GB2588755A - Methods and systems for characterising impedance discontinuities on electrical transmission lines - Google Patents

Abstract

Impedance mismatches or discontinuities in wire segments, particularly in a twisted metallic pair (TMP) transmission line, are located and characterised by launching a reference signal into one end of the line and using time domain windowing of the reflected response to isolate one or more reflections of the reference signal from impedance discontinuities on the line. The time windowing or time gating can isolate reflections from faults or discontinuities at different distances along the length of the line. The location of the impedance discontinuity and its impedance mismatch can be identified, e.g. signals reflected from the ends of the line can be characterised by their arrival times. The method may be used to select an impedance matching element for use on the line. The electrical transmission line may be used in a telephone system.

Description

METHODS AND SYSTEMS FOR CHARACTERISING IMPEDANCE DISCONTINUITIES ON ELECTRICAL TRANSMISSION LINES

Field

The present disclosure relates to finding impedance mismatches in wire segments.

More specifically, an aspect relates to a method of characterising impedance discontinuities on an electrical transmission line. Further aspects relate to a data processing system comprising a processor configured to perform such a method, a computer program comprising instructions which, when the program is executed by a computer, cause the computer to carry out such a method, a computer-readable data carrier having stored thereon such a computer program, a data carrier signal carrying such a computer program and a system comprising such a data processing system and the transmission line.

Background

When an electrical signal encounters an impedance discontinuity (e.g. due to a change in transmission medium) part of the signal is scattered, causing loss. Impedance matching is a well-known solution to this problem; there is commercial software available to automatically design a suitable matching circuit once an impedance mismatch is known.

One type of electrical transmission line is a twisted metallic pair (TMP), which can be used alone or in a cable bundle with other TMPs. There are three principal modes of communication between the two ports which terminate a TMP: differential mode, phantom mode and common mode. In all three of these modes the signal is transmitted (excited) and received (observed) as the (changing) potential difference (voltage differential) between two voltages (or equivalently between one "live" voltage and one "reference" voltage). In differential mode the signal is transmitted and observed as the difference in potential between two wires (typically between the two wires of the TMP). In phantom mode at least one of the voltages is the average voltage of a pair of wires. (Note that this average can vary without impacting on a signal carried in the differential mode across that same pair of wires -in this sense the phantom mode can be orthogonal to signals carried in the differential mode if carefully chosen.) The term pure phantom mode may be used to specify that both voltages being compared with each other are average voltages, each average voltage being the average or common voltage of at least one pair of wires. Second and higher order phantom modes can also be obtained by using the average voltage of two or more average voltages as one of the voltages to be compared, etc. Finally, common mode refers to the case where one of the voltages being compared is the "Earth" or ground reference voltage (or something substantially similar for telecommunications purposes). It is possible for various mixed modes to also be used for carrying signals. For example, one reference voltage could be a common ground and the other could be the average between the voltages of two wires in a TMP, to generate a mixed mode of phantom and common modes. Impedance matching can improve coupling to all TMP modes.

A network of TMPs was originally installed in many countries to provide a plain old telephone services (POTS) telephony connection, intended to carry signals using differential mode at frequencies of up to a few Kilohertz. However, such lines can often reliably carry signals at much greater frequencies. This has been exploited to make use of the POTS TMP network for digital subscriber line (DSL) data communication.

As access networks have evolved, telecommunications network providers have expanded their fibre optic infrastructure outwards towards the edges of the networks, making the length of the final portion of each connection to an end user subscriber (which is still typically provided by a TMP or a series of concatenated TMPs) shorter. The shorter the line, the greater the range of frequencies over which signals can be reliably transmitted, especially with the use of technologies such as discrete multi-tone (DMT), etc. Therefore, as the so-called "last mile" TMP connections used to connect subscribers to the network have become shorter, greater bandwidth potential has become available without having to bear the expense of installing new optic fibre connections to each subscriber.

Figure 1 is a schematic illustration of an example broadband deployment. The deployment comprises a distribution point unit (DPU) 10 which is connected to three user premises 31, 32, 33 (which in this example are flats within a single building 30) via respective TMP connections 21, 22, 23 which connect between an access node (AN) 16 (which may, for example, be a digital subscriber line access multiplexer, DSLAM) within the DPU 10 and respective customer premises equipment (CPE) modems 51, 52, 53 via respective network termination points 41, 42, 43 within the respective user premises 31, 32, 33. The DPU 10 additionally includes an optical network terminal (ONT) 14 which provides a backhaul connection from the DPU 10 to a local exchange building via an optical fibre connection such as a passive optical network (PON) and a controller 12 which coordinates communication between the AN 16 and the ONT 14, and which may perform some management functions such as communicating with a remote persistent management agent (PMA).

