Optical transceiver selection becomes increasingly complex as networks span from inside a data center rack to regional metro rings exceeding 100 kilometers. On one end, 400G VSR4 solutions like OSFP112-400G-VSR4 and QSFP56-DD-400G-VSR4 provide ultra-low-power intra-rack connectivity. On the other end, QSFP28 100G ZR4, QSFP28 100G BIDI 40KM/80KM, and specialized QSFP28 100G 100KM modules handle long-haul transport. This article bridges the gap between data center and telecom optics, offering clear guidance on when to deploy each technology, how to manage fiber scarcity with BIDI, and how to plan mixed 100G/400G networks.
For distances beyond 10km, the QSFP28 form factor offers several mature options. Understanding their differences in reach, fiber usage, and power is critical for metro and regional networks.
QSFP28 100G ZR4 is specified by the 100G ZR4 MSA for up to 80km over duplex single-mode fiber (SMF). It uses four wavelengths (1295.56, 1300.05, 1304.58, 1309.14 nm) in a CWDM grid, each carrying 25G NRZ (or 50G PAM4 in some implementations) to achieve 100G aggregate. The ZR4 specification includes tighter dispersion tolerance and stronger FEC (Reed-Solomon RS(528,514)) compared to LR4/ER4. Typical transmit power ranges from -2 to +4 dBm, with receiver sensitivity around -23 dBm (pre-FEC). This 21dB link budget allows 80km with standard SMF (0.25 dB/km loss) and multiple connectors. QSFP28 100G ZR4 is widely deployed in metro aggregation networks and regional data center interconnects (DCI) where 80km is sufficient.
When fiber pairs are expensive or unavailable, bidirectional (BIDI) technology provides a compelling alternative. QSFP28 100G BIDI 40KM and QSFP28 100G BIDI 80KM use two wavelengths (typically 1270nm/1330nm or 1310nm/1490nm) over a single strand of SMF. Each wavelength carries 50G PAM4 or 4×25G NRZ depending on the design, achieving 100G full duplex on one fiber. This effectively doubles fiber utilization compared to traditional duplex optics. However, BIDI modules have stricter link budgets due to the need for wavelength separation and higher reflection sensitivity. For 40km BIDI, the optical budget is typically 16-18dB; for 80km BIDI, it reaches 20-22dB. They are popular in fiber-constrained urban access networks and for connecting remote cell site aggregators.
Important: QSFP28 100G BIDI 40KM and QSFP28 100G BIDI 80KM must be used in matched pairs. You cannot mix BIDI with standard LR4 or ZR4. Also, BIDI modules often require cleaner connectors (APC recommended) to minimize back-reflection.
Standard QSFP28 100G ZR4 cannot reliably reach 100km due to dispersion and loss limits. Modules advertised as QSFP28 100G 100KM typically employ coherent detection (100G coherent DSP) or incorporate SOA (semiconductor optical amplifiers). Coherent 100G modules (sometimes called 100G ZR or ZR+) use advanced modulation (e.g., QPSK) and digital signal processing to compensate for chromatic dispersion and PMD, enabling reaches of 100km to 200km without external amplification. These modules are larger in power (6-8W) and more expensive. They are used in long-haul DCI and regional backbone links. When specifying QSFP28 100G 100KM, always verify if the module is coherent or amplified direct-detect, and confirm compatibility with your switch’s coherent support (e.g., Cisco NCS 1000 series).
While 100G handles long distances, 400G dominates high-density data center cores. The two main 400G categories for sub-500m links are VSR4 (very short reach) and DR4 (500m).
OSFP112-400G-VSR4 is designed for reaches up to 100 meters, typically using 4 parallel multimode fibers (MMF) with 850nm VCSELs. The OSFP112 form factor uses 4 electrical lanes at 112G each, resulting in lower power (7-8W) and latency compared to 8-lane designs. QSFP56-DD-400G-VSR4 achieves similar performance but in the QSFP-DD footprint, using 8×50G electrical to 4×100G optical. Both are ideal for top-of-rack (ToR) to server connections, or leaf-spine links under 100m. When power efficiency is paramount, OSFP112-400G-VSR4 leads the market.
