Patent ID: 12261978

DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention is described herein with reference to particular examples. The invention is not, however, limited to such examples.

Examples of the present invention present a method of detecting a loop length change in a digital subscriber line. An Uncalibrated Echo Response (UER) trace is obtained from the digital subscriber line by running a Single Ended Line Test (SELT) on the line. The UER is a frequency domain response obtained from a frequency sweep over the VDSL spectrum, which reflects from the end of the line, and results in a per-tone interference pattern detected at the DSLAM modem. A historical (baseline) UER trace from the same line is retrieved. A line that has experienced a small change in loop length will have the same overall shape, but be compressed or stretched in the frequency domain. Thus, to detect such a change, a comparison between the two traces is made to determine if the difference between the two traces is less than a threshold but non-zero. Then a range of scale factors are applied in the frequency domain on either one of the traces, before determining which of the scale factors results in the lowest difference between the un-scaled and scaled traces. The determined scale factor is above a certain threshold, then the line is determined to have had a loop length change. The value of the scale factor can be used to determine the amount the length has changed.

FIG.1is a simplified system diagram illustrating a telecommunications network100including a customer's premises102. The customer's premises102is connected to a DSLAM104located at a primary connection point PCP106, which is typically a roadside cabinet. The connection between the customer premises102and DSLAM104is provided by a telephone line108, made of a pair of twisted copper or aluminium wires. Specifically, a network termination equipment NTE110is at the customer premises102end of the line108. The NTE110is often referred to as a line box or master socket, and is the demarcation point between the telephone network and the customer wiring in the customer premises102. The line108runs from the NTE110to a junction box112, and then onto a distribution point DP114. In this example, the DP114is located on a telephone pole116. The line106then continues onto the PCP106and specifically the DSLAM104. Within the customer premises102, the NTE110is connected to customer premises equipment CPE118, which is typically a router or home hub that includes a modem.

It is envisaged that the line108can experience a change in its loop length as a result of the customer changing the cabling for a different length cable between the NTE110and the CPE118. For example, a customer may swap the cable for a longer one when moving the CPE118to a different location further from the NTE110. In examples of the invention, there are presented methods to determine that such a change in the length of the line108has occurred, and can further determine the amount the line has changed in length. Note, the loop length of the line108is usually measured as the total length of the metallic pair running from the DSLAM104to the CPE118.

A DSLAM is a network element that provides digital subscriber line (DSL) services to connected lines and associated customer premises. The line108is thus also referred to as digital subscriber line, or DSL line. In this example, the DSLAM104provides a VDSL service on the line108. The DSLAM104also has an onward connection, typically a fibre optic connection, to the local exchange120, and from there onto data provisioning networks122via suitable connections and equipment. The data provisioning networks may include the internet and other networks. A skilled person will appreciate that there are other elements in the network100that have been omitted for simplicity, such as elements that provide standard PSTN services to the line108.

Also shown connected to the data provisioning network is a test module or test server124. The test module124comprises a processor and a data store, such as hard disk array or similar. The test module124gathers data from the DSLAM104, and the processor can use that data, together with other data, to determine if there is a change in the loop length of the line108.

Whilst the present example shows a DSLAM residing in a PCP (roadside cabinet), the invention would still be applicable to configurations where the DSLAM is situated somewhere else. For example, the invention could still be applied to networks and services where the DSLAM is located in the local exchange120.

Furthermore,FIG.1only shows a single line and associated elements. However, in practice there will be a number of lines, each serving a respective customer premises, connected to the DSLAM. Moreover, there will be many DSLAMs connected to the exchange, and nationally there will be many exchanges. Any number of these lines can be tested by the test server124using the methods described below.

FIG.2is a flow chart summarising the steps of the present invention as performed by the test module124.

In step200, the test module124receives a SELT UER response (or test trace) for the test line108. The SELT UER response is obtained from the DSLAM104by performing SELT measurements using a frequency sweep over the VDSL spectrum on the line. A more detailed discussion of the SELT standard can be found in the International Telecommunication Union recommendation G.996.2, “Single-ended line testing for digital subscriber lines”. SELT measurements consist of sending wideband signals down a line, with the UER being the received signals reflected back from the line (over a frequency range).

SELTs can be run remotely and can run regardless of the line synchronisation condition, and when the line is faulty or undergoing repairs. In contrast, service layer tests require a line to be synchronised.

