Abstract:
A network interface device intended to be placed approximate to the customers equipment and remote from a central office of a telephone company (“TELCO”). The purpose of the network interface device is to allow the twisted pair transmission line between the central office and the customer&#39;s port to be selectively switched from a normal monitoring mode into a test mode. In the test mode, the NIU transmits a signal back to the central office so that the central office is able to determine whether there are any problems with the transmission line, the potential bandwidth for the transmission line and related parameters concerning the efficacy and suitability of the transmission line for a variety of communication services. A network interface device in accordance with the present invention has a monitor circuit which looks for a request signal from the central office. Upon receipt of the request signal, the detector circuit activates a relay to disconnect the customer equipment from remainder of the system and establishes a connection to a test signal generator and other components of the NIU. A test signal generator outputs onto the transmission line a predetermined signal for analysis. The duration of this signal is controlled by a reset control circuit which toggles the relay to break off connection to the remaining components of the NIU and re-establish the connection between the customer port and the network port for further ADSL communications.

Description:
TECHNICAL FIELD 
     The present invention is directed to remotely monitoring and evaluating a twisted pair transmission line, such as that used in telephone line communication. It is especially directed to communication systems, such as an asynchronous digital subscriber line (“ADSL”) which use a telephone line for communication between a customer and a central office. 
     RELATED APPLICATIONS 
     NONE 
     BACKGROUND OF THE INVENTION 
     Asymmetric Digital Subscriber Line (ADSL) is a technology which allows for simultaneous voice and data traffic to coexist over a communication channel comprising a standard telephone transmission line. Typically, the standard telephone transmission lines comprise an unshielded twisted pair of copper wire having a gauge of 22-26AWG. Twisted pairs, which can be used to connect a central telephone system (a ‘central’ unit) to a subscriber&#39;s telephone (a ‘remote’ unit) can support bandwidths of up to 2 MHz through the use of digital signal processing (DSP) technology. Thus, they can be used for bandwidth-intensive applications, such as internet access and video-on demand, as well as for carrying voice traffic. Frequency division multiplexing is used so that a plurality of signals, each occupying a different frequency band, can be simultaneously sent over the same transmission line. 
     The voice traffic band comprises a number of frequency sub-bands, or channels, ranging from DC to 20 KHz. The analog voiceband frequency is typically specified as 200-4000 Hz. Customer specified additions may include phone operation up to 8 KHz and 12-16 KHz billing tones. In addition, DC to 30 Hz frequencies are typically assigned for auxiliary analog signaling purposes, such as ringing the telephone, dial pulsing and on/off hook signaling. 
     ADSL data traffic bandwidth for CAP (carrierless amplitude and phase) modulation is typically from 35 KHz-1.5 MHZ. Of this, upstream data traffic (i.e., remote unit to central unit) uses the 35 KHz-191 KHz band, while the downstream traffic (i.e., central unit to remote unit) uses the 240 KHz-1.5 MHZ band. 
     Before providing the customer with ADSL service, the telephone company (“TELCO”) must determine whether the line is suitable for ADSL communication. This is typically done by testing the lines between the TELCO&#39;s central office or substation, and the customer&#39;s site. In cases where the ADSL service is provided to the customer&#39;s residence, this often means testing the line at the point of entry to the building in which the customer&#39;s dwelling is located. After initiation of ADSL service, there are occasions in which the customer experiences problems in ADSL communication. In those instances, then TELCO must determine whether the trouble lies with the TELCO&#39;s equipment or with the customer&#39;s equipment. To make this determination, a TELCO representative may visit the customer&#39;s site to test whether the twisted pair line from the customer&#39;s site to the TELCO central office or substation is working properly. This requires considerable expense due to the time taken by the repairman. What is desirable is an apparatus and method for remotely testing the TELCO&#39;s ADSL equipment, without having to travel to the customer&#39;s site. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an interface unit which is installed proximate to the customer&#39;s equipment, in which can be selectively operated from a remote location to test the TELCO&#39;s ADSL equipment. A device in accordance with the present invention is installed between the customer&#39;s port and the TELCO&#39;s port at the point where the line enters the customer&#39;s site. 
