alarm status. The switch corresponding to the switch setting SWOPT 1  detects loss of synchronization at the T point if the act bit of LTIH is 1 and the act bit of NTOH is 0. On the other hand, the switch corresponding to the switch setting SWOPT 2  detects loss of synchronization at the T point if the act bit of LTOH, the act bit of NTOH, and the sai bit are 1, 0, 0, respectively. Because of this, values as shown in  FIG. 7  are transmitted to let the switches properly detect loss of synchronization at the T point. 
   FIG.  8  through  FIG. 11  are tables showing ISDN alarm notification commands and values thereof transmitted in response to switch settings of the EOC-control unit of the EOC control card  16 .  FIG. 8  shows a case in which no ISDN channel card is in a card slot.  FIG. 9  shows a case in which synchronization is lost at the U point.  FIG. 10  demonstrates a case where the power supply to the NT 1  is disconnected.  FIG. 11  exhibits a case in which synchronization is lost at the T point. 
   When no ISDN channel card is in a card slot as shown in  FIG. 8 , for example, only the M Event Report for ISDN-line-termination purposes should be transmitted if the switch setting is SWOPT 1 . If the switch setting is SWOPT 2 , however, the M Event Report for ISDN-framing-path-termination-change purposes in the Overhead Bit Report should be transmitted in addition to the M Event Report for ISDN-line-termination purposes. The M Event Report for ISDN-framing-path-termination-change purposes in the Overhead Bit Report should include data of LTOH ad NTOH as shown in FIG.  8 . 
   FIG.  12  through  FIG. 14  are tables showing standards of ISDN alarm messages that are transmitted in response to switch settings of the SW-interface unit of the SW-interface unit  15 .  FIG. 12  shows ISDN alarm messages where the switch option is SWOPT 1  corresponding to the vendor A.  FIG. 13  illustrates ISDN alarm messages where the switch option is SWOPT 2  corresponding to the vendor B.  FIG. 14  exhibits ISDN alarm messages where the switch option is set to a default value SWOPT 3  corresponding to no existing vendor. 
   The subscriber-system transmission apparatus  1  operates as follows based on the mechanism as described above. In the following, it is assumed that the subscriber terminal  7  covered by the channel CH 1  of the ISDN channel cards  18  belongs to the service-provider network employing the TR303-type digital switch  2 A of the vendor A, and that the subscriber terminal  7  covered by the channel CH 2  belongs to the service-provider network employing the TR303-type digital switch  2 B of the vendor B. As a consequence, the alarm-control-switching unit  1854  of the U-point-termination unit  185  for the channel CH 1  is controlled so as to select the alarm-control unit  181  that corresponds to the switch setting SWOPT 1  of the vendor-A switch. Further, the alarm-control-switching unit  1854  of the U-point-termination unit  185  for the channel CH 2  is controlled so as to select the alarm-control unit  182  that corresponds to the switch setting SWOPT 2  of the vendor-B switch. 
   When the ISDN channel card  18  is taken out of the slot, for example, the EOC-control units  161  and  162  of the EOC control card  16  transmits alarm messages to the TR303-type digital switches  2 A and  2 B, respectively, thereby notifying the inoperative statuses of the channels CH 1  and CH 2 . 
   When synchronization is lost at the U point of the channel CH 1  in the ISDN channel card  18 , for example, the U-point-termination unit  185  of the channel CH 1  detects loss of synchronization at the U point. In response, the alarm-control unit  181  transmits an alarm-notification command that conforms to the switch option SWOPT 1 , and this command is supplied to the TR303-type digital switch  2 A from the internal-EOC-exchange unit  184  via the cross-connect card  14 , the EOC control card  16 , and the SW-interface unit  15 . This TR303-type digital switch  2 A corresponds to the switch option SWOPT 1 . When synchronization is lost at the U point of the channel CH 2 , for example, the U-point-termination unit  185  of the channel CH 2  detects loss of synchronization at the U point. In response, the alarm-control unit  182  transmits an alarm-notification command that conforms to the switch option SWOPT 2 , and this command is supplied to the TR303-type digital switch  2 B from the internal-EOC-exchange unit  184  via the cross-connect card  14 , the EOC control card  16 , and the SW-interface unit  15 . This TR303-type digital switch  2 B corresponds to the switch option SWOPT 2 . 
