GB2101751A

GB2101751A - Line circuit testing

Info

Publication number: GB2101751A
Application number: GB08120910A
Authority: GB
Inventors: Victor Teacher
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1981-07-07
Filing date: 1981-07-07
Publication date: 1983-01-19
Also published as: BE893781A; HK50087A; GB2101751B

Abstract

One of the known methods of testing the digital link (6) between a telephone exchange (1) and a subscriber's terminal equipment (7) in an "all-digital" system involves sending a bit stream over that link. At the subscriber's terminal equipment the "go" and "return" paths are looped together (switch 18) so that the bit stream is sent back to the exchange where it is compared in the test control unit (9) with what was sent. The result of this comparison indicates the condition of the link. In the present arrangement, the operation is improved by adding noise from a noise generator (10) at a known level to the received signal. The error rate is detected by the control unit (9) and the test repeated for increasing noise levels. This enables incipient features to be detected before the link fails completely. <IMAGE>

Description

SPECIFICATION Line circuit testing The present invention relates to automatic telecommunication exchange systems of the "all-digital" type, i.e. to systems in which communication is in digital form right up to the subscriber's apparatus.

A known method for the routine testing of digital transmission links between an exchange and subscriber's terminal equipments is for a test control unit located either at that exchange or at a centre linked to the exchange to send a command signal to the subscriber's terminal equipment. This signal causes the digital bit stream received by the terminal equipment to be re-transmitted back to the exchange, whereafter test equipment compares the bit stream received back from the subscriber's terminal equipment with the originally-transmitted test signal. The result of the comparison indicates whether the link is still operational.

Such a method is a simple way of ascertaining whether or not the transmission link is operational, since the presence of discrepancies or errors in significant amounts between the transmitted and received signals would indicate that the link is unserviceable. However, the method has the disadvantage that in many cases the operational margin above which errors begin to be significant is so large that faults which could indicate an imminent serious degradation may go undetected since they do not cause a noticeable error rate at the time at which a test is carried out.

According to the present invention there is provided a method of remote testing of the digital transmission links between an automatic telecommunication exchange and subscriber's terminal equipment, in which to test a said link a digital bit stream is transmitted from a test control unit at the exchange or connected thereto to the terminal equipment accessible via that link, in which at the terminal equipment the receive and transmit paths of the link are looped together so that the received bit stream is retransmitted back to the test control unit, in which the bit stream received back at the test control unit is modified during said reception by the addition thereto of a noise signal of known amplitude, in which the received bit stream as thus modified is compared with the bit stream as originally sent from the test control unit, and in which the number of discrepancies detected as a result of said comparison is representative of the error rate of the transmission link.

The figures for error rate, and the level of added noise required to produce it, are compared with a record of the figures exri d when the link is performing in a santlsratt . ç manner. Therefore the performance of the link is monitored so that warning is given that a link's performance is deteriorating before it actually fails.

An embodiment of the invention will now be described with reference to the accompanying highly schematic drawing.

In the present arrangement it is assumed that the test equipment is at the exchange at which the transmission link terminates. The exchange equipment is shown in the upper part of the drawing, and includes an exchange core 1, which consists of the exchange switching means and its control equipment.

This is connectable via change-over switches 2, 3 which may be, but are not necessarily, electronic switches, to the receiver 4 and transmitter 5 of the line circuit, which are connected to the link 6 to a subscriber's terminal equipment 7. The change-over switches in their alternative settings connect the link to a test highway 8 to which is connected a common test pattern generator and comparator 9. There is also a random noise generator 10 connected to a noise highway 11, which can be connected via a further electronic switch 12 to the incoming path of the link 6.

Note that the link 6 has separate "go" and "return" transmission paths. However, the link could utilize single physical path with the "go" and "return" transmission paths separated by suitable processing of the signals in the transmission link terminations.

The noise signal may be generated by the use of one or more random or pseudo-random sequencies of digital pulses of controlled period and amplitude, and applying them to a low-pass filter whose cut-off frequency is a small fraction of the pulse frequency. The output of such a filter is a suitable noise signal for the present purpose. Control of the RMS level of the noise signal is achieved by passing it through an amplifier and/or attenuator whose setting can be controlled either locally or remotely. It is not necessary for each subscriber's link to have its own noise generator; one such generator can serve a large number of links by a common highway connectable when needed to the chosen link, under control of the test equipment.

The subscriber's terminal equipment 7 includes the transmitter 1 5 and receiver 1 6 for the link 6, terminal unit 17, a switch 1 8 operable when the link is to be tested, and control means 1 9 for the switch 1 8.

A test on a subscriber's link is typically effected by the following procedure; (1) After the switches 2 and 3 have been set to connect the test highway 8 to the line circuit 4-5, a signal is sent from the test equipment 9 to the subscriber's apparatus 7, which causes the receive path to be looped to the transmit path by the change-over of the switch 18.

(9) A repetitive test pattern generated by the test equipment 9 in transmitted via the exchange (over highway 8) to the subscriber's apparatus, and is received therefrom in the reverse direction. Thus the test pattern has traversed both paths of the link under test.

(3) A control signal is sent from the test equipment 9 to the noise generator 10, causing it to set its output to a predetermined low level.

(4) A control signal is sent to the switch 1 2 of the exchange terminators of the link 6, at the receive side 4 of the line circuit, so that the noise generator is connected to that receive side of the line circuit.

(5) The test equipment compares the transmitted and received bit patterns, registering any discrepancies as errors and counting them over a set period of time. Some at least of the discrepancies thus noted may be due to the addition of the noise generator output.

(6) The test equipment 9 compares the error count with a predetermined reference number corresponding to an error rate substantially higher than the planned normal service figure. If the error count is higher still, a serious fault is indicated. and an appropriate alarm may be given.