     A device in accordance with the present invention operates in one of two modes: a monitor mode and a test mode. In the monitor mode a subcircuit of the device looks for a request signal asking the device to switch from the monitor mode to the test mode. When a valid request signal is received from the central office on the twisted pair, the device is switched into the test mode. In the test mode, a signal generator of the device transmits a test signal from the unit to the central station on the twisted pair. A timer circuit associated with the device returns the unit to the monitor mode from the test mode after a predetermined period of time. The device also includes a voltage regulator which provides an operating voltage for the remaining components and subcircuits of the device. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention can better be understood through the attached figures in which: 
     FIG. 1 presents a block diagram of ADSL system which incorporates a network interface unit in accordance with the present invention; 
     FIG. 2 presents a block diagram of the network interface unit of the present invention; 
     FIGS. 3 and 4 present block diagrams of the functional modes in which the network interface unit operates; 
     FIG. 5 presents a detailed diagram of the network interface unit of FIG.  2 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 shows an ADSL system  10  which incorporates a device in accordance with the present invention. The ADSL system  10  comprises a transmission line  12  which extends between a central office  14  and a customer&#39;s site  16 . In addition, in accordance with the present invention, the ADSL system  10  includes a network interface unit  18  (NIU) which is physically located near the customer&#39;s equipment  16  and interfaces the customer equipment to the transmission line  12 . 
     As shown in the figures, the network interface unit  18  is dual ported with one side connected to a network interface port  22  and the other side connected to the customer port  24 . The purpose of locating a network interface unit close to the customer&#39;s site is to allow the central office  14  to test whether its transmission equipment and the transmission line  12  are functioning properly without having to visit the customer&#39;s site. This testing is made possible by the design and function of the Network Interface Unit. 
     FIG. 2 shows a functional block diagram of the NIU. As seen in FIG. 2, the NIU comprises a request signal detector  30 , a pair of line-control switches  32   a ,  32   b , a voltage protection circuit  34 , a test signal generator  36 , a power supply  38 , and a reset control circuit  40 . As seen in FIG. 5, the NIU has a first port  22   a  comprising terminals  22   b ,  22   c  on the network side and a second port  24   a  comprising corresponding terminals  24   b ,  24   c  on the customer side. 
     The request signal detector  30  is a high impedance circuit connected across the network port which is configured to detect a valid request signal sent by the central office. The request signal may take any number of forms. For instance, it could be a coded sequence of pulses sent from the central office. Alternatively, it may be a signal having predetermined spectral and amplitude characteristics. In the preferred embodiment, however, the request signal is a battery reversal condition having a predetermined duration. Once the detector  30  recognizes that a valid request signal has been received, it outputs a first control signal on lines  42   a ,  42   b , in order to switch the NIU from the monitor mode to the test mode. 
     Switches  32   a  and  32   b  connect the voltage nodes  44   a ,  44   b  at the network side port  22   a  to the customer side port  24   a , when the NIU is in the monitor mode. Thus, in the monitor mode the presence of the NIU is transparent to the customer equipment and the central office only sees a high impedance monitor circuit  18   a  shunted across the transmission lines  12 . Upon receiving the first control signal on lines  42   a ,  42   b  from the request signal detector  30 , the switches  32   a ,  32   b  break the connection between the network port to the customer port, and instead establish a connection to the circuitry of the NIU to form a test circuit  18   b . Because switches  32   a ,  32   b  are activated at the same time in response to receipt of a valid request signal, the switches  32   a ,  32   b  may be implemented as a single double-pole double-throw electronic switch which selectively connects a pair of input lines to either of two pairs of output lines. Whether in the monitor mode or in the test mode, the terminals  22   b ,  22   c  of the network side port  22   a  serve as first and second voltage nodes having an input voltage therebetween. 
     When switches  32   a ,  32   b  are toggled into the test mode, other components of the NIU are activated. First, the voltage protection circuit  34  is activated and serves to prevent an over voltage condition across the transmission line which might otherwise destroy the NIU. In addition, the power supply  38  is activated and outputs a DC operating voltage V s  which is used to drive the test signal generator  36  and the reset control circuit  40 . The NIU&#39;s power supply  38  draws its power from the office battery voltage. Therefore, the power supply  38  does not need to be provided with a local battery or other power source. 
