Abstract:
An in-line data protector for preventing a test set from accessing a telephone line under a predetermined condition is disclosed. The in-line data protector selectively connects the test set and the telephone line responsive to detection of a data signal on the telephone line. The in-line data protector includes a power circuit for providing general operating power, a processor connected to the telephone line, and a relay serially connected between the telephone line and the test set. When general operating power is received, the processor analyzes the telephone line by emulating a single band filter. By using a single band filter, a very good determination can be made as to whether a signal on the telephone line is a data signal or a voice signal. If the signal is not a data signal, the processor instructs the relay to close, thereby connecting the telephone line to the test set.

Description:
TECHNICAL FIELD 
     This invention relates generally to telephone line interface devices, and more particularly, to a method and apparatus for preventing a test set from accessing a telephone line under a predetermined condition. 
     BACKGROUND 
     Telecommunications in general is advancing in many different directions. Such advancements can be divided into two categories: digital signals and analog signals. Analog signals provide conventional communication paths between two nodes in a telecommunication network. For example, a conventional home telephone may establish a connection with a central office through a pair of TIP and RING lines (collectively referred to as a telephone line) for the purpose of transmitting analog signals back and forth. The analog signals may include voice and/or analog computer modem traffic. 
     Digital signals also provide communication paths between two nodes in a telecommunication network. For example, a conventional home computer may establish a connection with a central office through a telephone line for the purpose of transmitting digital signals back and forth. A contemporary example would be a digital subscriber line, or DSL, for establishing a high speed data path between the computer and another node connected (directly or indirectly) to the central office. 
     Analog signals are defined to a predetermined bandwidth, called a voice band. The voice band typically ranges from a few hundred hertz (Hz) to about 3 kilo Hz (kHz), which coincides with a sound range for a human ear. Any signal carried on a telephone line that is in the voice band is deemed to be an “in-band” signal. Also by definition, analog signals must exceed a predetermined voltage threshold. The voltage threshold represents the amplitude of the signals, measured either in decibels (db) or volts peak-to-peak (Vp-p). 
     Digital signals are also defined to a predetermined bandwidth, called a data band. The data band typically ranges from 3 kHz to several thousand kHz. Any signal carried on a telephone line that is in the data band is deemed to be an “out-of-band” signal. Also by definition, data signals must exceed a predetermined voltage threshold. The voltage threshold represents the amplitude of the signals, measured either in decibels or volts peak-to-peak. 
     Telephone lines often require maintenance and repair. During such maintenance, a butt-in test set is often used to detect the presence of an audio signal on the line. Typically, the test set is connected across the TIP and RING lines and placed off-hook, thereby seizing the lines. If the telephone line is carrying an out-of-band digital signal, however, the person using the test set will not hear anything and inappropriately seize the line. As a result, the out-of-band digital signal will be corrupted by the test set. Specifically, the test set will provide an electrical load to the telephone line. After a period of time, this load will adversely affect the data signal, thereby corrupting the data. 
     Therefore, it is desired to accurately test for an out-of-band digital signal when a telephone line is to be seized. If the out-of-band digital signal is detected, it is further desired to prevent the line from being seized and to indicate such to the user. 
     It is also desired that the test set work properly with different types of telephone lines. For example, a central office will often supply a direct current (DC) voltage to the telephone line. This DC voltage can supply power to many conventional telephones, and has other conventional uses. However, some telephone lines do not have any applied DC voltage. It is important that a test set work with a telephone line, regardless of its type. 
     It is further desirable to support sequential automatic connection testing. Although the ideal method for connecting a test set to a telephone line is to first monitor the line with the test set in the on-hook mode , and then switch to the off-hook mode only after assuring that the line is not carrying data signals, it is common practice among telecom users to leave the test set in the off-hook mode and move the test set down a column of different telephone line terminals. This type of testing, called sequential automatic connection testing, can corrupt data signals in many types of telephone lines. 
     U.S. Pat. No. 5,617,466 describes a mechanism for controllably enabling a test set to assert off-hook condition on a telephone line if a prescribed voltage level is detected and no out-of-band digital signals are detected. However, this patent requires the existence of a DC voltage on the telephone line. Also, this patent does not support sequential automatic connection testing. Specifically, the patented circuit includes a rectifier bridge, similar to the rectifier bridge commonly found in most test sets. The additional voltage drop across the second bridge will degrade the performance of the test set in certain situations. 
