A method and system for monitoring channel eavesdropping

Abstract

This application provides a method and system for monitoring channel eavesdropping. The method includes: setting up two signal transmitting and receiving devices connected through an optical communication link under test; aligning the two signal transmitting and receiving devices with clocks; each signal transmitting and receiving device sending optical quantum signals to the other and receiving the optical quantum signals sent by the other; and each signal transmitting and receiving device confirming the security status of the optical communication link between the two signal transmitting and receiving devices based on the sent and received optical quantum signals. Applying this application can achieve real-time monitoring of channel eavesdropping activities.

Description

Technical Field

[0001] This application relates to the fields of quantum communication and information security technology, and in particular to a channel eavesdropping monitoring method and system based on optical quantum transmission. Background Technology

[0002] Currently, information security faces increasingly severe challenges, and traditional classic communication methods have fundamental flaws when facing eavesdropping. In traditional communication systems, security primarily relies on the complexity of encryption algorithms and key management mechanisms, such as symmetric encryption (AES), public-key encryption (RSA), and the TLS transport protocol. These methods mainly operate at the data layer, effectively preventing the decryption of communication content. However, attackers can directly perform side-channel eavesdropping on the transmission link, i.e., obtaining data content at the physical layer. Such eavesdropping methods include inserting couplers into fiber optic communications, creating micro-bends, and interfering with echoes, enabling data theft without significantly affecting communication quality. Encryption algorithms cannot prevent link eavesdropping because attackers can bypass encryption protocols, copy encrypted data packets within the link, and then perform offline analysis, side-channel cracking, or wait for future improvements in computing power to decrypt them. Furthermore, classic communication equipment struggles to accurately monitor channel status, cannot determine whether information has been eavesdropped on or tampered with during transmission, and cannot detect eavesdropping at the physical layer, especially lacking effective real-time detection mechanisms at the channel layer, resulting in a "passive defense" limitation. Therefore, the difficulty in determining whether a link is being monitored has become one of the weakest links in classic communication systems. Once the encryption mechanism is cracked or the key is leaked, the communication content will be at risk of being completely exposed.

[0003] Current network security protection systems primarily rely on physical protection (e.g., dedicated line deployment, enclosed pipelines) or passive methods (e.g., manual inspections) for link-layer security detection, lacking a deployable, quantifiable, and automated monitoring system. Furthermore, existing technologies often struggle to accurately monitor channel conditions, making it impossible to determine whether information has been eavesdropped on or tampered with during transmission, thus limiting them to a "passive defense" approach.

[0004] Furthermore, although quantum communication has inherent advantages in security and can detect eavesdropping through the principle of quantum impermeability, existing quantum communication systems still face challenges such as complex equipment, high costs, and stringent deployment requirements, making it difficult to achieve large-scale application in a wide range of scenarios. Especially in short-to-medium-range communication, low-cost edge devices, and resource-constrained secure communication environments, the deployment and maintenance costs of traditional quantum communication systems far exceed application requirements. Summary of the Invention

[0005] In view of this, this application provides a channel eavesdropping monitoring method and system, which can realize real-time monitoring of channel eavesdropping behavior.

[0006] Firstly, this application provides a method for detecting channel eavesdropping, including:

[0007] Two signal transmitting and receiving devices are set up, and the two signal transmitting and receiving devices are connected through the optical communication link under test;

[0008] The two signal transmitting and receiving devices synchronize their clocks.

[0009] Two signal transmitting and receiving devices send optical quantum signals to each other and receive optical quantum signals sent by each other;

[0010] The two signal transmitting and receiving devices confirm the security status of the optical communication link between them based on the transmitted and received optical quantum signals, respectively.

[0011] Furthermore, the two signal transmitting and receiving devices confirm the security status of the optical communication link between them based on the transmitted and received quantum signals, respectively, including:

[0012] Each signal transmitting and receiving device calculates the bit error rate of the qubit based on the optical quantum signals it transmits and receives.

[0013] When the bit error rate of any single quantum bit exceeds a preset security threshold, the optical communication link between the two signal transmitting and receiving devices is insecure.

[0014] When the bit error rate of both qubits is lower than or equal to a preset security threshold, the optical communication link between the two signal transmitting and receiving devices is secure.

