GB2105555A - Improvements in or relating to monitoring the margin of a digital transmission system - Google Patents

Abstract

A device 3 for monitoring the margin of a digital transmission system i.e. the capacity of the system to accommodate signal degradation, includes a data detector 4 connected in parallel with a system data detector 1. The sensitivity of detector 4 is varied by way of a D-A converter 5 which provides a signal which adjusts the detector threshold level or adds an offset to the detector input. The outputs of the two detectors are compared in exclusive-OR 8 to derive a signal indicative of the difference in the error rates of the two detectors. This signal is processed in control means 10 in such a manner as to make the error difference rate converge to a preset value which depends on the maximum acceptable error rate for satisfactory system operation. The amount by which the sensitivity of detector 4 has been adjusted is latched at 12 and provides a measure of the system margin. <IMAGE>

Description

SPECIFICATION Improvements in or relating to monitoring the margin of a digital transmission system The present invention relates to monitoring the margin of a digital transmission system.

In the present specification the term system margin is used to indicate a measure of the capacity of the system to accommodate degradation in the signal. For example, the system margin would allow the maximum extra noise that could be tolerated in the system before the error rate exceeds the predetermined maximum for effective performance to be assessed.

Current transmission system monitoring is primarily carried out by observing the error rate.

This gives an indication when the system is approaching failure, but gives no indication of the margin of a system with a low error rate.

A method of assessing the system margin is to consider the "eye opening" of received pulses as displayed on an oscilloscope. The eye opening is reduced in width by bandwidth limiations of the system and in height or amplitude by the presence of noise in the system.

Several parameters contribute to eye closure or a reduction in system margin, for example impulsive noise, radio pick up, crosstalk and thermal noise, intersymbol interference due to mis-equalisation and jitter introduced by previous regenerators along the line.

According to the present invention there is provided a device for monitoring the margin of a digital transmission system comprising a detector for connection in parallel with the system detector, means for producing a digital signal indicative of the difference in error rates of the two detectors, a feedback circuit responsive to said digital signal for adjusting the sensitivity of the device detector such that said difference signal is maintained substantially constant, and means for outputting an indication of the margin of the system according to said sensitivity adjustment, said feedback circuit including a control circuit for digitally processing to digital signal and arranged when said digital signal differs from a preset value, to produce a digital control signal for adjusting said sensitivity, said control signal being applied to said device detector via a digital to analogue conversion means.

The device of the present invention may be incorporated into a measuring device to be selectively attached in an individual line to provide a readout of the system margin associated with that line or may be permanently connected to continuously monitor a line in an operating transmission system. In either mode of operation it allows a rapid assessment of the system margin without interfering with the normal operation of the line. The device is capable of detecting degradation faults on a line before they become service affecting.

The invention also provides a method of monitoring the margin of a digital transmission system, characterized in that it comprises the steps of applying a signal to a detector arranged in parallel with a system detector, varying the sensitivity of the parallel detector until an error rate of the output thereof defined relative to the output of the system detector reaches a preset value, and outputting a value indicative of the sensitivity reduction of the parallel detector relative to the system detector to provide a measure of system margin.

The invention will be described now by way of example only with reference to the accompanying drawing which is a block diagram of a device for monitoring the margin of a digital transmission system.

The drawing shows a detector 1 which is part of a system receiver or regenerator. An input line 2 to the detector 1 carries a received digital signal which has passed through the amplification, automatic gain control and equalisation stages of the receiver or regenerator. The stages prior to detector 1 may vary according to the type of system involved but preceding states should provide an input signal having a constant peak amplitude.

In the present embodiment the detector 1 is a threshold detector which produces regenerated data signals. The threshold at which the system detector output changes state is fixed.

The device 3 for monitoring the system margin comprises a further threshold detector 4 connected in parallel with the system detector 1.

The detectors 1 and 4 are substantially identical but the detector 4 is arranged so that its sensitivity can be varied. The sensitivity of the detector 4 is varied under the control of a signal from a D-A converter 5 which varies the threshold level of detector 4. Alternatively the D-A converter 5 could be arranged to add an offset to the incoming signal.

The outputs 6 and 7 of the two detectors are compared in an exclusive OR gate 8 to derive a digital signal which represents the difference between the error rate of the detectors 1 and 4.