   When synchronization is lost at the T point of the channel CH 1  in the ISDN channel cards  18 , the NT 1  apparatus  6  connected to the channel CH 1  sends a m-bit message indicative of loss of T-point synchronization to the channel CH 1  of the ISDN channel cards  18  by passing the message through the U point. The alarm-control unit  181  then uses an alarm-notification command to transmit the EOC where the alarm-notification command conforms to the requirements of the vendor A. 
   In the following, operation of the U-point-termination unit  185  will be described in detail with reference to FIG.  4 . As was previously described, the U-point-termination unit  185  includes the U-point-synchronization-deviation-detection unit  1851 , the NT 1 -power-supply-disconnection-detection unit  1852 , and the T-point-synchronization-deviation-detection unit  1853 . It should be noted here that the number of m bits used for reporting loss of synchronization at the T point varies depending on the types of NT 1  apparatuses. 
   Some notify by use of the act bit that is 0, and others notify by use of the sai bit that is 0. There are still some others that notify by use of the act bit and sai bit that are both zero. In order to detect loss of T-point synchronization with respect to any variation of these, the T-point-synchronization-deviation-detection unit  1853  of the U-point-termination unit  185  detects loss of T-point synchronization by checking if either one of the act bit and the sai bit is 0. When the m-bit message is supplied to the switch without any processing thereof, the switch cannot properly detect the loss of T-point synchronization. Because of this reason, the alarm-control unit  181  attends to conversion into LTOH and NTOH values that conform to the requirements of the vendor-A switch. This ensures that any type of a switch can be properly notified of loss of T-point synchronization regardless of the type of the NT 1  that is connected to the ISDN channel card. 
   In the description provided above, the switches connected to the subscriber-system transmission apparatus  1  have been referred to as the TR303-type digital switches. If the switches of a network-service provider to which subscriber terminals belong are TR08-type digital switches, then, connection is established by using the TR08-type-switch mode. By the same token, if analog switches are used, connection is established by using the analog-switch mode. 
   For connection during the TR08-type-switch mode, none of the SW-interface unit  15  and the EOC control card  16  of the subscriber-system transmission apparatus  1  are used. Signals from the TR08-type digital switch are provided from the multiplexing/demultiplexing card  13  to the cross-connect card  14 , which has switched to the 3DS0 time-division-multiplexing scheme. The alarm-control unit of the ISDN channel card  18  in this case is comprised of only one type of alarm-control unit that conforms to the specifications of the TR08-type digital switch. 
   For connection during the analog-switch mode, the NT 1  apparatus  6  of the subscriber terminal  7  is connected via the U-point-termination unit  185  of the ISDN channel card  18  directly to the cross-connect card  14 , which has switched to the 3DS0 time-division-multiplexing scheme. This connection is established without having the alarm-control units  181  through  183  or the internal-EOC-exchange unit  184  as intervening units. Further, the connection is extended from the cross-connect card  14  to the center-station-system transmission apparatus  5  via an analog-switch multiplexing/demultiplexing unit  19  and the optical-fiber transmission line. 
   As described above, according to the present invention, no special work such as exchange of modules is necessary when there is a need to connect with switches operating based on the 4:1 time-division-multiplexing scheme. A relatively simple configuration includes the EOC-interface unit, the 4:1 TDM-cross-connect unit, the EOC/eoc-&amp;-EOC/I-bit-conversion unit, etc., which is all that is necessary to implement the ISDN service providing connections with digital switches based on 4:1 time-division-multiplexing scheme in addition to rendering of the 3DS0 time-division-multiplexing service. 
   Further, the same configuration as in the conventional art is used with regard to MUX/DMUX interface, U-point interface, line structures, etc., thereby making it possible to cope with switches of the 4:1 time-division-multiplexing scheme without requiring major changes that may affect existing services. 
   Further, the present invention is not limited to these embodiments, but various variations and modifications may be made without departing from the scope of the present invention. 
   The present application is based on Japanese priority application No. 2000-076034 filed on Mar. 17, 2000, with the Japanese Patent Office, the entire contents of which are hereby incorporated by reference.