[0015] Furthermore, the signal transmitting and receiving device includes: a photonic quantum transmitter and a photonic quantum receiver;

[0016] The optical quantum transmitter is used to generate optical quantum signals and send them to another signal transmitting and receiving device;

[0017] The optical quantum receiver is used to receive optical quantum signals sent by another signal transmitting and receiving device, and to confirm the security status of the optical communication link between the two signal transmitting and receiving devices based on the optical quantum signals sent by the optical quantum transmitter and the received optical quantum signals.

[0018] Furthermore, the method also includes:

[0019] When the optical communication link between two signal transmitting and receiving devices is insecure, the two signal transmitting and receiving devices locate the range of the eavesdropping point based on the time it takes for the received optical quantum signal to generate the bit error rate of the qubit.

[0020] Furthermore, locating the area of ​​the eavesdropping point includes:

[0021] Two signal transmitting and receiving devices measure the generation time of the bit error rate of the received optical quantum signal to generate qubits, and calculate the location of the eavesdropping point based on the length of the optical communication link between the two signal transmitting and receiving devices and the two generation times.

[0022] Furthermore, the optical quantum signal is a polarized optical signal, a phase optical signal, or a time-phase photon signal.

[0023] Secondly, this application also provides a channel eavesdropping monitoring system, comprising: two signal transmitting and receiving devices;

[0024] The two signal transmitting and receiving devices are connected through the optical communication link under test.

[0025] The two signal transmitting and receiving devices are used for clock alignment, and simultaneously send optical quantum signals to each other and receive optical quantum signals sent by each other; the security status of the optical communication link between the two signal transmitting and receiving devices is confirmed based on the sent and received optical quantum signals.

[0026] Furthermore, the signal transmitting and receiving device includes: a photonic quantum transmitter and a photonic quantum receiver;

[0027] The optical quantum transmitter is used to generate optical quantum signals and send them to another signal transmitting and receiving device;

[0028] The optical quantum receiver is used to receive optical quantum signals sent by another signal transmitting and receiving device, and to confirm the security status of the optical communication link between the two signal transmitting and receiving devices based on the optical quantum signals sent by the optical quantum transmitter and the received optical quantum signals.

[0029] Furthermore, the two signal transmitting and receiving devices are also used to locate the location range of the eavesdropping point based on the time it takes for the received optical quantum signal to generate a bit error rate when the optical communication link between the two signal transmitting and receiving devices is insecure.

[0030] Furthermore, the signal transmitting and receiving device includes: a computing unit;

[0031] The computing unit is used to locate the range of the eavesdropping point based on the time it takes for the received optical quantum signal to generate a bit error rate.

[0032] As can be seen from the above technical solution, in the channel eavesdropping monitoring method and system based on optical quantum transmission in this application, since two signal transmitting and receiving devices are set up and connected through the optical communication link under test, after the two signal transmitting and receiving devices are clock-aligned, they simultaneously send optical quantum signals to each other and receive optical quantum signals sent by each other. Then, the security status of the optical communication link between the two signal transmitting and receiving devices is confirmed based on the transmitted and received optical quantum signals respectively. Therefore, real-time monitoring of channel eavesdropping behavior can be achieved, and security alarms can be further issued. In addition, the location range of the eavesdropping point can be further located. Attached Figure Description

[0033] Figure 1 This is a flowchart illustrating a channel eavesdropping and monitoring method based on optical quantum transmission in a specific embodiment of this application.

[0034] Figure 2 This is a schematic diagram of the structure of a channel eavesdropping monitoring system based on optical quantum transmission in a specific embodiment of this application. Detailed Implementation

[0035] To enable those skilled in the art to better understand the present application, the technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present application, and not all embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative effort should fall within the scope of protection of the present application.

[0036] It should be noted that the terms "first," "second," etc., in the specification, claims, and accompanying drawings of this application are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate so that the embodiments of this application described herein can be implemented in orders other than those illustrated or described herein. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion; for example, a process, method, system, product, or apparatus that comprises a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus.

