Network architects today face a deluge of optical module choices, each with unique cost, reach, and power profiles. While 400G brings higher density, 100G remains the workhorse for distance-sensitive applications. This article provides an economics-driven comparison of key modules including OSFP112-400G-VSR4, QSFP56-DD-400G-VSR4, QSFP DD DR4, QSFP112 (compact 400G), and the full 100G lineup: QSFP28 100G LR4, ER4, ZR4, QSFP28 100G 100KM, QSFP28 100G BIDI 40KM, and QSFP28 100G BIDI 80KM. We will help you decide which module minimizes total cost of ownership for your specific link distance, fiber availability, and future upgrade path.
For distances under 2km, 400G modules offer exceptional value when amortized over high-bandwidth links. Two dominant types are VSR4 (Very Short Range) and DR4 (500m Datacenter Range).
Both OSFP112-400G-VSR4 and QSFP56-DD-400G-VSR4 target links up to 2km, typically over single-mode fiber (SMF) or, in some cases, OM4 multimode with limited reach. The key advantage of VSR4 modules is their low power consumption (typically 8–10W) achieved by using simplified DSP or direct CDR designs. They are ideal for data center interconnect (DCI) within a campus or between adjacent buildings. The OSFP version offers better heat dissipation for high-density faceplates, while the QSFP56-DD version benefits from wider switch compatibility. When deploying QSFP56-DD-400G-VSR4, ensure your switch supports the required FEC mode (often RS-FEC) to achieve the full 2km reach without bit errors.
For links up to 500m, QSFP DD DR4 (and its electrical equivalent QSFP112 400G-DR4) provides the lowest cost per gigabit. DR4 uses four parallel single-mode fibers with an MPO-12 connector, each lane running at 100G PAM4. The DR4 design eliminates expensive WDM optics, making it the default choice for spine-leaf connections inside a data center hall. Compared to VSR4, DR4 has slightly lower optical budget but also lower manufacturing complexity. If your inter-rack distance is ≤500m, choose DR4; if it exceeds 500m up to 2km, step up to VSR4.
When links stretch beyond 2km, 100G modules often provide better economics than 400G, especially for point-to-point spans where traffic does not require full 400G capacity.
The QSFP28 100G LR4 remains the gold standard for campus and metro edge connections up to 10km. Its LAN-WDM technology (4 wavelengths around 1310nm) keeps dispersion low and enables direct connection without optical amplification. For distances between 10km and 40km, QSFP28 100G ER4 adds an APD receiver and SOA pre-amplifier, raising power to ~4.5W but avoiding costly external amplifiers. When you need up to 80km, QSFP28 100G ZR4 further boosts sensitivity and uses stronger FEC. However, for links >40km, dispersion compensation becomes critical – many ZR4 modules assume standard G.652 fiber with dispersion ≤1600 ps/nm (roughly 80km). Exceeding that may require tunable dispersion compensation or coherent technology.
The QSFP28 100G 100KM segment is nuanced. True 100km non-coherent transmission in a QSFP28 form factor is challenging due to optical SNR limitations. Most solutions marketed as “100km” are either coherent modules in a larger package (e.g., CFP2-DCO) or enhanced non-coherent designs that require ideal fiber conditions (low loss ≤25dB, low dispersion). Enterprises should treat QSFP28 100G 100KM as a specialized product – always request link budget calculations and real-world test reports. If your backbone regularly exceeds 80km, consider migrating to 400G coherent (e.g., 400G-ZR) which offers better performance and scalability.
When dark fiber is scarce or leased fiber costs are high, QSFP28 100G BIDI modules become extremely attractive. BIDI transmits two directions on one fiber using different wavelength pairs.
This module uses two wavelengths (e.g., 1270nm TX / 1330nm RX) or four wavelengths in a 2x2 configuration to achieve full 100G over a single fiber. Typical reach is 40km on G.652 fiber. The main advantage is 50% fiber reduction compared to LR4/ER4, which require two fibers. For metro rings where fiber pairs are limited, QSFP28 100G BIDI 40KM can double the number of connections without laying new cable. However, BIDI modules have higher insertion loss on the WDM filter, so careful power budgeting is needed – avoid multiple patch panels.
