METHOD AND APPARATUS FOR TESTING A SUBSCRIBER LINE INTERFACE

Abstract: The invention relates to a method and apparatus for testing a subscriber line interface (12), a subscriber line (10) being disconnected from the subscriber line interface (12) and a testing unit (13) being connected to the subscriber line interface (12), in place of the subscriber line (10), without interrupting voltage supply to the subscriber line (10) for the duration of the testing. A continuous voltage supply to the subscriber line (10) during the testing of the subscriber line interface (12) is secured by connecting a separate voltage supply (14) to the subscriber line for the duration of the testing.

Description: METHOD AND APPARATUS FOR TESTING A SUBSCRIBER LINE INTERFACE BACKGROUND OF THE INVENTION The invention relates to a method for testing a subscriber line interface, a subscriber line being disconnected in the method from the subscriber line interface and a testing unit being connected to the subscriber line interface, in place of the subscriber line. Although digital exchanges are nowadays widely used in telephone networks, subscriber lines between an exchange and a subscriber are typically analog two-wire lines. Digital exchanges therefore need to be provided with a specific analog subscriber interface, i.e. a subscriber line interface, which interfaces a time-divisional signal from a digital exchange to an analog subscriber line. This is shown in Figure 1 , where a subscriber interface 25 of an exchange 24 is connected to a subscriber line 10 leading to a subscπber terminal 11. Figure 1 also shows an analog subscriber interface 26 which can be located in a multiplexing unit 27 arranged separately from the exchange 24, the multiplexing unit being connected to a digital subscriber interface 29 of the exchange via a PCM link 28 (of e.g. 2 Mbit/s). A remote multiplexing unit 30 can also be connected via a PCM link 20 (of e.g. 2 Mbit/s) to another multiplexing unit 21 located close to the exchange, an analog subscriber interface 22 of the other multiplexing unit being connected, in turn, to an analog subscriber interface 23 in a digital or an analog exchange. Multiplexers and digital connections used as described above support longer distances between subscribers and an exchange than a purely analog subscriber line. In addition, a multiplexer can be placed close to the subscribers, which allows a subscriber network to be implemented in a star configuration of short subscriber lines and, thereby, at relatively low cabling costs. The function of line testing (also known as test out) of a subscπber line interface of a telephone network is to measure the electric properties, such as capacitance and leakage resistance, of subscriber lines connecting a subscriber line interface to a subscriber. Functions known as test in functions are being added, some having already been added, to international specifications (European Telecommunications Standards Institute) relating to subscriber interfacing in telephone networks. These functions comprise measurements in which a testing unit measures the characteristics of a subscriber line interface from the direction of the subscriber. In the measurement the subscriber line is disconnected from the subscriber line interface and the testing unit is connected to the subscriber line interface, in place of the subscriber line. The testing unit is then capable of measuring the direct current/direct voltage and the ringing current/ringing voltage supplied by the subscriber line, and the operation of ring trip and off-hook detectors, for example. In prior art solutions testing is carried out by means of a specific test relay included in the subscriber interface card of the subscriber line interface, the test relay being used for disconnecting the subscriber line from the subscriber line interface and for connecting the testing unit in place of the subscriber line. This is a fairly simple solution and only requires a relay performing the disconnection and the connection, and a bus for connecting the subscriber line interface to the testing unit. A problem with the above described arrangement is that since the subscriber line is disconnected during the testing, it does not receive common battery voltage. The subscriber may have devices that comprise e.g. number dialling and abbreviated dialling memories and use common battery voltage. If a device such as this does not comprise a battery or an accumulator securing the memories, or if the battery or accumulator is empty or malfunctions, the interruption caused by the testing will empty the memories. BRIEF DESCRIPTION OF TH E INVENTION An object of the invention is thus to provide a method allowing the above mentioned problems to be solved. The objects of the invention are achieved by means of a method characterized in that voltage supply to the subscriber line is not interrupted for the duration of testing. The invention is based on the idea that a common battery voltage supplied to a subscriber line is kept unchanged during the testing, both as regards voltage and polarity. This can be realized by means of a separate voltage supply being arranged to the subscriber line for the duration of the testing. The separate voltage supply being connected at the same time as the actual voltage supply is disconnected, or earlier, allows a voltage supply interruption harmful to the subscriber device to be avoided. The invention also relates to an apparatus for testing a subscriber line interface, the apparatus comprising means for disconnecting a subscriber line from the subscriber line interface, a testing unit, and means for connecting the testing unit to the subscriber line interface, in place of the subscriber line, the apparatus being characterized in that it also comprises means for securing voltage supply to the subscriber line during the testing of the subscriber line interface. An apparatus such as this allows the advantages provided by the method of the invention to be achieved with a simple structure. BRIEF DESCRIPTION OF THE DRAWINGS The invention will be now described in greater detail in connection with an embodiment and with reference to the accompanying drawings in which Figure 1 illustrates different subscriber network implementations in an analog or a digital local exchange; Figure 2 is a block diagram illustrating an arrangement of the invention for the testing of a subscriber line interface. DETAILED DESCRIPTION OF THE INVENTION An arrangement according to Figure 2 for the testing of a subscriber line interface 12 functions as follows: in a normal situation a switching device 16 connects a subscriber terminal 11 to the subscriber line interface 12 through a subscriber line 10. The subscriber terminal 11 receives the supply voltage it possibly needs from the subscriber line interface 12. When the operation of the subscriber line interface 12 is to be tested, a testing unit 13 is connected by means of the switching device 16 to the subscriber line interface 12, in place of the subscriber line 10, the subscriber line interface 12 thus being disconnected from the subscriber terminal 11. At the same time when the above mentioned connection is made, or prior to it, a separate voltage supply 14 is provided to the subscriber terminal 11 by means of a switching device 15, via the subscriber line 10. The separate voltage supply 14 supplies a voltage which is substantially of the same level and polarity as the voltage supplied by the subscriber line interface 12. With this method the supply voltage received by the subscriber is not disconnected at any point. Correspondingly, when the testing is completed, return to the normal status is carried out by connecting the subscriber line 10 to the subscriber line interface 12 and disconnecting the testing unit 13, the separate voltage supply 14 being disconnected, at the same time or after a delay, from the subscribe line 10. The nature of the measurements carried out by the testing unit 13 is not essential for the invention. The testing unit can be arranged to measure the magnitude of the direct current/direct voltage and the ringing current/ringing voltage supplied by the subscriber line interface, and the operation of ring trip and off-hook detectors. Although in the above described preferred embodiment the invention is applied to an analog subscriber line interface, it can be applied, in a corresponding manner, also when a digital subscriber line interface is tested, without departing from the basic idea of the invention. It is apparent to a person skilled in the art that as technology advances, the basic idea of the invention can be implemented in various ways. The invention and its embodiments are therefore not restricted to the above described examples but they may vary within the scope of the claims.