[0037] Figure 1 This is a flowchart illustrating a channel eavesdropping and monitoring method based on optical quantum transmission in a specific embodiment of this application. Figure 1As shown, the channel eavesdropping and monitoring method based on optical quantum transmission in this application may include:

[0038] Step 101: Set up two signal transmitting and receiving devices, which are connected through the optical communication link under test.

[0039] In the technical solution of this application, if it is necessary to monitor the optical communication link under test, two signal transmitting and receiving devices can be set at both ends of the optical communication link under test, and the two signal transmitting and receiving devices can be connected through the optical communication link under test.

[0040] Step 102: The two signal transmitting and receiving devices perform clock alignment.

[0041] In the technical solution of this application, the two signal transmitting and receiving devices need to perform clock alignment operations first, so that the clocks of the two signal transmitting and receiving devices are aligned and synchronized with the same standard clock.

[0042] Step 103: The two signal transmitting and receiving devices send optical quantum signals to each other and receive optical quantum signals sent by each other.

[0043] In the technical solution of this application, after the two signal transmitting and receiving devices are clock aligned, when it is necessary to monitor the channel to be monitored, the two signal transmitting and receiving devices will send optical quantum signals to each other through the optical communication link to be tested, and receive optical quantum signals sent by each other.

[0044] For example, as an example, in a specific embodiment of this application, it can be assumed that the two signal transmitting and receiving devices are device A and device B, respectively. Device A will send a quantum light signal to device B through the optical communication link under test and receive the quantum light signal sent by device B; at the same time, device B will also send a quantum light signal to device A through the optical communication link under test and receive the quantum light signal sent by device A.

[0045] Furthermore, in the technical solution of this application, the aforementioned photonic quantum signal can be a variety of different signals.

[0046] For example, as an example, in a specific embodiment of this application, the optical quantum signal may be an optical signal such as a polarized state optical signal, a phase optical signal, or a time-phase photon signal, or other suitable optical quantum signals.

[0047] Step 104: The two signal transmitting and receiving devices confirm the security status of the optical communication link between them based on the transmitted and received optical quantum signals, respectively.

[0048] In the technical solution of this application, after the two signal transmitting and receiving devices respectively receive the optical quantum signal, they can confirm the security status of the optical communication link between the two signal transmitting and receiving devices based on the optical quantum signal they send to each other and the optical quantum signal they receive from each other. Therefore, through the above method, the two signal transmitting and receiving devices can confirm the security status of the optical communication link between them.

[0049] Therefore, through the above steps 101 to 104, the security status of the optical communication link between the two signal transmitting and receiving devices can be confirmed.

[0050] Furthermore, the technical solution of this application can confirm the security status of the optical communication link under test between two signal transmitting and receiving devices through various specific implementation methods. The following will use one specific method as an example to describe the technical solution of this application in detail.

[0051] For example, as an example, in a specific embodiment of this application, the two signal transmitting and receiving devices confirming the security status of the optical communication link between the two signal transmitting and receiving devices based on the transmitted and received optical quantum signals may include:

[0052] Each signal transmitting and receiving device calculates the quantum bit error rate (QBER) based on the transmitted and received quantum signals.

[0053] When any QBER exceeds the preset security threshold, the optical communication link between the two signal transmitting and receiving devices is insecure, abnormal, and may be monitored; at this time, a security alarm response can be further triggered.

[0054] When both QBERs are lower than or equal to the preset safety threshold, the optical communication link between the two signal transmitting and receiving devices is safe.

[0055] Therefore, the security status of the optical communication link between the two signal transmitting and receiving devices can be confirmed using the above method.

[0056] In addition, in the technical solution of this application, the value of the above-mentioned security threshold can be preset according to the needs of the actual application scenario.

[0057] For example, as an illustration, in one specific embodiment of this application, the security threshold can be 11%. Of course, other suitable values ​​are also possible, which will not be listed here.

[0058] Furthermore, the signal transmitting and receiving device described above can be implemented in various specific ways. The following will use one specific method as an example to describe the technical solution of this application in detail.

