A METHOD AND APPARATUS FOR TESTING LINES IN A TELECOMMUNICATIONS NETWORK

Abstract: The invention provides a method and apparatus for testing a telecommunications network (1 to 12) in order to determine an approximate location of a fault. The method comprises measuring a change in capacitance of a line in the network and then determining from a known ratio of capacitance to length of line a length of line that corresponds to that change. This length will give an approximate location of a fault from a subscriber's equipment located at premises (1).

Description: A METHOD AND APPARATUS FOR TESTING LINES IN A TELECOMMUNICATIONS NETWORK This invention relates to a method and apparatus for testing a telecommunications network. When describing a public telecommunications network it is usual to subdivide the network into two parts a first part being a core network and the other part being an access network. The access network is that part of the network which extends from a subscriber s equipment to a local telephony exchange. The access network includes telephone poles, drop wires, cables, junction boxes, primary cross connection points and secondary cross-connection points which are familiar features of the urban and rural landscapes. These are the components which are most exposed to the elements and a large number of faults can occur in this part of the network as a consequence. The core network includes the local and trunk exchanges. In the past network operators have relied upon subscribers to report faults. This may take place some time after the fault has occurred because some subscribers only use their equipment infrequently. According to the invention there is provided a method of testing a telecommunications network comprising determining a first capacitance value of at least one line of the network at a first time, storing the first value, determining a second capacitance value for the at least one line at a second time, determining from the first and second values a value for a change in capacitance and determining from the change in capacitance value, and a known capacitance length ratio of the line, a corresponding length of the line. The corresponding length of line will be a change in length of that part of the line which is operational. This change in length will be due to a fault occurring between the subscriber's equipment and the exchange. The fault may be caused by an increase in series resistance in the line due. for example, to corrosion, or the fault may be due to a break or disconnection in the line. Such faults have the effect of cutting-off the subscribers equipment. Where the series resistance increases gradually, the quality of transmission over the line will deteriorate gradually until the equipment is effectively cut-off . In the case of a break or disconnection, the cut-off will be immediate. The change in length will be indicative of the location of the fault. The rauit will be located by determining a position along the line from the subscnoer s equipment equal to the determined change in length of line. By monitoring the line in this way it is possible to detect and rectify faults before a subscriber discovers a fault exists. This greatly enhances the subscriber ' s perceived view of the service. According to a second aspect of the invention there is provided apparatus for testing a telecommunications networκ comprising: means for determining a change in capacitance value of a line in the network; and means for determining from the change in capacitance value and a known capacitance length ratio for the line a corresponding length of the line. A specific embodiment of the invention will now be described, by way of example only, with reference to the drawings in which:- Figure 1 shows in schematic form part of a telecommunications network from a local exchange to a subscriber's premises, the exchange including testing equipment in accordance with the invention; Figure 2 shows in schematic block diagram form the testing equipment shown in figure 1 ; and Figure 3 is a flowchart illustrating the operation of the testing equipment shown in figure 1 . Part of a public telecommunications network is shown in figure 1 and it comprises a subscriber ' s network terminating equipment located at a subscriber's premises 1 , a drop wire 2 connected between the subscriber's equipment and a pole mounted distribution point (DP) 3, a cable 4 extending from the DP 3 to a secondary cross connection point (SCP) 5, further cables generally indicated at 6 extending from other DPs (not shown) to the SCP 5, a cable 7 extending from the SCP 5 to a primary cross connection point 8, further cables generally indicated at 9 extending from other SCPs not shown and the PCP 8, a cable 1 0 extending from the PCP 8 to a local area exchange 1 1 , a trunk cable 1 2 linking the local area exchange 1 1 to other exchanges (not shown), and test equipment 1 3 located in the exchange 1 1 . The part of the network between customers' equipment and the local exchange is known as the access network. The drop wire 2 comprises a pair of copper wires held parallel and apart by an insulating sheath. The individual wires are called "legs " . There is an A leg and a B leg. The DP 3 usually serves up to eight or so houses. The wire pairs continue from the DP 3 as part of the cable 4. Thus the cable 4 could include eight or more wire pairs plus a further eight or more spare pairs making a total of typically twenty wire pairs. Similarly, each of the other cables 6 could include twenty wire pairs. In a similar way, the cable 7 will carry about eighty wire pairs and cable 10 about two hundred and forty wire pairs. The PCP 8 and the SCP 5 are in effect junction boxes which are usually in the form of streetside cabinets or posts. The access network may be conveniently split into two parts. An exchange side which is that part of the network situated between the PCP 8 and the exchange 1 1 and a distribution side which is that part of the network between the PCP 8 and the subscribers' network terminating equipment. The exchange side may extend for many kilometres. Thus the cable 10 may be many kilometres long. The distribution side is normally less than this distance say about one or two kilometres in length. Figure 2 shows in greater detail the test equipment 1 3 used to detect faults on the wire pairs. This includes a capacitance measuring device 14 connected to the wire pairs that form the cable 1 0 and a computer 1 5. The computer 1 5 includes a processor 1 6, an input/output device 1 7, an output device 1 8 and memory 1 9. The input/output device 1 7 is in this case an input/output port to which the capacitance