A method and system for detecting a connection of a pcm phone system

Abstract

The application discloses a kind of parallel machine detection method and system for pcm telephone system, it is related to communication security technical field, comprising the following steps: S1, when telephone is first opened, collect the reference pressure difference between A-B line in hands-free state and off-hook state, and the reference pressure difference is stored as health profile data, constructs health profile;S2, actual pressure difference between A-B line is continuously detected in the process of talking, sampling frequency cannot be lower than 1 time per second, and real-time dynamic monitoring is carried out to pressure difference value.The parallel machine detection method and system for pcm telephone system proposed in the application, by constructing health profile when telephone is first opened, accurate reference is provided for subsequent detection, effectively distinguishes normal communication state and illegal parallel machine eavesdropping behavior, in the process of talking, system real-time dynamic monitoring pressure difference between A-B line, in combination with line abnormality detection, can accurately identify and shield the misjudgment caused by fault, improve the reliability of detection.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of communication security technology, in particular to a method and system for detecting parallel connection of PCM telephone system. BACKGROUND

[0002] As the key infrastructure for realizing voice digital transmission, the communication security of PCM telephone system is directly related to the confidentiality and reliability of information transmission. With the continuous improvement of information security demand, the detection of parallel connection and eavesdropping behavior for telephone line has become an important part of the communication security protection system. Although the traditional parallel connection detection technology can identify abnormal access to a certain extent, its detection mechanism still faces many challenges in terms of integrity and anti-interference ability in complex communication environment, and it is urgent to improve the accuracy and reliability of detection through technological innovation.

[0003] In the prior art, some parallel connection detection methods have the problems of single detection logic and insufficient anti-line interference ability, which makes it difficult to accurately distinguish between fault state and parallel connection eavesdropping behavior when line faults such as short circuit and contact occur, and is prone to misjudgment or omission. At the same time, some schemes lack a linkage verification mechanism for line resistance and capacitance characteristics, and it is difficult to comprehensively confirm the parallel connection state by relying only on differential pressure monitoring, and the real-time performance and stability in dynamic communication environment need to be strengthened. In addition, the existing system is not perfect in the construction and dynamic updating mechanism of health records, which makes it difficult to adapt to the personalized operation state of different telephone equipment, affecting the accuracy of the detection result. In view of this, we propose a method and system for detecting parallel connection of PCM telephone system. SUMMARY

[0004] To solve the above technical problems, a method and system for detecting parallel connection of PCM telephone system are provided, which solves the problems of weak anti-line interference ability, difficulty in distinguishing between line fault and parallel connection eavesdropping, easy misjudgment and omission, lack of linkage verification of line resistance and capacitance characteristics, difficulty in comprehensively confirming the parallel connection state by relying only on differential pressure monitoring, imperfect health record construction and updating mechanism, difficulty in adapting to the personalized operation of equipment, and affecting the detection accuracy.

[0005] To achieve the above purposes, the technical scheme adopted by the present application is as follows:

[0006] A method for detecting parallel connection of PCM telephone system, comprising the following steps:

[0007] S1, when the telephone is first opened, the reference differential pressure value between A-B lines in the hands-free state and the off-hook state is collected, and the reference differential pressure value is stored as health record data to build a health record;

[0008] S2, continuously detect the actual differential pressure value between A-B lines during the call, and the sampling frequency should not be lower than 1 times per second, and the differential pressure value is monitored in real time and dynamically;

[0009] S3, judging the parallel state based on the real-time dynamic monitoring data, and judging in real time whether the parallel eavesdropping behavior exists according to the threshold range of the actual pressure difference value deviating from the reference pressure difference value;

[0010] S4, synchronously performing line abnormality detection in the pressure difference monitoring process, and the abnormality detection includes short circuit, contact electric detection and other mixed faults, and the parallel judgment result is shielded if the line fault exists;

[0011] S5, when the parallel pre-warning is triggered, the resistance-capacitance value of the telephone set is measured to perform resistance-capacitance verification when the user hangs up, and the reference resistance-capacitance value in the health record is compared, and if the difference exceeds a preset threshold, the parallel alarm is confirmed.

[0012] Preferably, the S1 step of constructing the health record specifically includes:

[0013] In the initial registration stage of the telephone set, the telephone set is controlled to enter the hands-free state and the off-hook state in sequence;

[0014] The hands-free reference pressure difference value and the off-hook reference pressure difference value associated with the telephone set identifier are stored as the health record;

[0015] The health record is synchronized to the gateway server database through the SNMP protocol.

