1.

Introduction

The transparent optical network (TON) is an attractive network paradigm offering high data rates without expensive O-E-O conversion, and will be more available to public users as fiber-to-the-home (FTTH) getting increasingly popular. However, transparency will also introduce attack threats to TON, e.g., malicious users can gain more chances to access to the network, and then inject a beam of light at a high power being 20 dB or even higher than a normal one, which will result in crosstalk attack on normal signals.^{1, 2, 3, 4, 5} Especially in the optical switch architectures, such as optical cross connect (OXC), a high-powered attack signal will leak significant power to normal channels working at the same wavelength, resulting in intrachannel crosstalk attack.

A model to describe the propagation of intrachannel crosstalk attack in a TON is proposed as shown in Fig. 1.^{1, 3} In this figure, a high-powered signal (Attacker) will leak power to the legitimate signal (User 1) through intrachannel crosstalk, and such significant leakage will enable User 1 the attack capability. The attack capability in User 1 will in turn affect User 2 in the next switch, therefore more stages of switches will be affected.^{1, 3, 5} The high-powered signal will also rob the gain of adjacent channels in an optical amplifier and become stronger,^{4, 6} which will make the attack propagation of intrachannel crosstalk more serious.

Fig. 1

The propagation of intrachannel crosstalk in an OXC.

In this letter, we present three scenarios of intrachannel crosstalk attack, attack propagation within the first OXC, the secondary attacker traversing successive OXCs, and the original attacker traversing successive OXCs. Via VPItransmaker^TM, analysis on bit-error-rate (BER) and eye diagram penalties imposed by attack signals with different switch crosstalk intensities and detection methods are presented. The penalties accompanied with gain competition attack are also given. The simulation proved that the original attacker will cause the propagation effect of intrachannel crosstalk attack within three successive OXCs but with a limited two stages of switches in each OXC. The results also proved that the polluted signals (i.e., the polluted secondary attacker) do not have enough attack capabilities to propagate intrachannel crosstalk attack to the next OXC. The simulation also indicates that if gain competition attack exists, the BER will be somewhat higher.

2.

Simulation Analysis and Setup

Figure 2 depicts the simulated TON system, in which four wavelengths are multiplexed in a fiber and all four laser transmitters (TxExtModLaser) transmit at a power of 1 mW. All the transmitted signals are nonreturn to zero non-return-to-zero (NRZ) formats at a rate of 10 Gbit/s and modulated on four wavelength channels λ₀, λ₁, λ₂, and λ₃ according to ITU 100 GHz grid from 193.00 to 193.30 THz at C-band. The grid spacing is wide enough to suppress four-wave mixing and cross-phase modulation (XPM)⁶ to focus on intrachannel crosstalk attack. However, the extremely high-powered signal will cause serious self-phase modulation (SPM), which causes the broadening of the signal spectrum and power degradation of the attack signal.⁷ The statistics of phase-difference between legitimate and crosstalk signal which dominates the BER performance⁸ is also difficult to determine due to serious SPM, thus multisimulations are carried out by adjusting phase shift in optical switch and we select the worst BER. All segment fibers are nonlinear dispersive fibers (NLS) with 0.2 dB/km attenuation and 2.6 × 10⁻²⁰ m²/W nonlinear index. The dispersion for all NLS segments is set identical 2.0 × 10⁻³ ps/nm/km also without compensation so that we can concentrate on the crosstalk attack. We employ erbium doped fiber amplifier (EDFA) (AmpSysOpt) as an amplifier with 16 dB fix gain to avoid gain competition attack and 4 dB noise figure. Multiplexer and switch are with 2 dB insertion loss. Node spacings are set to 400 km including five loops of 80 km NLS segments.⁹

Fig. 2

Simulation setup demonstrates propagation of intrachannel crosstalk attack.

We assume that the attacker injects a high-powered signal modulated at the same 10 Gbit/s NRZ format as legitimate signals on wavelength λ₁ at a power of 500 mW and the injection point is 15 km before OXC-1.² In each OXC, three cascaded 2 × 2 optical switches (SwitchDos-Y-Two) are set for channel λ₁. Let λ_{1, n} represent the legitimate signal on wavelength λ₁ passing the n’th stage of an optical switch. The high-powered attacker on wavelength λ₁ will attack legitimate signals λ_{1, 1}, λ_{1, 2} and λ_{1, 3} from OXC-1 to OXC-6, as shown in Fig. 2. The polluted signal in OXC-1 directly traverses OXC-7 and OXC-8. At the egress points of each switches (i.e., the point labeled as @), BERs and eye diagrams are detected using RxBERs and ViScopes.

