Tolerance in high-powered crosstalk attack of DPSK and NRZ in transparent optical networks

HUANG Qiong，YIN Peng－fei，SUN Ze－yu

(Key Laboratory of Mobile Communication Technology，Chongqing University of Posts and Telecommunications，Chongqing 400065，P.R.China)

1 Introduction

Transparentoptical network(TON)is an attractive network technique offering high－data－rates information delivery without complicated optical－electrical－optical conversion，and will getting increasingly popular in future several years.However，the ubiquitous transparency will introduce attack threats to TON，e.g.malicious users can gainmore chances to access to the net，and then inject a beam of light at high power being 20 dB or even 30 dB higher than normal one，which will result in crosstalk attack on normal signals［1－7］.Especially，in the optical switch architectures such as optical cross connect(OXC)，high－powered attack signal will leak significant power to normal channels working at the same wavelength frequency grid，resulting in intra－channel crosstalk attack.

A model to describe the propagation effect of intrachannel crosstalk attack in TON is shown in Fig.1［1，3－7］.In this model，high－powered signal(Attacker)will leak power to legitimate signal(User_1)through intrachannel crosstalk，and such significant leakage will heavily destroy User_1 and also enable it the attack capability.The attack capability in User_1 will in turn affect User_2 in the next switch;therefore，more stages of switcheswill be affected［3－7］.This intra－channel attack effect will cause severe damages to optical networks and due to the propagation effect，it is difficult to complement attack monitoring and localization［5－7］.

Fig.1 Model of intra－channel crosstalk attack propagation

The quantitative results of such attack effect have been proposed in our previous work［5－6］and we found that non－return－zero(NRZ)performs unexpectedly affronting the attack.In this paper，DPSK is employed to eliminate the attack effect due to its natural advantage of intra－channel crosstalk［8］.Via VPItransMakerTM，an optical communication system is simulated to investigate the BER penalty of both differential phaseshift－keying(DPSK)and NRZ signals imposed by high－powered intra－channel crosstalk attack.The results indicate that DPSK has about 9 dB higher tolerance of the attack than NRZ.

2 Simulation analysis and setup

At the receiver，the photodiodes convert a photo power into an electrical currentwhich is compared with decision threshold to determine whether the received bit is amark or space.Due to the power leakage from unintended channel in the optical switch，themain signal channel and crosstalk channel will result in a beat term at the receiverwhich reduces the photo power and dominates the BER penalty［9］.At the attack scenario，if only one interference source is considered，the input photo power of photodiode can be described as［9］

To investigate the worst beat noise，a communication system is simulated on VPItans－ MakerTMas illustrated in Fig.2.All laser transmitters transmit signals at rate of 10 Gbit/s and modulated on wavelength at 193.10 THz.Noiseless EDFAs(AmpSysOpt)is employed as optical amplifiers which are set to fix output average power at 0 dBm.Switch crosstalk intensity is set to －25 dB［4］at2 ×2 optical switches(SwitchDos_Y_Two).RX_OOK_BER is set to estimate BER for NRZ signal and RX_DPSK_BER with balanced detection is for DPSK.In this simulation，in order to simplified the system and focus on intra－channel crosstalk attack，the system impairment，including attenuation and dispersion in fiber，ASE in EDFA and insertion loss in optical component，are all approximated to be gauss white noise and NoiseWhiteGaussOpt component is employed as noise source with 192.10 THz center frequency and 3.4e－15 W/Hz power density.System impairment and crosstalk are respectively treated as gauss distribution at the receivers to achieve a conservative system design［10－11］.The BER of legitimate signalswithout attack is close to 1.0e－12 for DPSK and 1.0e－7 for NRZ modulated signal.To achieve the worst beat noise as described in equation(2)，three optical transmitters transmit signals at the same angle frequency.For NRZ signals，the same initial phase is set at the transmitters and for DPSK signals，the same bit sequence is used.180ophase shift is employed in two switches for the crosstalk，as shown in Fig.2.Polarization is set the same 0ofor all the transmitters to achieve the worst case polarization［12］.

Fig.2 Simulation setup demonstrates propagation of intra－channel crosstalk attack on VPI platform

3 Simulation results

Fig.3 illustrates the BER of signals on both NRZ and DPSK modulated signals under different attack powers detected by RX_1(1st order crosstalk attack)and RX_2(2nd crosstalk attack).In Fig.3，for 1st order attack，DPSK signal receives much less BER penalty than NRZ as the attack power is not strong enough.However，as the attack power increases，more negative beat noise is generated and both of the signals are damaged seriously.For the 2nd order crosstalk attack，the attack effect is notobvious and DPSK also receives lower BER than NRZ due to its nature advantage of DPSK modulation format［8］.

According to Fig.3，we can see that NRZ achieves its worst BER at the power of 22.42 dBm，and corresponding Pin，NRZis －11.96 dBm.For DPSK，at the attack power of 24.12 dBm，the BER is the worst and in case Pin，DPSKequals to － 20.81 dBm.So the tolerance advantage of DPSK under attack condition equals to Pin，NRZ－ Pin，DPSK，about9 dB than NRZ.

Fig.3 BER penalties of NRZ and DPSK signals imposed by intra－channel crosstalk attack with different attack powers

The reason for the 9 dB advantage is explained via constellations，which is showed in Fig.4.In Fig.4，the symbol distance of NRZ isand that of DPSK is 2，the distance of DPSK is increased bthan NRZ signal for the same average optical power［8］.This wider symbol distance makes DPSK more difficult to be attacked.Furthermore，when attack effect occurs，the symbol distance will be shorted by beat noise－，which induces BER penalty［8］.Since the peak power is 3 dB lower for DPSK than NRZ for the same average power，less beat noise is generated for DPSK.Wider symbol distance and less beat noisemake DPSK higher tolerance of intra－channel crosstalk attack than NRZ signal.

Fig.4 Constellation to explain higher tolerance of intra－channel crosstalk attack for DPSK than NRZ

The propagation effectmay be somewhat higher in case there is an amplifier after the attack source since the attacking signal will get stronger［5］.We will research it in our future work.

4 Conclusion

In conclusion，our work has investigated the BER penalty of DPSK and NRZ modulated signals imposed by high－powered intra－channel crosstalk attack via VPI－transMakerTM.The simulation results indicate that DPSK obtains about 9 dB higher tolerance of intrachannel crosstalk attack than NRZ.The work presented in this letter is expected to be useful to achieve the secure TON.

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Biography:

黄 琼(1971－)，女，四川西昌人，副教授，主要研究方向为个人通信、光互联网及交换技术。E－mial:huangqiong@cqupt.edu.cn。

(编辑:王敏琦)