AU717991B2 - A line testing arrangement - Google Patents

Description

w P/00/0i 128/5/91 Regulation 3.2

AUSTRALIA

Patents Act 1990

ORIGINAL

COMPLETE SPECIFICATION STANDARD PATENT Invention Title: "A LINE TESTING ARRANGEMENT" S S

S

S. The following statement is a full description of this invention, Including the best mnethod of performin~g it kniown to uis:- XO18 232 281196 Technical Field This invention relates to the remote testing of telephone lines, and may be used in such applications as the measuring of extension lines of a PABX or where the lines are multiplexed onto a high capacity line such as optical fibre or co-axial line, as in fibre in the loop or fibre to the curb applications.

Background Art In equipment providing telephone line interfaces it is often necessary to provide some form of line test facility. In telephone exchanges this facility often takes the form of a test unit which is shared over many lines, and each line interface is provided with test relay(s) which can connedct the line to be tested to the test unit. Typically the test unit can perform many different tests on the line under remote control.

A disadvantage of the above approach when applied to equipment with a small number of lines is the high cost of the shared test unit, and relay switching required.

In applications such as fibre in the loop (FITL), it is often necessary to provide small numbers of subscriber interfaces (as low as 1-2) in each network terminating unit In such applications the cost of traditional line test methods as described above may be unacceptably high. Because the NTs are usually located very close to the subscriber (eg. mounted on wall of subscriber premises or nearby street kerb) the lines to the subscriber are usually short, and many types of line tests relating to long lines are not necessary. In this application a reduced set of line tests may be used.

S" A known remote testirg arrangement is described in the specification of Australian Patent No. 654570. This know arrangement provides a relatively large number of line tests on subscriber lines at a network unit in response to test requests transmitted from a central test controller. The arrangement sends coded results of tests back to the controller or sends stored results from earlier tests if there is insufficient time to run a test.

disadvantage of this last mentioned known arrangement is that it is complex.

Disclosure of the invention It is an objedt of the present invention to provide a basic line test facility in a simple and very cost effedctive way. Basic line tests may include line to line feed voltage and line to line capacitance.

The feed voltage test i: used to verify the correct operation of the Subscriber Line Interface Circuit (SLIC).

The capacitance test is used to determine if one or more telephone sets are connected to the line.

An advantage of the present invention is that it may be applied to standard SLIC circuits, to add basic line testing capability at very low cost.

This specification therefore describes a remote telephone line testing arrangement for testing line conditions at a network termination unit, the arrangement including: control means responsive to a line test command signal to generate a line condition control signal; line condition variation means to alter one or more conditions of the telephone line in response to the line condition control signal; line voltage monitoring means to monitor the line voltage; line parameter test means responsive to the monitoring means to obtain a measure of one or more line parameters.

Brief Description of the Drawings Figure 1 shows examples of arrangements in which the invention may be used.

Figure 2 shows a first arrangement embodying the invention.

Figure 3 shows a modification of the arrangement of Figure 2.

Figure 4 shows a preferred embodiment of the present invention.

Figure 1 is a schematic illustration showing two of the possible arrangements for connecting subscribers via hybrid coaxial/optical cables or combinations of two wire with optical or coaxial or hybrid lines to an exchange 1. Other arrangements are possible without departing from the inventive concept.

~In a first arrangement optical fibre 2 is connected to a multiplexing interface 3 via which a plurality of co-axial ines are multiplexed onto the fibre 2. One of the co-axial lines 4 is shown connected to a network termination unit (NT) 5 installed at a subscriber's premises 6. A telephone 7 is connected by a two-wire line to NT In a second arrangement a second optical fibre 12 is connected to a multiline termination 13, as used eg in fibre to the curb, which multiplexes the output of a plurality of network termination units 15 onto fibre 12. The connection of one NT 15 to a subscriber's subset 17 at prei,,ises 16 via a two-wire line 18 is also illustrated.

From the exchange a control signal addressed to the subscriber's line may be sent via an optical fibre to a kerb or corner distribution point. A number of subscriber's lines are interfaced into the fibre by electro-optical and opto-eledctrical transducers controlled by a suitable multiplexing arrangement at the distribution point. The subscriber's phone may be connected to the distribution point either directly by a copper pair line, or the phone's two-wire line may be interfaced onto a co-axial line at the kerb or in the subscriber's premises.