The SELT UER comprises an array of complex values (at discrete tones or frequencies) representing the phase and amplitude of the reflected waves detected over the frequency range. This is encoded as two arrays, Real and Imaginary components and a data scaling factor. An example of a SELT UER trace illustrated in a graph of detected amplitude plotted against frequency tone (which ranges from 0 to 4095) as shown in plot302inFIG.3.

In step202, the test module124loads a baseline or historical SELT UER response associated with the line. The baseline SELT UER response is representative of what the response on the line should look like when in good working condition. One way in which to initially generate this baseline SELT UER response is by performing a number of SELTs over a period of time on the line. If these responses are fundamentally similar (i.e. largely the same shape), then it is assumed the line is in a stable good line condition, and a baseline response is generated from the responses—for example some average or weighted average of the responses. Note, if the responses differ significantly from each other, then no baseline is generated, and instead a potential fault might be diagnosed instead.

FIG.4shows an example of a test trace and a baseline trace. The test trace is shown by the solid line of plot402, and the baseline trace is shown by the solid line of plot404. In this example, the test trace represents a longer line, and the baseline trace a shorter line. Thus, the line has undergone a small loop length change. Note, under a small loop length change, the overall coarse shape of the trace is unchanged, but the frequency oscillations at the lower frequencies are different and indicate a small loop length change. Harmonic features in the lower frequencies indicate the reflection from the end of the line, decreasing in amplitude with increasing frequency. Tighter harmonic oscillations with frequency indicate a longer line, and vice versa.

These harmonics are caused by standing waves formed by the interference between the outward and reflected signals, and so the width of the oscillations are very sensitive to the line length. The wider features of the trace may be caused by other factors such as cross-talk, which would remain the same even when a line has become slightly longer such as due to a change in home wiring. Thus, when a line becomes slightly longer or shorter, the overall coarse shape of the trace remains the same, while the fine oscillations in the lower frequencies become stretched or compressed along the frequency axis.

In step204, the test module124checks to see if the baseline and test traces are fundamentally similar. This is to rule out cases where the line has not undergone any change in length, or indeed where the line is not suffering from any change in line condition. Note, this step is optional, or could be performed at a later point in the method.

The test can be done by calculating a delta Δ between the baseline trace and the test trace, where the delta is calculated as the sum of the squares of the differences between each tone value along the spectrum, as shown by equation (1) below:
Δ=Σi(Testi−Baselinei)2(1)

If the comparison results in a delta Δ=0 or near zero (or below some threshold near zero), then the traces are fundamentally the same, and processes passes to step205, where the line is classified as not having a loop length change, and processing ends. Otherwise processing passes to step206.

In step206, the test module124checks to see if the delta Δ is above a certain threshold. This threshold is set to some non-zero value that helps filter any actual line conditions or faults that would result in the traces being significantly different. This upper threshold for delta Δ may be determine from examining trace data from lines that have known line conditions or faults.

If the result of step206is that there is a difference above the set threshold, then processing passes to step207, where the line is classified as not having a small loop length change, and processing finishes. In practice, further tests can be conducted at this point to determine what line condition or fault might be present on the line.

If the difference is not above the threshold, then processing passes onto step208.

In step208, a scale factor SFnis defined. Application of the scale factor SFnto a trace will have the effect of either stretching (scale factor >1) or compressing (scale factor <1) the trace, as will be described below. A range for SFnis defined. In this example, SFn=0.8 to 1.2 in steps of 0.01.

Then each SFnis applied to the test trace by multiplying the frequency axis by SFn. Or more specifically, multiplying each tone/frequency for the data points that make up the trace by SFn. In this example, there are 4096 discrete tones/frequencies that make up the trace. The result is a scaled test trace, with data points at new (scaled) frequencies.

Described another way, for each scale factor SFn, the tone value X is scaled to Xi′, where:
Xi′=Xi·SFn(2)
for all tones i=0 to 4095.

Thus, a new scaled trace is generated for each scale factor SFn.

If the original points on the trace are represented by (Xi, Yi), then the application of the scale factor to Xito generate Xi′ will mean that the values of Xi′ will be stretched beyond (or compressed short of) the original Xito a new point (Xi′, Yi).

