Abstract:
The invention relates to a network station ( 1 ) comprising a selector switch ( 4 ) for coupling a switching center ( 2 ) to a POTS or ISDN telephone, and a detection circuit ( 3 ). After the detection circuit ( 3 ) has been made operative or after the mode change from the POTS mode to the ISDN mode, the detection circuit ( 3 ) controls the selector switch ( 4 ) such that this switch goes to the ISDN mode and thus couples the switching center ( 2 ) to an ISDN network terminator ( 11 ). When the ISDN mode is changed into the POTS mode, the selector switch ( 4 ) is actuated by the detection circuit ( 3 ) so that the switch couples the switching center ( 2 ) to the POTS telephone.

Description:
FIELD OF THE INVENTION 
     The invention relates to a network station to be operated by a subscriber. 
     BACKGROUND OF THE INVENTION 
     The telecommunication networks are nowadays still provided with both analog and digital terminals in the subscriber area. With analog operation, which is also referred to as the POTS mode (POTS=Plain Old Telephone Service), the analog terminal receives analog signals from an analog module in a switching center, and with digital operation, which is referred to as the ISDN mode (ISDN=Integrated Services Digital Network), the digital terminal receives digital signals from a digital module in the switching center. For the digital operation it is necessary to have an ISDN network terminator which is inserted between the digital terminal and the digital telephone. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a network station which, in the event of a mode change in the switching center, enables operation both in the POTS mode and in the ISDN mode. 
     The object is achieved by a network station comprising a selector switch for coupling a switching center to a POTS or ISDN telephone which network station contains a detection circuit which, after being put into operation or after a mode change from the POTS mode to the ISDN mode, is provided for coupling the switching center to an ISDN network terminator and, in the event of a mode change from the ISDN mode to the POTS mode, for coupling the switching center to the POTS telephone by actuation of the selector switch. 
     After being put into operation, the network station couples the switching center to an ISDN telephone via a selector switch. The network station is put into operation, for example, after a network failure or after it has been built-in at a subscriber station. When the detection circuit recognizes that there is no ISDN mode available, the detection circuit controls the selector switch so that it couples the switching center to an analog or POTS telephone. The advantage of the measures according to the invention is that a change-over from the analog to the digital mode of operation can be effected practically in a remote controlled fashion by the switching center and, therefore, no service need to be rendered at the subscriber end. 
     An embodiment of the detection circuit comprises a rectifier coupled to the input terminals of the selector switch, a threshold detector, a first and a second pulse shaper circuit and a bistable relay. The threshold detector compares the signal produced by the rectifier to a threshold value and the first pulse shaper circuit generates an actuation signal for the bistable relay to change the selector switch from the POTS mode to the ISDN mode when the signal produced by the rectifier has reached the threshold value. The second pulse shaper circuit generates an actuation signal for the bistable relay to change the selector switch from the ISDN mode to the POTS mode when the ISDN network terminator cannot carry out an ISDN operation with the switching center. 
     The detection circuit further includes a timing element which is provided for delaying the generation of an actuation signal of the first pulse shaper circuit. This suppresses noise pulses and the AC signaling voltage in the signal arriving from the switching center. 
     A microprocessor coupled to the ISDN network terminator evaluates status messages of the ISDN network terminator. The second pulse shaper circuit generates the actuation signal for the bistable relay after the microprocessor has generated an excitation signal. The microprocessor generates an excitation signal when the microprocessor receives a message from the ISDN network terminator about the ISDN mode that cannot be effected. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter. 
     In the drawings: 
     FIG. 1 shows a block diagram of a network station comprising a selector switch for coupling a switching center to an analog or digital telephone, and 
     FIG. 2 shows a detailed circuit diagram of the network station. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a network station  1  which couples a switching center  2  either to an analog or a digital telephone. The network station  1  comprises a detection circuit  3  which detects whether the switching center  2  can be coupled to the analog or digital telephone. By exchanging a module in the switching center  2 , the switching center can be converted from the analog to the digital mode. The detection circuit  3  included in the network station  1  automatically detects the change of a mode of operation and controls, in dependence on the detected mode of operation, a selector switch  4  also included in the network station. The analog mode is also referenced POTS mode (POTS=Plain Old Telecommunication Service) and the digital mode as ISDN mode (ISDN=Integrated Services Digital Network). 
     The selector switch  4  is coupled via two input terminals  5  and  6  to the switching center  2 , via two output terminals  7  and  8  to the analog telephone (not shown) and via two further output terminals  9  and  10  to an ISDN network terminator  11 . During the POTS mode the input terminals  5  and  6  are connected to the output terminals  7  and  8 . In the other case, in the ISDN mode, the input terminals  5  and  6  are connected to the output terminals  9  and  10 . The ISDN network terminator  11  supplies signals for the ISDN telephone which is not further shown to its S 0  interface  12 . 