[0016] Preferably, in the real-time dynamic monitoring of the S2 step, a differential amplification circuit is used to collect the analog voltage signal between A-B lines; a 12-bit ADC is used to convert the digital signal at a sampling rate of 1 kHz; a sliding window filtering algorithm is used to process noise, the window width is set to 10 sampling points, and the filtered pressure difference effective value is extracted once per second for parallel judgment.

[0017] Preferably, the S3 step of judging the parallel state specifically includes:

[0018] Setting a dynamic offset threshold ;

[0019] Calculating the absolute deviation of the current pressure difference sampling value and the health record value :

[0020]

[0021] If the condition is met for 3 times in a row , the parallel pre-warning is triggered;

[0022] The pre-warning signal is uploaded to the network management alarm station in real time through the Trap message.

[0023] Preferably, in the line abnormality detection of the S4 step, the parallel judgment logic includes short circuit detection, contact electric detection and other mixed detection.

[0024] The short circuit detection method is: if the first valid pressure difference sampling value after off-hook meets , then it is determined as a short circuit fault;

[0025] The collision detection method is: when the A-line voltage sampling value is rounded, it meets , and the longitudinal current sampling value is rounded, it meets , then it is determined as a collision fault;

[0026] The other mixing detection method is: when the A-line voltage sampling value is rounded, it meets , and the longitudinal current sampling value is rounded, it meets , then it is determined as an other mixing fault;

[0027] When any of the above faults is triggered, the on-hook determination process is interrupted and a line abnormal event is reported.

[0028] Preferably, the S5 step of resistance-capacitance verification further comprises:

[0029] A 1kHz sine test signal is injected to the telephone line after on-hook, and the line impedance and capacitance components are measured by four-wire method;

[0030] The equivalent capacitance value of the telephone is calculated, and the reference capacitance value in the health profile is retrieved;

[0031] If the difference between the equivalent capacitance value and the reference capacitance value is greater than a preset capacitance value, an on-hook confirmation alarm is generated.

[0032] An on-hook detection system for a PCM telephone system, for implementing the on-hook detection method for the PCM telephone system, comprising:

[0033] A health profile construction module configured in the telephone opening stage, for collecting A-B line reference pressure difference values in the hands-free and off-hook states, and storing the data to a non-volatile memory;

[0034] A dynamic monitoring module containing a voltage sensor and a microprocessor, for collecting A-B line actual pressure difference values at a frequency of more than 1 per second during the call;

[0035] An on-hook determination module with a comparator algorithm, for calculating the offset of the actual pressure difference value and the reference pressure difference value, and generating an on-hook early warning signal when the offset exceeds a preset threshold;

[0036] A line fault diagnosis module, for real-time analyzing A-line voltage and longitudinal current data, and disabling the on-hook determination module output if a line fault is diagnosed;

[0037] The resistance-capacitance verification module measures the resistance-capacitance value of the telephone set in response to the on-hook event and compares the value with the health record, and outputs the on-hook alarm confirmation signal to the network management interface.

[0038] Preferably, the dynamic monitoring module hardware component comprises:

[0039] The high-precision instrument amplifier has an input impedance of >10MΩ and a common-mode rejection ratio of ≥120dB.

[0040] The Σ-Δ type ADC has a resolution of 16 bits and a sampling rate of 10kSPS.

[0041] The FPGA preprocessing unit implements the FIR digital filter and the peak detection algorithm.

[0042] The SPI interface transmits the processed differential pressure data to the main control MCU.

[0043] Preferably, the line fault diagnosis module comprises:

[0044] The multiplexer switches the collected A-line-to-ground voltage, B-line-to-ground voltage and AB-line-to-ground voltage.

[0045] The current sensor is connected in series on the line ground end to measure the longitudinal current.

[0046] The fault analysis unit performs the following judgments:

[0047] When the AB-line resistance value is <2kΩ, the short-circuit flag is marked

[0048] When the absolute value of the A-line-to-ground voltage is >45V, the electric shock flag is marked

[0049] When the AB-line capacitance change rate is >±20%, the mixed flag is marked.

[0050] The flag register is output to the AND gate circuit to control the enable end of the on-hook judgment module.

