Abstract:
A method includes generating a pulse, transmitting the pulse on a first cable of a multi-cable communication system, monitoring a second cable of the multi-cable communication system for the pulse or a reflection of the pulse on the second cable, and, if the monitoring detects the pulse or the reflection on the second cable, outputting a notification of a short between the first cable and the second cable.

Description:
PRIORITY CLAIM 
       [0001]    This application is a divisional application of U.S. patent application Ser. No. 12/653,273, entitled “Cable Diagnostics for Base-T Systems,” filed on Dec. 9, 2009, now U.S. Pat. No. 8,461,848, which claims the benefit of U.S. Provisional Patent Application Ser. No. 61/201,519 filed on Dec. 10, 2008 and entitled “Cable Diagnostics for Base-T Systems.” The disclosures of both of the above-referenced applications are hereby incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates to cable diagnostics and in particular to a method and apparatus for cable diagnostics in a Base-T system. 
       RELATED ART 
       [0003]    Cable diagnostics for Base-T systems are useful for estimating cable length and identifying cable faults. Prior solutions for single-ended cable diagnostics use the method of time domain reflectometry (TDR), where a pulse of finite duration is transmitted and the reflected pulse (also known as the echo of the transmit pulse) is captured to analyze the cable, but the prior art system utilizes pulses which may be confused with other channel noise or signals. In addition, the prior art TDR operation occurs when the channel is disabled or when it is not yet in service. In addition, the prior art does not use an IEEE 802.3 standard compliant transmit pulse. If an active, standard compliant PHY is connected at the far end, a non-compliant pulse will cause errors on that PHY. 
         [0004]    In one prior art case disclosed in U.S. Pat. No. 7,245,129, the shape of the transmit pulse is optimized for the channel characteristics of the cable in order to maximize the signal to noise ratio of the reflected pulse. The reflected amplitude is analyzed for positive and negative peaks, and the relative locations of these are used to characterize the cable. 
         [0005]    In another case disclosed in U.S. Pat. 6,934,655, the transmit signal is chosen for good auto-correlation properties so that the timing and amplitude of its reflection can be easily determined. These properties are used to characterize the cable under test. Alternatively, in this method, an echo cancellation filter can be adapted to the reflected signal, and the echo canceller coefficients can be analyzed to characterize the cable. 
       SUMMARY 
       [0006]    Cable diagnostics for Base-T systems are useful for estimating cable length and identifying cable faults. Diagnostics can be deployed as part of an Ethernet PHY to allow diagnostics to be conducted in the field. As an interoperable PHY, the near end should be able to conduct its diagnostics with any IEEE 802.3 standard compliant vendor as the far end PHY. The diagnostics signal may be standard compliant. 
         [0007]    For a single-ended system to measure cable length, the best results typically come from TDR (Time Domain Reflectometry). Disclosed herein is a technique of transmitting a pulse of finite duration and measuring the amplitude and round-trip delay of the pulse&#39;s reflection, also known as its echo. In one embodiment, this innovation uses an IEEE 802.3 compliant transmit pulse, such as an auto-negotiation signal (AN pulse), to conduct cable diagnostics. 
         [0008]    The benefits of a standard compliant pulse are two-fold. First, the signal can be used with any vendor on the far-end without causing errors. Second, the signal requires no special hardware or software to produce and, therefore, reduces system complexity and cost. 
         [0009]    If there is another standard compliant PHY at the far-end, it will attempt to auto-negotiate with the near-end PHY via its own AN pulse(s). These pulses can resemble the echo pulses and can therefore cause incorrect length and termination measurements. 
         [0010]    To mitigate the likelihood of incorrect measurements, this invention measures the cable length and termination with multiple AN pulses. It then applies non-linear filters to these redundant measurements in such a way that it produces accurate cable diagnostics information even in the presence of a far-end signal, as long as the behavior of the far-end PHY is also IEEE 802.3 compliant. 
         [0011]    Other systems, methods, features and advantages of the invention will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. In the figures, like reference numerals designate corresponding parts throughout the different views. 
           [0013]      FIG. 1  illustrates a block diagram of an example embodiment of a two station communication system. 