     The detection circuit  3  includes a rectifier  13 , a threshold detector  14 , a timing element  15 , a first and a second pulse shaper circuit  16  and  17  and a bistable relay  18  and a microprocessor  19 . It is assumed that the selector switch has connected the input terminals  5  and  6  to the output terminals  7  and  8  (POTS mode). The detection circuit  3  detects, for example, after a card provided for the operation of the analog telephone has been exchanged for a card provided for the operation of the digital telephone, that the telephone works in the ISDN mode. The detection circuit  3  then works as follows: 
     The supply voltage applied to the input terminals  5  and  6  of the selector switch  4  is converted by the rectifier  13  into a defined polarity voltage. The supply voltage is a DC voltage whose polarity is unknown. The threshold detector  14 , when a threshold voltage in the output voltage of the rectifier  13  reaches or surpasses a threshold voltage, produces a signal which activates the next timing element  15 . The timing element  15  is deactivated when the output voltage produced by the rectifier  13  is smaller than the threshold voltage. After a time τ, after the timing element  15  has been activated, the timing element  15  produces a certain output signal which is converted into a brief current pulse by the subsequent first pulse shaper circuit  16 . The timing element enables the suppression of noise pulses and the AC signaling voltage on the input terminals  5  and  6 . The brief current pulse produced by the first pulse shaper circuit  16  is supplied to the bistable relay  18  which changes to the ISDN mode and connects in the selector switch  4  the input terminals  5  and  6  to the output terminals  9  and  10 . The detection circuit  3  further changes to a high-impedance state, so that the current flowing in the detection circuit  3  is much smaller than the current flowing through the input terminals  5  and  6 . 
     After the ISDN network terminator  11  has been coupled to the switching center  2  and during the operation in the ISDN mode, the ISDN network terminator  11  exchanges various messages with the switching center  2  (exchange of digital codewords). Such an ISDN network terminator  11  produces on a certain output a status message which gives information about whether data or messages are exchanged with the switching center  2 . Such an exchange of messages is not possible, for example, when a module for analog operation has been newly inserted in the switching center  2 . 
     The status message of the ISDN network terminator  11  is received by the microprocessor  19 . In the case where the microprocessor  19  finds out that the ISDN mode is not working, a signal is sent to the second pulse shaper circuit  17  which generates a brief current pulse. As a result of the brief current pulse applied to the bistable relay  18 , the selector switch will connect the input terminals  5  and  6  to the output terminals  7  and  8 . In this way the POTS mode is used, in which the analog telephone is coupled to the switching center  2 . 
     A detailed circuit diagram of the detection circuit  3  is shown in FIG.  2 . The detection circuit  3  shows, just like FIG. 1, the rectifier  13 , whose AC voltage terminals are connected to the input terminals  5  and  6  of the selector switch  4  and is arranged, for example, as a Graetz bridge circuit. The positive output terminal of the rectifier  13  is connected to a resistor  20  whose other terminal forms a common node with a respective first terminal of a capacitor  21 , a resistor  22  and resistor  23  and to the drain terminal of an n-channel MOS field effect transistor  24 . The second terminal of the resistor  22  is connected via two series-arranged further resistors  25  and  26 , the second terminal of the resistor  23  via a resistor  52  and a Z-diode  27  and the second terminal of the capacitor  21  to the negative output terminal of the rectifier  13 . The cathode of the Z-diode  27  and the terminal of the resistor  52  that is not connected to the resistor  23  are connected to the gate terminal of the transistor  24  and to a capacitor  28 . The other terminal of the capacitor  28  is connected to the negative output terminal of the rectifier  13 . 
     The common terminal of the resistors  25  and  26  is connected to the base of an NPN bipolar transistor  29  and to a resistor  30 . The emitter of the transistor  29  is connected to the cathode of a Z-diode  31  whose anode is connected to the negative output terminal of the rectifier  13 , and the collector of the transistor  29  is connected to a resistor  32 . The other terminal of the resistor  32  is connected to a first terminal of a resistor  33  and the base of a PNP bipolar transistor  34 . The emitter of the transistor  34 , the second terminal of the resistor  33 , a first terminal of a resistor  51  and the source terminal of the transistor  24  form a common node. The collector of the transistor  34  is connected to a capacitor  35  and to the terminal of the resistor  30  that is not connected to the base of the transistor  29 . A further connection is formed by the other terminal of the capacitor  35  and the gate terminal of an n-channel MOS field effect transistor  36  and of a resistor  37  whose other terminal is connected to the negative output terminal of the rectifier  13 . The source terminal of the transistor  36  is also connected to the negative terminal of the rectifier  13  and the drain terminal thereof to the negative terminal of a first winding of the bistable relay  18 . The first winding  38  actuates the switches of the selector switch  4 . The positive terminal of the winding  38  is connected to the emitter of an NPN bipolar transistor  39 . In parallel with the winding  38  is connected a diode  40 . The collector of the transistor  39  is connected to a resistor  41  and to the base of an NPN bipolar transistor  42 . A common terminal is formed by the base of the transistor  39 , the emitter of the transistor  42  and a first terminal of a resistor  43 . The collector of the transistor  42  and the other terminal of the resistor  41  are connected to the positive output terminal of the rectifier  13 . 
