Ring trip detection in a communication system

• US 6,219,417 B1
• Filed: 06/06/1997
• Issued: 04/17/2001
• Est. Priority Date: 06/06/1997
• Status: Expired due to Term

• Current Assignee: Microsemi Semiconductor (U.S.) Inc. (Microchip Technology Incorporated)
• Original Assignee: Advanced Micro Devices, Inc.
• Inventors: Zhou, Yan
• Primary Examiner(s): Isen, Forester W.
• Assistant Examiner(s): Saint-Surin, Jacques M.
• Application Number:
  US08/870,893
• Time in Patent Office: 1,411 Days
• Field of Search: 379/382, 379/412, 379/413, 379/377, 379/373, 379/379, 379/418, 379/252, 379/399, 381/57
• US Class Current: 379/377
• CPC Class Codes: H04M 3/005 Interface circuits for subscriber lines current supply H04M19/00 and subgroups; supervisory, monitoring or testing arrangements H04M3/22 and subgroups; in key telephone systems H04M9/006

Abstract

A line card integrates subscriber line interface circuitry, A/D and D/A converters, and digital signal processing technology. The digital signal processing technology performs many line card tasks such as switch hook detection, ground key detection, DC feed control, polarity reversal, ringing tests, fault detection, power cross detection, and ring trip detection. A line card having a digital signal processor supports both integral and external ringing signal generators. During application of a ringing signal, the line card detects ring trip conditions while discriminating against short duration short circuits. The line card includes programmable features that facilitate adaptation to varying regional ring trip detection and short duration short circuit discrimination specifications and subscriber loop characteristics.

Claims

1. A method comprising the steps of:

sampling a signal at a time, t0, and at a later time, t1;

calculating any change between an attribute of the signal sampled at time t0 and the signal attribute sampled at time t1;

comparing the change between an attribute of the signal sampled at time t0 and the signal attribute sampled at time t1 to a first predetermined threshold value;

if any change in the compared signal attribute exceeds the first predetermined threshold, preventing processing of the signal for at least a predetermined amount of time and

after elapse of the predetermined amount of time and if any change in the compared signal attribute does not exceed the first predetermined threshold, processing the signal with a signal processor to detect a change in an impedance of the transmission line.

2. The method as in claim 1 further comprising the step of:

comparing the processed signal with a second predetermined threshold to detect a minimum degree of change in the transmission line impedance.

3. The method as in claim 2 further comprising the step of:

prior to comparing the processed signal with the second predetermined threshold, determining if the signal is an integral ringing signal generated by a line card connected to the transmission line or an external ringing signal generated external to the line card; and

adjusting the second predetermined threshold according to the determined ringing signal type.

4. The method as in claim 2 wherein the signal attribute is a direct current bias level of the signal, and the step of comparing the processed signal with a second predetermined threshold further comprises the step of:

comparing the processed signal with a second predetermined threshold to detect an impedance change in the transmission line which indicates a switch hook on-hook to off-hook transition.

5. The method as in claim 1 wherein the step of preventing processing of the signal further comprises the steps of:

setting an input signal to a filter to zero.

6. The method as in claim 5 wherein the filter is an infinite impulse response digital filter, and the step of preventing further comprises the steps of:

resetting state variables of the infinite impulse response digital filter to obtain an approximately zero output signal from the infinite impulse response digital filter.

7. The method as in claim 1 wherein the step of sampling the signal comprises the step of:

sampling a signal on a subscriber loop with line card circuitry of a communication system.

8. The method as in claim 1 wherein the step of sampling the signal comprises the step of:

converting the signal from an analog signal to a digital signal.

9. The method as in claim 1 further comprising the step of:

determining if the signal is an integral ringing signal or an external ringing signal;

wherein, if the signal is an integral ringing signal, the step of sampling comprises the steps of;

sampling metallic currents on a transmission line; and

wherein, if the signal is an external ringing signal, the step of sampling comprises the steps of;

sampling a loop current on the transmission line.

10. The method as in claim 1 wherein the predetermined amount of time is an amount of time sufficient to discriminate against a short duration short circuit signal at a transmission line, the short circuit signal having a magnitude greater than the first predetermined threshold value.

11. The method as in claim 1 wherein the short duration short circuit signal endures for a maximum of twelve milliseconds, and the predetermined amount of time is approximately fourteen milliseconds.

12. The method as in claim 1 further comprising the steps of:

executing the steps of claim 1 respectively for a plurality of signals from respective subscriber loops with circuitry disposed on a line card of a communication system.

13. A method of detecting a ring trip in a communication system with subscriber loop short circuit discrimination comprising the steps of:

applying a ringing signal including a direct current bias voltage to a subscriber loop of the communication system;

calculating a current signal change between a current signal sampled at a first time and the current signal sampled at a second time in the subscriber loop;

determining if the calculated current signal change exceeds a first predetermined threshold during a predetermined amount of time;

if the current signal change exceeds the first predetermined threshold, preventing the current signal from being filtered by a low pass ring trip detection filter for a predetermined amount of time; and

after the predetermined amount of time and if the current signal change does not exceed the first predetermined threshold, determining if the current signal change exceeds a second predetermined threshold.