           [0014]      FIG. 2  illustrates a block diagram of an example embodiment of a communication system configured with channel analysis capability. 
           [0015]      FIG. 3  illustrates an exemplary plot of raw and low pass filtered echo. 
           [0016]      FIG. 4  illustrates an exemplary plot of magnitude of echo with near end limiting. 
           [0017]      FIG. 5  illustrates an exemplary plot of magnitude of echo after high pass filtering and a plot of far end threshold with the duration requirement. 
           [0018]      FIG. 6  illustrates an exemplary plot of comparison of cables with and without inter-pair shorts. 
           [0019]      FIG. 7  illustrates an exemplary plot of superimposed plots of open terminations for various cable lengths. 
           [0020]      FIG. 8  illustrates an exemplary high level operational flow diagram of a method for cable analysis for a terminated cable having intrapair (within each pair) opens or shorts, or no faults within each pair. 
           [0021]      FIG. 9  illustrates an exemplary operational flow diagram of an example method of performing analysis of an echo signal to estimate the length of the cable to a far end PHY or to the distance to an intra-pair fault, if a fault is present. 
           [0022]      FIG. 10  illustrates an exemplary operational flow diagram of a method for cable analysis for detecting shorts between wire pairs, including between one or more wires on one pair and one or more wires on another pair including across pairs, and the distance to the short, if a short is present. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]      FIG. 1  illustrates a block diagram of an example embodiment of a two station communication system. As shown a first set of transceivers  104 A- 104 D is part of Station A  102 , which is located at a first location. Station A  102  communicates over channels  108 A- 108 D with a second set of transceivers  112 A- 112 D that are part of Station B  110  and which are located at a second location. One exemplary communication system that adopts such a configuration is a network Ethernet communication system. Those of ordinary skill in the art understand communication systems of this nature and as a result an extensive discussion is not provided herein. 
         [0024]      FIG. 2  illustrates a block diagram of an example embodiment of a communication system configured with channel analysis capability. This is but one possible embodiment of hardware and as such it is contemplated that one of ordinary skill in the art may generate different embodiment that do not depart from the claims that follow. As shown and discussed below, the systems of  FIG. 2  may comprise hardware, software or a combination of software and hardware. 
         [0025]    As shown, one or more communication channels  204  connect to a communication device  208 . The channels  204  may comprise any type channel but in this example embodiment the channels comprise twisted pair copper, such as CATS or variations of this type channel. The communication  208  device may comprise any type communication device, but in this example embodiment the communication device  208  comprises an Ethernet communication device. The device may be configured as a communication card within a computer or on a motherboard or network board or other network device, or as part of a switch, router, hub, server or any other such network communication device. 
         [0026]    The channels  204  connect to a hybrid  212  which separates incoming and outgoing signals as is understood in the art. The hybrid isolates and sends incoming signals to an analog to digital converter ADC  220 . The output of the ADC  220  connects to a receiver  228 . The output of the receiver  228  provides the received and processed signal to upper layers  236  or other components of the communication device for use by a user or other processing elements. 
         [0027]    Likewise, outgoing signals from upper layers  232  or other processing elements are presented to the transmitter  224 . The transmitter  224  executes standard processing on the input to generate an output suitable for transmission over the one or more channels  204 . The DAC  216  converts the digital output from the transmitter to an analog format and a driver (not shown) may be present to amplify the outgoing signals to the proper power level for transmission over the channels  204 . 
         [0028]    A digital signal processor (DSP)  240  connects to or interfaces with the receiver  228 . The DSP  240  may also interface with the transmitter to coordinate operation of the system as described herein. Although shown as a DSP  240 , the DSP may comprise any processing element including a processor, ASIC, FPGA, DSP, control logic, or other such device. Memory  244  connects to and is accessible by the DSP  240 . The memory  244  may comprise any type read/write memory suitable for storing data and machine readable code. The machine readable code stored on the memory  244  may be configured to perform the routings and data processing disclosed herein. 
         [0029]    A user interface  248  connects to the DSP  240  to facilitate user interface to the DSP and the data and processing results stored in memory. The user interface  248  may comprise any type interface including but not limited to a computer bus, USB port, serial port, firewire, Ethernet connection, or another presently existing or future developed interface. In operation, the system of  FIG. 2  performs the processing operation described herein and discussed below. 
