WO2001001158A1 - Single ended measurement method and system for determining subscriber loop make up - Google Patents

Abstract

A method and system for determining the make-up of a subscriber loop (220) comprising sending pulses onto a loop and acquiring data based on received echo signals. Determining (7301) from the received echoes each discontinuity on the loop and, based on each discontinuity, determining a channel transfer function (7305) for each loop section preceding the discontinuity. The transfer function is then used to synthesize an inverse filter (7309). The inverse filter and acquired data are convolved for all the loop sections preceding the discontinuity (7313). The method may be further improved by modeling real and spurious echoes and subtracting these echoes from the echoes generated in the loop.

Description

SINGLE ENDED MEASUREMENT METHOD AND SYSTEM FOR DETERMINING SUBSCRIBER LOOP MAKE UP

RELATED APPLICATION

The present application claims the benefit of U.S. Provisional Application Number

60/141,121 filed on June 25, 1999 and entitled "An Algorithmic Approach to the Problem of

Loop Make-Up Identification Via Single Ended Measurements". The present application is

related to US Provisional Application No. 60/157,094 filed on September 30, 1999, entitled

"Method and Circuitry for Measuring Weak Echoes on Subscriber Loops" and which is assigned to the assignee of the present invention.

FIELD OF THE INVENTION

The present invention generally relates to determining the make-up of subscriber

loops in the public switched network and more specifically to methods and systems that

determine the make-up of subscriber loops via single ended measurements.

BACKGROUND The mainstay of the telephone company local network is the local subscriber loop.

The great majority of residential customers, and many business customers, are served by

metallic twisted pair cables connected from a local switch in the central office to the

subscriber's telephones. When customers request service, request a change in service, or

drop service, these facilities must be appropriately connected or arranged in the field,

referred to as the "outside plant," and telephone companies have specially trained craft

dedicated full time to this task. Obviously a company needs to have an understanding of its

subscriber loops including where they are connected and the location of the flexibility points such as junction boxes, etc. These records historically were kept on paper, called "plats," and

more recently are manually entered into a computer database. However, even when entered into a database there are still problems associated with keeping the records accurate and up- to-date.

Having accurate records of the loop plant is critically important to many aspects of a

telephone company's business. In addition to the need for accurate records to provide

traditional voice services, there will be a need for even more accurate and more detailed

records in order to deploy a whole new class of "xDSL" based services, including those based on integrated services digital network (ISDN), high-rate digital subscriber line

(HDSL), asymmetrical digital subscriber lines (ADSL) and very high rate digital subscriber

lines (VDSL) technology. These technologies are engineered to operate over a class of

subscriber loops, such as nonloaded loops (18 kft), or Carrier Serving Area (CSA) loops (9

to 12 kft). In fact, the need to be able to "qualify" a loop for provision of one of these technologies is becoming critical, as the technologies emerge and deployment begins. The

ability to easily and accurately qualify loops will allow telephone companies to offer a whole

range of new services; problems and high expenses associated with qualifying loops can potentially inhibit deployment and/or lower or forego associated new revenues. The

unscreened multipair cables in the existing subscriber loop network constitute the main

access connection of telephone users to the telephone network. Recently, the demand for

new services such as data, image and video has increased tremendously, and telephone

companies have planned to deliver broadband ISDN services via fiber optic local loops.

However, the deployment of fiber optic cables in the access plant will require at least twenty

years, so that, in the meantime, it is extremely important to fully exploit the existing copper

cable plant. Although there are many different digital subscriber line services, for example, ISDN

basic access, HDSL, ADSL, VDSL, and Synchronous DSL (SDSL), these services are not always be available to every customer since copper lines seem to present more problems

than expected. In fact, the cable length and the presence of load coils and bridged taps may

deeply affect the performance of DSL services. Unfortunately, loop records are unreliable

and often don't match the actual loop configuration, so that the existing databases cannot be

fully exploited.

Loop prequalification is an important issue not only because it can help an economic

deployment of DSL services, but also because it can help telephone companies in updating

and correcting their loop-plant records. From this point of view, the feasibility of accurate

loop make-up identification would have a much higher economic value than simple DSL

qualification.

One way to obtain accurate loop records is to manually examine the existing records

and update them if they are missing or inaccurate. This technique is expensive and time

consuming. Furthermore, new technologies such as xDSL require additional information that

was previously not kept for voice services, so there is the potential that new information

needs to be added to all existing loop records. Test set manufacturers offer measurement

devices that can greatly facilitate this process, but typically they require a remote craft

dispatch. Another way to obtain accurate loop records is by performing a loop prequalification

test. There are essentially two ways of carrying out a loop prequalification test: double

ended or single-ended measurements. Double-ended measurements allow us to easily

estimate the impulse response of a loop by using properly designed training sequences. Double-ended testing, however, requires equipment at both ends of the loop. Specifically, in

addition to equipment at the Central Office (CO) or near end of the loop, double ended

testing involves either the presence of a test device at the far end of the loop (Smart Jack or MTU), or dispatching a technician to the subscriber's location (SL) to install a modem that

communicates with the reference modem in the CO. An exemplary double ended system

and method that extrapolates voice band information to determine DSL service capability for

a subscriber loop is described in Lechleider, et. al., US Patent ,

entitled "Method and System for Estimating the Ability of a Subscriber Loop to Support Broadband Services" (which is assigned to the assignee of the present invention).

In contrast, single ended tests are less expensive and time consuming than double-

ended tests. Furthermore, single-ended testing requires test-equipment only at the CO. In

fact, no technician dispatching is required and the CO can perform all the tests in a batch

mode, exploiting the metallic access with full-splitting capability on the customer's line. An example of such a single ended test system is the "MLT" (Mechanized Loop Testing)

product that is included as part of the widely deployed automated loop testing system

originally developed by the Bell System. The MLT system utilizes a metallic test bus and

full- splitting metallic access relays on line card electronics. By this means, a given subscriber loop can be taken out of service and routed, metallically, to a centralized test

head, where single-ended measurements can be made on the customer's loop. The test head runs through a battery of tests aimed at maintaining and diagnosing the customer's

narrowband (4kHz) voice service, e.g., looking for valid termination signatures via

application of DC and AC voltages. This system is highly mechanized, highly efficient, and

almost universally deployed. In addition, the MLT system is linked to a Line Monitoring Operating System (LMOS) thereby providing a means to access and update loop records

which are useful in responding to customer service requests or complaints. However, because this system exclusively focuses on narrowband voice services, the system misses

important loop make-up features that will be deleterious to supporting broadband services

via DSL technologies.

Another well known single-ended measurement technique relies on the observation

of echoes that are produced by medium discontinuities to fully characterize the link.

Specifically, these single ended measurements typically rely on time domain reflectometry

(TDR). TDR measurements are analogous to radar measurements in terms of the physical

principles at work. TDR test systems transmit pulses of energy down the metallic cable

being investigated and once these pulses encounter a discontinuity on the cable a portion of

the transmitted energy is reflected or echoed back to a receiver on the test system. The

elapsed time of arrival of the echo pulse determines its location, while the shape and polarity of the echo pulse(s) provides a signature identifying the type of discontinuity that caused the

reflection or echo. Basically, if the reflecting discontinuity causes an increase in impedance,

the echo pulse's polarity is positive; if the reflecting discontinuity causes a decrease in

impedance, the echo pulse's polarity is negative. A bridged tap, for example, produces a

negative echo at the location of the tap and a positive echo at the end of the bridged tap.

