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IEEE Transactions on Instrumentation and Measurement

A Tampering Risk of Fiber-Based Frequency Synchronization Networks

Hongfei Dai, Yufeng Chen, Wenlin Li, Fangmin Wang, Guan Wang, Zhongwang Pang, Bo Wang

Published Jan 1, 2024• 2 citations
Publisher

Abstract

Fiber optic networks are used worldwide and have been regarded as excellent media for transmitting time–frequency (TF) signals. In the past decades, fiber-based TF synchronization techniques have been extensively studied. Instruments based on these techniques have been successfully applied. With the increasing application of TF synchronization instruments, their security has become an important issue. Unfortunately, the security risks of fiber-based frequency synchronization (FbFS) instruments have been overlooked. This article proposes a frequency tampering method called “frequency expander.” On a 200-km fiber link, we demonstrate a frequency tampering scenario using a frequency expander-enabled frequency tampering module (FTM). On the user side, the frequency value of the recovered 100-MHz signal can be stealthily altered within a range of 100 MHz ± 100 Hz, while the frequency dissemination stability of the system remains normal. Related to this tampering risk, potential hazards in two different application scenarios, which rely on precise frequency references, are analyzed. Two countermeasures are also proposed to solve this tampering risk.

DOI
10.1109/tim.2024.3414551

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Fiber optic networks are used worldwide and have been regarded as excellent media for transmitting time–frequency (TF) signals. In the past decades, fiber-based TF synchronization techniques have been extensively studied. Instruments based on these techniques have been successfully applied. With the increasing application of TF synchronization instruments, their security has become an important issue. Unfortunately, the security risks of fiber-based frequency synchronization (FbFS) instruments have been overlooked. This article proposes a frequency tampering method called “frequency expander.” On a 200-km fiber link, we demonstrate a frequency tampering scenario using a frequency expander-enabled frequency tampering module (FTM). On the user side, the frequency value of the recovered 100-MHz signal can be stealthily altered within a range of 100 MHz ± 100 Hz, while the frequency dissemination stability of the system remains normal. Related to this tampering risk, potential hazards in two different application scenarios, which rely on precise frequency references, are analyzed. Two countermeasures are also proposed to solve this tampering risk.
Fiber optic networks are used worldwide and have been regarded as excellent media for transmitting time-frequency (TF) signals. In the past decades, fiber-based TF synchronization techniques have been extensively studied. Instruments based on these techniques have been successfully applied. With the increasing application of TF synchronization instruments, their security has become an important issue. Unfortunately, the security risks of fiber-based frequency synchronization (FbFS) instruments have been overlooked. This paper proposes a frequency tampering method called "frequency lens". On a 200 km fiber link, we demonstrate a frequency tampering scenario using a frequency lens-enabled frequency tampering module (FTM). On the user side, the frequency value of the recovered 100 MHz signal can be stealthily altered within a range of 100 MHz-100 Hz to 100 MHz+100 Hz, while the frequency dissemination stability of the system remains normal. Related to this tampering risk, potential hazards in three different application scenarios, which rely on precise frequency references, are analyzed. Two countermeasures are also proposed to solve this tampering risk.
Fiber networks are utilized globally and are highly valued as effective channels for transmitting time-frequency (TF) signals. Over recent decades, considerable research has been dedicated to fiber-based TF synchronization technologies. Instruments developed from these technologies have proven effective in various applications. As these instruments become more widespread, their security issues have increasingly come to the forefront. This paper introduces a method known as the “frequency expander” to address frequency tampering. On a 200 km fiber link, we demonstrate a frequency tampering scenario using a frequency expander-enabled frequency tampering module (FTM). Additionally, we propose a hybrid fiber-based time synchronization and vibration detection method as a countermeasure. This hybrid method is exploring the idle light within the time synchronization system, i.e., Rayleigh backscattering, which is usually considered harmful.