measuring device 14 passes values of capacitance and from which the capacitance measuring device 14 receives instructions from the processor 1 6. These values pass to the processor 1 6 along a databus in a manner well known. Memory 1 9 is of a type that can be read and also written to such as Random Access Memory RAM. It holds instructions which can be accessed and acted on by the processor 1 6. (The instructions are held in accessible memory locations which can be addressed by the processor 1 6). The processor 1 6 is also able to store data in the memory by writing the data into a memory location. The memory 1 9 may take the form of a hard disc, silicon chips or other memory device. The data flows between the memory 1 9 and the processor 1 6 along a databus 21 in a well known manner. The output device 1 8 is a device for displaying information generated by the processor 1 6 and typically could include a Visual Display Unit (VDU) and/or a printer. Alternatively, it may be a device for transmitting the information to another location which accepts and then processes the information from a number of exchanges. The information may then be collated and used to generate instructions for service personnel. The capacitance measuring device 1 4 may take any convenient form but it must be capable of monitoring the large number of wire pairs on the cable. One way in which this could be achieved is by including means to sequentially or randomly connect the device to each wire pair. The instructions for which wire pair to test come from the processor 1 6 via the input/output device 1 7 or could be generated by the means to connect the device to each wire pair. If the latter alternative is adopted, capacitance values passed by the capacitance measuring device 1 4 would have to be accompanied by information identifying the wire pair being tested that is to say the wire pair having that capacitance value. (It may be convenient to do this even if the processor 1 6 instructs the capacitance measuring device 1 4 which wire pair to test). As earlier described, the processor 1 6 acts in accordance with processor instructions stored in memory locations of memory 1 9. A more detailed explanation will now be given with reference to an illustrative flowchart of the operation of the testing equipment which is shown in figure 3. A first step 30 in the operation is system initialisation. The next step 31 is for the processor 1 6 to instruct the capacitance measuring device 1 4 to measure the capacitance of the first wire pair. In a step 32, the capacitance measuring device 14 then measures the capacitance C of the wire pair. This value is passed via the input/output device 1 7 to the processor 1 6 where it is held in short term memory (not shown). In a step 33. the processor 1 6 then decides whether this is the first pass through the steps shown in Figure 3. Since it is the first pass, in a step 34 the value of C is stored in memory 1 9 as old C together with a wire pair identifier. In a step 35. the processor 1 6 considers whether that wire pair is the last wire pair. If it is not the last wire pair, then a return is made to step 31 and the next wire pair is selected. The series of steps described above are then separated. Thus, after measuring the capacitance C of the last wire pair, the memory 1 9 will hold wire pair identifiers and associated values of old C for all the selected wire pairs. Then, in a step 40, the processor 1 6 waits for a time internal T. T may be any time interval but, in this case it is twenty four hours. After twenty four hours, a return is made to step 31 and the first wire pair is again selected. Then, in step 32, the capacitance of the first wire pair is measured. Then, in step 33, since there is not the first pass, it is decided that the next step will be step 36. In step 36, the newly measured value of C is compared by the processor 1 6 with the old C stored in the memory 1 9 for the same wire pair. The difference of the values is determined that is to say, old C - C. The difference is then compared in a step 37 with a predetermined threshold value stored in memory 1 9 in order to determine whether it represents a significant difference. (A significant difference being one which is caused by a fault on the wire pair as opposed to a minor variation perhaps caused by climatic conditions which does not perceivably affect the service experienced by a subscriber). From the difference, in a step 38 a difference in the length of the wire pair is determined by the processor 1 6, the wire pair having a known relationship of capacitance per unit length. This difference in the length of the wire is the distance of the fault from the subscribers' terminating equipment. The processor 1 6 outputs this data along with a wire pair identifier such as a telephone number to the output device 1 9 where it is displayed. The newly measured value of C is then stored in a step 39 as old C by overwriting the previous old C value. The process is repeated for the next wire pair. Thus a printout of faulty telephone lines is printed out together with the distance of the fault from the subscriber's terminating equipment. Service personnel may then be dispatched to rectify the faults.

Claims: CLAIMS 1 A method of testing a telecommunications network comprising: determining a first capacitance value of at least one line of the networκ at a first time; storing the first capacitance value: determining a second capacitance value for the at least one line at a second time; determining from the first and second values a value for a change in capacitance value; and determining from the change in capacitance value, and a known capacitance length ratio of the line, a corresponding length of the line. 2. A method as claimed in claim 1 wherein the change in capacitance value is compared with a threshold value and if the threshold value is exceeded the step of determining the corresponding length of the line is then carried out. 3. Apparatus for testing a telecommunications network comprising: means for determining a change in capacitance value of a line in the network; and means for determining from the change in capacitance value and a known capacitance length ratio for the line a corresponding length of the line. 4. Apparatus as claimed in claim 3 wherein comparison means are provided to compare the determined change in capacitance with a threshold value.

Inventors: CHASKELL ANDREW DAVID, CHASKELL, ANDREW, DAVID
Applicants: BRITISH TELECOMM, CHASKELL ANDREW DAVID, BRITISH TELECOMMUNICATIONS PUBLIC LIMITED COMPANY, CHASKELL, ANDREW, DAVID
Application year: 1995
Main classification: H04M3
IPC codes: G01R31, H04M3