Croatian Single-Fiber Bidirectional NRZ

Single-fiber bidirectional NRZ enables simultaneous two-way data transmission over a single optical fiber using Non-Return-to-Zero line coding, optimizing fiber usage in high-speed networks.

Overview of Single-Fiber Bidirectional NRZ

Single-fiber bidirectional (BIDI) optical communication allows two independent data channels to share a single optical fiber, transmitting in opposite directions. This approach is particularly useful in scenarios where fiber resources are limited, such as edge access networks, enterprise campuses, or inter-city links . NRZ (Non-Return-to-Zero) is a line coding technique where binary '1' and '0' are represented by distinct voltage or light levels without returning to zero between bits, providing a simple and power-efficient signaling method .

Key Features

• Data Rates: Single-fiber BIDI modules using NRZ can support rates from 125 Mbit/s up to 25 Gbit/s per lane, with modern QSFP28 modules aggregating up to 100G using 4×25G NRZ lanes .
• Wavelength Division: BIDI modules often use different wavelengths for each direction (e.g., 1310 nm forward, 1330 nm reverse) to prevent interference and enable full-duplex operation on a single fiber .
• Applications: Ideal for data center interconnects, metro networks, and 5G convergence rooms, where fiber availability is constrained .
• Distance: Depending on the module and fiber type, single-fiber BIDI NRZ can achieve short-range (100 m on OM4 multimode) to long-range (up to 80 km on single-mode fiber) transmissions .

NRZ vs Other Line Coding

NRZ is widely used due to its simplicity and low power consumption, but it is more susceptible to inter-symbol interference (ISI) over long distances compared to RZ (Return-to-Zero) coding . NRZ is often combined with forward error correction (FEC) and high-sensitivity receivers to maintain signal integrity in long-distance BIDI links .

Practical Implementations

• QSFP28 BIDI Modules: Convert four 25G NRZ electrical signals into optical signals for single-fiber bidirectional transmission, supporting 100G links over distances up to 80 km with KR4 FEC .
• SFP/eSFP BIDI Modules: Support lower data rates (up to 2.67 Gbit/s) for enterprise or campus networks, providing cost-effective fiber utilization .
• Optical Compute Interconnects: Advanced implementations use NRZ with DWDM to achieve high-density, low-latency interconnects for AI and HPC clusters, supporting bidirectional operation over a single fiber with multiple wavelengths .

Advantages

• Fiber Efficiency: Reduces the number of fibers required for full-duplex communication.
• Cost Savings: Minimizes fiber deployment and maintenance costs.
• Scalability: Supports upgrades from 40G to 100G or higher without additional fiber installation.
• Compatibility: Works with standard optical modules and existing network infrastructure. In summary, Croatian single-fiber bidirectional NRZ systems leverage NRZ line coding and wavelength multiplexing to maximize fiber utilization, offering a practical solution for high-speed, cost-effective optical networks in environments with limited fiber availability .