         [0030]    In one embodiment of this method and apparatus, an IEEE 802.3 compliant auto-negotiation (AN) pulse is transmitted on a 4-pair UTP Ethernet cable. A buffer is used to capture the received signal samples from the time of the transmitted pulse to a predetermined time period, such as 5 microseconds, after the transmission. In one embodiment the AN pulse comprises a square pulse of 100 nanosecond duration, The received reflection signal is used to identify the near-end and far-end reflections of the AN pulse which are caused, respectively, by the cable connector at the near-end PHY and the termination at the far end of the cable. The far end of the cable may be connected to another PHY, or it maybe left disconnected. In case of a cable fault, such as a shorted wire or a broken (or “open”) wire, the far end echo will be a reflection of the pulse from the location of the fault. 
         [0031]    In one embodiment, the near-end echo is identified by applying an amplitude threshold proportional to the noise level of the signal. To identify the far end echo, the receive signal is first filtered to improve the signal to noise ratio, and then another amplitude threshold is applied. The number of samples between the near-end and far-end reflections may be proportional to the length of the cable or the distance to the nearest cable fault. The amplitude of the far-end echo may determine the termination and/or fault type. 
         [0032]    In one embodiment, this method may have a period of silence before any measurement is taken to ensure that a standard compliant far-end PHY will not transmit anything other than an auto-negotiation (AN) signal during the cable diagnostics measurements. If there is such a device on the far-end, it may be transmitting auto-negotiation pulses while attempting to establish a link. In order to avoid false detection of the far-end reflection due to the presence of these pulses in the received signal, the measurement process may be repeated multiple times at close intervals. Since the AN pulses may be configured to have a specific duration of silence (specific to a communication standard) in between each pulse, repeating the test multiple times with a much shorter interval between pulses ensures that, statistically, the majority of measurements will be taken on actual echo pulses. The measured lengths are median filtered, or other type filtering or analysis, to eliminate possible interference from far-end AN pulses. The termination type, i.e. the presence of a far-end PHY versus a cable fault, is determined by a majority rule across the multiple trials. 
         [0033]    The one or more embodiments of the method and apparatus described herein has numerous advantages over the prior art. These one or more advantages include but are not limited to:
       uses an IEEE 802.3 compliant transmit signal for cable diagnostics;   will work with any IEEE 802.3 compliant device on the far end;   no special hardware, software, or extra power is required;   uses redundancy and non-linear filtering to reduce the likelihood of erroneous results; and   does not require far-end silence or special, termination device.       
 
         [0039]    The following describes exemplary methods and apparatus of the innovation. 
       Auto-Negotiation TDR Method 
       [0040]    An auto negotiation method is disclosed which captures the time-domain reflection of Auto-Negotiation (AN) pulses by using an internal buffer. It then identifies the near and far end echoes and uses the distance between the signal echoes to estimate the cable length. It also uses the amplitudes of the far end echoes to determine whether the cable is terminated to a far-end PHY, or if it has an open or shorted wire. 
         [0041]    The presence of actual AN pulses from a far end device can cause incorrect measurements due to the resemblance between these pulses and the reflected pulses from the device conducting the test. In order to avoid this, each measurement is repeated multiple times and the results are calculated and saved for one or more or all iterations. Using all of the stored measurements, the cable length may be chosen by median filter to avoid spurious results. An improper termination is determined by whether a majority of the results were flagged for that type of cable fault. 
       Cable Length—PHY Terminated, Open Wire or Shorted Wire 
       [0042]    In one embodiment, the method disclosed herein calculates the length of the cable under test for good cables which are terminated by another PHY. For faulty cables with an open wire or a wire short between the two wires of any pair, this method calculates the distance to the location of the cable fault. 
