GB2572381A

GB2572381A - DSL line fault identification

Info

Publication number: GB2572381A
Application number: GB1805003.9A
Authority: GB
Inventors: Rohlfing David; Limb Patrick
Original assignee: British Telecommunications PLC
Current assignee: British Telecommunications PLC
Priority date: 2018-03-28
Filing date: 2018-03-28
Publication date: 2019-10-02
Anticipated expiration: 2038-03-28
Also published as: GB201805003D0; GB2572381B

Abstract

Prior art electrical line tests may have difficulty detecting intermittent faults such as bad or unstable joints. The present invention operates by gathering upstream and downstream line rate measurements over a period of time 200, and examining the line rates to determine a rate transition between successive samples 206. If the size of the rate transition exceeds a defined threshold 208, then it is regarded as a qualifying transition. If the number of qualifying transitions over a period of time is greater than a further defined threshold, then a fault condition is identified 210. A transition sensitivity factor (TSF) 204 is used to determine how large a relative rate transition on the line needs to be. The TSF can be obtained by observing known good lines. The line rate measurement may be a maximum attainable line rate and the rate transition threshold may be calculated from the minimum non-zero rate value for the day. The line rates may be obtained directly from a DSLAM or CPE, therefore no specialist test equipment or disruption of service is required. By using a TSF the invention may operate on DSL lines of differing line rates e.g. ADSL2+ or VDSL.

Description

DSL LINE FAULT IDENTIFICATION

Field of the Invention

This invention relates to a method of identifying a fault on a digital subscriber line in a telecommunications network.

Background to the Invention

Digital subscriber line (DSL) technology, often referred to as “broadband”, is a family of services that provides high speed digital data transmission over the metallic twisted copper pairs that form part of a local telephone network. DSL is commonly used to provide a customer’s home with a network connection, typically to the Internet via an ISP.

Broadband lines are prone to faults. These result in slow line speeds or line drop outs, affecting a customer’s service. Some of these faults are easily identified and rectified, such as missing micro-filters in the customer’s home. Others are more complex, such as faults relating to bad or unstable joints, which often result from a drop wire experiencing physical movement. Various techniques have been developed to help identify such faults.

One known method is to employ metallic line tests, where line test equipment at the telephone exchange runs various line tests. These are typically electrical tests, and the resulting measurements, such as resistance, capacitance and so on, are used to look for various line conditions on the metallic path. Such tests are intended to identify PSTN faults, and can lack sensitivity to fault conditions that affect broadband. Indeed, some of the testing will mask certain faults, as in certain situations the test itself can clear the fault condition as a result of the voltages being applied to the lines. Such testing also requires that specialist test equipment be connected to the line, requiring PSTN and DSL services to be temporarily disabled whilst the testing takes place. The test equipment typically requires some sort of relay to switch in and those relays tend to have a limited lifespan.

Moreover, metallic line tests also have difficulty with intermittent faults, which by their very nature, may not exhibit any fault characteristics at the time of testing.

In the Applicant’s European patent, EP2976839, a method is taught where DSL faults arising from unstable joints in the DSL line are identified. The method correlates nearend and far-end errors to determine if there is a match between the two sets of data. Matching data patterns are indicative of unstable or bad joints in the DSL line, and are typically intermittent and located near the customer's premises. However, this data is not always available.

Summary of the Invention

It is the aim of embodiments of the present invention to provide an improved method of identifying faults in a digital subscriber line in the telecommunications network.