     U.S. Pat. No. 4,939,765 describes an interlock circuit for preventing corruption of digital signals on a telephone line. However, this patent only checks for data during a limited test period. Therefore, this patent requires the existence of a DC voltage on the telephone line to perform sequential automatic connection testing. It will never know when it moves to a new telephone line that does not have a DC voltage present, and will automatically apply a data-corrupting load to the unpowered line. 
     SUMMARY 
     In response to the problems and needs described above, provided is an improved method and apparatus for preventing a test set from accessing a telephone line under a predetermined condition. In one embodiment, the apparatus selectively connects the test set and the telephone line responsive to detection of a data signal on the telephone line. The apparatus includes a power circuit for providing general operating power, a processor connected to the telephone line, and a relay serially connected between the telephone line and the test set. When general operating power is received, the processor analyzes the telephone line by emulating a single band filter By using a single band filter, a very good determination can be made as to whether a signal on the telephone line is a data signal or a voice signal. If the signal is not a data signal, the processor instructs the relay to close, thereby connecting the telephone line to the test set. 
     In some embodiments, the processor includes an audio output connected to the test set. If a data signal is determined to be on the telephone line, an audio output signal can thereby be provided to the test set. If no data signal is determined to be on the telephone line, no audio output signal is provided to the test set, thereby making the existence of the apparatus transparent to the test set. 
     In some embodiments, an amplifer and Schmitt trigger are connected between the telephone line and the processor for amplifying any signals on the telephone line. 
     In some embodiments, the power circuit includes a sense circuit to detect if the telephone line includes a DC voltage (is powered). In some embodiments, the power circuit also includes a test circuit, so that if the telephone line is unpowered, a user may manually activate the power circuit. A power hold circuit may also be provided for maintaining the activation of the power circuit for a predetermined period of time (e.g., 12 seconds) after initial user activation. 
     In some of the embodiments with the test circuit, in response to user activation, the processor can repeatedly analyze the telephone line to determine if a data signal exists. 
     As a result, several advantages are achieved over the prior art. One advantage is that the apparatus operates as a retrofit to an existing test set, requiring a reduced number of components and a very small package. 
     Another advantage of the present invention is that with unpowered lines, the apparatus works in a user-activated mode. If no data is detected, connection between the test set and telephone line is established. Furthermore, power-up may be maintained by an internal timer. If the apparatus is removed from the telephone line at anytime during power-up and then connected to another telephone line carrying data, it will quickly disconnect the test equipment and issue an alerting signal to the user. 
     Another advantage of the present invention is that with powered lines, the apparatus provides data protection that supports sequential connections to multiple telephone lines without unduly loading the telephone line. Furthermore, the apparatus allows the use of accustomed testing practices without the introduction of additional diode drops and their associated long loop performance and measurement accuracy degradations. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an illustration of a test set and an in-line data protector for testing a telephone line. 
     FIG. 2 is a schematic block diagram of one embodiment of the in-line data protector of FIG.  1 . 
     FIG. 3 is a detailed version of the schematic block diagram of FIG.  2 . 
     FIG. 4 is a flow chart illustrating the functionality of the in-line data protector of FIG. 1 
     FIG. 5 is a graph illustrating the performance of the in-line data protector of FIG. 1, as compared to the performance of other devices. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 1, a test set  10  is connected to a telephone line  12  through an in-line data protector  14 . The in-line data protector  14  accurately tests for an out-of-band digital signal when the telephone line  12  is to be seized by the test set  10 . If the out-of-band digital signal is detected, the in-line data protector  14  prevents the line  12  from being seized, and provides an audio indication thereof. It is understood that the following disclosure provides many different embodiments, or examples, for implementing different features of the present invention. Techniques and requirements that are only specific to certain embodiments should not be imported into other embodiments. Also, specific examples of integrated circuits, components, and voltage levels are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to limit the invention from that described in the claims. 
     The test set  10  is a conventional device, including a mouth piece  16 , an ear piece  18 , and a switch  20 . The switch allows the test set  10  to be selectively placed in either an on-hook or off-hook condition, for selectively opening or closing, respectively, a loop with the TIP and RING lines of the telephone line  12 . The test set  10  can operate in a talk mode while being connected (off-hook) with the telephone line  12 , or a monitor mode while being disconnected (on-hook) with the telephone line. 