Accordingly, a trained craftsperson is able to determine the type of fault based on the shape,

polarity, sequence of pulses.

Nevertheless, TDR methods (or, in general, single ended measurements that rely on

echo pulse signatures) are inaccurate and provide ambiguous results that even the most skilled craftsperson cannot interpret. Because the arrival of the echoes is dependent on the location of the discontinuities (or faults) one echo can be masked by another echo if the

echoes overlap. For example, FIG. 1A illustrates an exemplary loop having three

discontinuities, two of which are bridged taps 500 feet apart. In accordance with TDR

methods, a pulse 10 is sent from CO 13 to subscriber location 15 on the subscriber loop

having loop segments 20, 22, 24, and 26. As the pulse traverses the loop it encounters a

gauge change 30, a first bridged tap 32, and a second bridged tap 34 before arriving at SL 15.

FIG. IB depicts the echo response caused by the bridged taps 32 and 34 (the echoes

generated at the gauge change 30 and SL 15 were filtered from FIG. IB). As FIG. IB shows

it is not possible to detect the two bridged taps via prior art TDR methods. In fact, looking

at FIG. IB, it appears as if only one bridged tap 32 is present since there is only one negative

70 - positive 80 transition. However, since the positive echo 80 is not weaker than the

negative one 70 (as it usually is for bridged taps) it may be induced that either the bridged

tap is very short or several bridged taps are present. However, since the width of the positive

echo 70 looks quite large it is very unlikely that it was a short bridged tap since a short

bridged tap would introduce a small amount of distortion and, consequently, a narrower

pulse. Therefore, the case of several bridged taps may be the most probable, although we

cannot say how many there are. As such, TDR methods can produce ambiguous results.

In addition, prior art TDR methods do not take into account, more specifically, are

unable to measure, the effects of spurious echoes. That is, and with reference to FIG. 1A, as

pulse 10 arrives at gauge change 30, a portion of pulse 10 is reflected to generate a first real

echo, and the remaining portion (or refracted portion) travels toward bridged tap 32. At

bridged tap 32, reflection and refraction again occur in the process producing a second real

echo. This second echo pulse (travelling upstream to CO 13) is then reflected at gauge change 30 back to bridged tap 32 where a spurious echo pulse is then reflected to the CO 13.

Although spurious echoes will be more attenuated than real echoes, they are added to the real echoes causing the real echo signals between to be distorted. Accordingly, spurious

echoes enhance the ambiguity inherent in TDR measurements because the shape of the echo

is used to interpret the type of fault that caused the echo. In other words, a craft person

interpreting a TDR measurement analyzes a distorted trace that does not distinguish spurious echo distortion. More importantly, the effects of spurious echoes on the pulse shape cannot

be interpreted via human visual inspection.

Of utility then is a method and system for unambiguously and completely

determining a subscriber loop make-up including detecting the presence and location of load

coils, gauge changes, and bridged taps and the length of the loop including the length of each

bridged tap.

SUMMARY

Our invention is a method and system for unambiguously and completely

determining a subscriber loop make-up.

In accordance with an aspect of our invention the narrowband test head and

associated measurements that is included as part of the prior art mechanized loop test system

is replaced with a broadband test head and associated measurements. By using a broadband

test head and signal processing algorithms our invention advantageously allows automated

measurements of any subscriber loop that completely determine the loop make-up.

In accordance with an object of our invention, existing loop records can be checked.

Specifically, because our invention can be automated, loop make-ups can be methodically

obtained and then compared to existing database records. More likely, because we expect the information to be more accurate and detailed than the existing records, the new data will

simply replace existing data and populate the database. Thus, through an entirely

mechanized process, a telephone network or service provider can completely update its

records. Having an up-to-date database will help the provider in its day-to-day business

operation, and may also position it to cost-effectively meet requirements imposed as a result of "loop unbundling" rulings resulting from the Telecom Act of 1996.

In accordance with an additional object of our invention, loop make-ups can be

advantageously used by another mechanized system to calculate and qualify the ability of a

given loop to support advanced DSL services. A computer system can take the loop make- ups, models of the various DSL systems, and standards for spectral compatibility and

determine very precisely the service level of a given subscriber. Such determination is likely

to be achievable in real-time, for example in response to a service agent's query while on the

phone with a customer.

In accordance with another object of our invention, additional information will be

determined regarding the noise environment a particular loop is exposed to. This noise may

be analyzed and identified, further facilitating the service provisioning process, identifying

noise sources exceeding spectral compatibility requirements, and generally facilitating

maintenance and administration.

Our broadband test head method includes the steps of sending at least one pulse on

the loop, receiving the echoes generated when the pulse encounters a discontinuity, and

processing the received echoes to determine the type and location of each discontinuity. Our

processing method starts at the central office having access to the loop and moves along the

loop to the subscriber location. As we move out along the loop we compute the transfer function for each preceding loop section (excluding bridged taps), synthesize a filter based

on the transfer function, and convolve the synthesized filter with the echo data.

Our invention can be implemented in either a distributed or non-distributed

embodiment or architecture. In the distributed architecture the functions of data acquisition

and processing are done at different locations, whereas in the non-distributed architecture

each of these functions are done at the same location, preferably by the same equipment. In

either embodiment our method can be used to completely determine the loop make-up.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A depicts an exemplary subscriber loop being tested by time domain

reflectometry;

FIG. IB depicts the echo responses from the bridged taps of FIG. 1 A;

FIG. 2A depicts an illustrative embodiment of our broadband test head system for

determining a subscriber loop make-up in accordance with our invention;

FIG. 2B illustratively depicts another illustrative embodiment of a broadband test head in accordance with the present invention;

FIG. 3 illustratively depicts an exemplary subscriber loop having a gauge change

(labeled A) and a subsequent discontinuity (labeled B) present in the loop;

FIG. 4A illustratively depicts an exemplary subscriber loop having a gauge change in

the loop; FIG. 4B shows the real and spurious echoes produced by the discontinuity present in

the loop shown in FIG. 4A;

FIG. 5 depicts an exemplary subscriber loop having a bridged tap (labeled A) and a

subsequent discontinuity (labeled C) present in the loop; FIG. 6A illustratively depicts an exemplary subscriber loop having a bridged tap in

the loop;

FIG. 6B illustrates a simulation of the real echoes and spurious echoes for the loop

configuration in FIG. 6A; FIG. 7 A illustrates the method steps of our invention;

FIG. 7B illustrates the substeps of step 710 of FIG. 7A;

FIG. 7C illustrates the substeps of step 730 of FIG. 7A;

FIG. 8A depicts an exemplary loop used to demonstrate the advantages of our

invention;

FIG. 8B illustrates a simulation of the echoes generated by the loop of FIG. 8 A with

the real echoes and spurious echoes shown on separate curves;

FIG. 8C illustrates a simulation of the echoes generated by the loop of FIG. 8A with the real echoes and spurious echoes shown on the same curve;

FIG. 9A illustrates a simulation of the effects of filtering the echo signals with an

inverse filter in accordance with our invention for the loop of FIG. 8 A (the inverse filter

includes the transfer function of the section of the loop from the CO 13 to gauge change

805);

FIG. 9B illustrates a simulation of the effects of filtering the echo signals with an

inverse filter in accordance with our invention for the loop of FIG. 8A (the inverse filter

includes the transfer function of the sections of the loop from the CO 13 to bridged tap 807);

FIG. 10A illustrates an experimental setup used to verify our inventive method;

FIG. 10B illustrates the loop used in the setup of FIG. 10A; FIG. IOC depicts the echoes generated by the loop of FIG. 10B on the scope of FIG.