[0051] Preferably, the operation process of the resistance-capacitance verification module is:

[0052] In response to the interrupt signal of the on-hook detection circuit, the relay is switched to the resistance-capacitance test circuit, and the complex impedance is calculated by using the vector voltage and current method.

[0053] The imaginary part of the capacitance component is extracted, and the health record data is obtained through the I²C interface access EEPROM.

[0054] Compared with the prior art, the present application has the following beneficial effects:

[0055] The method and system for detecting parallel machine of the PCM telephone system provided by the application, by constructing a health record when the telephone is first opened, provide accurate reference for subsequent detection, effectively distinguish normal communication state from illegal parallel machine eavesdropping behavior, in the process of communication, the system dynamically monitors the pressure difference between A-B lines in real time, combined with line abnormality detection, can accurately identify and shield false positives caused by short circuit, short circuit, etc. Faults, improve the reliability of detection, through the verification of resistance and capacitance when the user hangs up, further confirm the parallel machine state, reduce false positives and false negatives, enhance the anti-interference ability of the system, through the cooperative monitoring and verification of multiple stages and multiple parameters, a comprehensive communication security protection system is constructed, ensuring the confidentiality and reliability of the PCM telephone system communication, providing a strong guarantee for communication security. BRIEF DESCRIPTION OF DRAWINGS

[0056] Figure 1 The method flowchart of the application;

[0057] Figure 2 The system module diagram of the application. DETAILED DESCRIPTION

[0058] The following description is used to disclose the application so that those skilled in the art can implement the application. The preferred embodiments in the following description are only as examples, and other obvious modifications can be thought of by those skilled in the art.

[0059] Reference Figure 1 As shown in the figure, a method for detecting parallel machine of PCM telephone system, the core of which is to accurately identify illegal parallel machine eavesdropping behavior through multi-stage, multi-parameter cooperative monitoring and verification.

[0060] The method first establishes a baseline health record in the initial stage of telephone opening, which is used as a reference standard for subsequent detection. Specifically, in the telephone completes physical installation and first power-on registration stage, the system will actively control the telephone to enter the speakerphone standby state and the simulated off-hook call state in turn.

[0061] In the speakerphone state, a high-precision voltage sampling circuit is used to measure the steady-state DC voltage difference between A and B lines when the telephone is not off-hook but powered on. The voltage difference in this state mainly reflects the standby power consumption of the telephone and the basic characteristics of the line.

[0062] Then, the system simulates the user's off-hook action, and measures the voltage difference between A and B lines again in the off-hook state. At this time, due to the connection of the telephone DC loop, the current increases, and the voltage difference will be significantly different from the speakerphone state. The system takes the voltage difference values in these two key states, i.e. the speakerphone baseline voltage difference and the off-hook baseline voltage difference, as the "fingerprint" characteristics of the telephone.

[0063] These two baseline values are strongly associated with the unique identifier of the phone (such as MAC address, physical port number or logical number) and packaged into an initial health profile data packet. This data packet is not only stored in the local phone or access device, but also immediately synchronized to the upper gateway server or network management system central database through the standard SNMP (Simple Network Management Protocol) protocol in a secure transmission manner for persistent storage.

[0064] This centralized storage ensures the reliability and manageability of the data, so that even if a single device fails, the health profile will not be lost, and it is convenient for network management personnel to centrally query and audit. The establishment of the health profile is the cornerstone of the entire detection process, providing an accurate reference for subsequent real-time comparison.

[0065] After the phone enters the normal call stage, the system starts the dynamic pressure difference monitoring process. This process requires continuous detection of the actual voltage difference between A-B lines, with a sampling frequency strictly set to no less than 1 per second to ensure real-time monitoring.

[0066] To achieve high-precision analog signal acquisition, a differential instrument amplifier with extremely high input impedance (greater than 10MΩ) and excellent common-mode rejection ratio (≥120dB) is used on the hardware. This amplifier can effectively suppress common-mode interference (such as 50Hz power frequency interference) and only amplify the small differential voltage signal between A-B lines. The collected analog voltage signal is then sent to a high-resolution, high-speed analog-to-digital converter (ADC) for digitization. The original text specifies the use of a 12-bit ADC, but in actual high-performance implementation, a Σ-Δ type ADC with higher sampling rate (e.g. 10kSPS) and better resolution (16-bit) is often chosen to achieve higher signal-to-noise ratio and more accurate digital representation.