     The test signal generator  36  outputs a signal and this signal is sent down the transmission line  12  back to the central office. The test signal maybe a narrow band tone, a broad band sweep or may take on any of an infinite number of spectral characteristics. The test signal&#39;s waveform may be sinusoidal, triangular, a square wave, or take on one of any number of different shapes. In the preferred embodiment, however, a 300 KHz square wave is generated by the test signal generator  36  and is received at the central office or other facility for analysis with known equipment such as spectrum analyzers and computers using established analytical techniques. This allows the central office to determine a number of parameters including 300 KHz insertion loss, harmonic insertion loss, return loss, phase distortion and predicted ADSL line data rate. 
     The reset control circuit  40  serves to return the switches  32   a ,  32   b  back to the monitor mode from the test mode after a predetermine lapse of time. Thus, the reset control circuit  40  limits the duration that the test signal is sent to the central office. This duration must be of sufficient length to allow the central office to assess the performance of the ADSL system between the central office to the customer&#39;s port. In the preferred embodiment, the reset control circuit allows the test signal generator to send a signal for the predetermined period of time of 30 seconds. At the end of this period, the reset control circuit  40  sends a reset control signal on lines  46   a ,  46   b  to switches  32   a ,  32   b  respectively. In response to the reset control signal, the switches  32   a ,  32   b  reconnect the network port to the customer port, and disable the test signal generator and other components of the NIU. 
     FIG. 5 shows the preferred embodiment of the NIU in accordance with the present invention. It is first noted in FIG. 5 that the switches of FIG. 2 are implemented by means of dual pole double throw latching relay whose contacts are designated K 1 /A and K 1 /B, both contacts being simultaneously activated in response to a signal received at one of two coils associated with the relay—a “set” coil and a “reset” coil. Each contact of the relay K 1  selectively connects one member of the twisted pair to a corresponding element. In the monitor mode, the relay connects the network side port  22   a  with the customer side port  24   a . Under these conditions the voltage protection circuit  34 , the test signal generator  36 , the power supply  38 , and the reset control circuit  40  are all disabled. However the request signal detector  30  is always enabled and monitors the signal coming on the transmission line. 
     As seen in FIG. 5, the request signal detector  30  provides an input impedance of R 7  plus the collective impedance of the remainder of a circuit comprising the detector  30 . As shown in Table 1, R 7  is preferably 2 MHz and so in the monitor mode, the input impedance of the NIU is at least 2 MΩ. Such an impedance level meets the requirements of some telephone companies, although a lower impedance level of, say, 500 KΩ meets the requirements of other telephone companies. Regardless of the input impedance due to R 7 , the time constants formed by R 7 , C 9  and RB, C 10  are selected to filter out a 20 Hz ringing signal so that the request signal detector does not false trigger due to the voltage reversal associated with a ringing signal. 
     As is known to those skilled in the art, the central office outputs a quiescent transmission line battery voltage of a predetermined polarity across the twisted pair  12 . The magnitude of the voltage is nominally 48 volts, although it may vary from between 42-65 volts. The transmission line battery voltage is used by the NIU both to trigger the request signal detector as well as power the remaining circuitry of the NIU. In the preferred embodiment, the request signal detector triggers on a reverse voltage condition, e.g., −48 volts, on the twisted pair for a predetermined period of time. The circuit of FIG. 5 requires a reverse voltage condition of at least 1.5 seconds, although the circuit can easily be configured to respond to some other minimum duration. Thus, to switch from the monitor mode to the test mode, the central office outputs a request signal comprising a reverse voltage of this duration on the twisted pair  12 . 
     When in the monitor mode, capacitors C 9  and C 10  charge up to the quiescent transmission line DC voltage (typically a 48 volt feed from the central office). Therefore, the voltage at PNP transistor Q 3 &#39;s emitter and base are also at the quiescent transmission line DC voltage and so Q 3  initially does not conduct. When a reverse battery condition occurs, C 10  cannot discharge due to diode CR 5 . However, under the reverse voltage condition, capacitor C 9  first discharges through R 7  and, due to the reverse voltage, then begins to charge with a polarity opposite to that of C 10 . C 10 , however, does not discharge under the reverse voltage condition because of bypass diode CR 9 . When the voltage difference between C 10  (which remains at about 48 volts during the reverse voltage condition) and C 9  exceeds the Zener voltage of CR 6  (51 volts in the preferred embodiment), Q 3  begins to go into the active region. Since Q 3 &#39;s collector is connected to Q 4 &#39;s base, Q 4  also becomes active. When transistor Q 4  turns on, its collector current causes Q 3  to turn on even harder, sending both transistors into saturation. This results in C 10  discharging through resistor R 11  and the SET coil of the relay K 1 . After approximately 2 ms, the relay K 1  sets, thereby disconnecting the customer from the transmission line  12  and engaging the test circuit. 
