800G DR8 vs FR4 vs ZR+: Reach, Power and Cost Tradeoffs for AI Fabrics

An 800G link can be built three ways, and the deciding variable is distance. DR8 is a short parallel single-mode standard, the FR class carries the same 800G over a duplex pair for a few kilometres, and ZR+ drives the signal with coherent DSP for metro and data center interconnect reach. Reach, fiber count, optical type and module power all move together, so the right 800G choice follows from where the link has to go.

Three Answers to 800G Over Different Distances

DR8 keeps the short-reach philosophy of the data center: eight 100G PAM4 lanes run in parallel, each on its own fiber, for up to 500m. The FR class takes the same lane rate but multiplexes several wavelengths onto a single duplex pair, trading a little optical complexity for a 2km reach over ordinary single-mode fiber. ZR+ abandons direct-detect PAM4 altogether and uses coherent modulation with a digital signal processor, which is what allows 800G to travel 80 to 120km or more without regeneration.

Reach and breakout are linked. A DR8 port can be fanned out to two 400G DR4 links or eight 100G FR links, which keeps short in-building runs on low-cost parallel optics and reuses the fiber already in the tray. The FR class cannot break out the same way, because its wavelengths are shared across the duplex pair, so it is used where a single 800G link over one pair is the goal. ZR+ is almost always deployed as a single point-to-point link between sites, with the coherent signal managed as a transport service.

Reach, Fiber and Connectors

The connector follows the fiber count. DR8 uses a multi-fiber MPO-16 assembly to break out eight transmit and eight receive fibers. The FR class uses a standard duplex LC or CS connector because the wavelengths share one pair of fibers; the four-wavelength version is designated 800GBASE-FR4, while an eight-wavelength variant is designated FR8. ZR+ also uses a duplex pair, but the light is encoded and decoded coherently at both ends.

Power: The Hidden Bill

Direct-detect 800G modules sit in the mid-teens of watts: roughly 14 to 16W for DR8 and 16 to 18W for the FR class, warmed by the extra laser and multiplexing optics. A coherent ZR+ module adds a DSP and tunable optics and typically draws 24 to 30W or more. Across a rack of dozens of ports, that difference compounds into power, cooling and switch-slot design, so ZR+ earns its cost only where coherent reach replaces a chain of regenerators or a separate transport layer.

A useful rule of thumb is to divide module wattage by the ports on the faceplate: two 400G QSFP112 ports that together deliver the same 800G as a single OSFP ZR+ port can sit in a very different power and cooling envelope. Direct-detect keeps that envelope flat and predictable across a rack, which is exactly what dense AI and storage fabrics depend on.

Direct-Detect or Coherent

The choice is not simply about metres. Direct-detect PAM4 sits inside the IEEE 802.3 Ethernet family, where 800GBASE-DR8 and the FR class are defined, and drops straight into a switch port. Coherent ZR+ follows the OIF 800ZR lineage, adds forward error correction and richer management, and behaves more like a transport function in a pluggable. When the link stays inside a building, direct-detect wins on power and simplicity. When it leaves the building, coherent usually wins on total cost of ownership.

How Apex Helps

Give us the link map, the longest and shortest spans, and whether the run stays in-building or crosses sites, and we will match DR8, FR4 or ZR+ optics to each span instead of over-speccing the whole fabric. Send your requirement list and we will quote within hours.