The example deployment illustrated in Figure 1, involving an optical fibre backhaul connection from a distribution point and a twisted metallic pair connection from the distribution point to the subscriber premises, is a type of deployment for which the G.FAST and G.MGFAST international telecommunication union (ITU) standards are intended to be applicable. In such a situation, the TMP connections may be as short as a few hundred metres or less, for example possibly a few tens of metres only, and because of this it tends to be possible to use very high frequency signals (e.g. up to hundreds of Megahertz) to communicate over the short TMPs because the attenuation of high frequency signals over such short distances is insufficient to prevent them from carrying useful information.

As data demands have increased, operating frequencies and system bandwidth have increased rapidly to match service demands. Whilst digital signal processing advanced DSL performance substantially, design of the analogue front ends which launch signals onto the lines on the other hand has not progressed apace. At the relatively low frequencies used for POTs and early generations of DSL, the electrical properties, in particular line impedance, of the TMPs were largely uniform. Impedance matching has therefore traditionally been provided by standard components, for example fixed impedance balun connectors. The ITU G.fast standard for example specifies a fixed termination impedance of 100 D. However, the electrical characteristics of transmission lines, in particular their impedance characteristics, are frequency-dependent. In the context of communication transmission lines, reflection losses due to impedance discontinuities limit line data rates and reach. They also make full duplex transmission (where a single transmission line carries signals in both directions simultaneously) difficult since the reflection of the outgoing signal can overwhelm the incoming signal. In TMPs, impedance perturbations are introduced at higher frequencies, such as those specified for the latest generations of DSL technology (G.fast and G.mgfast), due to radiation and crosstalk. In shorter transmission lines poor coupling can also create problematic resonances.

Eliminating losses due to radiation and resonance, and controlling crosstalk between adjacent TMPs, is increasingly important as we seek to increase line speeds and reach. Reducing radiation caused by reflections from impedance discontinuities may also mean that more data can be fitted under the power spectral density masks typically imposed by regulation.

What is needed is a way to determine what impedance matching is required for any given transmission line or type of transmission line.

Summary

According to a first aspect, there is provided a method of characterising impedance discontinuities on an electrical transmission line, the method comprising: using time-domain windowing of a response obtained at a first end of the transmission line, isolating from the response one or more reflections of a reference signal launched along the transmission line from the first end; and for each of the one or more reflections, characterising, from said reflection, an impedance discontinuity from which that reflection originated.

The isolating step can comprise: obtaining the response; then applying one or more time-domain windows to the response, each of the time-domain windows corresponding to one of the one or more reflections.

The response can extend from a time to the reference signal was launched to at least to + 2T, where T is the transmission line's propagation delay.

Alternatively, the isolation step could comprise obtaining the response for only one or more predetermined time windows following launch of the reference signal, each of the windows corresponding to one of the one or more reflections.

The characterising step can comprise, for each of the one or more reflections, one or more of: identifying the impedance discontinuity from which that reflection originated; locating the impedance discontinuity from which that reflection originated; and quantifying an impedance mismatch across the impedance discontinuity from which that reflection originated.

The isolating step can comprise identifying one of the reflections as originating from an impedance discontinuity at the first end by: identifying one of the reflections having an arrival time in advance of any of the others as originating from the impedance discontinuity at the first end; and/or identifying one of the reflections having a magnitude larger than any of the others as originating from the impedance discontinuity at the first end.

The method can further comprise: obtaining a source impedance Zs of a transmitter which launched the reference signal; and determining an input impedance Zi of the first end as: Z( = Z, , where S" is the magnitude of the reflection identified as originating from the impedance discontinuity at the first end, relative to the magnitude of the reference signal.

The isolating step can comprise identifying one of the reflections as originating from an impedance discontinuity at a second end of the transmission line by: obtaining a length L of the transmission line from the first end to the second end; obtaining a transmission speed v along the transmission line; obtaining an arrival time of each reflection relative to a time at which the reference signal was launched; and identifying one of the reflections having an arrival time closer to 2L/v than any of the others as the reflection originating from an impedance discontinuity at the second end of the transmission line.

