Abstract:
The invention provides a cost-effective system and method for testing telephone lines connected to transmission systems, such as Subscriber Line Multiplexer systems. The invention uses the same pair of wires for transmission and test. According to one embodiment of the invention, a system for allowing a test procedure to be performed on a communication system is provided. The communication system has a transmission line for connecting to telephone exchange and a subscriber line for connecting to a communication set. In the system of the invention, a detector is coupled to the transmission line for detecting test signals on the transmission line. A bypass circuit is coupled between the transmission and subscriber lines for allowing the test signals to bypass the communication system. Also, a discriminator is coupled to the subscriber line for discriminating a connection status of the communication set. Moreover, a controller is coupled to the detector, the discriminator and the bypass circuit for activating the bypass circuit in accordance with detection of the test signals and the connection status of the communication set. This allows the test signals to bypass the communication system and connect to the communication set and perform measurements. The system of invention additionally includes a status circuit, coupled to the controller, for reproducing, upon discriminating a predetermined connection status of the communication set, the predetermined connection status of the communication set for presenting to the test signals. The predetermined connection status includes off-hook and short statuses of the communication set.

Description:
BACKGROUND OF THE INVENTION 
     The present invention generally relates to telecommunication test systems and methods, and more particularly to telecommunication tests and methods that allow transmission systems, such as Subscriber Line Multiplexer Systems to effectively respond to automatic test procedures, such as the MLT (Mechanized Loop Test). 
     Transmission systems have been used to provide cost-effective connections between a central office and subscribers&#39; telephones. A Subscriber Line Multiplexer system is a type of transmission system that uses digitally multiplexed channels to connect two or more telephone subscribers to a central office. This system uses the ISDN U-interface technology, or xDSL technologies, such as HDSL (High Data Rate Subscriber Line), ADSL (Asymmetric Digital Subscriber Line) and their variations, to multiplex several voice and/or data channels over a single copper pair, in order to provide additional virtual telephone or data lines to one or more subscribers over the existing cable facilities. 
     When the personnel in the central office of a telephone company receives a complaint from a subscriber about telephone problems, the subscriber&#39;s line is remotely tested from the central office to verify the accuracy of the complaint and to locate defective network components. Thus, a repairing team may be dispatched to the appropriate location. This provides an efficient and economic solution for the telephone company in dealing with problems about telephone lines. 
     Typically, the telephone line tests are automated, and one of the most popular automated tests is known as the MLT procedure performed by an MLT system. In an MLT procedure, the MLT system applies known electrical signals on the subscriber&#39;s loop comprising the telephone line and the subscriber&#39;s telephone set and makes appropriate measurements. Based on the measurements, the MLT system can calculate the loop impedance, the parasitic impedances, as well as possible foreign voltages (i.e., voltages other than those supplied from the central office) present on the line. The MLT system reports back to the central office the results of the measurements, as well as the probable status of the line. The operator needs to know these results in order to understand the problems and take appropriate actions. For example, if the insulation between the wires of a telephone line is damaged, the impedance between wires may become very low. In this case, the transmission characteristics of the telephone line may become very poor, and the quality of the services could thus be impaired. Also, the presence of a foreign voltage due to, e.g., induction from defective high power electrical ducts, can impair the transmission characteristics of the telephone line and can even make it dangerous to use the telephone. 
