COMMUNICATIONS LINE TESTING

Abstract: A tester for a communications (e.g. telephone) line pair comprises a pair of different valued resistors (R1 and R2) each connected in series with a diode (D1 and D2). The diode/resistor pairs are connected in parallel between connectors (A and B) such that one of the pair provides a path for a DC voltage of one polarity and the other of the pairs provides a path for a DC voltage of the other polarity. The device is connected to the ends of a spare pair of telephone wires at a remote distribution point. The spare pair is provided with its exchange equipment termination and testing is carried out at the local exchange via the test access facility to check that the pair of wires is in working condition before sending installers out on site. The test procedure includes measuring the total resistance of the lines to determine whether the difference in resistance readings as determined are in accordance with the difference between the resistances (R1 and R2) in the device itself. If so, the lines are in working condition. If not, there is a fault and the lines cannot be used.

Description: COHHUNICATIONS LINE TESTING This invention relates to the testing of the working condition of a spare line pair of a communications cable using a device connected between remote ends of the line pair; Communications lines, such as telephone cables, are most commonly deployed with an over-capacity in the number of pairs of wires. The cable runs to a distribution point where the various wire pairs are mounted ready for connection to customer telephones and other telecommunication apparatus. The over-capacity is provided to accommodate new customer requiring connection, and is called "spare pairs" or "spare line pairs". Before a previously redundant (ie. spare) pair of wires constituting a telephone line can be used to connect a customer, it is necessary to test the wires to ensure they are both in working condition. To test the pairs before they are put to use, it is presently necessary to send an installation technician to conduct appropriate tests on the nominated wires at the distribution point local to a particular customer. In the event that a pair of wires corresponding to a particular designated telephone number are defective, a technician cannot connect the customer and he must return to the central exchange and have a further pair of wires designated for that customer's use. Once this is done, the technician must again visit the site and conduct the tests. It is clear that nominating a defective wire pair can lead to at least one wasted site visit before the fact is detected. It is clearly desirable to be able to test a given line pair remotely before the installer conducts the time consuming and expensive site visit. Line test systems are known which are able to diagnose various line faults, and line characteristics from a remote location. An example of such a proprietary line test system is the 4TEL system made by Teradyne Limited of Western Centre, Western Road, Bracknell, Berkshire, RG12 lRtf, England. These systems are sophisticated software test routines and test signals which exploit the inherent capacitance and resistance in a line pair to determine the nature and location of a fault and/or the parametric data associated with the line, for example, susceptance, resistance, leakance etc. These systems can also be used to determine the length of a line pair. However, they are not accurate enough to be able to determine whether an unter inated pair is in good working condition or whether there is a break in either wire in the vicinity of the distribution point. This latter condition would result in an indication of a pair of unterminated wires apparently in good condition. It is an object of the present invention to provide a method of testing the working condition of a spare line pair of a communications cable from a remote location, a test device for use in such a method, and a communications system arranged for such testing. According to the present invention there is provided a method of testing the working condition of a spare line pair of an electrically conductive communications cable which extends from a node to a remote distribution point, from which distribution point connection is to be made from the remote end of the line pair to a customer's premises or to a further distribution point; the method of comprising the steps of connecting a test device between the ends of the line pair at the remote distribution point, the test device having a first signal conditioning characteristic to a signal of one polarity applied to the line and a second signal conditioning characteristic to a signal of the other polarity applied to the line; providing at the node an exchange equipment termination for the line pair; applying signals of both polarities to the line pair via a test access facility associated with the exchange equipment termination; and determining from the relative conditions of the signals whether the line pair is in working order. Preferably, the signals are those of the proprietary line test system used and could be DC or or DC superimposed on an AC signal applied with opposite polarity across the line. The invention also extends to a test device for use in the above method, the device comprising means for connecting the device between the end of a line pair to be tested, one only first signal conditioning means which pass signals of the one polarity applied across the connecting means, one only second signal conditioning means which pass signals of the other polarity applied across the connecting means, and a housing for said one only first signal conditioning means and said one only second signal conditioning means. Preferably, the first and second signal conditioning means are