This difference is accumulated in counter 9. A control circuit 10 looks at the count of the counter 9 after a fixed integration period and compares it with a preset value. A signal dependent on the difference between the measured value and the preset value is fed along line 11 to the D-A converter 5 to provide further adjustment of the threshold of detector 4 to make the count of counter 9 converge to the preset value. If a low value is counted, the control circuit 10 generates a signal to cause the D-A converter output to increase and this moves the threshold of the detector 4 so that subsequent measurements of the error rate are made with a reduced effective eye opening in detector 4, hence increasing the subsequent count.If the measured value is too high the D-A converter 5 output is reduced increasing the eye opening of detector 4 and reducing the error rate count. If the count is within a specified range of the preset value the error rate is considered substantially equal to the preset value and the digital word sent to the D-A converter, which is representative of the level at which the detector threshold was set to achieve the preset error rate, is latched by device 1 2. It will be appreciated that if the error rate of the detector 1 varies an adjustment of the sensitivity of the detector 4 will be required to maintain the preset value. Thus the output of the circuit 10 which is indicative of the sensitivity level of the detector 4 can be used to provide an indication of the system margin. This signal is available at 1 3.

The D-A converter 5 receives an 8 bit input from line 11 and gives a resolution of 0.5%. A fixed output from the D-A converter 5 will displace the decision threshold of detector 4 in one direction only. A balanced measurement can be made by alternating the shift equally to both sides of the threshold. This can be achieved by driving the D-A converter 5 with a square wave reference from a source 14, or by complementing the D-A coefficient signal in the control circuit 10 and supplying the converter alternately with the normal and complemented values.

The device 3 may be implemented using a single chip microprocessor 1 5 (e.g. 8748) for the combined error counter 9, control circuit 10 and latch device 12 together with a single chip D-A converter 5 and duplication of the detection circuit 1 of the system for detector 4.

The preset value may be selected to correspond to the maximum acceptable error rate for satisfactory system operation. However, in cases where the transmission rate is low and the maximum acceptable error rate is relatively low it may take an unacceptably long sampling time to count such a preset value of errors. In such cases the preset value is chosen to correspond to a high error rate than is acceptable so that a sufficient number of errors may occur within a reasonable sampling time. This allows a fairly accurate determination of the true error rate for a particular threshold setting.

For example for a system transmitting at 80kbits/sec the error rate may be sampled over a period of half a second. The preset value may be selected as 40 errors in the sampling time which corresponds to an error rate of 10-3. Such an arrangement may provide an updated output of the system margin every four seconds. The actual permissible error rate for the system may in fact be 10-7 but for a preset value of 40 a sampling time of the order of 10 hours would be required which is clearly unacceptable. By making the assumption that the noise is Gaussian, the noise margin of a system at an error rate of 10-7 can be calculated given the noise margin at an error rate of 10-3.

Higher speed transmission systems could have the preset value set nearer to the system threshold without incurring excessive sampling and update times. For example a 2.048 Mbit/s system with a permissible system error rate of 10-7 could use a preset value of 100 errors in 0.5 seconds corresponding to a threshold error rate of 10-4.

Clearly the error threshold depends on various factors in particular the type of information being transmitted, for example more errors can be tolerated in speech in digital format than in data.

Claims (7)

Claims

1. A device for monitoring the margin of a digital transmission system comprising a detector for connection in parallel with the system detector, means for producing a digital signal indicative of the difference in error rates of the two detectors, a feedback circuit responsive to said digital signal for adjusting the sensitivity of the device detector such that said difference signal is maintained substantially constant, and means for outputting an indication of the margin of the system according to said sensitivity adjustment, said feedback circuit including a control circuit for digitally processing the digital signal and arranged, when said digital signal differs from a preset value, to produce a digital control signal for adjusting said sensitivity, said control signal being applied to said device detector via a digital to analogue conversion means.

2. A device as claimed in claim 1 wherein the detector is a threshold detector and the digital to analogue conversion means is arranged to vary the detector threshold.

3. A device as claimed in claim 1 wherein the digital to analogue conversion means is arranged to apply a variable offset to the incoming signal.

4. A device as claimed in claim 2 wherein conversion means varies the threshold of the device detector equally to both sides of the threshold of the system detector.

5. A device as claimed in any one of the preceding claims wherein the means for producing the difference signal includes a counter and the outputting means comprises a latch device operable when the count is substantially equal to the preset value to latch the signal to be applied to the digital to analogue conversion means.

6. A device as claimed in claim 5 wherein the counter, control circuit and latch device are formed as a single integrated circuit.

7. A device for monitoring the margin of a digital transmission system substantially as herein described with reference to the accompanying drawing.