[0067] The ADC converts the continuous analog voltage difference signal into a sequence of discrete digital sampling points at a sampling rate of 1kHz. Due to the complex line environment, there are random noise and transient interference, and directly using the original sampling value for judgment can easily lead to false positives. Therefore, a signal processing link is introduced in the digital domain. A sliding window filtering algorithm is used to smooth the continuous ADC sampling points.

[0068] The algorithm maintains a fixed-length data window (e.g. set to contain the last 10 sampling points), and slides forward one bit each time a new sampling point arrives, discarding the oldest point and adding the newest one. Mathematical operations (such as calculating the average, median or root mean square value) are performed on the 10 points in the window to generate a filtered effective value. This window filtering process continues, but in order to reduce processing burden and meet the frequency requirement of the decision, the system does not process 1000 points per second (corresponding to a 1 kHz sampling rate), but only extracts the final effective pressure difference value from this filtered data stream once per second (i.e. outputs one representative pressure difference value that has been sufficiently smoothed out per second) for subsequent parallel state determination. This design optimizes processing efficiency while ensuring data effectiveness.

[0069] Based on the effective pressure difference value obtained every second through real-time dynamic monitoring, the system performs parallel state determination, which is the core link of detecting illegal eavesdropping.

[0070] The core logic of the determination is to calculate the absolute deviation ( ) between the current actual pressure difference sampling value ( ) and the reference pressure difference value ( ) stored in the health record under the corresponding state (hands-free or on-hook).

[0071] The selection of this threshold is crucial, and needs to balance sensitivity and anti-interference ability. If the threshold is set too small, normal line fluctuations or slight changes in the device itself may trigger false alarms; if it is set too large, it may miss hidden eavesdropping devices. The typical value of 0.5V is an empirical value based on a large number of experiments and the characteristics of PCM telephone systems, which can effectively distinguish between normal fluctuations and pressure drops caused by abnormal access. The determination is not based on a single sampling deviation, but a continuous triggering mechanism is introduced to resist transient interference: only when the absolute deviation of the current pressure difference sampling value continuously exceeds the dynamic offset threshold ( ) for three times (i.e. for three seconds in a row), will the system confirm the triggering of the parallel pre-warning. This "three times in a row" determination strategy greatly improves the noise resistance and reliability of the system, avoiding false actions caused by occasional interference (such as switch power supply noise, lightning induction, etc.).

[0072] Once the triggering condition is met, the system will immediately generate a parallel pre-warning event. In order to ensure that the alarm information can be delivered to the network management personnel in time, the pre-warning signal is packaged into a standard SNMP Trap message, which is uploaded to the alarm console of the network management system in real time through the management network. The Trap message contains key information such as the telephone identifier that triggered the alarm, the detected pressure difference value, the reference pressure difference value, the deviation amount and the timestamp, etc., which facilitates the network management personnel to quickly locate and handle.

[0073] While the differential voltage monitoring and parallel machine determination are being performed, the system also synchronously performs the vital line abnormality detection. This is because the line itself failure (such as short circuit, contact electricity, and other mixed) will cause the A-B line differential voltage to change dramatically or abnormally, which will be misjudged as parallel machine eavesdropping, resulting in serious false positives. Therefore, the line fault diagnosis module operates independently, and its judgment logic is parallel to the parallel machine determination. The module analyzes more dimensional data collected from the line in real time, mainly including A-line-to-ground voltage, B-line-to-ground voltage, AB-line voltage, and longitudinal (usually refers to ground) current (Ia, Ib, Iab) ). Its core contains three kinds of fault detection logic: short circuit detection, contact electricity detection, and other mixed detection. Short circuit failure refers to the existence of an abnormal low resistance path between A and B lines. The detection method focuses on the moment of lifting the phone: when the phone is lifted, the system captures the first valid AB-line differential voltage sample value. In the normal lifting state, due to the internal resistance of the phone, the differential voltage will jump from the standby high voltage (such as -48V) to a stable lower differential voltage (such as a few volts to tens of volts, depending on the phone model and line length). If the absolute deviation of the first valid differential voltage sample value after lifting the phone ( ) and the lifting reference differential voltage value in the health record ( ) exceeds a larger threshold (such as 3V), it is highly likely that a serious short circuit has occurred between A and B lines, resulting in a differential voltage much lower than the normal lifting value (close to 0V) or abnormally high (if the short circuit point has a special voltage). The system will immediately mark the short circuit failure flag. Contact electricity failure refers to the accidental contact of the phone line (especially the A line, i.e., the power line) with an external abnormal high voltage source (such as the live wire of the mains).