[0059] For example, as an example, in one specific embodiment of this application, the signal transmitting and receiving device may include: a quantum light transmitter and a quantum light receiver;

[0060] The quantum light transmitter is used to generate and transmit quantum light signals (e.g., polarized light signals, phase light signals, time-phase photon signals, etc.), and to send the quantum light signals to another signal transmitting and receiving device;

[0061] The optical quantum receiver is used to receive optical quantum signals sent by another signal transmitting and receiving device, and to confirm the security status of the optical communication link between the two signal transmitting and receiving devices based on the optical quantum signals sent by the optical quantum transmitter and the received optical quantum signals.

[0062] For example, as an example, in a specific embodiment of this application, the optical quantum receiver can calculate the quantum bit error rate (QBER) based on the optical quantum signal sent by the optical quantum transmitter and the received optical quantum signal;

[0063] When any QBER exceeds the preset security threshold, the optical communication link between the two signal transmitting and receiving devices is insecure, abnormal, and may be monitored; at this time, a security alarm response can be further triggered.

[0064] When both QBERs are lower than or equal to the preset safety threshold, the optical communication link between the two signal transmitting and receiving devices is safe.

[0065] In addition, in the technical solution of this application, when the optical communication link between the two signal transmitting and receiving devices is insecure, the location range of the eavesdropping point can be further located.

[0066] For example, as an example, in a specific embodiment of this application, the channel eavesdropping monitoring method based on optical quantum transmission may further include:

[0067] Step 105: When the optical communication link between the two signal transmitting and receiving devices is insecure, the two signal transmitting and receiving devices locate the range of the eavesdropping point based on the time it takes for the received optical quantum signal to generate the bit error rate of the qubit.

[0068] In the technical solution of this application, when the optical communication link between the two signal transmitting and receiving devices is insecure (for example, when any QBER is higher than a preset security threshold), the two signal transmitting and receiving devices can further locate the location range of the eavesdropping point based on the time of the received optical quantum signal generating the quantum bit error rate.

[0069] Furthermore, the location range of the eavesdropping point can be determined through various specific implementation methods in the technical solution of this application. The following will use one specific implementation method as an example to provide a detailed description of the technical solution of this application.

[0070] For example, as an example, in a specific embodiment of this application, locating the area of ​​the eavesdropping point may include:

[0071] Two signal transmitting and receiving devices measure the generation time of the bit error rate of the received optical quantum signal to generate qubits, and calculate the location of the eavesdropping point based on the length of the optical communication link between the two signal transmitting and receiving devices and the two generation times.

[0072] In the technical solution of this application, two signal transmitting and receiving devices continuously transmit optical quantum signals to each other and simultaneously receive optical quantum signals transmitted by the other party. They measure each received optical quantum signal and continuously monitor the change in the bit error rate (BER) of the optical quantum signals. When there is no eavesdropping in the optical communication link, the BER will be lower than or equal to a preset security threshold; however, when there is eavesdropping in the optical communication link, the BER will be higher than the preset security threshold.

[0073] Therefore, under normal circumstances, the bit error rate (BER) of the qubits continuously monitored by both signal transmitting and receiving devices should be lower than or equal to a preset security threshold. So, when the BER is first detected to be higher than the preset security threshold, the time T at that moment can be recorded and called the generation time T of that qubit BER. Since the two signal transmitting and receiving devices are clock-aligned, the specific moment of the generation time T can be determined based on the aligned clocks. Then, based on the length of the optical communication link between the two signal transmitting and receiving devices and the two generation times, the location of the eavesdropping point can be calculated.

[0074] For example, as an example, such as Figure 2As shown, in a specific embodiment of this application, assuming that signal transmitting and receiving device B measures the first generation time Ta of the received quantum signal (i.e., the quantum signal sent by signal transmitting and receiving device A) to generate the qubit error rate, and signal transmitting and receiving device A measures the first generation time Tb of the received quantum signal (i.e., the quantum signal sent by signal transmitting and receiving device B) to generate the qubit error rate, then the location of the eavesdropping point C can be calculated using the following formula:

[0075] D = L / 2 + (Tb-Ta) V / 2;

[0076] Where D is the distance between the eavesdropping point C and the signal transmitting and receiving device A, L is the length of the optical communication link between the signal transmitting and receiving device A and the signal transmitting and receiving device B, and V is the optical quantum signal transmission rate.