10 A; and

FIG. 10D depicts our simulation of the echoes generated by the loop of FIG. 10B.

DETAILED DESCRIPTION 1. System and Broadband Test Head

Turning now to FIG. 2A, there is depicted a system for determining a subscriber loop

make-up in accordance with an embodiment of our invention. It should be noted that our

invention is the unambiguous determination of a loop make-up autonomously, e.g., without the aid of any human intervention. There may be more than one embodiment of a system

including our invention. Specifically, FIG. 2A is our invention in a distributed architecture

wherein the functions necessary to implement our broadband test head inventive concept is

distributed over a network having functional elements at different locations. FIG. 2B is our

invention in a non-distributed architecture wherein broadband test head functionality is done

at the same location, preferably, on the same equipment.

In accordance with the embodiment depicted in FIG. 2 A, a broadband test head

device 210 is selectively coupled through access module 211 by switch 215 to subscriber

loop 220. Switch 215 determines which subscriber loop 220 should be coupled to broadband test head device 210 in response to control messages transmitted to

communications module 222 from service/repair center 224 over a data communications

network 226. Data communications network is a typical packet data network using any of

the commercially available protocols. In accordance with this embodiment of our invention

customer loops 220 make-up and DSL service capability may be determined via a

mechanized process. In accordance with this embodiment a control message is sent from bureau 224 instructing switch 215 to provide loop access to broadband test head device 210.

Once broadband test head device 210 accesses the loop 220, the test head device 210 sends

at least one pulse of a predetermined duration onto the loop 220 thereby causing echoes at

the various discontinuities existing on the loop 220. The resulting echoes are subsequently

received at device 210 and used to determine loop make-up in accordance with our

invention.

As far as the choice of a probing signal is concerned, several choices are possible. For example, it is know in the prior art that a half-sine pulse is a good choice and this pulse

is used in today's high resolution TDRs. A square pulse may also be used, although it is

commonly claimed that the half-sine pulse leads to higher echo resolution than the square

pulse. However, this may not necessarily be true and both probing signals yield similar

responses. However, there may be a practical advantage in using a half-sine pulse instead of

a square pulse. In fact, a half-sine pulse has more energy at low frequencies than a square

pulse and this property has a twofold advantage. First, it may be more useful to detect gauge

changes since the reflection coefficient of a gauge change is characterized by a low-pass

behavior. Secondly, injecting low frequency pulses in the pair under test would cause less

crosstalk in adjacent pairs that, at the time of the test, may be supporting DSL services.

Another advantage of using a half-sine pulse is that it is easier, from an implementation

point of view, to generate "cleaner" high-amplitude half-sine pulses instead of high-

amplitude square pulses. However, except for the above mentioned practical advantages

there is no conceptual difference between the echo response to a square pulse or to a half-

sine pulse. In accordance with this embodiment of our invention, the processing necessary to determine the make up of the loop 220 is not done in central office 13 (the access CO) but

instead is done at service bureau 224. Specifically, device 210, in addition to having means

for transmitting and receiving pulses also includes means transmitting the acquired data to

communications module 222. The signals received at module 222 are transmitted over the data communications network to bureau 224. At bureau 224, a processor 230 running the

method steps below (and that are stored on memory 231) can completely determine loop

make-up. In accordance with this distributed processing embodiment of our invention, loop

make-up is determined in batch mode. That is, data may be acquired for a plurality of loops,

send to bureau 224, and processed at a later time. This embodiment is particularly advantageous for off-peak hour loop make-up determination so as not to disrupt customer

service.

At bureau 224 the loop make-ups, as determined in accordance with our invention,

are then used to replace or update existing records in a database 235, or if no records exist,

the new make-up is stored as a record in the database 235. It should be noted that the

system of FIG. 2A is meant to illustrate how our broadband test head invention may be

implemented as part of a mechanized loop testing system. In fact, as discussed below, the

essential components of our broadband test head and the method of our invention may also

be implemented in a metallic time domain reflectometer or in a non-distributed architecture.

Turning now to FIG. 2B, there is depicted a broadband test head 211 in accordance

with a second illustrative embodiment of our invention. The embodiment of FIG. 2B is a

non-distributed architecture wherein processing, and loop make-up, is done at the access

location. Here, broadband test head 211 comprises an input/output interface 2101 that couples the loop 220 tip and ring to a transmitter 2105 and receiver 2106. Transmitter 2105

and receiver 2106 are coupled to a micro-processor 2109. Memory 2111 is coupled to

micro-processor 2109 and is used to store the method steps, where applicable, of the present

invention and may be used to also store results. Although a display is not shown, broadband

test head 211 may also include a display, wherein echo results are displayed, and circuitry

for transmitting signals over a data network (as may be required by the embodiment of FIG.

2A). The micro-processor 2109 executes the method steps, where applicable, stored in

memory 2111 and is used in performing digital signal processing on the echoes received by receiver 2106. As detailed below, by processing the echoes received at the receiver in

accordance with our method the subscriber loop may be more accurately determined.

2. Modelline

In order that our method may be better understood, we will now describe a mathematical model for describing the echoes. This model is used to determine the effect

that spurious echoes caused by gauge changes and bridged taps have on determining the loop make-up. The effect of spurious echoes, based on the model, is then included in our method for determining loop make-up.

2.1 Echo Modelling It can be shown that the echo signals arriving at a Central Office (CO) (or more specifically the broadband test head device receiver) in response to a pulse being sent out on

a subscriber loop can be expressed as:

rit⁾ = ∑e^{m (}t-ξ_t ⁾ (1) where e^(l>(t) is the I^th echo response and ξi is the echo arrival time in the CO of the i-th echo.

The echo e^(l)(t) can be expressed as a function of its echo path impulse response h (t) :

e^w (t) = s(t) * h (t) , where λ£> (t) = F^"1 [κ{f)H$ (/) ⁽ (/)] (2)

In eq.(2), Hn(f) is the insertion loss pertaining to the round trip path from the CO to the

discontinuity and back to the CO again, the term K(f) takes into account the role of the

transmission coefficients τ(f) pertaining to previous bridged taps and the reflection

coefficient p(f) models the effects of the discontinuity on the incident signal. Each

discontinuity present in the loop will generate an echo whose shape will depend both on the kind of discontinuity and on the loop sections on which the echo has traveled. The observed signal r(t) will be the sum of a certain number of echoes that might be overlapping. In fact,

even if the probing signal s(t) is a very short pulse, the medium is very dispersive and will broaden the pulse. The longer the loop section, the broader will be the echo received in the

CO. The shape of the echoes generated by different discontinuities does not change significantly from discontinuity to discontinuity. The main difference consists in the width of

the echo: near discontinuities will generate narrower echoes, whereas far discontinuities will

generate broader echoes.