According to one aspect of the present invention, there is provided a method of identifying a fault on a digital subscriber line in a telecommunications network, said method comprising:

(i) gathering a plurality of samples of line rate measurements from the digital subscriber line for the upstream path and downstream path over a period of time;

(ii) determining a representative upstream line rate using the plurality of samples of line rate measurements for the upstream path, and determining a representative downstream line rate using the plurality of samples of line rate measurements for the downstream path;

(iii) determining a rate transition threshold for the upstream path in dependence on the representative upstream line rate and a transition sensitivity factor, and determining a rate transition threshold for the downstream path in dependence on the representative downstream line rate and the transition sensitivity factor;

(iv) determining the rate transitions from the line rate measurements for the upstream path that is greater than the rate transition threshold for the upstream path, and determining the rate transitions from the line rate measurements for the downstream path that is greater than the rate transition threshold for the downstream path;

(v) comparing a count of the rate transitions for the upstream path to a predetermined threshold, and comparing a count of the rate transitions for the downstream path to a predetermined threshold;

(vi) identifying a fault on the digital subscriber line in dependence on the result of the comparison step.

The plurality of samples may be taken at regular intervals. An example is every 15 minutes.

The line rate measurements may be maximum attainable rate measurements.

The representative upstream line rate may be one of the minimum, maximum, or median non-zero line rate measurement for the upstream path taken over the period of time, and the representative downstream line rate may be one of the minimum, maximum, or median non-zero line rate measurement for the downstream path taken over the period of time.

The rate transition may be calculated as the absolute difference between successive line rate measurement samples.

In examples of the invention, the period is 24 hours.

The invention differs from any known methods through its use of line rate to determine line faults. Line rate is a parameter that is generally always available from a DSLAM or CPE, allowing for continuous monitoring during normal operation, and also provides good sensitivity.

Brief Description of the Drawings

For a better understanding of the present invention reference will now be made by way of example only to the accompanying drawings, in which:

Figure 1 is a system diagram showing a telephone exchange and a digital subscriber line running to a customer premises;

Figure 2 is a flow chart summarising the steps of an example of the invention;

Figure 3 is a graph plotting the maximum attainable rate plotted over a 24-hour period for a good line;

Figure 4 is a graph plotting the maximum attainable rate plotted over a 24-hour period for a faulty line;

Figure 5 is a graph showing the rate transitions between successive samples plotted over a 24-hour period; and

Figure 6 is a graph showing occurrences of when a rate transition exceeds the rate transition threshold.

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 identifying faults in a DSL line using line rate measurements, which can be obtained directly from the DSLAM or CPE, thus requiring no specialist test equipment nor disrupting service. The upstream and downstream rates are examined to determine a rate transition between successive samples. If the transition is of sufficient magnitude (by comparing to some threshold), then it is regarded as a qualifying transition. If the qualifying transition count over a period of time is greater than a threshold, then a fault condition is identified. The rate transition threshold is calculated from the minimum non-zero rate value for the day.

Figure 1 illustrates a telecommunications network 100 including a customer’s premises 102. The customer’s premises 102 is connected to a telephone exchange 104 via a telephone line 106. The telephone line is a twisted copper or aluminium pair of wires. Specifically, a network termination equipment NTE 108 is at the customer premises 102 end of the line 106. The NTE 108 is 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 premises 102. The line 106 runs from the NTE 108 to a junction box 110, and then onto a distribution point DP 112. In this example, the DP 112 is a junction on a telephone pole. The line 106 then continues onto the exchange 104 where it terminates a digital subscriber line access multiplexer, DSLAM, 114. Within the customer premises 102, the NTE 108 is connected to customer premises equipment CPE 124, typically a router or home hub.

The DSLAM is a network element that provides digital subscriber line (DSL) services to connected lines and associated customer premises. The line 106 is thus also referred to as digital subscriber line, or DSL line. At the exchange is also a fault detection unit 118, connected to the DSLAM 114. The fault detection unit 118 comprises a processor 120, and a data store 122, such as hard disk array or similar. The fault detection unit 118 gathers various measurements made by the DSLAM 114, stores them in the data store 122, and the processor 120 use the stored measurements determine when a line is exhibiting a fault.

The DSLAM 114 also has onward connections 116 to data provisioning networks. A skilled person will also appreciate that there are other elements in the exchange 104, such as elements that provide standard PSTN services to connected lines. However, these have been omitted for simplicity.