[0074] The detection method is based on measuring the A-line-to-ground voltage (Va) and the longitudinal current (Ia) ). When the system detects that the A-line-to-ground voltage sample value (after appropriate filtering and rounding processing) is greater than a dangerous threshold (such as 10V, much higher than the normal range), and at the same time, the measured longitudinal current (after rounding) exceeds the safety limit (such as 15mA), it is determined that a contact electricity failure has occurred.

[0075] This combination of high voltage and large current is a typical feature of contact electricity. Other mixed failure refers to the error connection of the telephone line with other lines (usually adjacent lines). This fault will cause the AC impedance characteristics of the line, especially the capacitance, to change significantly. The detection method is achieved by monitoring the change of AB-line capacitance.

[0076] The system continuously calculates the relative change rate of the currently measured equivalent capacitance value between the A and B lines and the reference capacitance value stored in the health profile. If the change rate exceeds the pre-set reasonable range (such as ±20%), it is determined to be a mixed fault. Once the line fault diagnosis module detects any of the above faults (short circuit, contact electric shock, mixed), it will immediately generate a high-priority line abnormal event, and interrupt (mask) and machine determination module output through hardware or software logic. This means that as long as the line has these clear faults, even if the differential pressure monitoring triggers the parallel machine warning condition, the system will not generate a parallel machine warning, but will prioritize the line fault report. This design effectively prevents line faults from interfering with parallel machine detection, ensuring the accuracy of the alarm.

[0077] When the differential pressure monitoring continuously triggers the parallel machine warning (i.e. meets the condition of δV>ΔV for three times in a row), the system does not immediately confirm the parallel machine event, but enters a critical resistance and capacitance verification phase. This phase is automatically triggered when the user hangs up the phone. The hang-up detection circuit (usually by detecting the status of the fork spring switch or the sudden change of line current) will generate an interrupt signal. The system responds to the interrupt signal, first controls the relay or electronic switch to switch the phone line from the normal call circuit to the dedicated resistance and capacitance test circuit. The core goal of the resistance and capacitance verification module is to accurately measure the equivalent impedance of the phone in the hung-up state, especially its capacitive reactance component, because the parallel tapping device usually significantly changes the impedance characteristics of the original phone. In order to obtain high-precision measurement results that are not affected by line resistance, the system uses a four-wire method (Kelvin connection method) or a more advanced vector voltage and current method. This method injects a sine wave test signal of a specific frequency (such as 1 kHz) into the phone line. The voltage signal loaded on both ends of the phone is accurately measured through a pair of high-impedance detection lines, while the current signal flowing into the phone is accurately measured through a pair of precise current sampling resistors or current sensors in series. Since the tapping device is mostly capacitive load, the system pays special attention to the imaginary part of the impedance (i.e. capacitive reactance part). By measuring the amplitude ratio and phase difference between the injected voltage and the measured current, the complex impedance can be calculated. The standard formula for calculating complex impedance is:

[0078]

[0079] where, Z is the complex impedance, V is the injected test voltage signal, I is the measured current signal, j is the imaginary unit, φ is the phase difference between the voltage and current signals, ejφ is the complex exponential form, representing the phase rotation;

[0080] After obtaining the complex impedance Z, the imaginary part is extracted. Since the capacitive reactance is a negative imaginary part, the equivalent ground capacitance value (C) of the phone line can be deduced. The calculation formula is:

[0081]

[0082] wherein, C is the capacitance value to be solved, f is the frequency of the injected signal, is the imaginary part of the complex impedance ; and

[0083] The system accesses the local non-volatile memory (such as EEPROM) or remote database through the I2C or other serial bus interface to retrieve the reference on-hook resistance-capacitance value, especially the reference capacitance value, stored in the phone health profile. The equivalent capacitance value calculated by real-time measurement is compared with the reference value, and the absolute difference or relative change is calculated.

[0084] The system presets a capacitance difference threshold value (for example, several nanofarads to several tens of nanofarads, which is set according to the phone model and environment). If the difference between the equivalent capacitance value measured and the reference capacitance value in the health profile exceeds the preset threshold value, it is finally confirmed that there is an illegal on-hook eavesdropping behavior, and a highest-level on-hook confirmation alarm signal is generated. The confirmation signal is also reported to the network management system through the network management interface (such as SNMP). The resistance-capacitance verification is performed in the on-hook state, avoiding the interference of the call signal, and directly measuring the impedance characteristics of the phone body, which is a key and highly reliable secondary verification of the pressure difference monitoring and early warning, greatly reducing the false alarm rate of the entire system.