The method can further comprise: obtaining a transmission speed v along the transmission line; obtaining an arrival time ti of each reflection, indexed by i, relative to a time at which the reference signal was launched; and determining a length /i along the transmission line from the first end to each impedance discontinuity as: /i = (v * ti)/2. ;The method can further comprise: determining that a fault has arisen on the transmission line; and determining that the highest determined value of / is the fault's location. ;The isolating step can be performed in response to determining that a fault has arisen on the transmission line. ;The method can further comprise selecting an impedance matching element for one or more of the impedance discontinuities. ;The selecting step can be performed to: increase an objective function representing transmission efficiency over the transmission line; and/or decrease an objective function representing reflections along the transmission line; and/or match a predetermined function representing far end cross talk (FEXT) on the transmission line. ;The selecting step can be performed by means of an analytical impedance matching technique or an impedance matching simulation technique. ;The method can be: repeated for a plurality of frequency channels, the selecting step being performed according to average impedance characteristics as determined for the plurality of frequency channels; and/or repeated for a plurality of transmission lines, the selecting step being performed according to average impedance characteristics as determined for the plurality of transmission lines. ;The method can further comprise initiating launch of a communication signal along the transmission line through the impedance matching element. ;The launch of the communication signal can be from the first end on a first frequency channel; the method further comprising initiating launch of an additional communication signal along the transmission line through the impedance matching element from a second end of the transmission line on an additional frequency channel, such that the communication signal and the additional communication signal are transmitted in full duplex. ;The method can further comprise initiating launch of the reference signal, wherein the reference signal is optionally a data signal representative of communication signals to be transmitted over the transmission line. ;The reference signal can be launched on a channel having a frequency greater than 30 MHz, optionally between 300 and 500 MHz. ;The time-domain windowing can be performed using a window of less than 4 ns. ;The transmission line can be a twisted metallic pair (TMP). ;The method can be performed by a data processing system. ;According to a second aspect, there is provided a data processing system comprising a processor configured to perform the method of the first aspect. ;According to a third aspect, there is provided a computer program comprising instructions which, when the program is executed by a computer, cause the computer to carry out the method of the first aspect. ;According to a fourth aspect, there is provided a computer-readable data carrier having stored thereon the computer program of the third aspect. ;According to a fifth aspect, there is provided a data carrier signal carrying the computer program of the third aspect. ;According to a sixth aspect, there is provided a system comprising: the data processing system of the second aspect; and the transmission line; wherein the data processing system further comprises: a transmitter configured to launch the reference signal along the transmission line from the first end under the control of the processor; and a receiver configured to receive the response from the transmission line at the first end and pass it to the processor. ;Brief description of the figures ;Aspects of the present disclosure will now be described by way of example with reference to the accompanying figures. In the figures: Figure 1 is a schematic illustration of an example broadband deployment; Figure 2 schematically illustrates transmission and reflection over a transmission line with no impedance discontinuities along the length of the line: Figure 3 is a graph illustrating an idealised response measured at the port a signal transmitted over a transmission line is input to; Figure 4 is a graph comparing standard impedance matching to impedance matching performed according to the present disclosure; Figure 5 is a flowchart illustrating an example method of characterising impedance discontinuities on an electrical transmission line; and Figure 6 schematically illustrates an example system in which the method of Figure 5 could be implemented. ;Detailed description of the figures ;The following description is presented to enable any person skilled in the art to make and use the system, and is provided in the context of a particular application. Various modifications to the disclosed embodiments will be readily apparent to those skilled in the art. ;It is proposed to use time domain windowing (e.g. time-gated measurements) to isolate reflections on electrical transmission lines in order to characterise the impedance discontinuities which cause them. A test or reference signal is transmitted over the transmission line to be characterised and the reflection signals received over time measured. The first reflection can be used to characterise the mismatch between the source and input impedances. The location and magnitude of other impedance mismatches along the transmission line can respectively be determined from the arrival time and magnitude of the associated reflections (in the former case, provided the transmission speed is known). ;Impedance matching making use of this method can enhance the existing copper infrastructure by boosting the signal strength at higher frequencies. This can increase signal coverage throughout rural areas and help solve the last mile problem of modern communications, as well as providing energy savings by reducing losses due to reflections and/or radiation caused by impedance mismatches. Reducing reflections can also allow enough sensitivity and dynamic range in analogue front ends to enable them to operate in full duplex. ;Figure 2 schematically illustrates transmission and reflection over a transmission line 220 with no impedance discontinuities along the length of the line. A signal is directed from a transmitter 1 to a receiver 2. The connection between the transmitter 1 and the transmission line 220 presents an impedance discontinuity to the signal, with the transmitter having a source impedance Zs. which differs from the input impedance of the transmission line Z. This results in a fraction of the signal being almost immediately reflected back into the transmitter 1 via S". The remaining fraction of the signal continues to the receiver 2 via S21. The connection between the transmission line 220 and the receiver 2 presents another impedance discontinuity to the signal, with the transmission line having an output impedance 4 which differs from the load impedance of the receiver Z. This results in a fraction of the signal being reflected back to the transmitter 1 via S12. (Note