connected in parallel between the connecting means. Preferably, the first and second signal conditioning means are passive, for example electrical impedances. According to the present invention there is further provided a communications system having the capability for testing the working condition of spare line pairs, the system comprising an electrically conductive communications cable which extends from a node to a remote distribution point and comprises a plurality of spare line pairs; a plurality of identical test devices as described above connected between the ends of a respective plurality of spare line pairs at the remote distribution point; the other ends of the respective plurality of spare line pairs being terminated at the node in respective exchange equipment terminations; a test access facility associated with said exchange equipment terminations and means for applying signals of both polarities to a selected one of the spare line pairs connected to a test device; and means for determining from the relative conditions of the signals whether the line pair under test is in working condition. A test routine can be incorporated within the software of a known line test system. The test device is connected between a pair of wires at the distribution point. The signals are transmitted along the wires from the exchange and monitored in order to determine whether the condition of the signals is indicative of a pair of wires in working condition. Thus, it is possible to check whether the wires are in working condition before dispatching the installer to the site (distribution point). Furthermore, provided the device is connected according to a prescribed convention, it is also possible to identify the individual wires of the pair based on the different conditioned signals. In the case of optical fibre communication it may be necessary to rely on local conductive lines to connect up customers. Thus, an optical fibre will be terminated at a node from which the local lines will be distributed. This node may be the telephone exchange where the test signals are applied to the test access point or it may be a more local node in which case the lines will have their exchange equipment terminations at this local node and access to these terminations for testing will be via conductive wires in the optical fibre cable from the exchange to this local node. The present invention can be put into practice in various ways one of which will now be described by way of example with reference to the accompanying drawings in which:- Figure 1 is a circuit diagram of a test device in accordance with the invention; Figure 2 is a schematic diagram of one cable arrangement between an exchange and a remote distribution point; and Figure 3 is a schematic diagram of an alternative arrangement to that of Figure 2. Referring to Figure 1, a terminator test device comprises a first path 10 comprising a diode Dl serially connected to a 56 kohm resistor Rl and a second path 12 comprising a diode D2 serially connected to a 100 kohm resistor R2. The two paths 10 and 12 are connected in parallel between connecting terminals A and B which in use are each respectively connected to a corresponding end of one of the wires of the line to be tested. These are referred to as the A and B legs. The anode of the diode Dl is connected to terminal A and the anode of the diode D2 is connected to terminal B. The components are disposed within a sealed housing 14 from which extend the two terminals A and B. The terminals are used to connect the device between the wires of a pair at the distribution point. The housing 14 is a weatherproof rigid plastics moulding. For the sake of convenience, one outside face of the housing is provided with a surface suitable for writing on in pencil or some other erasible means. This may be used to note details of a particular line pair to which the device is connected, such as the corresponding telephone number. Although not shown in the circuit diagram of Figure 1, it is possible to protect the device from a potentially damaging overvoltage by connecting a discharge tube or similar device between the terminals A and B. A typical source of an overvoltage results from the accidental use of an insulating resistance meter producing potential differences in excess of 500 volts of random polarity. Without some form of overvoltage protection these may damage the circuit components. In accordance with the present invention the device is for use with a quiescent (spare) telephone line. The likely potential differences are in the ranges -48 volts to -58 volts in leg B and 0 volts to -5 volts in leg A. In this embodiment the resistors are 0.25 watt carbon resistors with a tolerance of + 5%. The housing 14 is conveniently made as small as possible. One practical version has dimensions of 80mm x 15mm x 10mm. Any suitable screw or spring displacement connectors may be used at the terminals A and B moulded into the housing. These are colour coded in order that the device can be connected up between the line pair according to an established convention such that not only the working condition of the wires can be determined, but also which wire is which. As an alternative, the device may be provided with colour coded fly leads, possibly having test clips for connection between the wires. In this case, it is necessary for the clips to be well insulated to prevent accidental electrical contact with surrounding surfaces or other wires. However, it may be more convenient to leave the fly leads bare so that an installer can fit clips of his preference. The device is connected between the ends of a pair of redundant (spare) wires at a distribution point. It is left in place until such time as the pair is required for use, and the near end of the line pair is provided