GB2269073A

GB2269073A - Remote telephone line tester

Info

Publication number: GB2269073A
Application number: GB9215657A
Authority: GB
Inventors: Christopher William Lewi Hobbs; John Hayward
Original assignee: Northern Telecom Ltd
Current assignee: Nortel Networks Ltd
Priority date: 1992-07-23
Filing date: 1992-07-23
Publication date: 1994-01-26
Anticipated expiration: 2012-07-23
Also published as: FR2694151B1; FR2694151A1; GB2269073B; DE4323781A1; GB9215657D0

Abstract

A telephone subscriber loop is tested by applying an electrical impulse across the legs of the loop and determining the nature of the resulting transient voltage due to current in the loop. The impulse may be provided by reversing the polarity of a power supply to the loop. The transient can be measured by a differential amplifier AMP coupled across a resistor R41 in series with the loop. Typically the maximum voltage, the time taken for the voltage to decay to a particular fraction of the maximum and the long term residual voltage are measured. These measurements are returned to the exchange to identify fault conditions in the loop. The arrangement can be used in systems in which part of the communications path between the subscriber and the exchange comprises a fibre optic waveguide. Apparatus can be installed in a subscriber line card at a street distribution unit. <IMAGE>

Description

2269073 REMOTE LINE TESTER This invention relates to equipment for testing

a telephone subscriber loop and to a method of testing the loop.

In a conventional telephone system a large number, usually several thousand, of subscribers are connected to a central office or exchange each by a pair of electrical conductors commonly referred to as a subscriber loop. In order to ensure maintenance of the quality of service it is necessary to provide some means of testing each subscriber loop in order to detect and identify line faults. For this purpose the exchange is provided with one or more test heads which are adapted to measure the electrical characteristics of subscriber loops whereby to diagnose faults. These test heads are relatively expensive pieces of equipment, but as each test head can service a large number of subscribers the cost per line is low.

Systems are now being introduced in which a small number, e.g. one hundred, of subscribers are coupled by relatively short subscriber loops to a local multiplexer/demultiplexer street unit and thence via a fibre optic path or highway to a remote switching centre. In such a system there is no direct electrical connection between the exchange and the subscriber, thus precluding the use of an exchange located test head. System installers have understandably been reluctant to incur the expense of providing a test head in every street unit. In addition, installation -2of a test head in a street unit requires the reservation of significant channel capacity on the fibre optic path to the exchange thus limiting the bandwidth available for subscriber traffic. The use of a test head also requires the installation of complex switches known as test access cards which allow the test head to be switched on to a particular channel.

Customer reported line faults thus require the attendance of an engineer equipped with portable test gear. This procedure is rather less than ideal. If the fault in fact lies in the subscriber's terminal equipment, that may not be the responsibility of the network provider and the engineer's journey thus represents wasted time and expense. Further,, the nature of the fault may be such that it is beyond the remit of the test engineer and requires the attention of a service engineer. Again this represents unnecessary expense and delay in the fault repair process.

The object of the invention is to minimise or to overcome this disadvantage.

According to one aspect of the invention there is provided apparatus for testing a telephone subscriber loop, including means for providing an electrical impulse across the legs of the loop, and means for sensing the pattern of a transient voltage signal responsive to said impulse whereby to determine the electrical condition of said loop.

According to another aspect of the invention there is provided a method of testing a telephone subscriber loop, the method including supplying an electrical impulse across the legs of the loop, the impulse causing a transient voltage signal to appear across the legs of the loop, and determining from the nature of the transient voltage signal the electrical condition of the loop.

In a preferred embodiment the electrical impulse is applied to the loop by reversing the polarity of the power supply thereto. The nature of the transient voltage signal is determined by a small number of discrete voltage measurements. These measurements may include the maximum voltage, the voltage at a fixed time following the impulse, e.g. the voltage after 1.2 m see, the time taken for the voltage to drop to a predetermined value, and the infinity' voltage condition which is the long term voltage decay limit. Other voltage/current measurements may also be made to characterise the transient signal.

Loop testing may be performed either as a routine procedure or in response to a fault report from a subscriber.

An embodiment of the invention will now be described with reference to the accompanying drawings in which:- Fig. 1 shows a telecommunications system in which a central switch is coupled to subscribers via a fibre optic path and a local street unit; Fig. 2 shows in schematic form a loop test apparatus coupled to a subscriber loop of the system of Fig. 1; and Fig. 3 illustrates a typical transient voltage signal obtained from a subscriber loop.

Referring to Fig. 1, the telecommunications system includes a switching centre or exchange 11 coupled via electrical conductor pairs to an interface unit 12. The interface unit provides multiplexing/demultiplexing facilities and also provides an interface between the switching centre 11 and a number of optical fibre paths or highways 13. Each fibre 13 services a number of subscribers and is terminated in a local street unit 14 comprising a local multiplexer/de multiplexer. Connection between the street unit and each subscriber terminal 15 is effected via a conductor pair or subscriber loop 16. Each subscriber loop being coupled to a -4corresponding line card 17 in the street unit 14. In addition to carrying telephone traffic the system may also carry e.g. cable television signals.

It will be appreciated that in the system of Fig. 1 there is no direct electrical connection between the switching centre 11 and the subscriber loops 16.

Referring now to Fig. 2, the subscriber loop is shown schematically as resistors Rll, R12 and R13, capacitors C11 and C12 and inductors L11 and L12. At the subscriber end, the legs of the loop are coupled across the subscriber's wall box represented by capacitor C21 and resistor R21. The subscriber's terminal equipment represented as resistor R31 and inductor L31 is coupled across resistor R21 of the wall box.