Spurious echoes occur because each discontinuity generates both a reflected and a refracted signal, so that a part of the signal travels back and forth on the line, bouncing

between discontinuities, before it arrives at the CO. Obviously, the more the spurious echo

travels on the line the more attenuation it will exhibit upon arrival at the CO. However, the

effect of these spurious echoes cannot be neglected since, in some instances, spurious echoes generated at a discontinuity may even be stronger than the real echoes generated by the

following discontinuities.

More importantly, an accurate analytical model of spurious echoes may be used to generate or synthesize spurious echoes and subtract such generated spurious echoes from the

observation data thereby resolving the ambiguities known in the prior art. The generation

and subtraction of the spurious echoes from the observation data could be performed step-

by-step and in parallel with our identification method described below.

In the following subsections we will analyze the main spurious echoes generated on a loop by a gauge change and a bridged tap. The spurious echoes are modeled as real echoes.

For this purpose, we will define their echo path and their reflection coefficient. The

transmission coefficient can then be expressed as a function of the reflection coefficient since the following relationship holds: τ(f) = 1 + p(f) . The determination of the echo path is

important because it allows us to build the insertion loss transfer function Hnif) in eq. (2);

the reflection coefficient will be constituted of several terms, each of which will account for

the consecutive bouncing.

2.1.1 Spurious Echoes: Gauge Changes Turning now to FIG. 3, there is depicted an exemplary subscriber loop wherein two

consecutive gauge changes (labeled A and B) are present in the loop. When a signal

transmitted from CO 13 arrives at A, an echo is generated and goes back to the CO 13; this

is a real echo and pertains to the discontinuity in A. After encountering A, the refracted part

of the signal travels on and arrives at B, where another echo is generated in accordance with

the reflection coefficient P_ι(f). When this second echo arrives back at A, part of it will be

refracted and will constitute the real echo pertaining to the discontinuity B and part will be reflected back towards B in accordance with the reflection coefficient Pι(f). Theoretically,

the part of signal going back towards B can bounce a infinite number of times between A

and B, and, at each bouncing, a refracted and a reflected wave will be generated again. For

this reason, an infinite number of spurious echoes will be received in the CO.

Following the notation of the model in eqs. (1) and(2), it is possible to prove that the

echo path and the reflection coefficient pertaining to the z^'-th spurious echo generated by two

consecutive gauge changes are given by the following (i > 0):

Path = Path from CO to A + 2(ι + l) + Path from A to CO (3.a)

/>(/) = [A (/)] /)]'⁺¹ (3-b)

Based on eq. 3, we have run simulations and found that spurious echoes due to gauge changes are much more attenuated than real echoes. For example, FIG. 4B shows the real and spurious echoes produced by the discontinuity present in the loop shown in FIG. 4A. In

the calculation of the spurious echoes we have considered the first five spurious echoes, i.e. i = 1, 2, ..., 5 as given by eqs. (3. a) - (3.b). As such, ignoring the spurious echoes produced by gauge changes will not have a major effect on the accuracy of our loop make-up method.

2.1.2 Spurious Echoes: Bridged Taps Turning now to FIG. 5, there is depicted an exemplary subscriber loop having a

bridged tap (labeled A) and a gauge change (labeled C) present in the loop. Actually, the last

discontinuity C can be either a gauge change or another bridged tap, since its behavior is

described by the reflection coefficient ?_?(/). When the signal arrives at A, an echo is

generated and goes back to the CO 13; this is a real echo and pertains to the discontinuity in A. After encountering A, part of the signal travels on and arrives at B and part travels on

arriving at C. In this case, there are three kinds of spurious echoes of importance:

1) the spurious echoes that bounce between A and B and go back to the CO;

2) the spurious echoes that bounce between A and B, travel on towards C and go back to the CO

3) the spurious echoes that bounce between A and C and go back to the CO.

Following the notation of the model in eqs. (1) and (2), the echo paths and the reflection coefficients pertaining to the above mentioned types of echoes are given by the following:

Case l) (i > 0)

Path = Path from CO to A + 2(ι + l)L_BT + Path from A to CO (4.a)

P(f) = [1 + A, ( )] & + A ( )] (A (/)] ' (4.b)

Case 2) (i , i > 0)

Path = Path from CO to A + 2 i L_BT + 2 j L + Path from A to CO (5. a)

/>(/) = [1 + A (/)][! + A (/)]& + (/)][A ⁽/⁾]'^"' Wf) ^X IPΛf))¹ (5.b)

Case 3) (i > 0)

Path = Path from CO to A + 2(i + \)L + Path from A to CO (6.a)

(/) = [l + A (/)][l + A(/)] (/)] ' ( )] '⁺¹ (6-b)

In the case where the consecutive discontinuities are constituted by the same kind of

gauge, the reflection coefficients in (4.b), (5.b) and (6.b) boil down to the following

expressions:

Case 2): p(/) = — 8

Case 3): >(/) = [ ₃(/)]'⁺¹ (7.c)

The spurious echoes produced in correspondence to a bridged tap are more harmful than those produced by a gauge change. FIG. 6B shows the real echoes and the first 35 spurious echoes for the loop configuration in FIG. 6A. The 35 spurious echoes we have

considered are: 5 echoes of case 1 (i = 1, 2, ..., 5 in eqs. (5.a) - (5.b)), 25 echoes of case 2 (i,

j = 1, 2, ..., 5 in eqs. (6.a) - (6.b)) and 5 echoes of type 3 (i = 1, 2, ..., 5 in eqs.(7.a) - (7.b)). The spurious echoes are still smaller than the real ones but much stronger than in the gauge change case and can change the peak sequence, causing the appearance of false peaks. If the bridged tap is longer, the spurious echoes are weaker but so are the real echoes and the peak sequence will experience a change.

3. Determining the Make-Up of a Loop

We now turn to our method for unambiguously determining (or identifying) the

make-up of a subscriber loop including identifying the number and location of load coils, bridged taps, and gauge changes.

In the course of our work we have found that if some simple assumptions are

satisfied, loop make-up identification may be possible without ambiguity. These

assumptions are here summarized: 1. The loop is well behaved, i.e., (a) the loop is constituted of cables deployed

following the regular gauging design rule and (b) only simple bridged taps may

be present and the length of each bridged tap is smaller that the length of the

following loop-section;

2. The loop is terminated on an on-hook telephone and the impedance of an on- hook telephone can be approximated by an open circuit; and

3. The loop does not have several discontinuities in close proximity concentrated at

the end of the loop.

