The chip inside a transceiver that equalises, retimes and regenerates the signal in both directions.
What Is a DSP in an Optical Module?
Every high-speed pluggable that is not explicitly linear contains a digital signal processor. On transmit it takes the electrical signal from the host, corrects for the distortion the circuit board introduced, recovers a clean clock, and drives the modulator from a regenerated waveform. On receive it does the reverse for the signal coming off the fibre.
This is why a pluggable module works reliably between a host and a fibre plant it has never met. The DSP isolates the two domains: whatever the host does, the fibre sees a clean signal, and whatever the fibre does, the host sees a clean signal.
The Four Jobs
Equalisation undoes the frequency-dependent loss of the electrical channel — continuous-time linear equalisation for the broad shape, decision-feedback equalisation for the residue. Clock and data recovery locks onto the incoming symbol timing so the sampler fires in the middle of each eye.
Retiming regenerates the waveform from the recovered decisions, which is what breaks the chain of accumulating distortion. And at PAM4 the DSP also handles level slicing and the correction of unequal level spacing, plus usually the forward error correction encode and decode.
What It Costs
All of that costs power — commonly half or more of the module's total — and latency, because a digital round trip is not free. Those two costs are precisely what LPO and LRO set out to avoid, and the trade they make is isolation for efficiency.
The power figure is worth quantifying rather than accepting as a rule of thumb: at 32 ports of 800G, half of 16 watts a port is 256 watts per switch, which the power calculator will turn into a rack number and an annual cost.
Where You Meet It
Indirectly, through what it enables: interoperability between vendors, tolerance of imperfect host boards, and the diagnostic data that surfaces in DDM. You also meet it whenever a module's CMIS revision matters, because the DSP firmware is what implements it.
Module DSP
| Share of module power | typically half or more |
|---|---|
| Latency added | tens of nanoseconds per direction |
| Functions | equalisation, clock recovery, retiming, FEC |
| PAM4 addition | four-level slicing and level correction |
| Removing it entirely | gives LPO |
| Removing it on receive only | gives LRO |
| What it provides | isolation between host and fibre domains |