         [0043]    For one or more length estimates, an AN pulse reflection is captured by an analog-to-digital converter which is connected to the cable under test, and the resulting digital samples are stored in digital memory. To improve the signal-to-noise ratio of the data, the DC component may be removed and a low-pass filter may be applied to the data.  FIG. 3  illustrates an exemplary plot of raw and filtered data, 
         [0044]    In the embodiment disclosed herein the near end echo location is determined by a threshold. The threshold may be set as double the maximum absolute value of the first portion of the vector, i.e., the peak noise value. In other embodiments other threshold values may be established.  FIG. 4  illustrates an exemplary plot of magnitude of echo with near end limiting. The start of the near end echo is identified by the circle at 100 nanoseconds. 
         [0045]    The far end echo location can be a little more difficult to identify because of attenuation as the pulse travels along the cable. The signal-to-noise ratio for this echo can be enhanced by applying a high-pass filter. The simplest such filter that suffices for accuracy is a one-point differential although in other embodiments other filters may be utilized. 
         [0046]    In this embodiment the far end echo is identified by searching the high-pass filtered signal backwards for two values which exceed a threshold and are a fixed distance apart. The distance between the two values can approximate the duration of the auto-neg pulse.  FIG. 5  illustrates an exemplary plot of magnitude of echo after high pass filtering and a plot of far end threshold with the duration requirement. The various plots are identified by the key shown in the upper right hand corner of the plot. 
       Cable Length—Inter-Pair Wire Short 
       [0047]    This method also calculates the cable length for a cable with a wire short that spans any of the four wire pairs (“inter-pair short”). The calculation has some minor differences from the previous case. In this case, the transmitted pulse reflection travels through the short to the termination of the other pair and then reflects back to the transmit pair so the length of travel is doubled. The extra distance combined with the short itself can cause a low-pass effect such that the one-point difference is not sufficient as a high-pass filter to identify the reflected pulse edges. Instead, a higher order high-pass filter may be applied that has a lower corner frequency, and the duration of the reflected pulse that needs to be past the threshold line is reduced.  FIG. 6  illustrates an example plot of a comparison of cables with and without inter-pair shorts. 
         [0048]    Cable Terminations 
         [0049]    In one variation the method may also indicate whether the cable under test is terminated correctly to another PHY, or whether the cable is faulty due to an open wire, a wire shorted across a single pair (“intra-pair short”), or a wire shorted across two different pairs (“inter-pair short”). The choice is determined by using the amplitude of the far end echo. For the case of intra-pair opens and shorts, in this embodiment the maximum absolute value of the unfiltered echo is compared to a fixed threshold. Other embodiments may use variable thresholds in proportion to the distance, and may use other amplitude measurements on the unfiltered echo or a filtered version of the echo. Other embodiments may also use a duration criterion to identify the echo. If the amplitude value exceeds the threshold, the polarity of the maximal value determines whether the cable fault is an intra-pair short or an open wire. Other embodiments may use a non-maximal value to determine the polarity of the far end echo. If the value is within the threshold, the cable is identified as good and connected to a far end PHY.  FIG. 7  illustrates an exemplary plot of superimposed plots of open terminations for various cable lengths. 
         [0050]    For the case of inter-pair shorts, the test is conducted by transmitting a pulse on one pair and analyzing the received samples on each of the other three pairs. The maximum absolute value on each of those receive pairs is checked against a fixed threshold, though other values and variable thresholds may be used. This threshold may be smaller than that of the intra-pair open/short case because any value that exceeds the noise floor indicates an inter-pair short cable fault. 
         [0051]      FIG. 8  illustrates a high level operational flow diagram of a method for cable analysis for a cable terminated to another PHY or having intra-pair (within each pair) opens or shorts. This method achieves robust measurements and avoids interference from far end signals, such as auto-neg type signal. This is but one possible method of operation and as such it is possible that one of ordinary skill in the art may arrive at different methods of operation without departing from the claims that follow. At a step  800  the operation starts and initializes. Initialization may occur at a step  804  which sets the channel being processed to channel 0. It is contemplated that any number of channels may be part of the communication system, and in this embodiment 4 channels (0-3) are present. 
         [0052]    Then, at a decision step  808  the operation determines if the channel count is less than four. Because this is a four channel embodiment, step  808  compares the channel count to four. If at decision step  808  the channel count is greater than or equal to four, the operation ends at step  812 . Alternatively, if the channel count is less than 4, then the operation advances to step  816 . 