     The in-line data protector  14  includes a plastic shell  22  having a removable opening  24  for receiving a battery such as a 9 Volt battery. The plastic shell  22  also includes a test switch actuator  26  for selectively activating an electric circuit, discussed in greater detail below. The in-line data protector  14  also includes two intermediate lines  28   a  and  28   b  for connecting to the telephone line  12  and the test set  10 , respectively. Connections to the telephone line  12  and the test  10  may be any type of conventional connection, such as a wire clip connection or a jack-type connection. For the present disclosure, when the intermediate line  28   a  and the telephone line  12  are connected, they can be considered as one and the same. Likewise, when the intermediate line  28   b  and the test set  10  are connected, they too are considered as one and the same. 
     Referring now to FIG. 2, the in-line data protector  14  of FIG. 1 includes a circuit  30  for implementing specific functions. FIG. 3 provides a schematic realization of one embodiment of the circuit  30 . The circuit  30  connects to the TIP and RING lines of the telephone line  12 , herein designated as lines  12   a  and  12   b , respectively. The circuit  30  also connects to the TIP and RING lines of the test set  10 , herein designated as lines  10   a  and  10   b , respectively. Although not shown, the circuit  30  receives power from the 9 Volt battery discussed above, with respect to FIG.  1 . 
     The circuit  30  includes a microcontroller  32 , such as a PICmicro MCU from Microchip Technology. The microcontroller  32  includes an internal timer/counter for improved power and space requirements. Although the following description uses the PICmicro microcontroller, it is understood that many different processors may be used, with appropriate voltage and/or signal modifications. It is further understood that the following description uses discrete components for the sake of clarity, and that alternative solutions may utilize different components, such as one or more application specific devices, to accomplish one or more of the features of the present invention. 
     The circuit  30  also includes a solid state relay  34  connected to a first output O 1  of the microcontroller  32  for selectively connecting one of the TIP and RING lines  12   a ,  12   b  to one of the test set lines  10   a ,  10   b , respectively. For the sake of example, the relay  34  selectively connects line  12   b  to line  10   b . The relay  34  includes an R-C bypass circuit  35  so that a small amount of alternating current (AC) signal is fed to the line  10   b , without the impedance being high enough to load the telephone line  12   b . In one embodiment, the relay  34  is a fast acting component specifically designed for telecom hookswitch applications. It is normally open (disconnected) and when closed, adds only about 30 Ohms of additional loop resistance without any diode voltage drop. 
     An amplifier  36  connects line  12   b  to a Schmitt triggered first input I 1  of the microcontroller  32 . As a result, an amplified version of any data signal or voice signal from the line  12   b  is provided to the microcontroller  32  for analysis according to the present invention. The combination of the signal amplification and the Schmitt triggering provides certain benefits discussed below. 
     The telephone line  12  may or may not include a DC voltage. As a result, different power up configurations are provided for the different types of telephone line. If the line  12   b  includes a DC voltage (the line is powered), it will be detected by a DC sense circuit  38 , which will instruct a power switch  40  to activate, thereby enabling the circuit  30 . This is referred to as Mode 1 operation, discussed in greater detail below. In Mode 1 operation, the power switch  40  provides regulated voltage to the various components of the circuit  30  as long as the DC sense circuit  38  continues to detect the DC voltage. 
     Alternatively, if there is no DC voltage on the line  12   b , a user can activate the circuit  30  by pressing the test switch actuator  26 . This is referred to as Mode 2 operation, also discussed in greater detail below. The pressed test switch actuator  26  will also instruct the power switch  40  to activate, thereby providing regulated voltage to the various components for as long as the test switch actuator is pressed. Furthermore, the test switch actuator  26  connects to a second input I 2  of the microcontroller  32  through a test switch circuit  42 . The test switch circuit  42  informs the microcontroller  32  that power up was responsive to activation of the test switch actuator  26 , and not by the presence of DC voltage on the line  12   b.    
     A second output O 2  of the microcontroller is connected to a power hold circuit  44 . The power hold circuit  44  activates the power switch  40  in test situations, as described above. In this way, the user can release the test switch actuator  26  and the power hold circuit  44  will continue to activate the power switch  40 , under control of the microcontroller  32 . 
