Optical Types, Wavelengths, and Reach

Transceiver optical variants are designed for specific distances and fiber types. Common Ethernet designations include:

• SR (Short Reach): Multimode fiber (MMF), usually at 850 nm. Examples: 10GBASE-SR up to typical MMF distances depending on OM grade.
• LR (Long Reach): Single-mode fiber (SMF), usually 1310 nm, around 10 km for 10GBASE-LR.
• ER/ZR: Extended reach on SMF, longer distances with higher link budgets.
• DR, FR, LR for 100G/200G/400G: Standardized reaches over SMF with 100G per wavelength or per lane (e.g., 100GBASE-DR 500 m, 100GBASE-FR 2 km, 100GBASE-LR 10 km).
• CWDM4/CLR4: Coarse WDM with 4 wavelengths on SMF for 2 km to 10 km classes.
• PSM4: Parallel SMF using 8 fibers (4 transmit, 4 receive) at 1310 nm for 500 m to 2 km classes.
• BiDi: Bidirectional over two fibers using different wavelengths on each direction to reuse duplex MMF plants.

Exact reach depends on fiber quality, connector loss, and standards compliance. Data center designs often use SR for short patching within rows, DR/FR for inter-row or building-scale links, and longer-reach variants for campus or metro extensions.

Multimode vs. Single-Mode Fiber

• Multimode (OM3/OM4/OM5): Larger core (50 μm) supports cost-effective SR optics with VCSEL sources. Reach depends on modal bandwidth; OM4 and OM5 support longer distances and higher lane rates than OM3.
• Single-mode (OS1/OS2): Smaller core (~9 μm) for long reach using lasers at 1310 nm or 1550 nm. OS2 is common in modern cabling for low attenuation.

Selection weighs initial transceiver costs, density, and future scalability. SMF increasingly appears within data halls due to simplified migration paths to 100G+ rates and consistent reach.

Connectors, Polarity, and Cabling Plant

Connector type aligns with optical interface:

• LC duplex: Two-fiber connections common on LR/FR/DR and BiDi modules.
• MPO/MTP: 12, 16, or 24-fiber connectors used for parallel SR, PSM4, and breakout applications. Polarity (A, B, C methods) ensures transmit aligns to receive; cassettes and trunks are selected accordingly.

Insertion loss budgets account for trunks, cassettes, and patch cords. Typical data center designs standardize on low-loss components to support higher-speed links with tight budgets, especially PAM4-based optics.