         [0053]    At step  816  the operation initializes the chan_open variable to zero and initializes the chan_short variable to zero. The chan_open variable comprises a value representing the number of times the routine results in a determination of an open circuit on the channel under analysis. The chan_short variable comprises a value representing the number of times the routine results in a determination of an open circuit on the channel under. 
         [0054]    Next, at a step  820 , the operation sets the trial variable to zero. The trial variable represents the number of times a pulse is transmitted and the corresponding received signal is analyzed. After setting the trial variable to zero, or other predetermined value, the operation advances to decision step  824 . At decision step  824  the operation determines if the trial variable is less than a threshold variable. The threshold may comprise a limit on the number of times to repeat or execute the operation. In one embodiment the threshold value comprises 5 while in other embodiments the threshold value may comprise a different value. High threshold values yield more accurate results but demand more time to complete the routine. 
         [0055]    If at decision step  824  the operation determines that the trial variable is less than the threshold, then the operation advances to a step  828 . At step  828  the operation processes the channel data as is set forth in detail in  FIG. 9  which follows. The processing of channel data comprises processing the reflected transmit pulse optionally in connection with other data. Thereafter, at a step  832  the operation may increment the chan_open variable or the chan_short variable based on the results of the processing at step  828 . 
         [0056]    At a step  836  the operation updates the cable length data. The cable length data comprises data regarding the length of the cable as determined by the time domain reflectometry routine of  FIG. 9 . Then at a step  840 , the operation increments the trial variable to thereby designate the routine as having been run. The trial variable represents the number of times the routine has run and the threshold at step  824  represents the number of times the routines is set to run, Thereafter, the operation returns to decision step  824  and at this decision step the routine again evaluates whether the trial variable is less than the threshold. If not, then the operation advances to step  844 . 
         [0057]    At a step  844  the operation sets the channel length to the median filter output length and then advances to a decision step  848 . At decision step  848 , the operation determines if the chap open variable is greater than a predetermined limit. The limit may comprise any number but is related to the number of trials. In summary, if the chap open variable is greater than the limit then the operation outputs a notice, such as via a user interface, that an open circuit condition exists. This occurs at a step  852 . As can be appreciated, the limit at which an open circuit is declared will depend on the number of trials. A larger limit value may be associated with a larger number of trials. 
         [0058]    If at decision step  848  the operation determines that the chap open variable is not greater than the limit, the operation advances to decision step  856 . At decision step  856  the operation determines if the chan_short variable is greater than the limit, A same or different limit value as used in step  848  may be used in step  856 . If the chan_short variable is greater than the limit value then the operation advances to step  860  and the operation outputs a notice that the channel has a short circuit. Alternatively, if at decision step  856  the chan_short variable is not greater than the limit value, the operation advances to step  864  wherein the system outputs a notice that no fault is detected. After steps  852 ,  860 , and  864  the operation advances to step  870  where the operation increments the chan variable, and thereafter returns to decision step  808 , which is described above. 
         [0059]      FIG. 9  illustrates an exemplary operational flow diagram of an example method of performing analysis of an echo signal to estimate the length of the cable to the distance to a fault, if a fault is present. This is but one possible method of operation and as such it is possible that one of ordinary skill in the art may arrive at different methods of operation without departing from the claims that follow. 
         [0060]    At a step  904 , the operation begins the data processing routine on the reflected signal, which may be referred to as the time domain reflected signal. Thereafter, at a step  908 , the operation low pass filters the received signal, and at a step  912  identifies and removes the near end echo. The removal of the near end echo may be done by setting the near end location as the first reflected value which is greater than the near end threshold. This accounts for reflection from the hybrid or other near end components or terminations. The near end threshold may be proportional to the noise level of the received signal. 
         [0061]    Next, at a decision step  916  the operation determines whether the processing operation is for an inter-pair mode. Inter-pair mode comprises analysis of a signal between two or more pairs and is in contrast to intra-pair mode which comprises analysis for signals within a pair. If at decision step  916  the operation determines the processing is not for inter-pair mode or data, then the operation advances to step  920 . At decision step  920 , the operation determines if there is a fault on the cable by analyzing the maximum absolute value of the time domain reflection data (hereafter TDR) to determine if it is greater than the unterminated threshold. The unterminated threshold comprises a predetermined value that indicates a fault in the cable. The unterminated threshold may also be a set of values associated with the distance to the fault, and the threshold is often in proportion to the distance to the fault. 