     A third output O 3  of the microcontroller  32  is connected to the line  10   b  through an R-C circuit  46 . The third output O 3  and R-C circuit  46  are used to provide tones to the test set upon detection of a data signal on the line  12   b . In one embodiment, the tones are provided to the test set ear piece  18  (FIG.  1 ), which can only be heard if the test set  10  is in the monitor (on-hook) mode. If the test set  10  is in the talk (off-hook) mode, the user will not hear the alert tone, but the line remains disconnected. The tones are provided only when the data signal is detected and the relay  34  is open. In this way, the in-line data protector  14  is transparent to a user of the test set  10  unless a data signal is present. 
     Referring also to FIG. 4, the in-line data protector  14  operates in accordance with a method  100  for implementing certain features of the present invention. The method  100  is always running, provided that the power source (the 9V battery) is connected to the circuit. It is understood, however, that various modifications can be made to the method  100  while still implementing the present invention. Certain modifications are discussed in detail, below. 
     The in-line data protector  14  has three modes of operation. In the first mode of operation (Mode 1), the in-line data protector  14  detects a DC voltage on the line  12   b  and powers up accordingly. The in-line data protector  14  checks for a data signal on the line  12   b  and continually remains powered up. In one embodiment, the in-line data protector  14  repeatedly checks for data on the line  12   b , while in another embodiment, the in-line data protector only checks for data once. The latter embodiment works well with certain test sets  10  that have keypads that produce harmonic tones that may falsely appear to be data signals. When disconnected from the line  12   b , the in-line data protector  14  powers down. 
     In the second mode of operation (Mode 2), the line  12   b  does not have a DC voltage associated therewith. In this mode, the in-line data protector  14  powers up responsive to the user pressing the test switch actuator  26 . Operation is similar to that of Mode 1, but after a predetermined period of inactivity (e.g., 12 seconds), the in-line data protector  14  powers itself down. 
     In the third mode of operation (Mode 3), the line  12   b  also does not have a DC voltage. In this mode, the in-line data protector  14  allows the test set  10  to be used to detect a tracing tone for line identification. The in-line data protector  14  remains unpowered in this mode, but passes a portion of the tracing tone signal through to the test set through the R-C circuit  35 . The tracing tone is provided through the R-C circuit  35 , which maintains the high impedance of the test set  10  while allowing the tone signal to pass, regardless of the test set. 
     The method  100  begins at step  102 , where the DC sense circuit  38  detects if any DC voltage appears on the line  12   b . A DC voltage only appears on some telephone lines, and it is desired that the present invention accommodate different types of telephone lines. It is further understood that no DC voltage will be detected until a user connects the in-line data protector  14  to the telephone line  12 . If a DC voltage is sensed, then at step  104 , Mode 1 is initiated. The power switch  40  is activated by the DC sense circuit  38 , power is provided to the entire circuit  30 , and the microcontroller  32  resets for operation. 
     If at step  102  no DC voltage was detected, the in-line data protector  14  can also be activated by a user pressing the test switch actuator  26 . At step  106 , if the test switch actuator  26  is pressed, Mode 2 is initiated. The pressed test switch actuator  26  will activate the power switch  40  thereby providing power to the entire circuit  30  and to the microcontroller  32 . At step  110 , the pressed test switch actuator  26  also informs the microcontroller  32  through input I 2  that the test switch was pressed. When the test switch actuator  26  is release by the user, the microcontroller  32  maintains power through the power hold circuit  44  attached to the second output O 2 . 
     If at step  106  the test switch is not pressed, execution returns to step  102 . This indicates an idle condition when the in-line data protector  14  is not powered up. It also may indicate Mode 3 operation, where there is no DC voltage on the line  12   b , the test switch actuator  26  is not pressed, but a portion of a signal from line  12   b  passes through the R-C circuit  35  and to the line  10   b . 
     At step  112 , the microcontroller  32  receives an amplified signal from the line  12   b  through the op amp  36 . The microcontroller  32  analyzes the amplified signal by emulating a single band filter. The microcontroller  32  provides the capability for single band frequency detection (instead of a multiple band frequency detector as used in the prior art) by measuring the number of cycles over a period of time and comparing the number to the frequency cutoff between the voice band and the data band (about 3 KHz). 