For distances between 100m and 500m, QSFP56-DD-400G-DR4 (also called QSFP DD DR4) is the de facto choice. It transmits four 100G PAM4 optical lanes over four parallel single-mode fibers (MPO-12 connector). Each lane uses a 1310nm DFB laser. Power consumption is around 10W. A key advantage is breakout capability: one QSFP56-DD-400G-DR4 port can be broken into four 100G DR1 links, directly connecting to QSFP28 ports on legacy switches. This makes DR4 a future-proof building block for data center spine-leaf architectures.
Though these families serve different roles, understanding their relative specifications helps in end-to-end network design.
| Transceiver | Form Factor | Reach | Fiber Type | Fiber Count | Power | Typical Use |
|---|---|---|---|---|---|---|
| QSFP28 100G ZR4 | QSFP28 | 80km | SMF | 2 strands | 5-6W | Metro regional |
| QSFP28 100G BIDI 40KM | QSFP28 | 40km | SMF | 1 strand | 4W | Fiber-saving access |
| QSFP28 100G BIDI 80KM | QSFP28 | 80km | SMF | 1 strand | 5-6W | Fiber-saving metro |
| QSFP28 100G 100KM | QSFP28 | 100km+ | SMF | 2 strands | 6-8W | Long-haul DCI |
| OSFP112-400G-VSR4 | OSFP112 | 100m | MMF | 8 strands (4 Tx, 4 Rx) | 7-8W | Intra-rack, ToR |
| QSFP56-DD-400G-VSR4 | QSFP-DD | 100m | MMF | 8 strands | 8-9W | Intra-rack (QSFP ecosystem) |
| QSFP56-DD-400G-DR4 | QSFP-DD | 500m | SMF | 8 strands | 10W | Spine-leaf, 500m DCI |
Real-world networks often require both: 400G inside the data center and 100G for metro/long-haul interconnects. Here are common hybrid scenarios.
A company has two data centers 75km apart. Inside each data center, switches run 400G using QSFP56-DD-400G-DR4 for spine-leaf. For the DCI link, the optimal solution is QSFP28 100G ZR4 on both ends, using a 100G DWDM mux or direct fiber. If fiber is scarce, two QSFP28 100G BIDI 80KM modules can carry 100G over a single fiber pair (one fiber per direction? Actually BIDI uses one fiber for both directions, so two modules would require two separate fibers if using two independent links). Alternatively, a single QSFP28 100G BIDI 80KM provides 100G bidirectional on one fiber, leaving the other fiber for another service.
For a remote office at 100km distance, standard ZR4 is insufficient. Use QSFP28 100G 100KM coherent modules on both ends. Ensure the switch supports coherent optics (e.g., Arista 7280CR with coherent support). The 100km link may still require inline amplifiers if fiber loss exceeds 25dB.
Existing campus backbone uses QSFP28 100G LR4 (10km) between buildings. To upgrade to 400G, you can replace the LR4 modules with QSFP56-DD-400G-DR4 only if the distance is under 500m. For 10km, you would need 400G LR4 (10km) which is expensive. A more practical approach: keep 100G for campus backbone and deploy 400G only inside data centers, using 100G uplinks from the data center leaf to campus core.
BIDI technology is a game-changer when dark fiber is limited or leased fiber costs are high. For example, a metro ring with only two available fibers can support 100G links using QSFP28 100G BIDI 40KM or 80KM on each fiber, effectively providing 200G aggregate (two independent 100G links). However, BIDI modules have downsides: they are less standardized, require matched pairs, and are more sensitive to reflections and connector quality. For new builds with abundant fiber, ZR4 remains simpler. For retrofits or fiber exhaust scenarios, BIDI is invaluable.
Note: QSFP28 100G BIDI 80KM achieves its reach using high-power lasers and APD receivers. Some modules also incorporate EDC. Always check the datasheet for chromatic dispersion tolerance; at 80km, dispersion can exceed 1300 ps/nm, which may require dispersion compensation for non-coherent BIDI.
Power per gigabit is a critical metric. A QSFP28 100G ZR4 consumes 5-6W, or 50-60mW/Gb. A QSFP56-DD-400G-DR4 consumes 10W, or 25mW/Gb—twice as efficient. OSFP112-400G-VSR4 at 7-8W gives 17.5-20mW/Gb, even better. However, long-haul optics always consume more power per bit due to amplification and DSP needs. For greenfield data centers, prioritizing VSR4 and DR4 over multiple 100G links reduces both power and port count.