     The MLT procedure is most effective for remote testing telephone lines without having any transmission system, such as a fiber optic transmission system, or a Subscriber Line Multiplexer System connected to the lines. If the subscriber&#39;s telephone set is not directly connected to the central office, but instead it is connected through a transmission system, the MLT system does not have direct access to the line. In such a case, when the transmission system detects the presence of an MLT signal, it feeds back to the MLT system the values of three resistors located at the subscriber&#39;s loop which represent the “signature” of the current state of the system. These three resistors are respectively connected between the ring and tip lines, the ring line and ground and the tip line and ground. 
     In order to present the proper signature to the MLT system, the transmission system must perform a series of self tests. If the number of lines serviced by the system is large, the remote equipment can be provided with a rather complex test head for performing elaborate tests of the subscriber&#39;s drop (i.e., the copper pair that connects the remote terminal to the subscriber). The test head can report back the results to the central office terminal for presenting the proper signatures. Because large number of lines are serviced, the per-line cost of the test head is relatively small. However, in the case of multiplexer systems with small number of lines, e.g., 2 or 4 lines, the per-line cost of the test head is very high. One solution would be to use a less sophisticated test head with simple self-test circuits. In such case, the system periodically performs (or at a time when the MLT is detected) simple tests which will establish signatures for presenting to the MLT system. However, the number of tests and the accuracy of the results obtained with these simple circuits are insufficient for detecting the problems of the telephone lines. Thus, complex circuits have to be used, which dramatically increases the cost. 
     Accordingly, there is a need to provide an improved and cost-effective system and method for testing telephone lines connected to transmission systems, such as Subscriber Line Multiplexer systems. 
     SUMMARY OF THE INVENTION 
     This invention provides a cost-effective system and method for testing telephone lines connected to transmission systems, such as Subscriber Line Multiplexer systems. By using the invention, the information returned by the multiplexer system is maximized when MLTs are run by the operating personnel for maintenance or failure detection purposes. The invention uses the same pair of wires for transmission and test. 
     According to one embodiment of the invention, a system for allowing a test procedure to be performed on a communication system is provided. The communication system has a transmission line for connecting to telephone exchange and a subscriber line for connecting to a communication set. The system of the invention comprises a detector, coupled to the transmission line, for detecting test signals on the transmission line; a bypass circuit, coupled between the transmission and subscriber lines, for allowing the test signals to bypass the communication system; a discriminator, coupled to the subscriber line, for discriminating a connection status of the communication set; and a controller, coupled to the detector, the discriminator and the bypass circuit, for activating, in accordance with detection of the test signals and the connection status of the communication set, the bypass circuit so as to allow the test signals to bypass the communication system and connect to the communication set and perform measurements. 
     According to another embodiment of the invention, the system of invention additionally includes a status circuit, coupled to the controller, for reproducing, upon discriminating a predetermined connection status of the communication set, the predetermined connection status of the communication set for presenting to the test signals. The predetermined connection status includes off-hook and short statuses of the communication set. The status circuit reproduces the predetermined connection status by emulating an impedance of a loop comprising the subscriber line and the communication set. The status circuit emulates a linear impedance if the connection status discriminated by the discriminator indicates a short status and a non-linear impedance if the connection status discriminated indicates an off-hook status. 