Claims: CLAIMS 1. A method for testing a subscriber line interface (12), a subscriber line (10) being disconnected in the method from the subscriber line interface (12) and a testing unit (13) being connected to the subscriber line interface (12), in place of the subscriber line (10), characterized in that voltage supply to the subscriber line (10) is not interrupted for the duration of the testing. 2. A method according to claim 1, characterized in that during the testing of the subscriber line interface (12), a continued voltage supply to the subscriber line (10) is secured by connecting a separate voltage supply (14) to the subscriber line (10) for the duration of the testing, when the subscriber line (10) does not receive voltage supply from the subscriber line interface (12). 3. An apparatus for testing a subscriber line interface (12), the apparatus comprising a testing unit (13), means (16) for disconnecting a subscriber line (10) from the subscriber line interface (12) and for connecting the testing unit (13) to the subscriber line interface (12), in place of the subscriber line (10), characterized in that the apparatus further comprises means (14, 15) for securing the voltage supply to the subscriber line (10) during the testing of the subscriber line interface (12). 4. An apparatus according to claim 3, characterized in that the means for securing the voltage supply to the subscriber line (10) during the testing of the subscriber line interface (12) comprise a separate voltage supply (14) and means (15) for connecting the voltage supply to the subscriber line for the duration of the testing, when the subscriber line (10) does not receive voltage supply from the subscriber line interface (12).

Inventors: HEIKKILAE JUHA, NORDSTROEM CAJ, HEIKKILAE, JUHA, NORDSTROEM, CAJ
Applicants: HEIKKILAE JUHA, NOKIA TELECOMMUNICATIONS OY, NORDSTROEM CAJ, HEIKKILAE, JUHA, NORDSTROEM, CAJ
Application year: 1998
Main classification: H04B
IPC codes: H04M3