with an exchange equipment termination. At this time, and before the installation personnel have been sent out to connect up a customer, a centralised diagnostic test is performed using a line test system of the type previously described. The test signals are applied to the test access point of the local exchange, but the control position for the line test system need not be at the local exchange and can be at a centralised maintenance and testing facility (test centre). Firstly, a first line test signal, i.e. a DC voltage of one polarity, is applied across the lines of the pair such that the diode Dl, for example, conducts. The line test system will derive a reading for the total line resistance, including the 56 kohms of the resistor Rl. Secondly, a second • line test signal, i.e. a DC voltage of the opposite polarity, is applied across the wires such that the other diode D2 conducts. The line test system will derive a reading for the new, different, total line resistance, including the 100 kohms of the resistor R2. Figure 2 shows schematically the arrangement when all of the local line from the local exchange is electrically conductive cable. A telephone cable 16 extends from a local exchange 18 to a cross-connection cabinet 20 from which a telephone cable 22 extends to a remote distribution point 24. Normal working line pairs are provided with exchange equipment terminations 26, as is known in the art, and the exchange has a test access facility 28, also as is known in the art, and a 4TEL line test system (LTS) 30. In a network of exchanges the control of the LTS 30 may be from a test centre (not shown) remote from exchange 28, to which the LTS 30 can report its test results via appropriate communications links. In Figure 3, the local exchange 18 is connected by an optical fibre cable 34 to a node 32 which extends to remote distribution point 24 by telephone cable 22. In this arrangement the working line pairs are provided with their exchange equipment terminations 26 at node 32, and signalling between the exchange 18 and the distribution point 24 is via the node 32 by means of the electrically conductive strength members 36 of the cable 34 and a switching matrix 38 at the node 32 for selecting the relevant exchange equipment termination. It will be appreciated that if node 32 is sufficiently large then the test access facility and the line test system can be situated at the node 32, and the line test system will communicate with its test centre via the optical fibre link. Having determined the two different total line resistances using the voltage of opposite polarity the line test system will display the relevant parametric data at the test centre. The line test system is arranged to display the parametric data created by the test device and thus indicate to a user by the values displayed whether a test device is fitted or not. The line test system may also be adapted to determine by means of a computer algorithm whether the difference in the line resistances is in accordance with that of the first and second paths of the device, and thereby to indicate whether a test device is fitted or not. In simple terms, this means that the line pair has been proved acceptable from the exchange to the remote distribution point so that a customer requesting telephone service by going off-hook will be recognised by the exchange equipment and sent dial tone. As mentioned previously, by adhering to a convention as to which terminal of the test device is connected to which wire of the pair, i.e. the A terminal to the A leg and the B terminal to the B leg, it is also possible for the line test system to identify the wires of each pair from one another, based on the higher resistance of the path in one direction compared to that in the other. Installers can hold stocks of the test device for connection between new pairs of wires at distribution points and for retro-fitting to existing pairs. When a pair of wires is brought into service the connected device can be retrieved for re-use if it is not damaged. The use of the device and the test procedure according to the invention represents an initial expenditure in the devices themselves. However, this is recoverable in the substantial reduction in unnecessary visits made to sites by installers. It is envisaged that all spare line pairs can be terminated with test devices in accordance with the invention such that remote testing of the line condition can be carried out. In any event, the test procedure can be conducted in order to see whether a pair or wires is terminated in a test device. In the absence of the receipt of signals indicating the appropriate differential resistance the line pair may be either faulty or simply not provided with a test device. In either case a further pair of wires may be adopted in preference in order to avoid a site visit without confirmation that a nominated line pair is in working condition. The device presents working conditions and line identity indications without external electrical connections. The line test system can, therefore, perform its diagnostic testing with little or no need for modification. It will be appreciated that the test device simulates a fault condition within the fault measuring range of the line test system, and that when the tested line pair is brought into use, either for connection to customer premises or for extending the line pair to a further distribution point, then the test device will be removed to avoid a continuous fault condition on the line pair. Whereas the present invention has been described with reference to a communications system intended for telephony, it will be appreciated that the present invention is equally applicable to the testing of line pairs of a communications system intended for data communications.