     According to a further embodiment of the invention, the communication system includes a second transmission line for connecting to the telephone exchange and a second subscriber line for connecting to a second communication set. In this communication system, the first transmission line transmits signals to the first subscriber line and the second transmission line transmits signals to the second subscriber line. The controller activates the bypass circuit in accordance with the connection statuses of both the communcation sets. 
     According to a still further embodiment of the invention, the system of the invention additionally includes a signature generator, coupled to the first and second transmission lines, for generating a signature, indicative of the communication system status, on one of the transmission lines on which the test signals are detected. 
     Other objects and attainments together with a fuller understanding of the invention will become apparent and appreciated by referring to the following description and claims taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings wherein like reference symbols refer to like parts: 
     FIG. 1 shows an overall block diagram of a 2-line subscriber line multiplexer system according to an embodiment of the invention; 
     FIG. 2 shows a detailed diagram of a line card (LC) according to an embodiment of the invention; 
     FIG. 3 shows the schematic diagram of an MLT detector according to an embodiment of the invention; 
     FIG. 4 shows the schematic diagram of a line status circuit according to an embodiment of the invention; 
     FIG. 5 illustrates a detailed diagram of a remote terminal according to an embodiment of the invention; and 
     FIG. 6 shows an alternative embodiment of a ROH discriminator according to an embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows an overall block diagram of a 2-line subscriber line multiplexer system according to one embodiment of the invention. It should be understood that the invention also applies to a multiplexer system with any number of lines. 
     As shown in FIG. 1, multiplexer system includes two terminals, a central office terminal (COT)  100  and a remote terminal (RT)  200 . COT  100  is installed in a central office (CO) and includes at least one line card (LC)  110 . LC  110  comprises MLT detectors  120  and  122 , LC circuitry  126 , line status circuits  130  and  132 , signature generation circuit  136 , and 2-pole line bypass relays K 1 , K 2  and K 3 . Analog lines from a telephone exchange (not shown) are connected to the analog inputs of COT  100 , i.e., lines  1  and  2 . An MLT system (not shown) is connected to the analog inputs of COT  100  via the telephone exchange. 
     In LC  110 , bypass relays K 1 , K 2  and K 3  in LC  110  are controlled by a microcontroller in LC circuitry  126 . When operated, relay K 1  connects line  1  to bypass bus  128 ; relay K 2  connects line  2  to bypass bus  128 ; and relay K 3  connects bypass bus  128  to a telephone line  160 , known as DSL (Digital Subscriber Line) and disconnects the DSL from LC circuitry  126 . 
     In the case where a signature has to be presented on one of the lines, an appropriate relay (K 1  or K 2 ) is operated, so that signature generator circuit  136  is connected to the bypass bus  128  without activating K 3 . Signature generation circuit  136  is connected to bypass bus  128  by the internal relays of signature generation circuit  136 . These internal relays are controlled by the microcontroller in LC circuitry  126 . 
     RT  200  is connected to COT  100  through DSL  160 . RT  200  comprises a RT circuitry  210 , ROH discriminators  220  and  230 , a 1-pole DSL bypass relay K 4 , 2-pole line bypass relays K 5  and K 6  and a 2-pole latching relay K 7 . Communication sets, such as telephone sets (not shown) are connected to RT  200  at its analog inputs, i.e., lines  1  and  2 . In FIG. 1, each of telephone lines  1  and  2  and DSL  160  includes a pair of analog lines, i.e., ring and tip lines, as will be illustrated in more detail later. 
     In RT  200 , bypass relays K 4 , K 5  and K 6  and latching relay K 7  are controlled by a microcontroller in RT circuitry  210 . Bypass relay K 4 , when operated, connects DSL  160  to bypass bus  240 . Bypass relays K 5  and K 6 , when operated, can respectively connect lines  1  and  2  to bypass bus  240 . Latching relay K 7  switches between lines  1  and  2  for connecting to DSL  160 . 
