RCDD Question #213: Real Exam Question with Answer & Explanation

The correct answer is D. Non-zero dispersion shifted (ITU-G655). For a 55 km link with planned DWDM expansion, ITU-T G.655 fiber is specified because it prevents four-wave mixing while remaining compatible with multi-wavelength transmission.

Question

You are placing PTP fiber between two locations 55 km (34 miles) apart. You are told that DWDM equipment will be installed within the next 5 years. Which of the following optical fiber types should you use?

Options

ADispersion unshifted (ITU-G652)
BDispersion shifted (ITU-G653)
CCut off shifted (ITU-G654)
DNon-zero dispersion shifted (ITU-G655)
EBending loss insensitive (ITU-G657)

Explanation

For a 55 km link with planned DWDM expansion, ITU-T G.655 fiber is specified because it prevents four-wave mixing while remaining compatible with multi-wavelength transmission.

Common mistakes.

A. ITU-T G.652 standard single-mode fiber has high chromatic dispersion at 1550 nm, requiring heavy dispersion compensation and making it non-ideal for planned DWDM deployments at this distance.
B. ITU-T G.653 dispersion shifted fiber moves the zero-dispersion point to 1550 nm, making it highly vulnerable to four-wave mixing crosstalk between DWDM channels and explicitly unsuitable for DWDM use.
C. ITU-T G.654 cut-off shifted fiber is optimized for ultra-low attenuation in long-haul submarine applications and is not designed for terrestrial DWDM multi-channel deployments.
E. ITU-T G.657 bending loss insensitive fiber is engineered for FTTH and indoor installations requiring tight bend radii, not for long-haul point-to-point DWDM links.

Concept tested. Optical fiber selection for DWDM compatibility

Reference. https://www.itu.int/rec/T-REC-G.655/en

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