To illustrate, reference is made toFIG.5, which shows two of the original data points, (XB, YB) at tone B and (XC, YC) at tone C, and their respective scaled points (XB′, YB) at tone B′ and (XC′, YC) at tone C′.

However, the scaled values of Xi′ are now not at the same tone as Xi, which means comparing the scaled (in this example test) trace with the unscaled (baseline) trace is challenging as the data points are not at the same tones. To address this, interpolation of the scaled data can be done to generate data points at the same tones as the unscaled trace.

There are many different ways in which interpolation can be done, but there now follows one example using linear interpolation.

We want to determine an interpolated value of Y′ at location (Xi, Yi′), where Xiis the required tone. For example, referring toFIG.5, original data point (XC, YC) is scaled to (XC′, YC). However, we need to calculate the difference between the points at frequency of the tone C, so we will need to calculate the interpolated point (XC, YC′). To do this, Yi′ is interpolated at x=Xias a new y-axis value between Yi−1and Yi(or between Yiand Yi+1for compression, as the original Yivalue has been compressed short of the Xifrequency and so Yi+1is needed to interpolate the Yi′ value).

The result of step208is a set of adjusted (scaled and interpolated) test traces, one corresponding to each of the scale factors SFnused.

Once the test trace has been scaled and interpolated, processing moves to step210where each of the adjusted test traces are compared to the baseline trace to calculate a corresponding delta Δ(SFn), one for each scale factor. The same technique as described above with reference to equation (1) could be used, but instead we compare the adjusted test trace with the baseline trace as follows:
Δ(SFn)=(AdjustedTesti−Baselinei)2

Where AdjustedTestiis Yi′, and Baseline; is Yi, for all tones i=0 to 4095.

The result is a delta Δ for each scale factor SFn.

FIG.6shows an example plot of the resulting delta plotted against the respective scale factor used. It can be seen that there is a minimum delta around scale factor=0.86 in this plot.

Note, steps208and210where a scaling (and interpolation) are performed can be done on the baseline trace instead, resulting in an adjusted baseline trace, which can in turn be compared to the original test trace.

Turning back to the flow chart, in step212, the scale factor that results in the lowest delta Δ is identified. This can be done by generating a list of deltas which corresponds to a list of scale factors, and then finding the minimum value in the deltas list, and returning the corresponding scale factor that generated it. This resulting scale factor is indicative of the translation required to match the adjusted test trace with the baseline trace. A resulting scale factor of 1 indicates that there has not been a small loop change, whereas a scale factor of, for example, 1.02 may indicate a small lengthening of the line as the test trace has been squashed and has needed stretching to match it back to the existing baseline. Thus, an additional check can be made here to determine if the resulting scale factor is 1 or some small threshold either side of 1, such as 1.01, both of which could indicate that the loop length change is not significant to flag a potential loop length change. [Feiyu, is this last part correct?]

A scale factor >1 means that the test trace needed to be stretched to match the baseline trace. As tighter harmonics represent longer lines, this means that the test trace represents a longer line than the baseline and that the line has become longer.

A scale factor <1 compresses the trace, meaning that the test trace needed to be compressed to match the baseline trace. This means that the test trace represents a shorter line than the baseline and that the line has become shorter.

It should also be noted that tighter, more squashed harmonics also represent a lengthening of the line.

The relationship between the scale factor and the actual percentage change in the line length is observed experimentally to be linear, as shown inFIG.7. This linear relationship may then be used to convert a scale factor to an estimated percentage change in line length in step214.

The percentage change can also be converted to an absolute change in metres, the percentage can be multiplied by a measure of the line length obtained from some other technique, such as from inventory data, frequency-domain reflectometry, or attenuation measurements.

Examples of the invention are realised, at least in part, by executable computer program code which may be embodied in an application program data. When such computer program code is loaded into the memory of the processor in the test module124, it provides a computer program code structure which is capable of performing at least part of the methods in accordance with the above-described examples.

A person skilled in the art will appreciate that the computer program structure referred to can correspond to the flow chart shown inFIG.2, where each step of the flow chart can correspond to at least one line of computer program code and that such, in combination with the processor in the test module124, provides apparatus for effecting the described process.

In general, it is noted herein that while the above describes examples of the invention, there are several variations and modifications which may be made to the described examples without departing from the scope of the present invention as defined in the appended claims. One skilled in the art will recognise modifications to the described examples.