     Once relay K 1  has been set by detector  30 , the remainder of the NIU is activated. The voltage protection circuit  34  comprises clamping circuit VR 1  which acts as an open circuit at voltages below ±100 volts. When the voltage between the twisted pair  12  exceeds ±100 volts, clamping circuit acts as a short circuit between relay contacts K 1 /A and K 1 /B. When the current through the relay drops below the holding current, the clamping circuit once more becomes an open circuit. 
     The power supply  38  includes a bridge circuit CR 1  arranged in electrical parallel with the voltage protection circuit  34 . When the relay K 1  is set, bridge circuit CR 1  begins to conduct and provides a current through inductor L 1 . The current then flows through resistor R 1  and into base lead of NPN transistor Q 1 . Because the base-collector voltage is reversed biased, transistor Q 1  begins to conduct with a current flowing through capacitor C 3  and inductor L 2  back to the bridge circuit CR 1 . In the course of charging up C 3 , the positive node of C 3  connected to the emitter of Q 1  reaches a level of +2 volts. This level of +2 volts is maintained so long as the relay K 1  is set and some voltage of either polarity is provided at the first port. Thus, the emitter voltage of Q 1 , where it connects to capacitor C 3 , is kept steady at +2 volts, and this voltage is tapped to provide the input voltage to drive the remainder of the NIU circuit. CR 2  is preferably implemented as a TL431 and serves to ensure that the base-emitter voltage of Q 1  never exceeds a predetermined value, thereby regulating the voltage and Q 1 &#39;s emitter. Inductors L 1  and L 2  isolate the remainder of the power supply  38  from the signal generator  36  by effectively blocking AC signals. Thus, in the present invention, the power supply is run off of the line voltage of the twisted pair, onto which a signal from the test signal generator is placed. 
     The test signal generator in the preferred embodiment is implemented by means of a LMC555 timer U 1 . This timer is configured to output a square wave having a frequency of 300 KHz. The frequency output by the timer is determined selected by the specific values of R 12 , R 5  and C 5 . In the preferred embodiment, the 300 KHz frequency is used because it is a standard frequency used to benchmark ADSL system performance. However, if desired, other frequencies above 35 KHz can also be used as the test signal. 
     The output of the LMC555 timer U 1  is sent to an output driver formed by transistors Q 5 , Q 6 , current limiting base resistors R 13 , R 14  and speed-up capacitors C 11  and C 12  which preserve charge to quickly turn on/turn off Q 5  and Q 6 . The output driver is used to insulate the output pin of the 555 timer from the impedance load of transistor T 1 . 
     The output driver acts as a trigger which outputs a signal acceptable for transmission back to the central office. In this instance, the test signal generated by the output driver passes through R 6  and C 6  before it is applied to transformer T 1  prior to transmission. The test signal then passes through AC coupling capacitors C 1 , C 2  which block DC, before the test signal is applied to the transmission line via relay contacts of relay K 1 . Voltage protection element VR 2  ensures that the signal applied to coil T 1  stays below a predetermined value, in this case 5 volts. 
     The reset control circuit  40  is implemented using a second LMC555 timer U 2 , and so the reset control circuit and the reset control circuit can be formed from a single 556 dual-timer. In the preferred embodiment of FIG. 5, the LMC555 timer U 2  triggers the RESET coil of the relay K 1  after 30 seconds. It should be noted, however, that the 30 second value is determined by the specific values used for R 4  and C 7 . After 30 seconds the OUT pin on the 555 timer U 2  outputs a signal which passes through base resistor R 3  and into the base lead of transistor Q 2 . This turns on transistor Q 2 , thereby resetting relay K 1 . Once relay K 1  has been reset, the effect of the signal generator  36  no longer impacts the signal on the transmission line and the transmission line itself returns to the monitor mode in which the request signal detector  30  further awaits a valid request signal. 
     Table 1 lists the component values of the inductors, resistors and capacitors in the circuit of FIG. 5, while Table 2 provides information about the remaining components. 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Component Values for Inductors, Resistors &amp; 
               