Whilst the present example shows a DSLAM residing in the exchange 104, the invention would still be applicable to configurations where the DSLAM is situated somewhere else. For example, in a fibre to the cabinet (FTTC) arrangement, the DSLAM 114 would be located in a roadside cabinet, which is typically located nearer the customer premises than the exchange. In an alternative network arrangement, DSLAM like functionality can be provided by an MSAN (multi services access node), which also provides other capabilities such as voice.

Figure 2 is a flow chart summarising the steps of the present invention.

In step 200, the fault detection unit 118 collects line rate measurements associated with the line 106 over a period of time. Specifically, the line rate measurements are the maximum attainable line rates for both the upstream and downstream paths, taken at intervals from the DSLAM 114 and the CPE 124 respectively. In this example, the interval is every 15 minutes, which has been found to be effective, though a shorter or longer interval could be used instead. The fault detection unit 118 can store these measurements in the data store 122. Measurements are preferably collected over a 24 hour period or longer.

In step 202, a representative daily rate RDR is calculated for the upstream and downstream paths using the respective upstream and downstream line rate measurements. This could be the minimum, maximum, or median non-zero line rate over the previous 24-hour period. In this example, the maximum line rate is used for both upstream and downstream paths, resulting in two values - one for the upstream path, RDR U, and one for the downstream path, RDR D. Note, the same representative rate must be used for the determination of thresholds later.

Then in step 204, a transition sensitivity factor (TSF) is determined. TSF is a number that is used to determine how large a relative rate change in the line rate needs to be. It can be set to some predetermined value, which in this example is 0.1, but it can be adjusted or set in other ways. For example, it can be determined by running the present invention across a collection of good lines where line faults are not reported, and looking for looking for maximum observed values for rate transition divided by the daily maximum rate.

Use of TSF also allows the invention to be applied to DSL lines across a variety of DSL products (e.g. ADSL2+, VDSL etc), which operate with differing line rates, because the TSF allows for a relative threshold to be set for rate transitions as set out in the next step. Indeed, even good lines within the same DSL product could have very different line rates e.g. long vs short lines.

Then in step 206, a rate transition threshold is determined for the upstream and downstream paths, RTTU and RTT D respectively. The rate transition threshold for the upstream is equal to the representative daily rate for the upstream multiplied by the transition sensitivity factor. Thus:

RTT U = RDR U * TSF (1)

Similarly, for the rate transition threshold for the downstream is given by:

RTT U = RDR U * TSF (2)

Thus, the rate transition thresholds are a relative measure that varies as a function of the representative daily rate. This is advantageous compared to using absolute thresholds, as the rate transition thresholds are set appropriately for the rate of the line.

Then in step 208, the fault detection unit 118 determines a count of rate transitions that meet predetermined criteria over a given period. In these examples, the predetermined criteria are based on the rate transition thresholds from step 206. Specifically, within the given period (e.g. 24 hours), a rate transition is calculated as the absolute difference between successive line rate measurements for each of the upstream and downstream paths. Then for each of the upstream and downstream paths, a transition marker is generated for each rate transition that is greater than the respective upstream or downstream rate transition threshold, RTT U or RTTD. A count of transition markers, TMU, is the number of transition markers for the upstream path. A count of transition markers, TMD, is the number of transition markers for the downstream path. In an example implementation, with line rate measurements taken every 15 minutes (=900 seconds), a 24 hour period would result in 96 line rate measurements for each of upstream and downstream paths.

In step 210, the count of transition markers TMU and TMD are compared to a transition marker threshold, TMthreshold. Both the upstream and downstream transition markers are examined, and the fault state of the line classified accordingly. One example of classification is as follows:

1. A line is classified as good (no faults) when TMU < TM threshold, and TMD < TMthreshold

2. A line is classified as fair (some faults) when TMU >= TM threshold, or TMD >= TMthreshold

3. A line is classified as faulty or exhibiting significant faults when TMU >= TM threshold and TMD >= TM threshold

The value of TM threshold determines how many transition markers are required to signify line instability. In our example we use a value of 4. It can be determined in several ways, including by comparison with other fault classification algorithms, or by running the algorithm across a collection of good lines and looking for maximum observed values for TMU and TMD.