(7) If. as is usually the case, the error count is lower than a reference number, the test equipment 9 sends a control signal to the noise generator 10 to cause it to increase its output by a set amount, whereafter steps (5) and (6) are repeated.

(8) Steps 7. 5 and 6 are repeated until an error count which just exceeds the reference number is obtained.

(9) At this point the test equipment 9 compares the set output level of the noise generator 10 with a record of the same test sequence made at the time of installation or last servicing or testing of the link 6.

(10) The difference between the two levels indicates the degree of degradation in the link performance, and is used to provide data for decision on whether or not maintenance action is needed.

(11) All circuits are reset for normal working.

Thus the essence of the present arrangement lies in the appiication of the noise generator techniques referred to the routine testing of the links to the subscribers' terminals in a way which indicates the presence of faults.

This can give warnings of impending service failure before the degradation in service quality can be detected by users. and without the use of parametric measurement of the electrical characteristics of the links. In an "alldigital network such an arrangement results in a lower combination of equipment and maintenance costs than known arrangements for line testing and maintenance.

Claims

1. A method of remote testing of the digital transmission links between an automatic telecommunication exchange and subscribers' terminal equipment, in which to test a said link a digital bit stream is transmitted from a test control unit at the exchange or connected thereto to the terminal equipment accessible via that link, in which at the terminal equipment the receive and transmit paths of the link are looped together so that the received bit stream is retransmitted back to the test control unit, in which the bit stream received back at the test control unit is modified during said reception by the addition thereto of a noise signal of known amplitude, in which the received bit stream as thus modified is compared with the bit stream as originally sent from the test control unit, and in which the number of discrepancies detected as a result of said comparison is representative of the error rate of the transmission link.

2. A method as claimed in claim 1, in which the tests are repeated with increasing amplitudes of the added noise until the number of errors detected exceeds a preset level above which the link is held to be unserviceable, and in which the level of noise at which said preset error level is exceeded is indicative of the condition of the link.

3. A method as claimed in claim 2, in which the results of said tests are compared with the results obtained when the link was installed, or on the latest occasion on which the link was serviced or tested.

4. A method of remote testing of the digital transmission links between an automatic telecommunication exchange and a subscriber's terminal equipment. substantially as described with reference to the accompanying drawing.

CLAIMS (6 Oct 1981)

5. An automatic telecommunications system, in which the links between the exchange and the subscribers' terminal equipments are tested by the method of claim 1, 2, 3 or 4.

6. An automatic telecommunications exchange system which includes a fully digital telecommunications exchange serving a number of subscribers each of whose terminal equipments is coupled to the exchange by a digital link, in which to test a said digital link a test control unit at or associated with the exchange causes a repetitive digital bit stream to be transmitted from the exchange to the terminal equipment at the end of the link, in which a control signal from the test control unit operates switching means at the terminal equipment which causes the receive and transmit paths to be coupled together so that the received bit stream is retransmitted back to the test control unit, in which the test control unit causes a noise generator to be connected to the receive path at the exchange so that a noise signal of known amplitude is added to the digital signal before it is received at the test control unit, in which a comparator compares the received bit stream as modified by the noise signal with the bit stream as originally sent from the test control unit, and in which the number of discrepancies detected as a result of said comparison is representative of the error rate of the transmission link.

7. A system as claimed in claim 6, in which the test control unit repeats the tests with increasing amplitudes of the added noise signal until the number or errors detected exceeds a preset level above which the link is held to be unserviceable, and in which the level of noise at which said preset error level is indicative of the conditions of the link.

CLAIMS (15 Dec 1981)

8. A method of remote testing of the digital transmission links between an automatic telecommunication exchange and subscribers' terminal equipment, in which to test a said link a digital bit stream is transmitted from a test control unit over that link to a said terminal equipment connected to that link, in which at the terminal equipment the receive and transmit paths of the link are looped together so that the received bit stream is retransmitted via the link to the test control unit, in which the bit stream received back at the test control unit is modified during its said reception by the addition thereto of a noise signal of known amplitude, in which the received bit stream as thus modified is compared with the bit stream as originally sent from the test control unit, in which the number of discrepancies detected as a result of said comparison is representative of the error rate of the transmission link, in which the tests are repeated with increasing amplitudes of the added noise until the number of errors detected exceeds a preset level above which the link is held to be unserviceable, and in which the level of noise at which said preset error level is exceeded is indicative of the condition of the link.

9. A method as claimed in claim 8, in which the results of said tests are compared with the results obtained when the link was installed, or on the latest occasion on which the link was serviced or tested.

1 0. An automatic telecommunications exchange system which includes a fully digital telecommunications exchange serving a number of subscribers each of whose terminal equipments is coupled to the exchange by a digital link, in which to test a said digital link a test control unit causes a repetitive digital bit stream to be transmitted over the link to be tested to the terminal equipment at the end of that link, in which a control signal from the test control unit operates switching means at the terminal equipment which causes the receive and transmit paths to be coupled together so that the received bit stream is retransmitted via that link to the test control unit, in which the test control unit causes a noise generator to be connected to the receive path at the exchange so that a noise signal of known amplitude is added to the digital signal before it is received at the test control unit, in which a comparator compares the received bit stream as modified by the noise signal with the bit stream as originally sent from the test control unit, in which the number of discrepancies detected as a result of said comparison is representative of the error rate of the transmission link, and in which the test control unit repeats the tests with increasing amplitudes of the added noise signal until the number of errors detected exceeds a preset level above which the link is held to be unserviceable, and in which the level of noise at which said preset error level is indicative of the conditions of the link.

11. A system as described in claims 6, 7 or 10, and in which the test control unit is located within the said exchange.