     In the normal operation, when no MLT procedure is performed on the telephone lines, lines  1  and  2  are connected to subscribers via LC circuitry  126  and RT circuitry  210 , and communications among the subscribers are as usual. 
     When a test person starts an MLT procedure with the MLT system on one of the telephone lines, one of MLT detectors  120  and  122  detects the MLT signals and reports the detection to the microcontroller in LC circuitry  126 . If the multiplexer system is in good operating condition, with all the telephone sets at the subscriber&#39;s end being on hook, LC circuitry  126  sends a message to the microcontroller in RT circuitry  210  in RT  200  to request the telephone line under test to go into a bypass state. Upon receiving the message, the microcontroller in RT  200  switches latching relay K 7  on the position that corresponds to the requested line and sends an acknowledge message back to LC  110 . 
     After receiving the acknowledgment message from RT  200 , the microcontroller in LC circuitry  126  causes selected relays in LC  110  to go into a bypass state so as to connect the line under test to a corresponding bypass bus  128  or  129 . In this way, the MLT system is directly connected to the subscriber&#39;s premises through DSL  160  and various tests can thus be performed to detect faults. 
     In FIG. 1, RT circuitry  210  is provided with a SLIC (Subscriber Line Interface Circuit) (not shown) on each telephone line. The SLIC reports to the local microcontroller in RT circuitry  210  whether there is a current flowing through the loop on the associated line. If no current is detected, it indicates that either the telephone receiver is on hook or the subscriber&#39;s drop is interrupted. In such case, when an MLT signal comes on the line, the multiplexer system will go into bypass state at both LC  110  and RT  200  so that the MLT signal can make measurements and detect whether a telephone set is present (on hook) or whether the subscriber&#39;s drop is interrupted. On the other hand, if a current is detected by the SLIC in a loop, an associated ROH discriminator  220  or  230  will measure the loop impedance and report the measurement to the local microcontroller in RT circuitry  210 . The local microcontroller then reports back the measured loop impedance to the microcontroller in LC circuitry  126 , which will configure an associated line status circuit  130  or  132  to emulate the measured impedance at LC  110 . The emulated impedance will be linear in case of a short or nonlinear in case of a ROH(the telephone impedance when its receiver is off hook). In such case, if an MLT signal comes on the line, the MLT signal will measure the emulated impedance and reports back the proper result (short or ROH) to the MLT system. This emulated impedance represents the status of the line under test. When a fault is detected by the MLT, the telephone company may follow up with necessary repairs on the line. The details will be described later. 
     In the case where RT  200  is remotely powered, it loses power when LC  110  goes into bypass state. This causes bypass relays K 4 , K 5  and K 6  to switch to the bypass positions, while latching relay K 7  remains at the position before the power was down and operates to select the line for bypassing. 
     If, at the time the MLT procedure is performed, the multiplexer system or the DSL is not in good operating condition, then signature generation circuit  136  will present an appropriate signature, i.e., a set of electrical signals, to the MLT system, indicating the system status, as will be described in detail below. 
     FIGS. 2,  3  and  4  are next described. FIG. 2 shows a detailed diagram of LC  110 . FIGS. 3 and 4 show, respectively, the schematic diagrams of the MLT detector  120 ,  122  and the line status circuit  130 ,  132  illustrated in FIG.  2 . 
     In FIG. 2, signature generation circuit  136  uses four relays Ka 2 , Ka 3 , Ka 4  and Ka 5  to select a desired signature (a group of three resistors). Relay Ka 1  is used to connect the signature to bypass bus  128 , as described above. Signature generation circuit  136  is designed to accommodate the applicable signatures standardized by the Bellcore TR-909 recommendation. The resistor values in circuit  136  are those specified by this recommendation and are tabulated in Table 1 below. 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Resistance values in signature generation circuit 136 
               