     A further parallel circuit is formed by a second winding  44  of the bistable relay  18  and a diode  45  is arranged between the other terminal of the resistor  43  and the collector of an NPN bipolar transistor  46 . The positive terminal of the winding  44 , which is provided for actuating the switches of the selector switch  4 , is connected to the resistor  43  and the positive terminal of the first winding. The emitter of the transistor  46  is also connected to the negative output terminal of the rectifier  13  just like the first terminals of two resistors  47  and  48  and the emitter of an NPN bipolar transistor  49 . The base of the transistor  46 , the second terminal of the resistor  47  and the collector of the transistor  49  are connected to the emitter of the transistor of an optocoupler  50 . The collector of the transistor of the optocoupler  50  is connected to the second terminal of the resistor  51 . The anode of the diode in the optocoupler  50  is switched to the microprocessor  19  via a resistor  53 . The cathode of the diode in the optocoupler  50  is connected to ground. The second terminal of the resistor  48  and the base of the transistor  49  are further connected, via two series-arranged Z-diodes  54  and  55 , to the first terminal of a resistor  56 , whose second terminal is connected to the positive output terminal of the rectifier  13 . 
     The resistor  20  and the capacitor  21  form the timing element  15 . The threshold detector  14  comprises, in essence, the transistors  29  and  34 , the resistors  30 ,  32  and  33  and the Z-diode  31 . The first pulse shaper circuit  16  is formed by the transistor  36 , the capacitor  35  and the resistor  37 . The second pulse shaper circuit  17  comprises, in essence, the transistor  46 . When, for example, after a module has been built-in for the operation of a digital telephone in the switching center, or after the network station  1  has been taken into operation, a current flows from the switching center to the network station  1 , the capacitor  21  will be charged. When the charging state of the capacitor  21  reaches a certain threshold value, the transistor  29  becomes conductive and so does, as a result, the transistor  34 . The threshold value is determined by the base-emitter voltage of the transistor  29  and the zener voltage of the Z-diode  31 . After the transistor  34  has become conductive, the capacitor  35  is charged. If the charging state of the capacitor  35  indicates a certain threshold value, the transistor  36  is briefly rendered conductive and thus delivers a brief current pulse to the first winding  36 , so that the first winding  38  puts the switches in the selector switch into the ISDN mode. 
     Both the first and the second winding  38  and  44  respectively, receive the current from a current source which is formed by the transistors  39  and  42  and the resistors  41  and  43 . The diodes  40  and  45  connected in parallel to the windings  38  and  44  have a protective function for the transistors  36  and  46 . 
     After the switches of the selector switch  4  have been actuated by the first winding  38 , the circuit becomes a high-impedance circuit, so that only a very small current flows into the detection circuit  3 . The high impedance is reached, more particularly, by the resistors  22 ,  23 ,  25  and  52  which are selected to be high-value resistors. A renewed current pulse cannot be received by the winding  38  until the capacitor  21  is recharged after being discharged. The transistor  24  and the Z-diode  27  have the function of keeping the voltage for the elements  35  and  37  at a constant level, so that the first pulse shape does not become voltage-dependent. The capacitor  28  has a smoothing function. 
     When the microprocessor  19  takes from the status message sent by the ISDN network terminator  11  the information that the ISDN mode is no longer present, the microprocessor delivers a pulse to the optocoupler  50 . This pulse is delivered to the transistor  46  by the transistor of the optocoupler  50  which electrically isolates the microprocessor  19  from the second pulse shaper circuit  17 . The transistor  46  briefly becomes conductive and leads a current pulse to the second winding  44 , so that the switches in the selector switch  4  change to the POTS mode. 
     Together with the two series-arranged Z-diodes  54  and  55  and the resistor  56 , the resistors  47  and  48  and the transistor  49  prevent the transistor  46  becoming conductive when the voltage on the output terminals of the rectifier surpass a voltage of  80  volts. In that case, t is impossible for the POTS mode to be used.