that the transmission fractions are frequency-dependent, and that in reality there will be losses along the line.) Figure 3 illustrates an idealised response measured at the port a signal transmitted over a transmission line is input to, where the transmission line has impedance discontinuities at its input and output as well as at three locations along the line. The ordinate axis represents the response amplitude (A) and the abscissa axis represents time (t) from transmission of the signal (to). Very soon after to, at time ti, reflection from the input discontinuity registers. Sequentially smaller reflections register at times t2, t3 and lit from impedance discontinuities along the line. At 2T, i.e. twice the propagation delay of the transmission line, a reflection from the output discontinuity registers. ;It can be seen from Figure 3 that the various reflections can be decoupled from one another by means of time-domain windowing. A suitable window size to capture a single reflection at a time is shown as At. Once the other reflections are decoupled from the initial reflection at ti, the amplitude of that initial reflection can be expressed as a fraction of the amplitude of the reference pulse to find the self-coupling coefficient of the input port, Su, which for any given frequency obeys: ZrZ, (1) Sll Z,+Zs which shows that the initial reflection goes to zero when Zt and Zs are matched. ;To calculate the target matching impedance, i.e. 21, rearrange equation (1) to obtain: z, = zs (1'11) (2) Once 2, is obtained, classical impedance matching design can be performed analytically or via simulation. ;The above analysis can be repeated for each decoupled reflection in sequence to characterise the other impedance discontinuities, for example as might arise at the transition from a bundled cable to the "final drop" of a line to subscriber premises. The locations of impedance discontinuities along the line can also be determined from the arrival times of their associated reflections (t2, t3 and t4). ;Figure 4 illustrates an example of the gains which have been achieved using this method. The fraction of the transmitted signal received at the output port is shown on the ordinate axis for frequencies on the abscissa axis from 0.28 to 0.50 MHz. The dot-dash line shows results for a TMP with standard baluns for termination impedances, while the solid line shows results for the same TMP terminated with matching circuits designed to optimise matching in the 300 to 500 MHz range by making use of the method described herein. (The method was repeated for several reference signals, each reference signal having a different frequency in the 300 to 500 MHz range. In this way, the impedance discontinuities on the line were characterised across the frequency range of interest, to inform suitable impedance matching for that frequency range.) It can be seen that the transmission efficiency was significantly improved by tailoring the termination impedances according to the present method. More generally, it has been found that fixed impedance terminations in DSL cause a bottleneck on transmission efficiency for frequencies beyond around 30 MHz. ;Figure 5 is a flowchart illustrating an example method 500 of characterising impedance discontinuities on an electrical transmission line. The core steps are steps 510 and 520. At step 510 one or more reflections of a reference signal launched along the transmission line (initiated at step 504) from the first end are isolated using time-domain windowing of a response obtained at a first end of the transmission line. At step 520 an impedance discontinuity from which each of the reflections originated is characterised. ;The time-domain windowing can for example be performed using a window of a few picoseconds to a few nanoseconds, e.g. less than 4 ns, depending on how far the signal source is from the location of the target impedance discontinuity. ;Step 510 can for example comprise obtaining the response at step 511, then applying one or more time-domain windows to the response at step 512, each of the time-domain windows corresponding to one of the one or more reflections. The time-domain windowing in this case acts as a post-filter on the response. In order to capture all impedance discontinuities from the input to the output of the transmission line, the response extends from a time to the reference signal was launched to at least to + 2T, where T is the transmission line's propagation delay. ;Alternatively step 510 could comprise obtaining the response for only one or more predetermined time windows following launch of the reference signal, each of the windows corresponding to one of the one or more reflections. The time-domain windowing in this case acts as a pre-filter on the response. This approach may require less memory resource than that of steps 511 and 512, but requires prior knowledge of the locations of the discontinuities so that the windows can be positioned correctly in advance. If an iterative approach to impedance matching is used, then a first iteration could implement step 510 using steps 511 and 512, and the knowledge of when to expect reflections from each discontinuity thus gained could be used to inform subsequent iterations using the pre-windowing approach. ;Step 510 could comprise identifying a first one of the reflections, which arrived at the first end in advance of any of the others, as originating from an impedance discontinuity at the first end. In that case, the method could further comprise obtaining a source impedance Zs. of a transmitter which launched the reference signal, for example from a memory of a data processing device performing the method, by referring to technical specifications or by measuring. The input impedance zi of the first end can then be determined from equation (2) above, where Si, is the magnitude of the first one of the reflections divided by the magnitude of the reference signal. ;Step 510 could comprise identifying a reflection as originating from an impedance discontinuity at a second end of the transmission line by obtaining a length L of the transmission line from the first end to the second end, a transmission speed v along the transmission line and an arrival time of each reflection relative to a time at which the reference signal was launched, then identifying one of the reflections having an arrival time closer to 2/1v than any of the others as the reflection originating from an impedance discontinuity at the second end of the transmission line. The values of L and v can for example be obtained from a memory of a data processing device performing the method, by referring to technical specifications or by measuring. ;Optionally, a length I along the transmission line from the first end to each impedance discontinuity, indexed by 1, can be calculated as: L = (v * t3/2 at step 522, where ti is the arrival time of the ith reflection relative to the time at which the reference signal was launched. This could for example be done if the method is being performed in response to determining that a fault has arisen on the transmission line at step 502. The fault's location can then be found at step 524 as the highest determined value of /.