Extending BIDI to 80km requires even more sensitive optics and advanced FEC. QSFP28 100G BIDI 80KM often incorporates SOA pre-amplifiers on the receive side, increasing power to ~6W. It is best suited for greenfield deployments or long-haul upgrade projects where adding new fiber is prohibitively expensive. Always verify the module’s dispersion tolerance: at 80km, standard SMF accumulates ~1360 ps/nm, which may exceed the capability of some BIDI designs. Look for modules with integrated EDC (electronic dispersion compensation).
To simplify decision making, we present three common distance bands and the optimal module type from an economics perspective.
≤500m (inside data center): QSFP DD DR4 or QSFP112 400G-DR4 – lowest $/Gbps, future-proof for 400G switches. Only use 100G if switch ports are 100G-only.
500m – 2km (campus or adjacent DCs): OSFP112-400G-VSR4 or QSFP56-DD-400G-VSR4 – moderate cost, single-fiber pair per link. If traffic per link is<100G, consider QSFP28 100G LR4 with 4x aggregation.
2km – 40km (metro edge): QSFP28 100G ER4 is the workhorse. For fiber conservation, use QSFP28 100G BIDI 40KM. 400G is rarely cost-effective at this reach unless you have extremely high bandwidth demand (>2x400G).
40km – 80km (metro core): QSFP28 100G ZR4 or QSFP28 100G BIDI 80KM. For new builds, evaluate 400G-ZR (QSFP-DD) coherent modules if future growth justifies the higher upfront cost.
>80km (regional DCI): QSFP28 100G 100KM (if available and link loss<25dB) or move to coherent 100G/400G (CFP2-DCO or QSFP-DD ZR).
Avoid common pitfalls with these actionable guidelines:
Check switch FEC support: Many 400G VSR4 modules rely on RS-FEC (Reed-Solomon Forward Error Correction). If your switch only supports RS-FEC for 400G but not for breakout modes, you may need to disable breakout.
BIDI wavelength planning: When using QSFP28 100G BIDI 40KM or 80KM on a WDM system, ensure no other service uses the same wavelength pair (e.g., 1270/1330nm). BIDI is not DWDM-tunable.
Connector hygiene: For MPO-based DR4 modules, a single dirty ferrule can bring down all four lanes. Use certified cleaning tools and perform insertion loss tests.
Thermal derating: High-density 400G line cards may exceed thermal limits when fully populated with 12W+ modules. Prefer OSFP112-400G-VSR4 (lower power) for dense configurations.
A: No. QSFP56-DD has double the electrical lanes (8 lanes vs. 4) and a different pinout. However, some switches support “port-splitting” using a QSFP-DD to 2x QSFP28 fanout cable, but that would give 2x200G not 400G.
A: ER4 is guaranteed to 40km on standard SMF with a link loss budget of ~23dB. ZR4 extends to 80km with a budget of ~27dB. However, ZR4 requires better fiber quality (lower PMD) and may need dispersion compensation beyond 60km.
A: For four 100G links under 500m, a single 400G DR4 module + breakout cable is almost always cheaper than four 100G LR4 modules (both in module cost and port consumption). But for links >10km, four 100G LR4 modules are more economical than any 400G solution because 400G long-haul modules remain expensive.
A: Most genuine 100km+ solutions in QSFP28 size are non-coherent but use EDFA pre-amps and high-power boosters. True coherent 100G typically requires CFP2 or QSFP-DD due to DSP power. Always ask the vendor for the technology type and link budget.
A: No, because VSR4 and DR4 have different optical specifications (transmit power, receive sensitivity, and FEC requirements). Both ends must be identical or explicitly interoperable per MSA.
A: It depends. At 80km on G.652 fiber, dispersion is ~1360 ps/nm. Some advanced BIDI modules have integrated EDC (Electronic Dispersion Compensation) and can handle up to 1600 ps/nm. Others may need external DCM. Check the datasheet’s dispersion tolerance.
Selecting the optimal module among OSFP112-400G-VSR4, QSFP56-DD-400G-VSR4, QSFP DD DR4, QSFP28 100G LR4/ER4/ZR4, QSFP28 100G 100KM, and BIDI 40/80KM variants requires balancing distance, fiber count, power budget, and future traffic growth. Our team offers free link engineering – including dispersion and loss calculations – and provides sample modules for validation in your actual environment.
Contact our optical specialists today to receive a customized procurement roadmap and volume pricing. We also offer trade-in credits for legacy 10G/40G modules when upgrading to 100G or 400G. Optimize your network economics now.
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