[0085] In order to effectively support the implementation of the above method, a dedicated on-hook detection system is designed.

[0086] Referring to Figure 2 , an on-hook detection system for a PCM phone system adopts modular design, and each functional module works cooperatively.

[0087] The health profile construction module is the core of system initialization, deployed in the handset opening configuration stage. It contains a state control unit, a precision voltage measurement unit and a data storage management unit. The state control unit accurately controls the switching timing of the handset between the hands-free and on-hook states to ensure that the measurement is performed in a stable state. The precision voltage measurement unit uses the aforementioned high-performance differential amplifier and ADC to ensure that the reference voltage difference value collected is accurate and reliable. The data storage management unit is responsible for binding the collected hands-free reference voltage difference, on-hook reference voltage difference and the unique identifier of the handset (such as the serial number, logical port number), and writing it into the local non-volatile memory (such as Flash or EEPROM), and at the same time uploading the health profile data packet to the central server database through the integrated network communication unit (supporting SNMP protocol stack) for backup and centralized management.

[0088] The dynamic monitoring module is the core perception unit of the system that continuously works during the call. Its hardware components include a high-precision instrument amplifier responsible for collecting weak differential voltage signals, with key parameters such as high input impedance (>10MΩ) and excellent common-mode rejection ratio (≥120dB) to ensure the accuracy of signal acquisition; a Σ-Δ type ADC (such as 16-bit resolution, 10kSPS sampling rate) responsible for high-speed and high-precision digitization; and an FPGA (Field Programmable Gate Array) unit responsible for real-time signal preprocessing. The FPGA internally implements an efficient FIR (Finite Impulse Response) digital filter that performs real-time filtering and noise reduction on the raw data output by the ADC, and may run peak detection or effective value calculation algorithms, ultimately outputting a processed and stable A-B line actual voltage difference value every second. The processed data is transmitted to the host MCU (Microcontroller Unit) through high-speed SPI (Serial Peripheral Interface) for subsequent analysis.

[0089] The parallel machine determination module runs on the host MCU or in a dedicated digital logic circuit. It has a built-in core comparator algorithm that continuously receives the voltage difference effective value per second from the dynamic monitoring module. The algorithm calculates the absolute deviation of the current actual voltage difference value from the reference voltage difference value stored in the health profile under the current state (hands-free or on-hook). The module maintains a state machine or counter to implement the "three consecutive times δV> dynamic offset threshold ΔV (such as 0.5V)" determination logic. Once the condition is met, the module generates a parallel machine warning signal. The signal is usually a digital level jump or a software event flag, and is reported through a network interface (such as an Ethernet PHY chip combined with a TCP / IP protocol stack or directly an SNMP Trap).

[0090] The line fault diagnosis module is a separate and parallel running safeguard unit, aiming to identify and exclude the interference of line faults to the parallel operation decision. Its hardware basis includes a multiplexer (MUX) for periodically or event-triggered switching of the voltage collected at different points under the control of the microprocessor: A-line-to-ground voltage (Va), B-line-to-ground voltage (Vb) and AB-line-to-line voltage (Vdiff).

[0091] In addition, a precision current sensor (such as a Hall effect sensor or a precision sampling resistor combined with an amplifier) in series on the line ground path is responsible for measuring the longitudinal current. The collected multi-channel voltage and current signals are sent to the ADC for digitization. The fault analysis unit (which can be implemented in the MCU or special logic) executes the preset fault detection algorithm: continuously calculates the AB-line resistance, if the resistance value is lower than an extremely low threshold (such as <2kΩ), marks the short circuit flag; real-time monitors the absolute value of A-line-to-ground voltage (Va), if it exceeds a dangerous high voltage threshold (such as >45V), marks the electric shock flag; continuously calculates the AB-line capacitance (which can be measured by AC impedance measurement or special capacitance measurement circuit), and calculates its change rate relative to the reference capacitance value in the health profile, if the change rate exceeds the preset range (such as ±20%), marks the mixed flag.