             
          
           
               
                   
                 Resistor 
                 Resistance value (in ohms) 
               
               
                   
                   
               
               
                   
                 R1 
                 295k 
               
               
                   
                 R2 
                 175k 
               
               
                   
                 R3 
                 175k 
               
               
                   
                 R4 
                 130k 
               
               
                   
                 R5 
                  70k 
               
               
                   
                 R6 
                  70k 
               
               
                   
                 R7 
                  19.8k 
               
               
                   
                 R8 
                 189.1k 
               
               
                   
                 R9 
                 189.1k 
               
               
                   
                 R10 
                  17.8k 
               
               
                   
                 R11 
                 296.1k 
               
               
                   
                 R12 
                 296.1k 
               
               
                   
                   
               
             
          
         
       
     
     In FIG. 3, each MLT detector comprises a buffer  123 , two diodes D 1  and D 2 , and three resistors Rd 1 , Rd 2  and Rd 3 . Diodes D 1  and D 2  are used to bias the relative polarities of the ring and tip leads. When the MLT system sends MLT signals on a telephone line, the associated MLT detector measures the voltage across the ring and tip lines of the telephone line under test. If the voltage value is within a predetermined range, e.g., &lt;5V, buffer  123  outputs an MLT detection signal to microcontroller  127  in LC circuitry  126  to indicate that an MLT signal has been detected on the line. 
     If the multiplexer system is in good operating condition as detected by its detectors and a signaling transmission channel (both of which are not shown), and if an MLT signal is detected on one line, then whether the MLT system has fill access to the line under test depends on the connection statuses of the telephone sets connected to the two lines. 
     If the connection status of the telephone set on the line under test is on hook as detected by an associated SLIC and the connection status of the telephone set on the other line is either on hook or shorted, then after receiving the MLT detection signal from the MLT detector, microcontroller  127  in LC circuitry  126  will send a message to the microcontroller in RT  200  to request the telephone line under test to go into a bypass state. In such case, the system will go into a bypass state and the MLT system will have full access down to the subscriber&#39;s end on the line under test and be able to make necessary measurements, as described in the above. 
     If the multiplexer system is in good operating condition, and if the connection status of the telephone set on the line under test is on hook, but the connection status of the telephone set on the other line is off hook, i.e., busy, the multiplexer system will, through signature generation circuit  136 , apply a signature “System Busy” on the line under test. In this case, no MLT procedure is performed in order not to disturb communication on the other busy line. 
     In the case where the multiplexer system is in a good operating condition and the telephone connected to the line under test is off hook as detected by an associated SLIC, the subscriber loop at the COT is closed and the system does not do anything in response to detection of an MLT signal. This is the case regardless of the connection status of the telephone set on the other line. In this case, the associated ROH discriminator discriminates whether the off-hook status is due to a real ROH or a short since the SLIC does not differentiate between the two conditions. The ROH discriminator informs the microcontroller in the LC of the discriminated connection status and then the microcontroller configures an associated line status circuit at LC  110  to emulate either the ROH or short impedance, as the case may be. This will allow the MLT signal to measure the emulated impedance and report to the MLT system the appropriate status of the line under test. 
     If the system is not in operating condition because of a fault in the DSL (disconnected or shorted DSL), and if the system is provided with the “fail to copper” capability (i.e., the system will cause a selected line (usually line  1 ) to go into a bypass state when a failure occurs) and an MLT signal is detected on that line, then the system will do nothing. Thus, the MLT system will be allowed to measure the line up to the failure point, and detect the nature and location of the failure. If the MLT signal comes on the other line instead, signature generation circuit  136  will generate an appropriate signature and the MLT system will display a message “Bad DSL”, which is also provided for under a different name by Bellcore TR-909 specifications. 
     If the system is not in operating condition due to failure in one of the terminals (COT  100  or RT  200 ), then the signature generation circuit will generate an appropriate signature and the MLT system will display a message “Bad RT” or “Bad COT” to indicate where the failure is located. 
     If the system is not in operating condition because no RT has yet been installed. Most multiplexer systems still provide service to the subscriber&#39;s telephone when the COT is installed but the RT is not installed. This is a “COT bypass” situation where, as an example, relays K 1  and K 3  in FIG. 1 are not operated, and the subscriber&#39;s telephone is directly connected to the CO, via the DSL and the contacts of these relays (which bypass the LC circuitry), so that the subscriber can use the telephone on line  1  as usual, until a RT is installed and line  2  is added. If an MLT signal comes on line  1  which is connected to the subscriber&#39;s telephone, the system will do nothing and the MLT system will have full access up to the subscriber&#39;s telephone. If, instead, the MLT signal comes on line  2 , which is not connected to anything, the system will generate an appropriate signature and the MLT system will display a message “CPE not provisioned”, where CPE stands for Customer Premises Equipment. 
     If the system is in the process of start up, the signature generation circuit will generate an appropriate signature and the MLT system will display a message “System busy, try later”. 
     The various signatures which comply with Bellcore TR-909 specifications and are generated by signature generation circuit  136  under different circumstances are next described. 
     As illustrated in FIG. 2, each of lines  1  and  2  can be connected to signature generation circuit  136  by activating selected bypass relays, so that a signature can be generated and presented on the appropriate line. 
     A signature generated by generation circuit  136  complies with Bellcore TR-909 specifications and includes a set of impedance values. These impedance values include the impedance between the tip line and the ground RTG, the impedance between the ring line and the ground RRG, and the impedance between the tip line and ring line RTG. Table 2 shows a summary of various signatures generated by the system in response to MLT signals under different circumstances. 
     