         [0062]    If at decision step  920  the maximum absolute value of the reflection is less than the unterminated threshold, then a no fault result is the outcome and the operation advances to step  924 . A no fault result occurs because the TDR reflection indicated a channel which is terminated to another PHY or electrically equivalent device. 
         [0063]    Next, at a step  924 , high pass filtering occurs to improve the signal to noise ratio of high frequency content, such as pulse edges, which in turn improves detection of the far end echo. Then at a step  928 , the operation processes the filtered TDR data to identify the location of the far end echo. The processing equation in step  928  of  FIG. 9  is one exemplary operation that may occur, but other equations or routines are contemplated. In the operation of step  928  the system searches for reflections which match the shape and duration of an auto negotiation pulse. The variable n represents the start of the far end echo in nanoseconds of time, the value n+100 represents the time value plus one hundred nanoseconds, and the value n+99 represents the time value plus ninety-nine nanoseconds. In other embodiments, other values may be utilized. Stated another way, this represents a position in time plus a predetermined additional time period. This additional period may be a single value or a set of two or more values that span a time less than the duration of the transmitted pulse. In this embodiment the time period is approximated to the duration of a standard compliant AN pulse which is 100 nanoseconds, as well as one nanosecond less than this duration. This step may be considered as analyzing a range of values to locate the actual echo signal to distinguish it from noise. 
         [0064]    Next, the operation advances to a step  932  to calculate the cable length. In this embodiment the cable length is determined by calculating the far end location minus the near end location and multiplying the result by the signal velocity within the cable. Thereafter the cable length is output or returned to the respective routine of  FIG. 8  or  FIG. 10 . The operation ends at step  936  which may include return to another routine. 
         [0065]    Alternatively, if at step  920  a determination is made that an intra-pair fault is present on the channel being analyzed, the operation advances to step  940 . At step  940  the operation sets the far end location as the first absolute TDR data value that is greater than the unterminated threshold. The unterminated threshold is discussed in step  920 . Next, the operation advances to step  944  and the operation determines if the maximum TDR data value is greater than zero. This serves as a polarity check. If the maximum TDR data value is greater than zero then the operation advances to step  952  at which the operation designates the fault as an open circuit. Thereafter the operation advances to step  932  and  936  as shown. 
         [0066]    If at decision step  948  the polarity check reveals that the maximum TDR data value is less than zero, then at step  948  the operation designates the fault as a short circuit and then advances to step  932 ,  936 . 
         [0067]    Returning to step  916 , if the processing is for identifying an inter-pair fault, then the operation advances to step  956  and high pass filtering of the TDR signal occurs. This filter provides the same benefit as in step  924 . Next, at step  960 , the operation processes the filtered TDR data to identify the location of the far end echo. . The processing equation in step  960  of  FIG. 9  may be similar to the processing of step  928 . In one exemplary embodiment the equation shown in step  960  occurs, but other equations or routines are contemplated. In the operation of step  960  the system searches for reflections which match the shape and duration of an auto negotiation pulse. The variable n represents the start of the far end echo in nanoseconds of time, the value n+90 represents the time value plus ninety nanoseconds, and the value n+89 represents the time value plus eighty-nine nanoseconds. Stated another way, this represents a position in time plus a predetermined additional time period. This additional period may be a single value or a set of two or more values that span a time less than the duration of the transmitted pulse. In this embodiment the time period is approximated to slightly less than the duration of a standard compliant AN pulse which is 100 nanoseconds, as well as one nanosecond less than this duration. This step may be considered as analyzing a range of values to locate the actual echo signal to distinguish it from noise. 
         [0068]    Next, at decision step  964  the operation performs a peak value analysis by determining if the maximum absolute value of the TDR signal is greater than the inter-pair short threshold. The inter-pair short threshold represents a value beyond which is an indicator of a short between two different pairs. If the peak value is greater than the threshold, then the operation advances to step  968  and the operation designates the fault as an inter-pair short for the particular channel. After step  964  and  968  the operation advances to steps  932  and  936 . 