If these assumptions are satisfied, it is possible to identify the loop make-up by just looking

at the position and sign of the peaks of the echoes. The rationale or physical principle underlying the above assumption is the realization that the spatial relationship of the

discontinuities on the loop manifest themselves in a temporal manner via our measurement

or any TDR measurement. Note however, that even if the above assumptions are satisfied,

echoes may overlap and it may be possible that some peaks are hidden so that some discontinuities may not be detected. Our method will be able to perform loop make-up

identification without any ambiguity if the loop under investigation satisfies the above

mentioned four assumptions. For those loops that do not satisfy these assumptions, it will be

necessary, as described below, to perform additional steps to resolve the ambiguities, i.e., it

may be necessary to resort to sophisticated signal processing techniques.

Our identification process is performed in several steps. At each step some partial

information on the loop make-up is calculated and exploited in a subsequent step. At a high

level, and as indicated in FIG. 7, the identification process can be divided into four main

phases or steps: 1. Phase 1 (Step 710, FIG. 7 A): Acquire Data;

2. Phase 2 (Step 720, FIG. 7A): Check Consistency of the Loop Record;

3. Phase 3 (Step 730, FIG. 7A): Perform Active Loop Make-up

Identification;

4. Phase 4 (Step 740, FIG. 7A): Resolve Ambiguities.

Once the loop make-up is identified, the loop may then be qualified for DSL service.

However, where the loop make-up cannot determined without ambiguity, it is still possible to exploit the information given via our method to determine the quality of DSL services

achievable by the loop. In fact, in the case of ambiguous identification, the possible

configurations that can be attributed to the loop under test are limited and can be used for

DSL qualification purposes. In this scenario, a worst-case quality of service can be

determined.

In the following sub-sections, we will describe the functions or steps performed in

each phase.

3.1 Phase 1 (Step 710): Acquire Data

As FIG. 7A shows at the same time data is being acquired, step 710, it is also

possible, but not required in accordance with our invention, to detect the presence of load

coils and estimate the noise on the loop under tests, step 712. Step 712 is optional.

Nonetheless, given that the commercial driver behind unambiguously determining the make-

up of a loop is deployment of DSL services step 712 will be required in most cases and by

almost all users of the embodiments depicted in FIG. 2. However, in implementing our inventive concept load coil detection and noise estimation are not to be considered necessary

steps. Turning to FIG. 7B, there is illustrated the main sub— steps performed in this phase,

including the optional steps of load coil detection and noise estimation. Specifically, FIG.

7B illustrates the substeps of detecting of load coils, step or block 7101, estimation of the

noise present on the loop, step 7105, and repetitive transmission of suitably designed

probing signals into the loop and sampling of the received echoes, step 7109. In addition,

and in accordance with our method, the echoes received at step 7109, may be stored in

memory, step 7119. In the embodiment depicted in FIG. 2A, the received echoes may be

transmitted back to the service bureau and stored (step 7119) for later processing. Or in the

embodiment of FIG. 2B, the received echoes may be stored (step 7119) in local memory 2111 on the broadband test head 210.

In order to detect loading coils along the line, we measure the input impedance of the

loop from a few hertz to a few kilohertz; measuring the impedance of a loop is well known

in the art and any prior art method may be used to perform this step. It is known that the

presence of loading coils is detected if the behavior of the input impedance versus frequency

exhibits peaks. If the input impedance of the loop is a decreasing monotonic function of

frequency, the loop is unloaded. If load coils are detected, the loop cannot support DSL

services (as is known in the art).

The prior art contains methods for estimating noise on a loop and any of those

methods may be used here. Noise estimation is important because the knowledge of the

noise level will determine the number of independent snapshots required via repetitive

probing, step 7109, in order to achieve the desired Signal-to-Noise Ratio (SNR). In fact,

given that safety, service, and customer satisfaction requirements place an upper limit on the

energy that can be transmitted on a loop, repetitive probing allows us to maintain energy levels below safety requirements and, at the same time, allows to achieve desired Signal-to-

Noise Ratio. The process of noise estimation can be limited to a second order description of the noise process which may be useful when processing the acquired data for the resolution

of the echoes. A second order description is sufficient because of the Gaussian nature of the

noise and may be necessary since crosstalk may introduce colored components in the noise

spectrum; crosstalk may be present because DSL services might be running on pairs adjacent to the pair under test.

As far as the choice of the probing signal is concerned, a compromise on the width of

the probing signal will need to be made: short pulses give rise to narrow echoes, whereas

long pulses are able to reach longer distances.

Those of ordinary skill in the art will also note that metallic access to the cable pair

or subscriber loop is needed only in this step, step 710. Therefore, the loop is physically

disconnected from the customer only during step 710 and then typically only for a number of

seconds. Once the data is acquired, the line can be given back to the customer since all the

further processing can be made in a batch mode off line.

3.2 Phase 2 (Optional Step 720): Compare Acquired Data to Loop Record Step 720 is an optional step. In step 720 we compare available loop records to the

previously acquired data. This proves to be a useful way of verifying the reliability of a pre¬

existing loop record for the loop under test.

In particular, at step 720 the correctness of the loop record is verified by comparing

the loop record with acquired data. Accordingly, we process the data and use accurate time

windowing to detect if a certain discontinuity is located where the records indicate. The

verification process will analyze all the discontinuities indicated in the records starting with the ones nearer to the CO. This consistency check will provide the following phase with

some partial information on the loop make-up. In fact, even though most of the records are

not updated and, therefore, unreliable, it is reasonable to assume that at least some of the

information in the database is correct.

3.3 Phase 3 (Step 730):Perform Active Loop Make-up Identification

During active loop make-up identification, step 730, the data gathered during step

710 is analyzed and a first attempt to identify the loop make-up is made. If the consistency

check of the records was able to provide some a priori knowledge on the loop, this information is used during the identification process.

The basic parameters to be identified in this phase are the estimation of the time of

arrival of the echo and its amplitude. The information on the time of arrival is necessary for

the determination of the location of the discontinuity. Electric signals propagate in a loop at

a speed of approximately 1.5 μs per kilofeet, so the time of arrival of an echo allows us to

locate the distance of the discontinuity from the CO (or broadband test head). The

knowledge of the amplitude of the echo is necessary to identify the type of discontinuity that

caused the echo. Specifically, the sign of an echo allows us to determine whether the

probing signal has passed from a lower to a higher characteristic impedance, or vice versa.

In addition, the absolute value of the peak of the echo is useful in determining the kind of

gauges present in the loop. In fact, we can state the following rules that provide a signature

of the discontinuity:

• when a signal passes from a loop section with a higher (lower) characteristic impedance to a loop section with a lower (higher) one, the echo generated at the discontinuity is always negative (positive);

• a bridged tap always produces a negative echo followed by a positive echo; • in the case of bridged tap, the absolute value of the negative peak is always higher than the absolute value of the positive peak;

• the bigger the difference between the characteristic impedance of two loop sections, the higher the absolute value of the peak generated at the discontinuity.

Basically, our identification method attempts to detect the discontinuities nearer to

the CO and, then, moving forward out into the loop detecting, in turn, each discontinuity.