Claims: CLAIMS 1. A method of testing the working condition of a spare line pair of an electrically conductive communications cable which extends from a node to a remote distribution point, from which distribution point connection is to be made from the remote end of the line pair to a customer's premises or to a further distribution point; the method of comprising the steps of connecting a test device between the ends of the line pair at the remote distribution point, the test device having a first signal conditioning characteristic to a signal of one polarity applied to the line and a second signal conditioning characteristic to a signal of the other polarity applied to the line; providing at the node an exchange equipment termination for the line pair; applying signals of both polarities to the line pair via a test access facility associated with the exchange equipment termination; and determining from the relative conditions of the signals whether the line pair is in working order. 2. A method as claimed in claim 1, wherein the signals are applied via a test access facility at the said node. 3. A method as claimed in claim 2, wherein the said node is constituted by a communications exchange. 4. A method as claimed in claim 1, wherein the signals are applied via a test access facility provided at another node connected to the said node by an optical fibre communications link. 5. A method as claimed in any of one of claims 1 to 4, in which the signals are DC voltages 6. A test device for use in the method of any of claims 1 to 5, the device comprising means for connecting the device between the end of a line pair to be tested, one only first signal conditioning means (Rl/Dl) which pass signals of the one polarity applied across the connecting means, one only second signal conditioning means (R2/D2) which pass signals of the other polarity applied across the connecting means, and a housing for said one only first signal conditioning means and said one only second signal conditioning means. 7. A device as claimed in claim 6, in which the first and second signal conditioning means are connected in parallel between the connecting means. 8. A device as claimed in claim 6 or claim 7, in which the first and/or second signal conditioning means are passive elements. 9. A device as claimed in claim 8 when dependent on claim 7, in which the first and/or second signal conditioning means comprise a resistor (Rl, R2) in series with a diode (Dl, D2). 10. A device as claimed in any of claims 6 to 9, in which connecting means are fly leads. 11. A device as claimed in any of claims 6 to 9, in which the connecting means are electrical connectors mounted on the housing. 12. A communications system having the capability for testing the working condition of spare line pairs, the system comprising an electrically conductive communications cable (16, 22) which extends from a node (18, 32) to a remote distribution point (24) and comprises a plurality of spare line pairs; a plurality of identical test devices (Rl, Dl, Rl, R2) as claimed in any one of claims 6 to 11 connected between the ends of a respective plurality of spare line pairs at the remote distribution point (24); the other ends of the respective plurality of spare line pairs being terminated at the node in respective exchange equipment terminations (26); a test access facility (28) associated with said exchange equipment terminations (26) and means (30) for applying signals of both polarities to a selected one of the spare line pairs connected to a test device (Rl, Dl, R2, D2); and means (30) for determining from the relative conditions of the signals whether the line pair under test is in working condition. 13. A system as claimed in claim 12, wherein the test access facility (28) is provided at the said node (18, 32). 14. A system as claimed in claim 13, wherein the said node is constituted by a communications exchange (18). 15. A system as claimed in claim 12, wherein the test access facility (28) is provided at another node (32) connected to the said node (18) by an optical fibre communications link.

Inventors: WEBBER ALAN, WEBBER, ALAN
Applicants: BRITISH TELECOMM, BRITISH TELECOMMUNICATIONS PUBLIC LIMITED COMPANY
Application year: 1990
Main classification: H04M3
IPC codes: G01R31, H04M3