Cooling: In a 1U switch with 32 ports of QSFP56-DD-400G-DR4, total optical power 320W requires high airflow (≥15 CFM per module). Long-haul QSFP28 100G 100KM coherent modules generate more heat per port (6-8W) but are typically used in low-density routers or dedicated transport gear, not in high-density data center switches.
When connecting a 400G DR4 port to a 100G ZR4 link, direct connection is impossible due to different fiber counts and signaling. Two methods exist:
Breakout cable: A QSFP56-DD-400G-DR4 port can break out to four 100G DR1 signals, but those DR1 signals are not compatible with QSFP28 100G ZR4 (different wavelength and modulation). You would need a media converter or a switch that supports 100G ZR4 on its breakout ports.
Transponder/muxponder: Use an external device that takes a 400G DR4 input and converts it to a 100G ZR4 output (e.g., a 4×100G to 100G muxponder). This adds cost but is necessary for long-haul integration.
For most operators, it’s simpler to keep 400G for intra-DC and 100G long-haul as separate domains, interconnected via router ports that support both.
Yes, but only one fiber strand is used. You can connect a BIDI module to a duplex LC cable by using only one of the two fibers. Ensure the other end also has a BIDI module with the complementary wavelength pair.
Depending on fiber quality and connector loss, 80km is safe. Some modules can reach 90-100km with low-loss fiber (0.2 dB/km) and clean connectors, but this is not guaranteed. For 100km, use a certified QSFP28 100G 100KM coherent module.
Yes. VSR4 is designed for OM3/OM4 multimode fiber. For 100m reach, OM4 is recommended. OM3 may limit to 70-80m.
Yes, as long as the switch’s port configuration matches each module’s requirements. However, they cannot directly connect to each other because one uses SMF (DR4) and the other typically uses MMF (VSR4).
When you have only one fiber available (or want to use one fiber for 100G and the other for a different service), BIDI is the only choice. ER4 requires two fibers. If fiber is abundant, ER4 is simpler and more standard.
Check the switch vendor’s compatibility list. Coherent 100G modules use different DSP and sometimes require special software licensing (e.g., Cisco’s coherent OS). Most general-purpose switches like Arista 7280CR support certain coherent modules. When in doubt, use a dedicated coherent transponder.
Per gigabit, 400G DR4 is cheaper (roughly $0.5-0.8 per Gb) vs 100G ZR4 ($1-1.5 per Gb) because of higher volume and simpler optics. However, ZR4 includes longer reach and dispersion compensation, which adds cost.
As 400G becomes common in data centers, the telecom industry is standardizing 400G ZR (coherent) for up to 120km and 400G ZR+ for longer distances. These will eventually replace 100G ZR4 in metro DCI. However, 100G long-haul will remain relevant for low-bandwidth regional links and as backup circuits. Meanwhile, OSFP112-400G-VSR4 and QSFP56-DD-400G-VSR4 are being extended to 800G VSR8 using 112G lanes. Network planners should design fiber infrastructure (both MMF and SMF) to accommodate future speed upgrades.
Successful optical networking requires matching transceiver technology to each link’s unique distance, fiber availability, and bandwidth needs. For intra-rack and ToR connections under 100m, OSFP112-400G-VSR4 or QSFP56-DD-400G-VSR4 offer the lowest power and latency. For spine-leaf up to 500m, QSFP56-DD-400G-DR4 is the proven workhorse. For campus backbones up to 10km, QSFP28 100G LR4 suffices. For metro regional up to 80km, QSFP28 100G ZR4 or BIDI 80KM (when fiber is scarce) are excellent choices. And for true long-haul 100km+, only coherent QSFP28 100G 100KM modules will deliver reliable performance.
Our team offers end-to-end optical design consulting, from 400G VSR4 breakout cables to 100G ZR4 link budget calculations. We stock all the modules discussed in this guide, pre-qualified for major switch brands, and provide free link testing before deployment.
Contact us to discuss your specific distance and fiber constraints. We will help you build a cost-optimized, future-ready optical layer that seamlessly integrates 100G long-haul and 400G data center technologies.
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