The determination of the first echo(es) does not normally present a problem since it (they)

will arrive in the CO without being overlapped by previous echoes and because the first

spurious echoes will arrive later. However, as the identification process moves forward,

detection of the echoes is more difficult. In fact, the echoes will be increasingly weaker,

broader and overlapping with each other and with spurious echoes. The main problem of

detecting far discontinuities is due to the broadening of the echo caused by the dispersive

nature of the medium. In fact, a broad echo could hide a subsequent and weaker echo,

making the detection of discontinuities occurring after other discontinuities extremely difficult. A possible way to limit widening effect of the medium is to process the received

signal or echoes in the order received to compensate for the dispersive behavior of the

medium. In the case of digital communications, the effect of a dispersive channel is to

broaden the transmitted pulse thus leading to inter-symbol interference. In digital

communications, the solution to the problem of inter-symbol interference is an equalizer, i.e.

a device that compensates for the dispersive behavior of the channel. In our case, we have to

process a signal that is not digital in nature and this limits our freedom in the choice of the

equalizer. In fact, since we cannot act on the digital structure of the signal, the only thing we

can do is to act on the channel characteristics. There is only one equalizer that does not take into account the digital structure of a received signal: the zero-forcing equalizer, that in our

case turns out to be a filter with a transfer function equal to the inverse of the transfer

function of the channel.

On the basis of the above considerations, the identification method should proceed as

indicated below and by FIG. 7C:

1. identify the location and type of the i^Λ discontinuity on the loop by processing the received echoes in pairs, step 7301 (processing includes using the echo time of arrival and amplitude and signature rules to determine, respectively, the location and type of discontinuity, step 7302) ; 2. compute the transfer function of the first through i^th loop sections identified up to now, step 7305 (for i > 1, the i^th loop section is the loop section between the (i - 1)* and i^th discontinuities and for i = 1, the i^th loop section is the loop section between the CO and first discontinuity);

3. synthesize an inverse filter for the i^th loop section based on the computed transfer function, step 7309; and

4. operate the convolution between the previously synthesized inverse filter and the observation data, step 7313 (this operation would be best performed in the frequency domain by means of FFT/TFFT techniques);

The above steps are then repeated until the last discontinuity in the loop is found; in

other words, if there are N echoes i is incremented from 1 to N. In principle, this procedure is equivalent to "moving" the CO along the line and towards the subscriber's location, thus

reducing the distance between the CO and the discontinuity to be identified. It also should

be noted that at step 2, bridged taps should not included in the calculation of the transfer

function. This is the case because the bridged tap distorts only signals travelling on it and

the signals do not travel on the bridged tap. In other words, the cumulative transfer function is meant to take into account the effect each loop section has on signals traversing and

signals reflected at discontinuities after the bridged tap do not travel on the bridged tap.

The main drawback of this procedure is the noise enhancement. Due to the

characteristic of the transfer function of the twisted-pair channel, the inverse filter will enhance the high frequency components of the noise. This effect should be carefully taken

into account when fixing the minimum SNR required. On the other hand, since the

correlation introduced in the noise samples is known, we can resort to standard noise

reduction techniques.

An example of the results achievable with this technique is shown in FIGS. 8 and 9.

Let us consider the well behaved loop shown in FIG. 8A; the real and spurious echoes are shown in FIG. 8B (separately) and in FIG. 8C (the superposition of real and spurious

echoes). The first three echoes pertain to the gauge change 805 and the bridged tap 807.

The first two echoes are negative, so that we can affirm that the first discontinuity is gauge change 805. The second and third echo show a negative/positive sequence, so that we can

say that after the gauge change there is bridged tap 807. However, the last small positive

peak is due to spurious echoes and not to the echo generated at the end of the loop (see

FIGS. 8B and 8C), because the echo generated at the end of the loop is hidden by the

positive echo of the bridged tap. In fact, the echo paths of these last two echoes differ by

1000 feet only and this corresponds to a time delay of 1.5 jus, a time delay that is too small

with respect to the time duration of the positive echo pertaining to the bridged tap. From the

previous considerations, we can easily detect the first two discontinuities, so that we can

build the inverse transfer function pertaining to the first two loop sections. FIGS 9A and 9B

show the effect of inverse filtering on the identification process. The normalized observed realization filtered with the inverse transfer function of the first loop section, i.e. the loop

section from the CO up to the gauge change, is shown in FIG. 9A. The first two negative

echoes are more separated, but we still cannot detect the echo pertaining to the end of the

loop. In fact, we "moved" our reference point only 1000 feet forward. However, if we filter

the observation data with the inverse transfer function of the first two loop section, i.e. the

loop section from the CO up to the junction of the bridged tap, we are finally able to detect

the last echo due to the end of the loop. In fact, as FIG. 9B shows, we have two peaks

approximately 1.5 μs apart, and this time difference corresponds to the 1000 feet difference

between the echo path of the positive echo of the bridged tap and of the end of loop. Around

11.5 μs, we can also appreciate the spurious echoes.

FIGS. 8B, 8C, 9A, and 9B demonstrate the feasibility of using our method to

determine loop make-up. Though these figures are simulations, we have verified, within the

limits of commercially available equipment, our method via actual measurement, as

illustrated in FIGS. 10A, 10B, 10C, and 10D. Our measurement set up is illustrated in FIG.

10A and consists of a pulse generator 1001 that is coupled through a balun 1005 to a loop under test 1020. The output of the pulse generator was a square wave of 5 Volts (on 50

ohms) with a width of 200 ns and a 5 ns rise and fall time. The balun 1005 provides a 50

ohms unbalanced to a 100 ohms balanced conversion. An oscilloscope 1025 is used to

receive echoes generated on the loop under test 1020. In FIG. 10B, the loop under tested is

illustrated. It is understood that the equipment, namely generator 1001, balun 1005, and

scope 1025, illustrated in FIG. 10A is located in CO 13. Turning to FIG. 10C, there is

depicted an oscilloscope traces of the echoes generated by the loop of FIG. 10B. In FIG.

10D the loop is simulated in accordance with our method. As FIG. 10D shows the simulation and trace match up almost perfectly. The region 1037 of FIG. 10D is generated

by the balun 1005.

We have done other measurements and simulations to verify our measurement

method and have concluded that our method can be used to determine the make-up of a

loop. Our measurements, however, also demonstrated that under certain conditions gauge

changes cannot be detected using the simple set up of FIG. 10A. For distances on the order

of 5,000 ft (5 kft) the amplitude of the reflected echo is on the order of micro- volts. Assuming that the white noise has power density spectrum of -120 dBw Hz over 100 ohms,

for a bandwidth of 5 MHz the noise would be in the mircovolt range as well. Nonetheless, these shortcomings may be overcome by using high density pulses and repetitive probing

followed by noise averaging.

It is worth pointing out that the use of our method allows us to simplify the model of the echoes given in eqs. (1) and (2). In fact, since our method compensates for the distortion of the medium, the arriving echoes will be much more "similar" to the probing signal

transmitted in the loop, as if we were transmitting on a non-dispersive channel. Using the

inverse of the transfer function of the first n* loop sections to detect the echo generated in

the (n + 1)* loop section does not eliminate all the distortion; however, we may model the

residual distortion as a simple attenuation. This allows us the following simplification of the

model (see eq.(2)):

e (t) = s(t) *h_e ⁽ _p ^,) (t) ≡ s(t)* (a_lδ(t)) = a,s(t) (8)

On the basis of (8), the model given by eq. (1) can be rewritten as:

r( = ∑α,s(t- ) (9) This simplification allows us to state the problem of time of arrival and amplitude estimation in a simple way.

