RU2236758C1 - Method for detecting attempt of reading information off fiber-optic transmission line using quantum noise - Google Patents

Abstract

FIELD: monitoring fiber-optic restricted-access data transfer systems using quantum noise method.

SUBSTANCE: proposed method includes generation of optical transmission signal incorporating dc optical radiation and data signal, transmission of optical signal over fiber-optic line, reception, detection, amplification, and separation of data signal and fixed level, and check-up for variations in inserted loss. Pending transmission check signal is introduced in optical signal and upon reception and detection it is separated and its amplitude is compared with threshold level found in advance from proposed formula. Attempt of reading information off fiber-optic transmission line is recognized by reduction of check signal amplitude relative to threshold level.

EFFECT: enhanced probability of detecting attempt of reading information off fiber-optic line; reduced malfunction probability.

1 cl, 2 dwg

Description

The invention relates to methods for detecting attempts to retrieve information from fiber-optic transmission lines and can be used as a method of continuous monitoring of a limited access fiber-optic information transmission system using the quantum noise method.

The known method of quantum noise from the patent “Safe optical information system” (UK Patent No. 2060869 from 05/05/1981).

The method consists in the fact that information is transmitted by an information optical signal modulating a constant signal of large amplitude. This causes the appearance of additional quantum or shot noise in the channel. The appearance of this noise causes a deterioration in the parameters of the receiver of the fiber optic system and the interception receiver in case of access to the optical fiber. The dominant component of the noise is not the thermal noise of the receiver, but the shot noise in the transmitted signal. In this way, an increase in the power required to intercept information from the fiber optic transmission line is achieved. It is assumed that the receiver sensitivity of the fiber optic system is lower than the receiver sensitivity of the interception. Therefore, a deterioration in the sensitivity of the receiver of the system will be lower than a decrease in the sensitivity of the receiver of interception.

The disadvantages of the method include the following.

1. The deterioration of the sensitivity of the interception receiver does not guarantee the impossibility of information retrieval, since it is known that a sufficiently large optical signal power sufficient to intercept can be derived from the side surface of the optical fiber.

2. The optimal ratio between the level of constant radiation and the information signal is unknown so that the deterioration of the sensitivity of the receiver of the system is minimal, and for the receiver of interception - maximum.

It is known to develop a method of quantum noise in combination with a control system from the patent "Equipment for detecting the removal of light energy from optical fiber" (US patent No. 4636029 from 01/13/1987).

In an information system where the computer side and the terminal side are connected using a fiber optic cable, an unauthorized connection is detected using the integrated monitoring system. The transmitted signal is a superposition, where the majority is the constant optical radiation used for monitoring, and the smaller part is informational signals.

The computer and terminal sides contain threshold devices that are sensitive to changes in the level of optical radiation and are attached to the receiving modules. According to its technical essence, the method of monitoring a fiber optic transmission line from the present patent is closest to the claimed method and therefore is taken as a prototype.

The disadvantages of this method of control include the following provisions:

1. The optimal ratio between the parameters of constant radiation and the optical information signal is unknown.

2. It is not known what accuracy the control of changes in introduced losses should be for guaranteed detection of attempts of unauthorized access to information.

The technical problem to be solved is an increase in the probability of detecting attempts to extract information from a fiber-optic transmission line and a decrease in the probability of false positives.

The technical result is to determine the requirements for setting the threshold level of the control signal change at the optimal value of the quantum noise component.

This technical result is achieved by a method for detecting attempts to retrieve information from a fiber optic transmission line using quantum noise, which consists in generating a transmission signal consisting of a constantly radiated optical power and an information signal, transmitting a signal over a fiber optic line, receiving, detecting, amplification, separation of information signals and a constant level, control of changes in introduced losses.

What is new is that before transmission, a control signal is introduced into the optical signal, after reception and detection it is isolated, the amplitude of the control signal is compared with a threshold level, which is previously determined by the formula

where K _n is the transmission coefficient of optical radiation, showing which part of the information radiation lost in the fiber-optic transmission line during removal, got on the interception receiver;

Q is the maximum signal-to-noise ratio specified for the interception receiver;

B is the frequency band of information signals;

L _n is the Persian integral;

A - loss of the optical signal in the fiber optic transmission line, dB;

S is the conversion coefficient of the photodetector of the receiver of the system, A / W;

M - modulation coefficient - the ratio of the power of the information signal to the power of a constant level;

W _n is the power of the information signal at the input of the receiver of the system, necessary to ensure the required quality of information reception;

e is the electron charge,

by reducing the amplitude of the control signal relative to the threshold level, an attempt is made to retrieve information from the fiber optic transmission line.

The set of essential features allows you to effectively detect attempts to remove information transmitted over fiber-optic transmission lines.

Figure 1 presents a functional diagram of a fiber optic channel that implements the proposed method.

1 - optical transmitting module;

2 - formation device;

3 - source of constant bias and control signal;

4 - fiber optic transmission line;

5 - receiving optical module;

6 - amplifier-driver of information signals;

7 - pilot signal filter;

8 - control device.

Figure 2 presents the dependence of the required control level for two cases:

1 - without the use of quantum noise,

2 - using quantum noise and the optimal modulation coefficient.

