Reach of 400G and 800G coherent modules: a comparison for typical carrier routes

Why the range of a coherent module is a variable, not a constant

A classic SFP+ module has a fixed range of 10 km or 40 km. You buy a module for a specific distance, and that’s it. A coherent module works differently because the same hardware supports multiple modulation modes, each representing a different trade-off between throughput and range.

The modulation mode determines how many bits are encoded in a single optical symbol. 16QAM encodes four bits per symbol, providing maximum information density, but the symbols are close together, and noise causes errors over long distances. QPSK encodes two bits per symbol; the symbols are far apart, so the signal tolerates much greater degradation and travels much further. 8QAM sits in between. A network designer selects the modulation mode for a route just as an engineer chooses a speed for a road: maximum speed on a straight highway, but safety over speed on a mountain pass.

400G modules, ranges by mode

GBC Photonics QSFP-DD 400G OpenZR+ modules operate in three primary modulation modes, each corresponding to a different type of route in a real-world operator network.

400G DP-16QAM mode, OIF 400ZR standard

Maximum throughput with a range of up to 120 km. This mode is designed for DCI connections between data centers in the same city or between cities separated by several dozen kilometers. It requires good OSNR and a path without an excessive number of passive elements. For routes under 80 km using high-quality G.652.D fiber, it is the optimal choice; beyond 80 km, the result depends on the number of ROADM nodes and path quality, which is why we always calculate the optical budget before deployment. Typical applications include connections between data center nodes in Warsaw, Krakow, or Wroclaw, metro networks handling urban and suburban traffic, and DCI between an organization's facilities within a 100 km radius.

300G DP-8QAM mode

Three-quarters of the throughput with a range of up to 600 km under good path conditions, or realistically 300 to 500 km for routes with intermediate nodes. 8QAM provides a significantly higher OSNR margin than 16QAM while maintaining 75 percent of the throughput. For regional routes where 16QAM falls short and 100G is not enough, 8QAM is the right answer. In production tests on the Poznan-Frankfurt route—nearly 1,000 km on a Polish backbone operator's infrastructure—GBC Photonics 400G modules achieved stable 300G transmission in 8QAM mode, directly from the router, without external transponders. Applications include backbone routes between major nodes in Poland, connections to neighboring CEE countries, and regional networks serving multiple provinces.

200G DP-QPSK mode

Half the throughput with a range exceeding 1,000 km under favorable path conditions. QPSK has the highest OSNR margin among standard coherent modulation modes, which translates into resistance to signal degradation when passing through multiple ROADM nodes and EDFA amplifiers. Applications include international connections, routes with many intermediate nodes, and securing critical backbone links where reliability is more important than throughput.

800G modules with PCS, ranges by mode

The GBC Photonics QSFP-DD 800G OpenZR+ module with PCS (Probabilistic Constellation Shaping) represents the next generation. PCS allows for smooth adjustment of the effective modulation mode between standard values, so instead of jumping between QPSK and 8QAM, the module moves along the entire scale.

The 800G DP-16QAM mode provides maximum throughput for short links and DCI, with a range of 80 to 120 km given good OSNR. This mode is for connections between nearby locations where the priority is maximum throughput density per port, such as DCI within the same metropolitan area, campus connections within a few dozen kilometers, and high-density GPU data centers.

The 600G DP-8QAM mode provides three-quarters of 800G throughput with a range of 300 to 500 km. Thanks to PCS, the module automatically optimizes the trade-off between throughput and range based on measured OSNR, adapting smoothly to path conditions. This is a solution for regional routes with modern ROADM nodes and connections between large cities where the required throughput exceeds the capabilities of 400G.

For longer routes, an 800G module with PCS can operate at 400G throughput or lower, maintaining a significantly higher range than in 800G mode. The flexibility of PCS allows for continuous optimization without hard thresholds between modes, as you have a smooth curve of range versus throughput instead of a step-by-step choice between predefined options. This is the choice for backbone routes where you need 400G today, but are designing the infrastructure for future expansion to 800G without replacing modules.

The impact of path quality on real-world range

The stated ranges are values under standard path conditions. In practice, two factors most often reduce the real-world range below nominal values.

The first is the number of ROADM nodes. Each reconfigurable optical add-drop multiplexer introduces signal loss, so a 500 km route with five ROADM nodes has a completely different effective optical budget than a 500 km point-to-point route. In networks with many intermediate nodes, the range of higher modulation modes drops, and that is precisely when 8QAM or QPSK becomes the right choice.

The second is fiber quality and the number of splices. The catalog attenuation of new G.652.D fiber in the C-band is around 0.18 to 0.20 dB/km, but old fiber, numerous high-loss splices, and connectors after years of operation increase the effective path attenuation and shorten the range. Data from installation documentation from ten years ago is not a reliable basis for planning the range of a 400G coherent module, which is why we always measure current path parameters using an OTDR.

Reach for typical operator routes in Poland and CEE

400ZR vs. OpenZR+, a difference not visible in the name

The OIF 400ZR standard defines a single mode: 400G DP-16QAM up to 120 km. A module compliant only with 400ZR will not provide more without replacing the hardware. The OpenZR+ MSA standard defines many modes, from 100G to 400G, with QPSK, 8QAM, and 16QAM modulations and reaches of 600 km and beyond. One of the required modes is compatible with OIF 400ZR, so an OpenZR+ module supports everything 400ZR does, and much more.

GBC Photonics modules are compliant with both standards. The 400ZR module is a good solution for DCI networks where all routes are under 120 km and will remain so. The OpenZR+ module is the right choice for networks with routes of varying lengths, as you keep one type of module in stock and configure it differently for each route.