Once the impedance discontinuities have been characterised at step 520, an impedance matching element for one or more of them can be selected at step 550. Step 550 can for example be performed with the aim of improving transmission efficiency over the transmission line and/or decreasing reflections along the transmission line and/or controlling far end cross talk (FEXT) on the transmission line. One or more objective functions could be developed for these purposes. Step 550 could for example be performed by means of an analytical impedance matching technique or an impedance matching simulation technique.

The method could be repeated for a plurality of frequency channels, with step 550 being performed according to average impedance characteristics as determined for the plurality of frequency channels. To this end, at query 530 it can be determined whether or not all channels have been tested. If the determination is negative then the flow cycles back through step 532 to step 504, where the method is repeated for the next channel, while if it is positive it continues on to step 550.

Alternatively or additionally, the method could be repeated for a plurality of transmission lines, with step 550 being performed according to average impedance characteristics as determined for the plurality of transmission lines. To this end, at query 540 it can be determined whether or not all transmission line samples have been tested. If the determination is negative then the flow cycles back through step 542 to step 504, where the method is repeated for the next sample, while if it is positive it continues on to step 550.

The averaging described above could be used to arrive at tailored standard termination impedances for various types of deployments. The averages could be weighted according to the expected use and/or importance of various channels and/or types of transmission line in a deployment type.

Finally, at step 560 launch of a communication signal along the transmission line through the impedance matching element can be initiated. The launch of the communication signal can be from the first end on a first frequency channel. The method can further comprise contemporaneously initiating launch of an additional communication signal along the transmission line through the impedance matching element from a second end of the transmission line on an additional frequency channel, such that the communication signal and the additional communication signal are transmitted in full duplex.

The reference signal launched at step 504 can optionally be representative of communication signals to be transmitted over the transmission line according to step 560. The reference signal can for example be launched on a channel having 10 a frequency greater than 30 MHz, for example between 300 and 500 MHz.

The transmission line can for example be a TMP.

Method 500 has been described with reference entirely to the time domain.

However, in some implementations it may be necessary or expedient to perform some steps in the frequency domain, with Fourier transforms being used to convert between the time and frequency domains.