Figure 2 illustrates relevant components of a first embodiment of a network termination unit, to which a subscriber's subset 7 is connected via a two-wire line b).

The termination unit includes a Subscriber Line Interface Circuit (SLIC) 23 to which the lines a b are connected on the subscriber side. On the exchange side of SLIC 23 there are analog output and input ports, a signalling output port and a control and signalling input port which are connected to interface 30 which may for example include analogto-digital, and digital-to-analog conversion means, coding/decoding, and multiplexing/demultiplexing means to enable the various signals to be applied to and extracted from an optical fibre or co-axial line connected to the network. Also shown in Figure 2 is a line voltage measuring circuit, a differential amplifier connected to an analog-to-digital converter 24, via optional switch array 25. Power for subset 7 is provided via SLIC 23 from power supply 28 which may be derived from mains or o battery. The network termination unit includes power switching means 29 shown in SLIC 23 to enable the power to lines a b to be interrupted in response to a control signal received from the network. Connected across lines a b is a discharge path illustrated as a resistor 22 in series with a switch 26, shown as a bipolar transistor, though any suitable switch could be used. The discharge path may optionally be a resistor alone or .9o alternatively it may consist of a current sink.

Line voltage measuring means 21 are connected across lines a b, the output of ":0'i30 measuring means 21 being connected to an analog-to-digital converter 24.

Preferably measuring means 21 is a differential amplifier.

A processor 20 can be initiated by control commands from the exchange to implement one or more test routines from software stored in ROM 20a. Test results can be stored in RAM 20b or transmitted to the exchange via the SLIC 23 signalling output. The processor 20 may be substituted by a suitable control circuit eg in ASIC form.

The operation of the arrangement will be described with reference to the drawings.

From the exchange 1 a line test command is sent to a subscriber's NT 5 via line 2. The line test command is incorporated in the appropriate channel of the TDM or WDM structure to ensure it reaches the intended subscriber's equipment.

The line test command is passed to the processor 20 and the processor commences the test routine. The processor 20 causes the line switch 29 to disconnect the subset 7 from the power supply of the order of 50V, and the discharge path (e.g.

is applied across lines a b. The major contribution to capacitance is from the ringing circuit of subset 7.

A typical ring load may be modelled as a capacitor and a resistor in series. In Australia Austel specifies a standard ring load of 1 uF and 4000 ohms in series. This network is called a 1 REN (Ringer Equivalence Number) load.

A typical discharge recistor value would be 50K. This would result in a decay time with a 1 REN load of approx The line voltage measuring means 21, a differential amplifier, provides on its output a continuous measure of the line voltage. This is sampled and digitized in A/D converter 24 at an appropriate rate and at a sufficient level of accuracy e.g. (6 bits) to enable the required information discharge rate, capacitance across line) to be 25 extracted. The digitized readings may be stored in a RAM 20b for later transmission to the exchange or may be transmitted directly to the exchange.

The line voltage can be monitored while the line is idle (phone 7 on-hook) or when the line is active (phone 7 off-hook).

Optionally the switching means 29 may be a change-over switch arrangement which reverses the line polarity at a b so the phone is not without power for the duration of the test. Telephone 7 would include a bridge rectifier so the phone remains powered. When the line voltage reversal method is used, the discharge circuit 22, 26 is not required.

Given a 1 REN load, a current pulse of 25mA amplitude and 4ms decay time would be expected. This pulse could in fact be detected by the "loop detector" output of the SLIC which has a typical threshold of 1 0mA. The width of the pulse at the loop detector output would be related to line capacitance and resistance.

To perform a line capacitance measurement the following procedure is used.

The capacitance measurement is made only when the subscriber line is not in use, i.e. phone is on-hook and no significant loop current flows from a to b. The voltage between a and b is typically 40 50 volts in this state (as supplied by SLIC 23).

To start the capacitance test SLIC 23 is set to the high impedance state i.e. both line terminals of the SLIC become open circuit and the line capacitance is discharged via discharge means 22. The discharge decay characteristic may then be monitored via the digital output of A/D converter means 24. The time for the line voltage to decay may be then used to estimate line capacitance. The control of the SLIC 23 and interpretation of the digital decay characteristic will typically be by use of a microprocessor.

In a preferred embodiment discharge means 22 may take the form of a simple resistor. In an alternative embodiment discharge means 22 may be a current sink.