[0092] These fault flags (short circuit, electric shock, mixed) are written into a flag register. The output of the register (usually a logical "or" relationship, any fault set will make the total fault flag valid) is connected to one input of an AND gate circuit. The other input of the AND gate circuit receives the pre-warning enable signal from the parallel operation decision module. Only when the total fault flag is invalid (i.e. no line fault), the AND gate will allow the pre-warning signal of the parallel operation decision module to pass through; once any line fault is detected, the AND gate immediately blocks the pre-warning signal output, while the system reports the specific line abnormal event. This hardware or firmware level interlocking mechanism ensures that no parallel operation false alarm will occur when the line fault is known.

[0093] The RC verification module is the final confirmation step in the parallel alarm process, operating after an on-hook event is triggered. Its operation is highly automated: an interrupt signal generated by the on-hook detection circuit (e.g., monitoring the fork spring status or line current drop) wakes the module. The module first controls a relay or solid-state switch to switch the phone line from the call bus to a dedicated precision RC test circuit. This circuit includes a highly stable sine wave signal generator (typically 1kHz) and a high-precision four-wire measurement unit. The test signal is applied to the phone line through a pair of "Force" lines. Simultaneously, a pair of independent "Sense" lines accurately measure the actual voltage applied to the phone terminals. A precision current detection circuit (e.g., an IV converter circuit composed of a low-drift operational amplifier) ​​connected in series in the test signal loop measures the current flowing into the phone. At the core of the measurement unit is a circuit or digital signal processing (DSP) unit with phase detection capability. It accurately measures the amplitude ratio and phase difference between the test voltage and test current.

[0094] For determining whether a phone is in parallel operation (which typically introduces an additional capacitive load), the most crucial step is extracting the imaginary part. This module accesses the EEPROM storing the health profile via I²C or another interface to read the baseline on-hook capacitance value measured when the phone is in a healthy state. The equivalent capacitance value calculated in real-time is compared to the baseline value. If the absolute difference exceeds a preset confirmation threshold, the module generates a final parallel operation confirmation alarm signal.

[0095] The confirmation signal is reported to the network management system via a network management interface (such as SNMP Set / Response or a dedicated alarm interface), providing the most conclusive evidence of parallel eavesdropping and completing the entire detection and confirmation process. This multi-detection mechanism, combining dynamic differential pressure monitoring, line fault troubleshooting, and on-hook impedance verification, constitutes a powerful technical barrier against parallel eavesdropping in PCM telephone systems.

[0096] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the claimed invention. The scope of protection claimed by the appended claims and their equivalents is defined.

Claims

1. A method for detecting a connection of a PCM telephone system, characterized in that The method comprises the following steps: S1, collecting the reference pressure difference value between A-B lines in the hands-free state and the off-hook state when the telephone is first opened, and storing the reference pressure difference value as health record data to build a health record; S2, continuously detecting the actual pressure difference value between A-B lines during the call process, and the sampling frequency should not be lower than 1 time per second, and the pressure difference value is dynamically monitored in real time; S3, based on the data of real-time dynamic monitoring, the parallel machine state is determined, and whether there is a parallel machine eavesdropping behavior is determined in real time according to whether the actual pressure difference value deviates from the threshold range of the reference pressure difference value; S4, line anomaly detection is performed synchronously during the pressure difference monitoring process, and if there is a line fault, the parallel machine determination result is shielded; S5, when the parallel machine early warning is triggered, the resistance and capacitance value of the telephone is measured when the user hangs up to perform resistance and capacitance verification, and the resistance and capacitance value is compared with the reference resistance and capacitance value in the health record, and if the difference exceeds the preset threshold value, the parallel machine alarm is confirmed.

2. The method for detecting the connection of a PCM telephone system according to claim 1, wherein The health record built in the S1 step specifically comprises: In the initial registration stage of the telephone, the telephone is controlled to enter the hands-free state and the off-hook state in turn; The hands-free reference pressure difference value and the off-hook reference pressure difference value are associated with the telephone identifier and stored as the health record; The health record is synchronized to the gateway server database through the SNMP protocol.

3. The method of claim 1, wherein the PCM phone system is a PCM phone system of a digital TV receiver. In the real-time dynamic monitoring of the S2 step, the differential amplification circuit is used to collect the analog voltage signal between A-B lines; the 12-bit ADC is used to convert the digital signal at a sampling rate of 1 kHz; the sliding window filtering algorithm is used to process noise, the window width is set to 10 sampling points, and the filtered pressure difference effective value is extracted once per second for parallel machine determination.