Another improvement that may be useful in combating the overlapping of

consecutive echoes would be to subtract a "synthesized" echo from the observation data as

indicated by block 735 of FIG. 7A. Specifically, and with reference to FIG. 7A, once a discontinuity has been identified (step 730), the echo caused by that discontinuity could be

generated via software and subtracted during processing, step 735. This "onion peeling"

technique will ensure that the later arriving echoes will not be hidden by previously arriving

echoes. In addition our "onion peeling" approach may also be used to remove the effect of

spurious echoes. It should be noted that step 735 would be done in parallel with identifying

the loop makeup, step 730. That is, as the method moves out onto the loop identifying each discontinuity via the observation data, each discontinuity and spurious echo will be removed

from the observation in data before identifying the next discontinuity. It should be noted

that "onion peeling" is optional and our underlying method would work well for a large

majority of loops currently deployed in the Public Switched Telephone Network.

On the basis of the above considerations, we can now summarize the main steps of

the identification algorithm.

1. Compute the arrival times of the i^th and (i + 1)^Λ echoes and determine the correspondent distances from the CO (this step is representing by block 7302 of FIG. 7C);

2. Determine the sign sequence of the i^th and (i + l)^th echoes according to the following (block 7302 of FIG. 7C):

• Negative-Negative: the i^Λ discontinuity is a gauge change, from a thinner to a thicker cable; • Negative-Positive: the i^ώ discontinuity is a bridged tap or the i^th and (i + l)^th discontinuities are two consecutive gauge changes from thinner to thicker to thinner cable;

• Positive-Positive: the i^ώ discontinuity is a gauge change from thicker to thinner cables; or

• Positive-Negative: the i^Λ discontinuity is a gauge change from a thicker to a thinner cable.

Note that if the loop is well behaved then the only possible transitions are:

• Negative-Negative: i^th discontinuity is a gauge change from thinner to thicker; or

• Negative-Positive: the i^th discontinuity is a bridged tap. 3. Compute the absolute value of the peak of the i^th echo and compare it to simulation results in order to determine the kind of gauge constituting the i^th loop section, step 7303 (if a bridged tap was detected, proceed in the same way to identify the (i + i ¹ loop section also);

4. Compute the transfer function of all the loop sections up to the i^th discontinuity (step 7305) (if a bridged tap was detected, do not include the bridged tap in the transfer function because the echo generated by the discontinuity following the bridged tap does not "travel" on it and, thus, is not distorted by it);

5. Synthesize the inverse filter (step 7309); 6. Operate the convolution between the inverse filter and the observation data

(step 7313);

7. Compute the next echo arrival time (i.e., compute the (i + 2)^th echo arrival time), however, if a bridged tap was previously detected, compute the next two echoes arrival time (i.e., compute the (i + 2)^ώ and (i + 3)^th echo arrival times); and

8. Set i = i +1 if a bridged tap was not detected or set i = i + 2 if a bridged tap was detected and repeat items 1 through 7 from the list above until the last discontinuity is determined.

Note from the list above and FIG. 7C discontinuities are determined by processing

the echoes in pairs. Note also that bridged taps are afforded special attention because the echo generated by a bridged tap does not travel on the loop section comprising the bridged

tap. The above seven steps illustrate the main steps followed during Phase 3 of our method.

However, these steps may not be sufficient to achieve unambiguous loop make-up

identification if the four assumptions pointed out earlier are not satisfied. Below we describe an approach that can be used to resolve ambiguities.

3.4 Phase 4 (Step 740): Resolution Of Ambiguities This method step is the most sophisticated and delicate part of the identification

process. The problem of resolving an unknown number of closely spaced, overlapping and noisy echoes of a signal with a priori known shape is a problem that arises in many

applications such as radar and sonar processing, geological sounding, etc. In principle, this is

a combined detection-estimation problem since we have to determine first the number of returning echoes and, then, apply an estimation procedure to determine their location in time.

The usual approach is to assume the availability of an array of M sensors located in the far field of the sources, so that the waves generated by each of the D radiating sources behave like plane waves.

The signal received by the M sensors is usually modeled as follows:

r t)

: [Λ(ι?, ) ^{• • •} A(ϋ_D)] (10) r_M (t)

or, more compactly, as: r = As + n (11)

where η(t) is the signal received at the i-t sensor ( = 1, ..., M), S_j(t) is the signal generated

by the j^'-th sensor (j = 1, ..., D), A(ϋ) is the "signature" of a source in the direction and

«,(/) is an independent noise affecting the i-th sensor. The set of signature vectors A(ϋ is also called the array manifold, since it characterizes the directional properties of the sensor

array. The array manifold may be obtainable in a closed form for simple spatial geometries (e.g., linear, circular), or can be measured through field calibration procedures. By carefully choosing the array geometry, it is possible to ensure the following property: for any set of

parameters ϋ with D<M elements, the array manifold vectors are linearly independent.

There have been several approaches to such problems. First of all we can recall

Capon's Maximum Likelihood method and Burg's Maximum Entropy method. These

methods have been widely used in the past but their main limitation is due to the fact that

they use an incorrect model of the measurements: an autoregressive model instead of an

autoregressive-moving-average model. The first one to exploit the structure of the data model using a covariance approach was Pisarenko, even though he considered a very particular case: parameter estimation of cissoids in additive noise. The covariance approach to the case of sensor arrays of arbitrary form was first developed by Schmidt and Bienvenu

and Kopp. Their algorithm, widely known as Multiple Signal Classification (MUSIC)

algorithm, was the first algorithm to exploit the eigenstructure of the covariance matrix of

the observed data. The basic principle of this sub-space approach lies in the fact that, when

there are more measurements than signals (D<M), the signal component of the received data (As) is confined to at most a D-dimensional subspace (the signal subspace) of the M-

dimendional space of the observations. Since the noise is typically assumed to possess energy in all the dimensions of the observation space, the problem is to extract a low-rank

signal observed in a full-rank noise. There are two other more recent subspace methods that

are worth mentioning because they are able to provide performances superior to MUSIC: the Estimation of Signal Parameters via Rotational Invariance (ESPRIT) algorithm and the Weighted Subspace Fitting (WSF) algorithm.

It would certainly be useful to exploit this vast literature for the resolution of the

overlapping echoes in the problem of loop make-up identification via single ended

measurements. The main difference between our case and the model in eqs. (10) - (11) is

that we do not have the availability of a sensor array. However, it is possible to reformulate our observation model in order to provide it with a structure identical to the model in eqs.

(10) - (11). Our observation model is:

r(t) = a,s(t-ξ,) + n(t) (12)

Assuming that there are D main echoes and sampling the received waveform r(i) at

M instants, we can write:

r(t,) = T «,*(?, - ) + n(t,)

(=1 D r(t₂) = a,s(t₂ -ξ,) + n(t₂)

(13)

t_M) = _ls(t_M -ξ_l) + n(t_M)

and, using a matrix notation, we obtain:

^' s(t₎ - ,) s(t_x-ξ₂) ^■■■ s(t,-ξ_D) r(t₂) s(t₂-ξ,) s(t₂-ξ₂) ^■■■ s(t,-ξ_D)

(14) ri_u) s(t_M-ξ,) s(t_M-ξ₂) s(t_λ -ξo)

or, equivalently,

r = Sa + n (15) The model in eq. (15) has the same structure as the model in eq. (11), with the only difference that now the array manifold for the signal resolution problem is obtained from the

signal shape. Furthermore, the desired array manifold (linear independence of the rows of S

for any set of D<M delays) is satisfied provided that we work with finite-span pulse type

waveforms.