Discharge means 22 may also incorporate a control means 26 to enable discharge during capacitance testing and disable discharge at other times.

The invention may optionally include switching means 25, to enable A/D means 24 to be shared over a number of line interfaces. Switching means 25 would typically take the form of a CMOS switch array and would typically be controlled by a microprocessor.

Figure 3 shows an alternative embodiment of the invention: In this embodiment, a D/A converter means 36 and comparator means 37 are used to replace the A/D converter means 24 of figure 1.

This approach allows cost savings as D/A converter devices are generally considerably less expensive than A/D converter devices of comparable resolution and speed. The potential cost saving can be quite significant were the D/A converter means 36 takes the form of a simple resistor network (for example as shown at 42 in Figure 4).

In this embodiment, to perform a voltage measurement the control means e.g.

7 processor 20 must step the digital input to D/A converter means 36 until the output of comparator 37 changes state.

When performing a capacitance measurement, the D/A converter means may be programmed with a fixed threshold and the output of comparator means 7 changes state when the line voltage decays to this threshold point.

Again, switching means 25 may be included to allow D/A means 36 and comparator means 37 to be shared over a number of lines.

Figure 4 shows a typical embodiment of the invention.

Differential amplifier means is based on an operational amplifier 40 and four resistors in a standard configuration.

Optional line protection means 44 is connected to line terminals a,b and ground.

D/A means 42 may typically be a network of resistors in a very low cost variant. In typical applications, high resolution and accuracy is not necessary and as few as six bits resolution may be used. The network of resistors may be a hybrid film network.

Output ports of said control means are connected to D/A means 42 and switching means 25. The output of comparator 37 indicates when the output of D/A converter 42 becomes greater than the line voltage.

too* I09** *9e* a.

Claims (14)

1. A remote telephone line testing arrangement for testing line conditions of a two-wire line connecting at least one subset to a network termination unit interfacing said two-wire line with a multiplexed high capacity line, the arrangement including: control means responsive to a line test command signal to generate a line condition control signal; line condition variation means to alter one or more conditions of the two-wire line in response to the line condition control interrupt signal; line voltage monitoring means to monitor the line voltage; line parameter test means responsive to the monitoring means to obtain a measure of one or more line parameters.

2. An arrangement as claimed in claim 1 wherein the line condition variation means removes or reverses the power supply to the subset. 15

3. An arrangement as claimed in claim 1 or claim 2 wherein the line condition variation means includes line discharge means connected across the two-wire line.

4. An arrangement as claimed in any one of claims 1 to 3, wherein the line parameter test means measures the discharge time for the line to discharge.

5. An arrangement as claimed in claim 3 or claim 4 as appended to claim 3, 0 20 wherein the discharge means is a current sink.

6. An arrangement as claimed in claim 3 or claim 4 wherein the discharge •go• means is a resistor.

7. An arrangement as claimed in claim 3 or any one of claims 4 to 6 as appended to claim 3, wherein the discharge means is controllable to discharge the line only during a line test.

8. An arrangement as claimed in any one of the preceding claims wherein the line parameter test means estimates the line capacitance from the line discharge time.

9. An arrangement as claimed in any one of claims 1 to 7 including transmitter >t i.eans to transmit test results to a test centre.

K Th 9 A line testing arrangement substantially as herein described with reference to the accompanying drawings.

11. A system for testing a telephone subset connected to a network via a hybrid line, comprising test signal generator means at a test centre connected to the network, the test signal generator generating a line test command signal and transmitting the line test command signal to a network termination unit including a testing arrangement as claimed in any one of claims 1 to

12. A PABX including a line testing arrangement as claimed in any one of claims 1 to

13. A method of remotely testing line conditions of a two-wire line connecting at least one subset to a network termination unit interfacing said two-wire line with a multiplexed high capacity line, including the steps of: generating a line test command signal; varying one or more of the line conditions in response to the command 15 signal; monitoring the line voltage to determine the response of the line voltage to the variation of line conditions; determining one or more line parameters from the monitored line voltage.

14. A method or remotely testing telephone lines substantially as herein described 20 with reference to the accompanying drawings. see. o*o* DATED THIS THIRTY FIRST DAY OF JANUARY 2000 ALCATEL AUSTRALIA LIMITED 000 005 363)