14. The method as in claim 13 wherein the first predetermined threshold corresponds to a current magnitude which is less than a short duration subscriber loop short circuit current change and greater than a ring trip current change during the predetermined amount of time.

15. The method as in claim 14 wherein the first predetermined threshold is 50 mA and the predetermined amount of time is 14 milliseconds.

16. The method as in claim 13 wherein the second predetermined threshold is 1 mA.

17. The method as in claim 13 further comprising the step of:

setting the second predetermined threshold in accordance with a source of the applied ringing voltage.

18. The method as in claim 17 wherein if the source of the applied ringing voltage is a line card of the communication system, the method further comprises the step of:

setting the second predetermined threshold to 10 mA.

19. The method as in claim 17 wherein if the source of the applied ringing voltage is a central office ringing signal generator, the method further comprises the step of:

setting the second predetermined threshold to a value less than a ring trip current change.

20. The method as in claim 13 wherein the low pass ring trip detection filter is a two pole infinite impulse response filter.

21. The method as in claim 13 wherein the step of preventing the detected current signal from reaching a low pass ring trip detection filter comprises the step of:

setting an input signal to the low pass ring trip detection filter to about zero and setting state variables of the low pass ring trip detection filter to obtain about a zero output signal.

22. The method as in claim 13 wherein the steps of calculating a current signal change, determining if the calculated current signal change exceeds a first predetermined threshold, preventing a detected current signal from reaching a low pass ring trip detection filter, and determining if the detected current signal exceeds a second predetermined threshold are implemented as code in a ring trip detection module stored in a memory coupled to a processor, the method further comprising the step of:

executing the ring trip detection module with the processor.

23. A communication system comprising:

circuitry for successively sampling a signal on a subscriber loop at a time, t0, and at a later time, t1;

a line card having a processor and a memory, the memory storing code for execution by the processor, the code including instructions for calculating a change in successive samples of the signal to detect a short circuit of the subscriber loop while generally discriminating against a ring trip of the subscriber loop and for low pass filtering the sampled signal to detect a ring trip unless the short circuit of the subscriber loop is detected.

24. The communication system as in claim 23 further comprising:

terminal equipment having a switch hook in the subscriber loop which when off hook corresponds to a ring trip.

25. The communication system as in claim 23 wherein the code further includes instructions to compare a calculated change in successive samples of the signal with a predetermined threshold to detect the short circuit of the subscriber loop.

26. The communication system as in claim 23 wherein the code further includes instructions to compare an output result of the low pass filter operation with a predetermined threshold to detect a DC bias current change in the subscriber loop indicating the ring trip condition.

27. The communication system as in claim 26 wherein the code further includes instructions to adjust the predetermined threshold in accordance with integral and external ringing signals.

28. A communication system comprising:

a subscriber line interface circuit having input terminals coupled to respective terminal pairs of a subscriber loop;

a loop current sensing circuit coupled to a ring generator terminal and a subscriber loop terminal;

an analog to digital converter circuitry coupled to output terminals of the subscriber line interface circuit and coupled to an output terminal of the loop current sensing circuit;

a processor coupled to the analog to digital converter for receiving subscriber loop signal data; and

a memory coupled to the processor and storing code for execution by the processor, the code including instructions for calculating any change between an attribute of the signal sampled at time t0 and the attribute in the signal sampled at time t1 comparing the calculated change to a first predetermined threshold value, the signal being sampled by (i) the subscriber line interface circuit and analog to digital converter circuit if the ringing signal is an integral ringing signal or (ii) the loop current sensing circuit if the ringing signal is an external ringing signal, and, if any change in the compared signal attribute exceeds the first predetermined threshold, preventing processing of the signal for at least a predetermined amount of time;

terminal equipment coupled to the subscriber loop, wherein the code further included instructions for digitally filtering the signal after the predetermined amount of time if the signal attribute does not exceed the first predetermined threshold and comparing output data from the digital filter with a second predetermined threshold to detect a ring trip on the subscriber loop.

29. The communication system of claim 28, further comprising:

a plurality of subscriber line interface circuits having respective input terminals coupled to respective subscriber loop terminal pairs;

a plurality of loop current sensing circuits coupled to respective ring generator terminals and to respective subscriber loop terminals, wherein the respective pluralities of subscriber line interface circuits and loop current sensing circuits are coupled to the analog to digital converter circuitry.

30. The communication system of claim 28 wherein the subscriber line interface circuit, loop current sensing circuit, analog to digital converter circuitry, processor, and memory are integrated on a line card.

31. The communication system of claim 29 wherein the code further includes instructions for scaling the second predetermined threshold to detect a ring trip on the subscriber loop and not detect subscriber loop current changes attributable to non-ring trip events.