         [0069]      FIG. 10  illustrates an operational flow diagram of a method for cable analysis for detecting shorts between wire pairs, including between one or more wires on one pair and one or more wires on another pair including across pairs. This is but one possible method of operation and one of ordinary skill in the art may derive other methods of operation which do not depart from the claims that follow. In this embodiment, the operation starts at a step  1004 . Next, at a step  1008  the operation sets the transmit channel to 0. In other embodiments, the operation could start with other channels. Then, at a decision step  1012 , the operation determines if the transmit channel is less then four. In embodiments with other than 4 channels, a different comparison would occur. If not, then the operation ends and step  1016 . In this method of operation a pulse is transmitted on one channel, and the received signal data on the other channels are monitored for the presence of a pulse. Because only one channel transmits at a given time, a pulse on the other channels during that time indicates a short between the channel transmitting the pulse, and a monitored channel. 
         [0070]    If the transmit channel number is less than four, then the operation advances to step  1020  and the counters or variables are initialized. The counters or variables are set forth below. Then, at a step  1024 , the operation sets the channel equal to zero as a starting point for the routine. In one embodiment this comprises initializing the receiver channel loop. At decision step  1028  the operation determines if the channel is less than four. As the routine loops through each channel analysis, the channel number is incremented. If the channel is less than four, the operation advances to step  1050  and sets the trial variable to zero. The trial variable represents the number of times the routine has executed. Each routine execution generates additional data which is used to populate the median filter and the majority rule data. In general, the routine is run multiple times to generate multiple sets of measurement data, each of which is analyzed. Through use of multiple sets of data, abnormal data may be discarded or ignored to thereby yield more consistent and accurate results. 
         [0071]    Thereafter, at decision step  1054  the operation determines if the trial variable is less than a variable threshold X, where X can equal any number of times the loop routine is to execute. The larger the value of X the more accurate the results, but the longer the operation takes to complete. In one embodiment, the value of X is 5. If the result of decision step  1054  is true or yes, the operation advances to step  1056  and processing of channel data occurs. One example embodiment of channel data processing is set forth in  FIG. 9 . 
         [0072]    The operation then advances to a decision step  1062  during which the operation determines if the transmit channel (tchan) is equal to the channel variable. If it is, then the operation ignores or discards any indication of an inter-pair short at step  1066  because the channel upon which the signal was transmitted is also the channel upon which the reflection was received. Therefore, it can not indicate a short that spans two channels as is the purpose of the routine. Next, at a step  1070 , the operation adds the cable length to the median filter, and the operation increments the trial variable and returns to step  1054 . If at step  1062  the transmit channel does not equal the channel variable value, then the operation returns to step  1054 . 
         [0073]    If at step  1054  the trial variable is not less than the value X, then the operation advances to step  1064  at which time the operation increments the channel value as part of the loop operation. The operation then returns to step  1028 . 
         [0074]    If after one or more loop routines, the channel value is not less than four or the number of channels in the system or under analysis, then the operation advances to decision step  1032 . At decision step  1032  the operation determines if the inter-pair flag counter for any channel is greater than a predetermined limit. The limit may comprise any number and may be related to the number of trials, X. As can be appreciated, the limit at which an open circuit is declared will depend on X. A larger limit value may be associated with a larger X. If any inter-pair flag counter is greater than the limit, then the operation advances to step  1036 , and the routine sets the results for the inter-pair operation to indicate a short. Then at step  1040  the operation sets the cable length to the median value output divided by two. Division occurs because the reflection traveled to the short, through the short, to another channel&#39;s termination and then back through the short to the originating channel for detection. Therefore, the distance to the short is only half of the distance traveled by the reflected pulse. 
         [0075]    After step  1040  or if the result of decision step  1032  is no, then the operation advances to step  1044  at which time the operation increments the transmit channel variable and returns to decision step  1012  as shown. 
         [0076]    While various embodiments of the invention have been described, it will be apparent to those of ordinary skill in the art that many more embodiments and implementations are possible that are within the scope of this invention, In addition, the various features, elements, and embodiments described herein may be claimed or combined in any combination or arrangement.