Note, the third classification above requires both the upstream and downstream transition marker counts to exceed the threshold. This requirement helps eliminates noise events, which tend to affect one path more than the other. In order to correctly correlate upstream and downstream transitions, use of a small interval between line rate measurements is advantageous. Too large an interval, and transitions can be correlated between the upstream and downstream that do not actually relate to the same event. In testing a maximum interval of 15 mins has been found to be effective.

The method then cycles back to step 200, where line rate measurements continue to be collected. The method can then be repeated every 24 hours, resulting in a line fault classification every 24 hours, but with line rate measurements taken continuously at the predetermined interval (e.g. every 15 minutes). It will be appreciated that the invention could also be repeated over some longer or shorter period than 24 hours, or indeed a sliding window can be used instead. The period over which the method is repeated (and counts made) should be reflected in the TMthreshold, with a larger TMthreshold used for larger periods. Thus, another way to set the threshold is to set it as a proportion (some fraction) of the total number of line rate measurements (from either one of the upstream ordownstream paths).

To understand how the invention works, first consider the situation where a line is operating normally without any faults. One such line is shown in Figure 3, which shows a graph of the maximum attainable rate plotted over a 24-hour period. The maximum attainable rate varies only by a very small amount over the 24-hour period.

In contrast, another line shown in Figure 4 shows notable transitions in the max attainable rate between some successive samples. One explanation is that mechanically unstable joints can cause a temporary reduction in the quality of the signal path, which has a negative impact on the max attainable rate.

Note, the graphs in Figures 3 and 4 show the case for the downstream max attainable rates, but similar variations can be seen in upstream max attainable rate. Similar variations could also be seen for actual rates, but an accompanying retrain may also be observed.

Figure 5 shows a graph of the rate transitions between successive samples plotted over time, and includes a plot of the rate transition threshold.

Finally, Figure 6 plots occurrences of when a rate transition exceeds the rate transition threshold. In this example, there were eight transitions within the day. This is greater than the example value for TM threshold of 4, so this line would be classified as showing a potential line fault.

Although mechanically unstable joints could, in principle, occur at any joint along the circuit the most usual location is at the customer premises especially where there are external drop wires or jointing boxes.

In an alternative example, the fault detection units 118 can reside in the CPE 124 for use in performing the operation of steps 200 to 210 to identify a fault on a line. The fault detection unit may need to be provided some of the data from the DSLAM and store measurements and threshold values locally.

Exemplary embodiments 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 120 in the fault detection unit 118, it provides a computer program code structure which is capable of performing at least part of the methods in accordance with the above described exemplary embodiments of the invention.

A person skilled in the art will appreciate that the computer program structure referred to can correspond to the flow chart shown in Figure 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 120 in the fault detection unit 118, 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.

Claims

1. A method of identifying a fault on a digital subscriber line in a telecommunications network, said method comprising:

2. A method as claimed in claim 1, wherein the plurality of samples are taken at regular intervals.

3. A method as claimed in claim 1 or 2, wherein the line rate measurements are maximum attainable rate measurements.

4. A method as claimed in any preceding claim, wherein the representative upstream line rate is one of the minimum, maximum, or median non-zero line rate measurement for the upstream path taken over the period of time, and the representative downstream line rate is one of the minimum, maximum, or median nonzero line rate measurement for the downstream path taken over the period of time.

5. A method as claimed in any preceding claim, wherein a rate transition is calculated as the absolute difference between successive line rate measurement samples.

6. A method as claimed in any preceding claim, wherein the period is 24 hours.