Figure 6 illustrates an example data processing system 610 which could comprise a processor 611 configured to perform the method 500 of Figure 5. A memory 612 of the data processing system 610 could to this end store a computer program comprising instructions which, when executed by the data processing system 610, cause it to carry out the method 500. The memory 612 could be a computer-readable data carrier, or a transitory memory used to implement the computer program following receipt on a data carrier signal. The data processing system 610 could for example be a network analyser such as a vector network analyser.

The data processing system 610 is illustrated as part of a system 600 which also comprises a transmission line 620 extending between a first end 601 and a second end 602. The data processing system 610 further comprises a transmitter 613 configured to launch the reference signal along the transmission line from the first end 601 under the control of the processor 611 and a receiver 614 configured to receive the response from the transmission line 620 at the first end 601 and pass it to the processor 611 for analysis.

Other embodiments will be apparent to those skilled in the art from consideration of the specification and practice of the embodiments disclosed herein. It is intended that the specification and examples be considered as exemplary only.

In addition, where this application has listed the steps of a method or procedure in a specific order, it could be possible, or even expedient in certain circumstances, to change the order in which some steps are performed, and it is intended that the particular steps of the method or procedure claims set forth herein not be construed as being order-specific unless such order specificity is expressly stated in the claim. That is, the operations/steps may be performed in any order, unless otherwise specified, and embodiments may include additional or fewer operations/steps than those disclosed herein. It is further contemplated that executing or performing a particular operation/step before, contemporaneously with, or after another operation is in accordance with the described embodiments.

The methods described herein may be encoded as executable instructions embodied in a computer readable medium, including, without limitation, non-transitory computer-readable storage, a storage device, and/or a memory device.

Such instructions, when executed by a processor (or one or more computers, processors, and/or other devices) cause the processor (the one or more computers, processors, and/or other devices) to perform at least a portion of the methods described herein. A non-transitory computer-readable storage medium includes, but is not limited to, volatile memory, non-volatile memory, magnetic and optical storage devices such as disk drives, magnetic tape, compact discs (CDs), digital versatile discs (DVDs), or other media that are capable of storing code and/or data.

Where a processor is referred to herein, this is to be understood to refer to a single processor or multiple processors operably connected to one another. Similarly, where a memory is referred to herein, this is to be understood to refer to a single memory or multiple memories operably connected to one another.

The methods and processes can also be partially or fully embodied in hardware modules or apparatuses or firmware, so that when the hardware modules or apparatuses are activated, they perform the associated methods and processes. The methods and processes can be embodied using a combination of code, data, and hardware modules or apparatuses.

Examples of processing systems, environments, and/or configurations that may be suitable for use with the embodiments described herein include, but are not limited to, embedded computer devices, personal computers, server computers (specific or cloud (virtual) servers), hand-held or laptop devices, multiprocessor systems, microprocessor-based systems, set top boxes, programmable consumer electronics, mobile telephones, network personal computers (PCs), minicomputers, mainframe computers, distributed computing environments that include any of the above systems or devices, and the like. Hardware modules or apparatuses described in this disclosure include, but are not limited to, application-specific integrated circuits (ASICs), field-programmable gate arrays (FPGAs), dedicated or shared processors, and/or other hardware modules or apparatuses.

User devices can include, without limitation, static user devices such as PCs and mobile user devices such as smartphones, tablets, laptops and smartwatches.

Receivers and transmitters as described herein may be standalone or may be comprised in transceivers. A communication link as described herein comprises at least one transmitter capable of transmitting data to at least one receiver over one or more wired or wireless communication channels. Wired communication channels can be arranged for electrical or optical transmission. Such a communication link can optionally further comprise one or more relaying transceivers.

Claims (9)

