400ZR and ZR+

What 400ZR Actually Specifies

400ZR is an OIF Implementation Agreement, and it is unusually prescriptive for coherent transport: one line rate, one modulation format, one FEC. DP-16QAM at 59.843750 GBd, cFEC at 14.8% overhead for 10.8 dB of net coding gain, 26 dB of required OSNR in 0.1 nm at end of life, 2400 ps/nm of dispersion tolerance.

Fixing all of that is the point. Modules from different vendors interoperate on the line, which is rare in coherent transport, and the application is deliberately narrow: an amplified, point-to-point DWDM span of up to about 120 km, or 30 to 40 km unamplified.

What ZR+ Means, and What It Does Not

OpenZR+ is a separate MSA rather than an OIF document. It keeps the form factor and the management model and swaps the FEC for oFEC at 15.3% overhead, which buys roughly another decibel of coding gain, then adds lower-rate modes: 100G and 200G DP-QPSK, 300G DP-8QAM, 400G DP-16QAM. Required OSNR runs from 12.5 dB up to 24 dB across that ladder, and reach with it, from a few spans to well beyond 480 km.

OpenROADM defines its own ZR+ profiles for OpenROADM line systems, and several vendors sell modules labelled ZR+ that are neither. The word on the box is not a specification — match the mode name at both ends. ZR vs ZR+ and the DCI guide lay the variants out.

800ZR

OIF published OIF-800ZR-01.0 in October 2024: DP-16QAM at about 118.2 GBd with oFEC, on a 150 GHz grid with 32 defined channels, aimed at the same single-span amplified DWDM application of 80 to 120 km.

Doubling the symbol rate rather than the constellation keeps the format familiar but leaves less margin, so 800G is a shorter-reach proposition than 400G on identical fibre. Where more distance is wanted the answer is the same as it was at 400G: drop the mode, or move to an 800G OpenZR+ profile built for line systems rather than point-to-point spans.

Power, Heat and How to Buy

A 400G ZR pluggable draws about 18 W and an 800G one 20 to 30, against roughly 9 for a 400G DR4. Not every port on a switch can host that, and platforms commonly qualify a subset of cages. Check the port map and model the faceplate in the power and thermal calculator before the design assumes a full row.

Then settle the line side: amplified or not, what the EDFA plan is, whether the module tunes to the channels the mux expects, and whether both ends run the same mode. The architectural gain — deleting a transponder shelf and its rack, power and management — is real, but it is a DCO transponder living inside a switch, and it wants to be engineered as one.

Related Terms

Coherent optics, DCO and ACO, OSNR, DWDM and EDFA.