3.

Simulation and Discussion

3.1.

Intrachannel Crosstalk Attack Propagation Within OXC

We assume the crosstalk to be Gaussian distribution to achieve the upper bound of system BER (Ref. 10) and the statistics of the received optical signal will follow one of a family of Chi-squared probability densities,^{11, 12} and then the Chi-squared method is set to estimate the BER at the receivers. As a comparison, the Gaussian method is also implemented, in which the statistics of the received optical signal are assumed to be Gaussian distribution. Switch crosstalk intensity is a parameter representing the amount of power leakage between two channels in an optical switch, which determines how much crosstalk noise will be added in the received optical signals. Figure 3 illustrates the BER of signals λ_{1, 1}, λ_{1, 2}, and λ_{1, 3} in OXC-1. As switch crosstalk intensity decreases from −20 to −35 dB in four grades, i.e., −20, −25, −30, and −35 dB,⁴ the BERs for λ_{1, 1} are affected between 0.1 and 0.5, and those for λ_{1, 2}, are distinguishingly distributed. However, BERs for λ_{1, 3} are all kept lower. Eye diagrams for λ_{1, 2} with different switch crosstalk intensities under Chi-squared detection are also given in Fig. 4. The result reveals that the extent of intrachannel attack propagation within an OXC is different with switch crosstalk intensities but with limited two stages of switches. The result also indicates that as the crosstalk increases, there will be less difference between detection methods. Compared to the Gaussian method, the Chi-squared method will overestimate the system BER.

Fig. 3

BER for intrachannel crosstalk attack within OXC-1 with different switch crosstalk intensities and crosstalk detection methods.

Fig. 4

Eye diagrams for λ_{1, 2} within OXC-1 under the Chi-squared detection method with different switch crosstalk intensities of optical switches (−20, −25, −30, and −35 dB).

3.2.

Original Attacker Traversing Successive OXCs

To investigate the intrachannel crosstalk attack spanning multiple OXCs, the original attack signal is set to traverse from OXC-1 to OXC-6. The switch crosstalk intensity is set to be identically −25 dB at all switches. Figure 5 shows the BERs of λ_{1, 1}, λ_{1, 2}, and λ_{1, 3} in OXC-2 to OXC-6, respectively. The BERs of λ_{1, 1} at OXC-2 and OXC-3 are 0.3 and 1.05  ×  10⁻⁵, respectively. The BER of λ_{1, 1} at OXC-4, -5, and -6 are all quickly dropped. Those BERs of λ_{1, 2} and λ_{1, 3} at five OXCs are all less than 1.0  ×  10⁻¹⁰. Eye diagrams of λ_{1, 1} in OXC-2 to OXC-5 are shown in Fig. 6. By setting EDFA to power model, a gain competition effect can be achieved. Figure 5 shows the BER penalties accompanied with gain competition attack, in which we can see that the robbed gain in EDFA will make the BER of λ_{1, 1} in OXC-2 to OXC-6 a little worse for the reason that −25 dB of the robbed power will be leaked to the legitimate channels. The results also indicate that only the first stage of switches will be affected worse if gain competition attack exists. The results indicate that the high-powered original intrachannel crosstalk can propagate its attack effect to successive three OXCs and the BER will be somewhat higher in case there is gain competition attack.

Fig. 5

BER of signals λ_{1, 1}, λ_{1, 2}, and λ_{1, 3} in OXC-2 to OXC-8: (a) with fix gain in EDFA and (b) accompanied with gain competition attack.

Fig. 6

Eye diagrams of λ_{1, 1} in OXC-2 to OXC-5.

4.

Conclusions

In this letter, three attack scenarios of intrachannel crosstalk accompanied with gain competition attack have been presented and investigated via VPItransMaker^TM. We found that the high-powered signal will propagate an intrachannel crosstalk attack to successive three OXCs but with limited two stage switches within each OXC, and this propagation extent depends on switch crosstalk intensity and detection method. The secondary attacker does not have enough attack capability to propagate this attack. We also found if gain competition exists, the system BER will be somewhat higher.