Whether the coherent DSP lives inside the pluggable module (DCO) or on the host line card (ACO).
Where the DSP Physically Sits
Coherent transmission needs a large digital signal processor to run the FEC, shape the constellation and unwind what the fibre did. DCO and ACO answer one question about it: is that chip inside the pluggable, or on the line card the pluggable goes into?
In a DCO module it is inside, and the host sees an ordinary electrical Ethernet interface — the same lanes it would drive into a grey optic. In an ACO module the pluggable holds only the optics and the analogue front end, and the DSP lives on the host board.
What an ACO Interface Actually Carries
The distinction is more physical than it sounds. A coherent modulator needs an in-phase and a quadrature drive for each of two polarisations, and the receiver produces the same four signals coming back. An ACO connector therefore carries four wideband analogue rails in each direction, with no retiming and no digital decision anywhere in the path.
Those rails have to survive a connector and a stretch of host PCB with their linearity intact, which makes the electrical channel part of the optical specification. Module and host DSP end up designed as a pair, and a CFP2-ACO from one vendor is not a drop-in for another's.
Why DCO Won
ACO had a real argument when coherent DSPs were too large and too hot for a pluggable: put the chip where there is board area and airflow, and let the system vendor differentiate on it. Process shrinks removed the premise. A 400G DSP now fits in a QSFP-DD alongside a tunable laser and runs on about 18 W.
Operators wanted the other property anyway. A DCO is a self-contained transponder that plugs into any compliant port, is managed through CMIS like any other module, and can be spared as a single part number. Essentially every 400ZR and 800ZR pluggable shipping today is DCO, and ACO survives only in installed CFP2 line cards.
What to Check on a Datasheet
If it says DCO, the host just sees Ethernet and the questions that remain are about the line side: which operating modes, which OSNR each requires, and what the tuning range is. See ZR vs ZR+ for how those modes are grouped.
The remaining trap is thermal rather than logical. Putting a transponder inside a cage concentrates 18 to 30 W where a grey optic put 4, so check the platform's supported port positions and run the numbers in the power and thermal calculator before assuming a full faceplate. Lower-rate coherent optics such as 100G ZR4 are far easier to host.
DCO and ACO reference
| DCO | digital coherent optics — DSP inside the module |
|---|---|
| ACO | analogue coherent optics — DSP on the host card |
| DCO host interface | standard electrical Ethernet lanes |
| ACO host interface | four analogue rails each way: I and Q, two polarisations |
| Typical ACO form factor | CFP2-ACO |
| Typical DCO form factors | CFP2-DCO, QSFP-DD, OSFP |
| Management | CMIS on the QSFP-DD and OSFP generation |
| Power | about 18 W for a 400G DCO; 20 to 30 W at 800G |
| Shipping today | essentially all ZR and ZR+ pluggables are DCO |
Related Terms
Coherent optics, 400ZR and ZR+, DSP, CMIS and DWDM.