     Referring now to FIG. 5, the operation of the microcontroller  32  is illustrated on a graph  200 . A vertical axis  202  of the graph  200  designates peak-to-peak voltage of a signal (voice or data) on the line  12   b  and a horizontal axis  204  represents the frequency of the signal. A first region  210  designates a voice band, i.e., a range of possible voltages and frequencies for voice signals. The voice band  210  resides between about 30 Hz to about 3 kHz on the horizontal axis  204  and may include a wide range of voltage Vp-p on the vertical axis  202 . A second region  212  designates a data band, i.e., a range of possible voltages and frequencies for data signals. The data band  212  includes, for example, a T1 frequency of about 1.5 MHZ. 
     A first test area  214  illustrates the operation of the microcontroller  32  according to one embodiment of the present invention. By emulating a single band filter, the microcontroller  32  can detect a significant portion of the data band  212 , without detecting any portion of the voice band  210 . In comparison, a second test area  216  and a third test area  218  illustrate the operation of conventional units, such as those described in U.S. Pat. Nos. 4,939,765 and 5,617,466. It is noted that both of these patents teach multi-band pass filters to detect signals in the data band  212 . However, as can be clearly seen from the graph  200 , neither of these test areas  216 ,  218  provide adequate coverage of the data band  212 . Furthermore, the test area  218  also overlaps the voice band  210  and therefore is likely to produce false data signal readings. 
     Referring again to FIG. 4, upon completion of step  112 , execution proceeds to step  114  where a determination is made if the signal from line  12   b  is a data signal. If not, then at step  116  the relay  34  (which is open by default) is closed, thereby connecting the line  12   b  to the line  10   b  and closing the loop. 
     At step  118 , a determination is made as to which mode (Mode 1 or Mode 2) is being implemented. If Mode 1 is being implemented, execution proceeds to step  120  where the microcontroller  32  waits to be power down. If Mode 2 is being implemented, execution proceeds to step  122 , where the microcontroller  32  waits a predetermined period of time (e.g., 12 seconds) after the test switch actuator  26  is pressed and no data has been detected. While it is waiting, it returns to step  114  and repeatedly checks for a data signal. After the predetermined period of time has elapsed, the in-line data protector  32  will power down and execution will return to step  102 . The user can press the test button at any time during the predetermined time period for an additional 12 seconds. 
     If at step  114  a data signal is detected, execution then proceeds to step  124  and the relay is not activated. The user is informed that data is detected and no connection is made between lines  12   b  and  10   b . Execution then returns to step  112  where the microcontroller  32  continues to analyze the amplified signal. A power down of the circuit  30  will, of course, terminate operation of the method  100 . 
     An advantage of the present invention is that it operates as a retrofit to an existing test set, requiring a reduced number of components and a very small package. 
     Another advantage of the present invention is that with unpowered lines, the present invention assumes Mode 2 operation. In this mode, power-up and data test are initiated by pressing the test switch actuator  26 . If no data is detected, connection of the test set  10  to the line  12  is established. 
     Furthermore, in Mode 2 operation, power-up is maintained by an internal timer, which allows a specific amount (12 seconds) of operation at a time. Unlike the prior art, in Mode 2 operation, the in-line data protector  32  tests for data continuously. If the in-line data protector input is removed from the line  12   b  at anytime during power-up and then connected to another line carrying data, it will disconnect the test equipment from the line in less than 10 milliseconds and issue an alerting signal to the user. Should the power-up timer expire before connection to another line, the test switch actuator  26  must be pressed again to initiate another test operation. 
     Another advantage of the present invention is that with powered lines, the present invention assumes Mode 1 operation. This supports sequential automatic connection testing because the in-line data protector  14  powers-up automatically when a DC voltage is detected and powers-down automatically when the DC voltage is removed. The DC voltage is sensed at the line  12   b  without introducing an additional rectifier bridge into the phone line loop. Furthermore, the in-line data protector  14  allows the use of accustomed testing practices without the introduction of additional diode drops and their associated long loop performance and measurement accuracy degradations. 
     Although illustrative embodiments of the invention have been shown and described, other modifications, changes, and substitutions are intended in the foregoing disclosure. Also, different considerations may require different circuit components and arrangements. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the disclosure.