Fiber optic communication links have long been touted as providing a secure communications channel for sensitive information. This reputation is largely based on the absence of electromagnetic emanations from the fiber. However, unauthorized access to data in optical fibers is a realistic possibility. Unfortunately it is not always possible to encrypt data transmitted via fiber optic links due to the lofty cost of high data rate encrypters. Therefore, on non-encrypted fiber communications channels, some form of tamper detection scheme is desirable to assure security. The limitations of conventional tamper detection techniques suggests that more sophisticated methods be used. Fiber optic links, like all communications channels, have a band-limited frequency response which produces a characteristic distortion of the signals that propagate down the fiber. A technique has been developed to perform a sensitive characterization of the fiber's frequency response function. This characterization can be exploited for detecting tamper in fiber optic communications links.>
In this study, we demonstrate the possibility to protect, with Quantum Key Distribution (QKD), a critical infrastructure as the fiber-based one used for time and frequency (TF) dissemination service. The proposed technique allows to disseminate secure and precise TF signals between two fiber-optic-connected locations, on a critical infrastructure, using both QKD and White Rabbit technique. This secure exchange enables the secret sharing of time information between two parties, allowing the synchronization of distant clocks with a stability of [Formula: see text] at 1 s, traceable to the Italian time scale. When encrypted, the time signals provide no useful information to a third party regarding the synchronization status, resulting in a time stability degraded by two orders of magnitude.
Optical clocks are considered as promising candidates for redefining the second in the International System of Units. Compared with microwave clocks, optical clocks are powerful tools for the fundamental research such as the constancy of the fundamental constants, the validity of Einstein’s theory of general relativity, and the predictions of quantum electrodynamics. Recently two research groups have demonstrated the optical clocks with an unprecedented precision level of 10-18, which is two orders better than the present primary frequency standard. Using two Sr optical clocks and three Cs fountain clocks, SYRTE group has demonstrated the definition of second with optical clocks.#br#For redefining the second with optical clocks in the future, the optical clocks from the remote laboratories should have a high precision and the frequency of the optical clocks need to be transferred over a long distance, with extremely high precision. Unfortunately the conventional means of frequency transfer such as two-way satellite time and frequency transfer can reach a 10-16 level in one day which is far below the requirement for an optical clocks. Various methods have been developed to transfer optical frequency signal via optical fibers. Especially a research group from Germany has achieved a frequency transfer stability of 10-19 level in hundreds of seconds with a fiber length of 1840 km.#br#We demonstrate the recent development of optical frequency transfer over a 70-km fiber spool at National Time Service Center. The measurement shows that the compensation for the fiber noise is close to the limitation induced by the fiber delay for the Fourier frequency from 1 Hz to 250 Hz. The transfer stability (Allan deviation) of the fiber link is 1.2×10-15 in 1 s averaging time, and 1.4×10-18 in 10000 s. A preliminary test of the optical frequency transfer over a 100-km spooled fiber is achieved with a stability of roughly one order worse than the 71 km result, 5×10-15 in 1 s.#br#We demonstrate a new scheme of remote compensation for optical frequency transfer via fibers against conventional local compensation method. This new scheme has the advantage of great simplification of the local site, which can find applications in massive extension of star network. The key feature is that we transfer the mixture of the round-trip signal and local reference to the remote user’s end via an auxiliary fiber. At remote site, the fiber noise is measured and compensated by AOM2 accordingly.#br#Transfer stabilities of 13×10-15 in 1 s averaging time and 4.8×10-18 in 10000 s are achieved with the remote fiber noise compensation via a 25 km fiber spool. The demonstrated transfer stability is comparable to that obtained by the local fiber noise compensation method.#br#The future star fiber network of optical frequency transfer can benefit from this method, because the simpler local setup is required and even can be shared in the central site for multitudinous remote users.
In recent years, the use of optical frequency dissemination through telecommunication fibers has steadily grown both in the context of stateof-the-art metrology like optical clocks and in the use of ultrastable reference signals for precision measurements in other fields of physics or earth science.Evaluating the performance of the fiber link can be difficult, as outliers and dysfunctioning regions lead to missing data.In this article, we discuss these challenges and investigate three different approaches to dealing with the missing data of a 1410-km-long optical link.We find a benefit in replacing the missing data with simulated noise with a statistical behavior similar to that of the original signal.