       
         
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Summary of Various Signatures in Different Situations 
               
             
          
           
               
                   
                   
                   
                   
                 Signature Generated 
                   
                   
                   
                   
               
               
                   
                   
                   
                   
                 in Response to an 
               
               
                   
                   
                   
                 Connection 
                 MLT Signal Detected 
                 Relays 
               
               
                 LC 
                 RT 
                 Line 
                 Status 
                 on the Line 
                 Operated 
                 RTG 
                 RRG 
                 RTR 
               
               
                   
               
               
                 Good 
                 Good 
                 1 
                 On-hook 
                 Generate Signature 
                 Ka1, 
                 175k 
                 175k 
                 295k 
               
               
                   
                   
                   
                   
                 “System Busy, Try 
                 Ka2, K3 
               
               
                   
                   
                   
                   
                 Again Later” 
               
               
                   
                   
                 2 
                 Off-hook 
                 None 
                 K3 
                 Open 
                 Open 
                 Emulated 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 Nonlinea 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 r at LC 
               
               
                 Bad 
                 Good 
                 1 
                 Any 
                 Generate Signature 
                 Ka1, 
                 110k 
                 110k 
                 17.8k 
               
               
                   
                   
                   
                   
                 “COT Failure” 
                 Ka5, K3 
               
               
                   
                   
                 2 
                 Any 
                 Generate Signature 
                 K8, K9, 
                 110k 
                 110k 
                 17.8k 
               
               
                   
                   
                   
                   
                 “COT Failure” 
                 Ka1, 
               
               
                   
                   
                   
                   
                   
                 Ka5, K3 
               
               
                 Good 
                 Bad 
                 1 
                 Any 
                 Generate Signature 
                 Ka1, 
                 90.9k 
                 90.9k 
                 17.8k 
               
               
                   
                   
                   
                   
                 “RT Equipment 
                 Ka2, 
               
               
                   
                   
                   
                   
                 Failure” 
                 Ka4, K3 
               
               
                   
                   
                 2 
                 Any 
                 Generate Signature 
                 K8, K9, 
                 90.9k 
                 90.9k 
                 17.8k 
               
               
                   
                   
                   
                   
                 “RT Equipment 
                 Ka1, 
               
               
                   
                   
                   
                   
                 Failure” 
                 Ka2, 
               
               
                   
                   
                   
                   
                   
                 Ka4, K3 
               
               
                 Good 
                 Not 
                 1 
                 Any 
                 None 
                 None 
                 MLT 
                 MLT 
                 MLT 
               
               
                   
                 pre- 
                 2 
                 Any 
                 Generate Signature 
                 K1, K2, 
                 50k 
                 50k 
                 130k 
               
               
                   
                 sent 
                   
                   
                 “CPE Not 
                 Ka3 
               
               
                   
                   
                   
                   
                 Provisioned” or 
               
               
                   
                   
                   
                   
                 “Subscriber Channel 
               
               
                   
                   
                   
                   
                 Unit Missing” 
               
               
                   
               
             
          
         
       
     