               
                 Capacitors in Filter Circuit of FIG. 5 
               
             
          
           
               
                   
                 COMPONENT 
                 VALUE 
               
               
                   
                   
               
               
                   
                 L1, L2 
                 470 μH 
               
               
                   
                 R1 
                 20 KΩ 
               
               
                   
                 R2 
                 50 KΩ 
               
               
                   
                 R3 
                 1200 Ω 
               
               
                   
                 R4 
                 1 MΩ 
               
               
                   
                 R5 
                 18 KΩ 
               
               
                   
                 R6 
                 100 Ω 
               
               
                   
                 R7 
                 2 MΩ 
               
               
                   
                 R8 
                 100 KΩ 
               
               
                   
                 R9 
                 249 KΩ 
               
               
                   
                 R10 
                 249 KΩ 
               
               
                   
                 R11 
                 100Ω, ½W 
               
               
                   
                 R12 
                 1 KΩ 
               
               
                   
                 R13, R14 
                 5 KΩ 
               
               
                   
                 C1, C2 
                 0.068 μF 
               
               
                   
                 C3 
                 100 μF 
               
               
                   
                 C4 
                 0.1 μF 
               
               
                   
                 C5 
                 100 pF 
               
               
                   
                 C6 
                 0.1 μF 
               
               
                   
                 C7 
                 20 μF 
               
               
                   
                 C8 
                 100 μF 
               
               
                   
                 C9 
                 1 μF 
               
               
                   
                 C10 
                 47 μF 
               
               
                   
                 C11 
                 200 pF 
               
               
                   
                 C12 
                 200 pF 
               
               
                   
                 C13, C14 
                 0.1 μF 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Components in Circuit of FIG. 5 
               
             
          
           
               
                 COMPONENT 
                 DESCRIPTION 
               
               
                   
               
               
                 CR1 
                 KBP204G from Diodes, Inc. 
               
               
                 CR2 
                 TL431 
               
               
                 CR3, CR8 
                 1N5229B, 4.3 V Zener 
               
               
                 CR4, CR5, CR7, CR9 
                 1N4454 
               
               
                 CR6 
                 1N4757A 51 V Zener 
               
               
                 VR1 
                 Part no. P0800EC from Teccor 
               
               
                 VR2 
                 1.5KE10C from Motorola 
               
               
                 T1 
                 651-K427 from Midcom 
               
               
                 K1 (dual relay) 
                 TQ2E-L2-3 V from Aromat 
               
               
                 Q1, Q2, Q4, Q6 
                 MPS8099 from Motorola 
               
               
                 Q3, Q5 
                 MPS8599 from Motorola 
               
               
                 U1, U2 
                 555 CMOS timers 
               
               
                   
               
             
          
         
       
     
     A device in accordance with the present invention can be used whenever it is desirable to temporarily isolate equipment connected to a twisted pair transmission line for testing purposes. Thus, instead of only being useful for ADSL services, the present device can be used in conjunction with a variety of narrowband and broadband communication systems which use a twisted pair. In such case, changing the signal generator to output different test signals may allow ones to test the suitability of the twisted pair for a wide variety of communication services. 
     A device in accordance with the present invention may be implemented on a printed circuit board using discrete components and portions may even be implemented as an application-specific integrated circuit (ASIC). In addition, as is known to those skilled in the art, portions of the signal generator circuit and reset control circuit, among other components of the device, can be replaced by a microprocessor or other programmable unit, albeit at greater expense using current technology. 
     Therefore, while the above invention has been described with reference to certain preferred embodiments, it should be kept in mind that the scope of the present invention is not limited to these. One skilled in the art may find variations of these preferred embodiments which, nevertheless, fall within the spirit of the present invention, whose scope is defined by the claims set forth below.