[0077] In practical applications, L and V are usually constant values. Therefore, after knowing Ta and Tb, the location of the eavesdropping point can be calculated according to the above formula, thus allowing the location range of the eavesdropping point to be determined.

[0078] In addition, the technical solution of this application also proposes a channel eavesdropping monitoring system based on optical quantum transmission.

[0079] like Figure 2 As shown, the channel eavesdropping monitoring system based on optical quantum transmission in a specific embodiment of this application may include: two signal transmitting and receiving devices;

[0080] The two signal transmitting and receiving devices are connected through the optical communication link under test.

[0081] The two signal transmitting and receiving devices are used for clock alignment, and simultaneously send optical quantum signals to each other and receive optical quantum signals sent by each other; the security status of the optical communication link between the two signal transmitting and receiving devices is confirmed based on the sent and received optical quantum signals.

[0082] Additionally, as an example, in one specific embodiment of this application, the two signal transmitting and receiving devices can also be used to locate the eavesdropping point's location range based on the time it takes for the received quantum signal to generate a bit error rate when the optical communication link between the two signal transmitting and receiving devices is insecure.

[0083] Additionally, as an example, such as Figure 2 As shown, in one specific embodiment of this application, the signal transmitting and receiving device may include: a quantum light transmitter and a quantum light receiver;

[0084] The quantum light transmitter is used to generate quantum light signals (e.g., polarized light signals, phase light signals, time-phase photon signals, etc.) and transmit the quantum light signals to another signal transmitting and receiving device;

[0085] The optical quantum receiver is used to receive optical quantum signals sent by another signal transmitting and receiving device, and to confirm the security status of the optical communication link between the two signal transmitting and receiving devices based on the optical quantum signals sent by the optical quantum transmitter and the received optical quantum signals.

[0086] Additionally, as an example, in one specific embodiment of this application, the signal transmitting and receiving device may further include: a computing unit (not shown in the figure); the computing unit is used to locate the location range of the eavesdropping point based on the time it takes for the received optical quantum signal to generate a bit error rate.

[0087] In summary, the technical solution of this application, by setting up two signal transmitting and receiving devices connected through the optical communication link under test, allows the two devices to synchronize their clocks and simultaneously transmit and receive optical quantum signals from each other. The security status of the optical communication link between the two devices is then confirmed based on the transmitted and received optical quantum signals. Therefore, real-time monitoring of channel eavesdropping can be achieved, and security alarms can be further issued. Furthermore, the location and range of the eavesdropping point can be further pinpointed.

[0088] By using the technical solution of this application, it is possible to detect eavesdropping and locate the location range of the eavesdropper. Therefore, it is possible to efficiently and accurately identify weak eavesdropping behavior in optical communication links and trigger security alarm responses in real time. This can significantly improve the sensitivity and accuracy of communication channel monitoring detection, effectively reduce the false alarm rate, and improve the security and reliability of the system. In addition, it can also provide effective clues for locating eavesdropping points, which can help to proactively investigate and repair eavesdropping points (and eavesdroppers) in the next step.

[0089] The technical solution of this application, by transmitting quantum-state optical signals and transmitting them back to the receiving end, combined with a quantum bit error rate (QBER) analysis mechanism, can detect the presence of third-party probing or interference at the physical layer, thus achieving physical-level channel eavesdropping detection. Compared with traditional communication technologies in the prior art, which cannot determine the security of the channel at the physical layer, the technical solution of this application provides proactive and quantifiable eavesdropping detection capabilities.

[0090] In addition, the technical solution of this application does not require the construction of a complete quantum communication system, nor does it rely on complex entanglement sources. It can achieve security monitoring and location by using two sets of quantum state transmitters, receivers and reflective terminal nodes.

[0091] Furthermore, in the exemplary embodiments of this application, any of the embodiments in the foregoing method embodiments can be applied to the device embodiments, and will not be described in detail here.

[0092] The device embodiments described above are merely illustrative. The units described as separate components may or may not be physically separate. The components shown as units may or may not be physical units; that is, they may be located in one place or distributed across multiple network units. Some or all of the modules can be selected to achieve the purpose of this embodiment according to actual needs. Those skilled in the art can understand and implement this without any creative effort.