An interface between the street unit and the subscriber loop is provided by the line card. In addition to coupling speech signals to the loop, the line card also provides a power supply, typically 48 volts, across the loop to operate the subscriber terminal. In order to perform its normal function a line card incorporates a switch whereby the feed voltage polarity to the line may be reversed. This is an engineering requirement. The effect of applying polarity reversed to a quiescent line is to subject the line to a voltage step of twice the supply voltage, i.e. a step of 96 volts for the UK standard supply voltage.

The subscriber loop is coupled to the line card via output resistors R41 and R42 across one of which (R41) the inputs of a differential amplifier AMP are coupled. Optionally a switch SW1 is provided to allow coupling of the amplifier AMP across the legs of the loop. The output of the amplifier is connected to an analogue to digital converter A/D whose digital output is transmitted to the switching centre via the fibre path e.g. on a supervisory channel. Advantageously the amplifier AMP and the analogue to digital converter A/D are mounted on the line card.

When a subscriber loop is to be tested a signal is sent from the switching centre to the corresponding card causing the polarity of the power supply to the subscriber loop to be reversed. This provides an electrical impulse to the loop resulting in a corresponding transient current whose nature is a function of the electrical condition of the loop. A voltage signal corresponding to this transient current appears across resistor R41 thus producing a corresponding output from the amplifier AMP. This output is converted to a corresponding digital signal by the analogue to digital connector, this signal being returned to the switching centre e.g. over a supervisory channel. The switch SW1 allows a similar measurement to be made across the legs of the loop.

Analysis of the transient signal provides an indication of the electrical condition of the subscriber loop and also provides, where appropriate, fault information. This is illustrated in Fig. 3 which shows a typical transient waveform for a subscriber loop.

Analysis of the transient waveform is effected by relatively few measurements of characteristic features of the waveform, preferably with the current limited to a particular value. Suitable characterising parameters are listed in Table 1 below, but it should be understood that these parameters are given by way of example only and are in no way to be considered limiting.

Table Z. Characterising Parameters Symbol Parameter vmx73 Maximum Voltage measured across feed resistor with current limit set to 75mA Vw35 Long-term quiescent voltage measured across the feed resistor with thecwtUmit set to 35mA Vw75 Long-term quiescent voltage measured across dre feed resistor with dwt- limit set to 75mA V1.2 Voltage measured across the feed resistor 1.2mseconds after the application of the step t75 Tune required for the voltage across the fed resistor to drop to 37% of V1.2 V2 Voltage measured across die line terminals before the step is applied When the subscriber's line is first installed these parameters are measured and stored e.g. at the network manager. Typical values of these parameters are listed in Table 2 below for both new-style and old-style telephone instruments.

Table 2_. Initial Line Characterisation Periodic testing of the loop may be performed to detect major changes in these parameters possibly indicating a fault condition. Alternatively the loop can be tested in response to a request from the subscriber. Typical fault conditions and their associated parameters are listed in Table 3 below.

Condition ymax75 V.35 V.75 V 1.2 t75 New-Style Telephone 5-10V Not checked OV 5-10V 3-7ms Old-Style Telephone.5V Not checked OV 0.2-2.5V 13-30ms Table 3: Fault Conditions Fault. Condition Vmax75 V.45 V-75 V1.2 t75 V2 Short Circuit x lan 15V 7V 15V 15V 0 from linecard (x < I.Skm) 18 v Short Circuit x km from 10-ISV 7V 10-ISV 10-ISV linecard Okm>xm.5km) Short-Circuit Capacitor in 2OmV 2OmV 2OmV 20taV o wall-box, no telephone attached Short-Circuit Capacitor in 10-15V 7V 10-15v 10-ISV 00 wall-box. telephone attached Open Circuit on Line ov ov ov ov 0 Open Circuit on Sub4OmV OV ov 4OmV 850ms scribers side of wall-box The results of these measurements can provide an indication of a fault condition at the switching centre. Using a suitable decision table (Fig. 4) it is possible to determine from a few simple measurements whether a fault is present in the subscriber loop., in the wall box or in the subscriber's terminal equipment.

It will be appreciated that the cost of the equipment mounted on the line card is relatively low. It is thus economically feasible to provide the test facility described above for every subscriber loop.

Claims

CLAIMS:

1. Apparatus for testing a telephone subscriber loop, including means for providing an electrical impulse across the legs of the loop, and means for sensing the pattern of a transient voltage signal responsive to said impulse whereby to determine the electrical condition of said loop.

2. Test apparatus as claimed in claim 1 wherein said impulse means comprises means for reversing the polarity of a power supply to the subscriber loop.

3. Test apparatus as claimed in claim 1 or 2, wherein said sensing means comprises a differential amplifier whose inputs are coupled across a resistor in series with the loop.

4. Test apparatus substantially as described herein with reference to and as shown in Fig. 2 of the accompanying drawings.

5. A method of testing a telephone subscriber loop, the method including supplying an electrical impulse across the legs of the loop, the impulse causing a transient voltage signal to appear across the legs of the loop, and determining from the nature of the transient voltage signal the electrical condition of the loop.

6. A method as claimed in claim 5, wherein said impulse is applied by reversal of the polarity of a power supply coupled to the loop.

7. A method as claimed in claim 5 or 6. wherein the transient voltage is characterised by measurement of its maximum voltage, the time taken for the voltage to decay to a predetermined function of the maximum voltage and of the limit towards which the voltage decays.

-9

8. A method of testing a telephone subscriber loop substantially as described herein with reference to and as shown in Figs. 2 to 6 of the accompanying drawings.

9. A telephone subscriber line card incorporating test apparatus as claimed in any of claims 1 to 4.

10. A telecommunications street distribution unit incorporating a plurality of line cards as claimed in claim 9.