1. Claims 1. A method of characterising impedance discontinuities on an electrical transmission line, the method comprising: using time-domain windowing of a response obtained at a first end of the transmission line, isolating from the response one or more reflections of a reference signal launched along the transmission line from the first end; and for each of the one or more reflections, characterising, from said reflection, an impedance discontinuity from which that reflection originated.
2. 2. The method of claim 1, wherein the isolating step comprises: obtaining the response; then applying one or more time-domain windows to the response, each of the time-domain windows corresponding to one of the one or more reflections.
3. 3. The method of claim 2, wherein the response extends from a time to the reference signal was launched to at least to + 2T, where T is the transmission line's propagation delay.
4. 4. The method of claim 1, wherein the isolation step comprises obtaining the response for only one or more predetermined time windows following launch of the reference signal, each of the windows corresponding to one of the one or more reflections.
5. 5. The method of any of claims 1 to 4, wherein the characterising step comprises, for each of the one or more reflections, one or more of: identifying the impedance discontinuity from which that reflection originated; locating the impedance discontinuity from which that reflection originated; and quantifying an impedance mismatch across the impedance discontinuity from which that reflection originated.
6. 6. The method of any preceding claim, wherein the isolating step comprises identifying one of the reflections as originating from an impedance discontinuity at the first end by: identifying one of the reflections having an arrival time in advance of any of the others as originating from the impedance discontinuity at the first end; and/or identifying one of the reflections having a magnitude larger than any of the others as originating from the impedance discontinuity at the first end.
7. 7. The method of claim 6, further comprising: obtaining a source impedance Zs of a transmitter which launched the reference signal; and determining an input impedance Zi of the first end as: Z, = Zs where S" is the magnitude of the reflection identified as originating from the impedance discontinuity at the first end, relative to the magnitude of the reference signal.
8. 8. The method of any preceding claim, wherein the isolating step comprises identifying one of the reflections as originating from an impedance discontinuity at a second end of the transmission line by: obtaining a length L of the transmission line from the first end to the second end; obtaining a transmission speed v along the transmission line; obtaining an arrival time of each reflection relative to a time at which the reference signal was launched; and identifying one of the reflections having an arrival time closer to 2 Yv than any of the others as the reflection originating from an impedance discontinuity at the second end of the transmission line.
9. 9. The method of any preceding claim, further comprising: obtaining a transmission speed v along the transmission line; obtaining an arrival time t, of each reflection, indexed by 1, relative to a time at which the reference signal was launched; and determining a length /, along the transmission line from the first end to each impedance discontinuity as: /, = (v * t,)/2.;10. The method of claim 9, further comprising: determining that a fault has arisen on the transmission line; and determining that the highest determined value of is the fault's location.;11. The method of any preceding claim, further comprising selecting an impedance matching element for one or more of the impedance discontinuities.;12. The method of claim 11, wherein the selecting step is performed to: increase an objective function representing transmission efficiency over the transmission line; and/or decrease an objective function representing reflections along the transmission line; and/or match a predetermined function representing far end cross talk, FEXT', on the transmission line.;13. The method of either of claims 11 or 12, wherein the selecting step is performed by means of an analytical impedance matching technique or an impedance matching simulation technique.;14. The method of any of claims 11 to 13: repeated for a plurality of frequency channels, the selecting step being performed according to average impedance characteristics as determined for the plurality of frequency channels; and/or repeated for a plurality of transmission lines, the selecting step being performed according to average impedance characteristics as determined for the plurality of transmission lines.;15. The method of any of claims 11 to 14, further comprising initiating launch of a communication signal along the transmission line through the impedance matching element.;16. The method of claim 15, wherein the launch of the communication signal is from the first end on a first frequency channel; the method further comprising initiating launch of an additional communication signal along the transmission line through the impedance matching element from a second end of the transmission line on an additional frequency channel, such that the communication signal and the additional communication signal are transmitted in full duplex.;17. The method of any preceding claim, further comprising initiating launch of the reference signal, wherein the reference signal is optionally a data signal representative of communication signals to be transmitted over the transmission line.;18. The method of any preceding claim, wherein the reference signal is launched on a channel having a frequency greater than 30 MHz, optionally 20 between 300 and 500 MHz.;19. The method of any preceding claim, wherein the time-domain windowing is performed using a window of less than 4 ns.;20. The method of any preceding claim, wherein the transmission line is a twisted metallic pair, 'IMP'.;21. A data processing system comprising a processor configured to perform the method of any of claims 1 to 20.;22. A computer program comprising instructions which, when the program is executed by a computer, cause the computer to carry out the method of any of claims 1 to 20.;23. A computer-readable data carrier having stored thereon the computer program of claim 22.;24. A data carrier signal carrying the computer program of claim 22.;25. A system comprising: the data processing system of claim 21; and the transmission line; wherein the data processing system further comprises: a transmitter configured to launch the reference signal along the transmission line from the first end under the control of the processor; and a receiver configured to receive the response from the transmission line at the first end and pass it to the processor.