On the basis of the new model structure in eq. (15), we are now able to exploit the MUSIC, the ESPRIT and the WSF algorithms previously mentioned to detect the number of

echoes and estimate their arrival time.

The array manifold S in eq. (15) requires the knowledge of the shape of the echo and

that all the echoes have the same shape. This is impossible to obtain in our case but it is possible to define a "reasonable" shape of the echo. In fact, as previously pointed out above,

the shape of all the echoes is similar and they differ only by their width which is a function

of their distance from the CO. Moreover, we can make the CO "nearer" to far discontinuities by filtering the data with an appropriate filter. This would ensure that the use of an array

manifold containing echoes of the same shape is a less problematic approximation. A possible choice for a "reasonable" shape of the echo could be:

e(t) = F-^l{cx_V(- f )} (16)

where is a parameter that has to be optimized. The choice of an exponential function is

due to the fact that it is a good approximation for the transfer function of the twisted pair

channel. The choice of a signal of the kind of (16) has another advantage. It preserves the

linear independence of the array manifold, whereas the piecewise probing signal would not. When defining a value for the number of samples M, we have to make sure that we

have D<M. Fortunately, the real echoes generated by a loop are limited to a maximum of ten

to fifteen, but the spurious echoes are virtually infinite. In this sense, the value for M should

be chosen very high and this could compromise the computational efficiency of the algorithm.

With respect to the arrival estimation problem, the use of a sensor array and the

linear independence of the array ensure that the subspace algorithms can detect the emitting sources and separate them from spurious reflections due, for example, to multipath

phenomena. In fact, a linear combination of two or more direction signatures will never provide the "phantom" signature of some different direction. However, this property is not

maintained in the single-sensor case. In fact, the spurious echoes generated in a loop will be

considered as real echoes and the algorithm will not be able to isolate them.

The above description has been presented only to illustrate and describe the

invention. It is not intended to be exhaustive or to limit the invention to any precise form disclosed. Many modifications and variations are possible in light of the above teaching.

The applications described were chosen and described in order to best explain the principles

of the invention and its practical application to enable others skilled in the art to best utilize the invention on various applications and with various modifications as are suited to the

particular use contemplated.

Claims

CLAIMSWe claim:

1. A method for determining the make-up of a subscriber loop, said method comprising the steps of:

transmitting at least one pulse on the loop;

receiving echoes based on said transmitted pulse, said received echoes being caused

by discontinuities on the loops; and determining the make-up of the loop based on said received echoes.

2. The method in accordance with claim 1 wherein said step of determining

comprises the sub-steps of: identifying the location and type of the i^l discontinuity on the loop based on the

arrival times and amplitudes of the received echoes; computing a transfer function of all loop sections that precede the i^th discontinuity

based on said identification of the i^th discontinuity to produce the i^th loop section transfer

function;

synthesizing an inverse filter for the i^th loop section based on said i^th loop section

transfer function; convolving said synthesized filter and said received echoes for all loop sections that precede the i' discontinuity; and

determining the loop make-up based on said convolution wherein each echo pulse represents a discontinuity.

3. The method in accordance with claim 2 wherein said step of identifying

comprises the substeps of: WO 01/01158 PCTYUSOO/16865

estimating the arrival times of the i' and (i+1)¹ echoes; determining the distance of the first discontinuity based on said estimation; and detecting the sign sequence of the i^lh and (i+l)^th and if said sign sequence is -

negative - negative, then record the i' discontinuity as a gauge change from a thinner to a thicker cable, or

negative - positive, then record the i^th discontinuity as a bridged tap or record

the i^th and (i + l)^th discontinuities as two consecutive gauge changes from thinner to thicker to thinner cable, or

positive - positive, then record the i^th discontinuities as a gauge change from thicker to thinner cables, or positive - negative, then record the i⁽ discontinuity as a gauge change from a

thicker to a thinner cable.

4. The method in accordance with claim 3 wherein said step of computing a transfer

function further comprises the sub— steps of: estimating the absolute amplitude value each echo pulse;

simulating said echo pulse; and

comparing said estimate to said simulation to determine the gauge of the i^th loop section.

5. The method in accordance with claim 4 further comprising generating the echo of

the i^th discontinuity and subtracting said generated echo from said received echoes.

6. The method in accordance with claim 5 further comprising generating a spurious

echo and subtracting said generated spurious echo from said received echoes.

7. The method in accordance with claim 6 further comprising, prior to said step of transmitting a pulse, the step of detecting load coils present on the loop.

8. The method in accordance with claim 7 further comprising updating the loop record corresponding to the loop make-up based on said determined loop make-up.

9. The method in accordance with claim 8 further comprising the step of qualifying the loop for digital subscriber line service.

10. A system for determining the make up of a subscriber loop, said system comprising: a broadband test head device; access circuitry connecting said broadband test head device to the subscriber loop;

a switch for controlling the access circuitry; and a communications module for connecting said broadband test head device to a

micro-processor and a memory, and wherein said micro-processor performs the method steps included on said memory

of, estimating the arrival time of two successive echoes from the subscriber loop when a pulse is applied to said loop through said access circuitry,

determining the sign sequence of the two echoes as negative - negative,

negative - positive, positive - positive, or positive - negative, and

estimating the absolute value of the peak of an echo.

11. The system in accordance with claim 10 wherein said micro-processor also performs the functions of:

computing the transfer function of a first section of said subscriber loop; synthesizing an inverse filter; and operating the convolution between said inverse filter and a received echo.

12. The system in accordance with claim 11 wherein said micro-processor also performs the function of estimating the arrival time of a subsequent echo.

13. The system in accordance with claim 12 wherein said broadband test head device

comprises a pulse generator and an echo receiver.

14. A system for determining the make up of a subscriber loop, said system

comprising: a transmitter for sending pulses onto the loop;

a receiver for receiving echo signals generating by the loop in response to said

transmitted pulses; a micro-processor and a memory, said microprocessor coupling and controlling said

transmitter, receiver, and memory, and wherein said micro-processor performs the method steps included on said memory

of, estimating the arrival time of two successive echoes from the subscriber loop

when a pulse is applied to said loop through said access circuitry,

determining the sign sequence of the two echoes as negative - negative,

negative - positive, positive - positive, or positive - negative, and estimating the absolute value of the peak of an echo.

15. The system in accordance with claim 14 wherein said micro-processor also

performs the functions of: computing the transfer function of a first section of said subscriber loop;

synthesizing an inverse filter; and operating the convolution between said inverse filter and a received echo.

16. The system in accordance with claim 15 wherein said micro-processor also

performs the function of estimating the arrival time of a subsequent echo.