4. The method of claim 1, wherein the PCM phone system is a PCM phone system of a digital TV receiver. The parallel machine state determination of the S3 step specifically comprises: Setting dynamic offset threshold ; Computing the current pressure difference sample value Absolute deviation from the health profile value : If the following condition is met for 3 times continuously Then trigger the early warning of parallel machine; The early warning signal is uploaded to the network management alarm station in real time through the Trap message.

5. The method of claim 1, wherein the PCM phone system is a PCM phone system of a digital TV receiver. In the line anomaly detection of the S4 step, the parallel judgment logic comprises: short circuit detection, collision detection and mixed detection; The short circuit detection method is: if the first valid pressure difference sampling value after the off-hook satisfies , then the short circuit fault is determined. The method for detecting the electric shock is: when the A-line voltage sampling value is rounded, the following condition is met , and when the longitudinal current sampling value is rounded, the following condition is met , then the electric shock fault is determined. The method for detecting the fault is: when the A-line voltage sampling value after rounding off satisfies , and the longitudinal current sampling value after rounding off satisfies , the fault is determined as a fault of the other mixing. When any of the above faults is triggered, the parallel machine determination process is interrupted and the line anomaly event is reported.

6. The method of claim 1, wherein the method further comprises: The resistance and capacitance verification of the S5 step further comprises: After hanging up, a 1 kHz sine test signal is injected into the telephone line, and the four-wire method is used to measure the line impedance and capacitive impedance components; The equivalent capacitance value of the telephone is calculated, and the reference capacitance value in the health record is called; If the difference between the equivalent capacitance value and the reference capacitance value is greater than the preset capacitance value, a parallel machine confirmation alarm is generated.

7. A hunt detection system for a PCM telephone system, characterized by The method for implementing the parallel machine detection method for the PCM telephone system according to any one of claims 1-6 comprises: A health record construction module is configured in the telephone opening stage, which is used to collect the reference pressure difference value between A-B lines in the hands-free and off-hook states, and store the data to the non-volatile memory; A dynamic monitoring module comprises a voltage sensor and a microprocessor, which is used to collect the actual pressure difference value between A-B lines at a frequency of more than 1 time per second during the call process; A parallel machine determination module comprises a comparator algorithm, which is used to calculate the offset between the actual pressure difference value and the reference pressure difference value, and generate a parallel machine early warning signal when the offset exceeds the preset threshold value; A line fault diagnosis module analyzes the A-line voltage and longitudinal current data in real time, and disables the output of the parallel machine determination module if a line fault is diagnosed. The resistance-capacitance verification module measures the resistance-capacitance value of the telephone set in response to the on-hook event and compares the resistance-capacitance value with the health record, and outputs the on-hook alarm confirmation signal to the network management interface.

8. A hunt detection system for a PCM telephone system as claimed in claim 7, wherein The dynamic monitoring module hardware composition comprises: A high-precision instrument amplifier with an input impedance of >10MΩ and a common-mode rejection ratio of ≥120dB; A Σ-Δ type ADC with a resolution of 16 bits and a sampling rate of 10kSPS; An FPGA preprocessing unit implementing an FIR digital filter and a peak detection algorithm; An SPI interface transmitting the processed differential pressure data to the main control MCU.

9. A hunt detection system for a PCM telephone system as claimed in claim 7, wherein The line fault diagnosis module comprises: A multiplexer switching the collection of the A-line-to-ground voltage, the B-line-to-ground voltage and the AB-line-to-line voltage; A current sensor measuring the longitudinal current in series on the line ground end; A fault analysis unit performing the following judgments: When the AB-line resistance value is <2kΩ, a short-circuit flag is marked When the absolute value of the A-line-to-ground voltage is >45V, a collision flag is marked When the AB-line capacitance change rate is >±20%, a mixed flag is marked; The flag register is output to an AND gate circuit to control the enable end of the on-hook judgment module.

10. A hunt detection system for a PCM telephone system as claimed in claim 7, wherein The resistance-capacitance verification module operation process is: In response to the interrupt signal of the on-hook detection circuit, the relay is switched to the resistance-capacitance test circuit, and the vector voltage current method is used to calculate the complex impedance; The imaginary part of the capacitive reactance component is extracted, and the health record data is obtained through the I²C interface access EEPROM.

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