[0093] Through the above description of the embodiments, those skilled in the art can clearly understand that each embodiment can be implemented by means of software plus necessary general-purpose hardware platforms, and of course, it can also be implemented by hardware. Based on this understanding, the above technical solutions, in essence or the part that contributes to the prior art, can be embodied in the form of a software product. This computer software product can be stored in a computer-readable storage medium, such as ROM / RAM, magnetic disk, optical disk, etc., and includes several instructions to cause a computer device (which may be a personal computer, server, or network device, etc.) to execute the methods described in the various embodiments or some parts of the embodiments.

[0094] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the scope of protection of this application.

Claims

1. A method for detecting channel eavesdropping, characterized in that, The method includes: Two signal transmitting and receiving devices are set up, and the two signal transmitting and receiving devices are connected through the optical communication link under test; The two signal transmitting and receiving devices synchronize their clocks. Two signal transmitting and receiving devices send optical quantum signals to each other and receive optical quantum signals sent by each other; The two signal transmitting and receiving devices confirm the security status of the optical communication link between them based on the transmitted and received optical quantum signals, respectively.

2. The method according to claim 1, characterized in that, The two signal transmitting and receiving devices confirm the security status of the optical communication link between them based on the transmitted and received quantum signals, including: Each signal transmitting and receiving device calculates the bit error rate of the qubit based on the optical quantum signals it transmits and receives. When the bit error rate of any single quantum bit exceeds a preset security threshold, the optical communication link between the two signal transmitting and receiving devices is insecure. When the bit error rate of both qubits is lower than or equal to a preset security threshold, the optical communication link between the two signal transmitting and receiving devices is secure.

3. The method according to claim 1, characterized in that, The signal transmitting and receiving device includes: a quantum optical transmitter and a quantum optical receiver; The optical quantum transmitter is used to generate optical quantum signals and send them to another signal transmitting and receiving device; The optical quantum receiver is used to receive optical quantum signals sent by another signal transmitting and receiving device, and to confirm the security status of the optical communication link between the two signal transmitting and receiving devices based on the optical quantum signals sent by the optical quantum transmitter and the received optical quantum signals.

4. The method according to claim 1, characterized in that, The method also includes: When the optical communication link between two signal transmitting and receiving devices is insecure, the two signal transmitting and receiving devices locate the range of the eavesdropping point based on the time it takes for the received optical quantum signal to generate the bit error rate of the qubit.

5. The method according to claim 4, characterized in that, The process of locating the area of ​​the eavesdropping point includes: Two signal transmitting and receiving devices measure the generation time of the bit error rate of the received optical quantum signal to generate qubits, and calculate the location of the eavesdropping point based on the length of the optical communication link between the two signal transmitting and receiving devices and the two generation times.

6. The method according to claim 1, characterized in that: The optical quantum signal is a polarized optical signal, a phase optical signal, or a time-phase photon signal.

7. A channel eavesdropping monitoring system, characterized in that, The system includes: two signal transmitting and receiving devices; The two signal transmitting and receiving devices are connected through the optical communication link under test. The two signal transmitting and receiving devices are used for clock alignment, and simultaneously send optical quantum signals to each other and receive optical quantum signals sent by each other; the security status of the optical communication link between the two signal transmitting and receiving devices is confirmed based on the sent and received optical quantum signals.

8. The system according to claim 7, characterized in that, The signal transmitting and receiving device includes: a quantum optical transmitter and a quantum optical receiver; The optical quantum transmitter is used to generate optical quantum signals and send them to another signal transmitting and receiving device; The optical quantum receiver is used to receive optical quantum signals sent by another signal transmitting and receiving device, and to confirm the security status of the optical communication link between the two signal transmitting and receiving devices based on the optical quantum signals sent by the optical quantum transmitter and the received optical quantum signals.

9. The system according to claim 7, characterized in that: The two signal transmitting and receiving devices are also used to locate the eavesdropping point's location range based on the time it takes for the received quantum signal to generate a bit error rate when the optical communication link between the two signal transmitting and receiving devices is insecure.

10. The system according to claim 9, characterized in that, The signal transmitting and receiving device includes: a computing unit; The computing unit is used to locate the range of the eavesdropping point based on the time it takes for the received optical quantum signal to generate a bit error rate.

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