     FIG. 4 shows a detailed diagram of the line status circuit  130 ,  132 . As shown, the MLT status circuit comprises a resistor Rp 1  connected in series with zener diodes Z 1  and Z 2 . The series combination of Rp 1 , Z 1  and Z 2  is connected to resistor Rp 2  in parallel. A relay KROH is connected to the parallel circuit. The line status circuit reproduces the status of the line under test as discriminated by an associated ROH discriminator  220  or  230 . Detection of the line status by the ROH discriminator will be described in connection with FIGS. 5 and 6. 
     When a ROH discriminator discriminates the status of the line under test as short and reports back to LC  110 , microcontroller  127  operates KROH to cause the MLT system to connect to resistor Rp 2 . Thus, the MLT system will measure a linear resistance Rp 2  between tip and ring lines, and will display a message indicating a probable short. 
     In the case that ROH, i.e., the line resistance value when the telephone set is off hook, is detected, microcontroller  127  operates KROH to cause the MLT system to connect to the series combination of resistor Rp 1 , and zener diodes Z 1  and Z 2 . Therefore, the MLT system will measure a nonlinear impedance between tip and ring lines, which is a characteristic of an off-hook telephone. The MLT system will display the message “ROH”. 
     In both cases, when impedance emulation is performed, the bypass relays are in the positions that would not cause the system to be bypassed. 
     FIG. 5 illustrates RT  200  in more detail. In FIG. 5, each of ROH discriminators  220  and  230  comprises the same components and both operate in the similar manner. When an MLT signal comes on line  1 , for example, latching relay K 7  is switched by microcontroller  212  to connect the ring and tip lines of line  1  to bypass bus  240 . Microcontroller  212  also causes a current to be provided from SLIC (Subscriber Line Interface Circuit)  214 , e.g., Erricsson SLIC, to an associated ROH discriminator  220 . 
     ROH discriminator  220  comprises a buffer  222  which functions as an amplifier with balanced input impedance, a comparator  223  with hysteresis and a plurality of resistors. When the telephone connected to line  1  is off-hook, SLIC  214  delivers a constant current to the loop comprising line  1  and the telephone. The line voltage of line  1  amplified by buffer  222  is proportional to the line resistance and is fed to comparator  223 . Comparator  223  compares the line voltage with a predetermined reference value Vref The reference voltage Vref corresponds to a line resistance of 200 ohms. Therefore, if the output of the comparator indicates that the line voltage is greater than Vref, this indicates that the line resistance is greater than 200 ohms. 
     According to the current telecommunication standards (Bellcore TR-57), the line impedance without having a telephone set connected should less than 100 ohms for line multiplexers. The telephone impedance is generally greater than 450 ohms. A 200 ohm threshold makes a good differentiation between a short and a ROH (receiver off-hook). Thus, if the line impedance is greater than 200 ohms, it indicates a ROH, but if the line impedance is less than 200 ohms, it indicates a short. If the line impedance is an open circuit, the SLIC will report the status of the telephone as on hook. The comparator output is polled by microcontroller  212 , and an appropriate message is sent to COT  100  indicating the status of the line based on the comparator output. 
     FIG. 6 shows an alternative and more accurate embodiment of the ROH discriminator. The ROH discriminator comprises a current generation circuit  302 , a SLIC  304 , an analog-to-converter (A/D) converter  306 , an amplifier buffer  310  and resistors Ra to Rd. Current generation circuit  302  includes resistors RDC 1 , RDC 2  and RDCA and a transistor Q. The gate electrode of transistor Q is coupled to microcontroller  212 . By turning the transistor on and off, two different currents may be applied to the subscriber&#39;s loop comprising the telephone line and the telephone. The line voltage between the ring and tip lines is amplified by buffer  310 . The amplified line voltage is provided to A/D converter  306 , which can be read by the microcontroller, via the data bus, to obtain the digital value of the line voltage. 
     By measuring the line voltage and thus the line impedance for two different currents, it is possible to determine whether the line impedance is linear (i.e., shorted line) or non-linear (i.e., ROH). Specifically, if the line impedance RL, as measured, is in the range of 200-600 ohms and the two measured values of RL are different by more than 15%, then ROH is indicated. if RL, as measured, is less than 200 ohms, the assumption is that the line is shorted. The line status circuit emulates the line impedance only when the telephone is off hook or shorted. In the on hook situation, the impedance is not emulated. 
     While the invention has been described in conjunction with several specific embodiments, it is evident to those skilled in the art that many further alternatives, modifications and variations will be apparent in light of the foregoing description. Thus, the invention described herein is intended to embrace all such alternatives, modifications, applications and variations as may fall within the spirit and scope of the appended claims.