The essence of the proposed method consists in the fact that initially the optimal ratio between the noise components at the input pole of the receiver of the fiber optic transmission system is selected.

Thermal noise is generated in the input circuits of the receiving optical module and is caused by the appearance of current carriers due to thermal fluctuations. An accurate calculation must be carried out separately for each source of thermal noise, which requires knowledge of a large number of parameters of the input circuits of the receiver. As a rule, these parameters are unknown and the noise value is usually estimated by the equivalent noise resistance R _equiv .

The square of the mean square value of the thermal noise current is:

where K is the Boltzmann constant equal to 1.37 · 10 ^-23 J / K;

T is the absolute temperature, K;

B is the frequency band of the received signals;

R _equiv - equivalent noise resistance.

Quantum (shot) noise is determined by the amount of current flowing through the photodetector of the receiver. The square of the mean square value of the current of quantum noise is:

where e is the electron charge equal to 1.6 · 10 ^-19 C;

I _m is the dark current of the photodetector;

I _f - the background current of the photodetector, that is, the photocurrent from constant optical radiation;

I _n - the Personnel integral - coefficient taking into account the nature of the transmitted information (see. "Optical communication technology. Photodetectors." Ed. Tsanga, Moscow, Mir, 1988).

It can be seen from formula (4) that noise has two components: intrinsic noise due to dark current, and the external component due to the power of external optical radiation (the so-called "noise in the signal").

Thus, if the signal of the optical transmitter will be constant optical radiation modulated by an information signal, the level of constant optical radiation will determine the noise in the channel, and the modulation depth coefficient will determine the signal-to-noise ratio at the receivers of the fiber-optic system and interception.

It is necessary to choose the ratio of the parameters of optical radiation in such a way that, with quantum noise, the degradation of the parameters for the receiver of the system is small, and for the receiver of interception, maximum.

The square of the mean square noise current of the photodetector of the receiver of the fiber optic system will be:

The most optimal ratio between the noise components, that is, when the total noise increases slightly, and the quantum noise component is maximum, should be considered the condition for their equality, i.e.

Then I _w = 1.41 I _sq , that is, the sensitivity deterioration for the receiver of the fiber optic system is 1.5 dB with respect to its maximum value, which fits into the normal operating margin (3 dB). The sensitivity threshold of the interception receiver in the best place for interception (at the beginning of the fiber-optic line) will become equal

where W ₀ = W _n / M is the power of constant radiation at the input pole of the receiver of the fiber optic system;

M is the modulation coefficient;

S is the conversion coefficient of the photodetector of the receiver of the system, A / W.

If quantum noise is not used, then the required control level of the control device will be:

where Q is the signal-to-noise ratio for the interception receiver;

To _n is the optical transmission coefficient for the interception receiver;

W _pop - sensitivity threshold of the interception receiver at a signal-to-noise ratio of 1;

A - introduced losses in the fiber optic transmission line;

W _n is the sensitivity threshold of the receiver of the fiber optic transmission system for a given error rate.

When using quantum noise, the required control level is:

Figure 2 presents the results of calculating the value of the control level from the value of the introduced losses A according to two formulas (7) - curve 1 and (8) - curve 2 with the same initial data: Q = 1; K _n = 0.015; B = 110 MHz; I _n = 0.55; S = 0.6 A / W; W _nop = 3 · 10 ^-9 ; M = 0.87; W _n = 2 · 10 ^-6 .

From figure 2 it follows that the application of the proposed method allows you to use a lower control level with the same amount of loss in the fiber optic transmission line, which increases the likelihood of detecting attempts to unauthorized connections.

To confirm the correctness of the proposed technical solution, a mock fiber optic channel was assembled in accordance with figure 1 using the following elements: optical transmitting module - POM-514, a device for generating information signals on transistors 2T372, a constant current source on an operational amplifier 1407 UD2, receiving optical module PROM-364, amplifier-driver on a comparator 597CA1, a low-pass filter on an RC circuit, a control device on an operational amplifier 1401UD2. Measurements confirmed the correct choice of the optimal ratio (5).

Claims (1)

A method for detecting attempts to retrieve information from a fiber optic transmission line using quantum noise, which consists in generating an optical transmission signal consisting of constant optical radiation and an information signal, transmitting an optical signal through a fiber optic line, receiving, detecting, amplifying, separating an information signal and a constant level, controlling changes in the introduced losses, characterized in that before the transmission, a control signal is introduced into the optical signal, after ma and detecting release it, comparing the amplitude of the pilot signal with a threshold level which is predetermined by the formula

where K _p is the transmission coefficient of optical radiation, showing how much of the information signal lost in the fiber-optic transmission line during removal, got on the interception receiver; Q - maximum value set for the interception receiver signal to noise ratio; B is the frequency band of information signals; I _p - the integral of Person; A - loss of the optical signal in the fiber optic transmission line, dB; S is the conversion coefficient of the photodetector of the receiver of the system, A / W; M - modulation coefficient - the ratio of the power of the information signal to the power of a constant level; W _p - the power of the information signal at the input of the receiver of the system, necessary to ensure the required quality of receiving information; e is the electron charge, by reducing the amplitude of the control signal relative to the threshold level, an attempt is made to retrieve information from the fiber optic transmission line.

Images