NRZ, PAM4 and the DSP: retimers, gearboxes, LPO, coherent

Up to 25G per lane a transceiver moved bits by switching a laser on and off and reading the result with a comparator. Beyond that, the optics could not get faster cheaply, so the industry made each symbol carry more — PAM4 — and put a digital signal processor in the module to make a noisy four-level waveform readable. The DSP now dominates power, cost and latency of 400G+ modules, which is why the newest designs try to remove it again (LPO) or move it deeper (coherent). This page explains what the signal-processing chain does and what each variant means for hosts and diagnostics.

Modulation formats

Format	Bits per symbol	Lane examples	Receiver needs	Penalty vs NRZ
NRZ (on/off, 2 levels)	1	1G, 10G, 25G, 40G (4×10), 100G (4×25)	limiting amplifier + CDR	—
PAM4 (4 levels)	2	50G, 100G, 200G per lane; 400G (8×50 or 4×100), 800G	linear front end, ADC, DSP equaliser; RS-FEC mandatory	≈ 9.5 dB less eye opening → needs FEC and equalisation
Coherent (QPSK, 8QAM, 16QAM on two polarisations)	4–8 per symbol	100G–800G per wavelength (ZR, ZR+)	local-oscillator laser, 90° hybrid, balanced PDs, 4-channel ADC + heavy DSP + FEC	huge OSNR advantage over long spans

PAM4 eye quality is measured as TDECQ (transmitter dispersion eye closure quaternary) rather than a simple mask — a DSP-derived figure of how much a reference equaliser must work to recover the levels. Standards and FEC pairing: FEC, AN & link training.

What the DSP does

host lanes ─► SerDes Rx ─► (retime) ─► gearbox ─► DAC ─► driver ─► laser/modulator
fibre ─► PD/TIA (linear) ─► ADC ─► CTLE/FFE/DFE equaliser ─► slicer ─► (FEC in coherent) ─► gearbox ─► SerDes Tx ─► host

Function	Purpose	Notes
Retiming	clean clock on both sides — the CDR's job, absorbed into the DSP	CDR
Equalisation — CTLE, FFE, DFE, sometimes MLSE	undo bandwidth limits of laser, photodiode, fibre and PCB traces so four levels separate again	adaptive; converges in ms at link-up
Gearbox	change lane count: 8 × 50G PAM4 from the host ↔ 4 × 100G PAM4 to the optics (400G DR4/FR4); 4 × 25G NRZ ↔ 2 × 50G PAM4 (100G DR)	why a 400G module has 8 electrical and 4 optical lanes
Pre-distortion / driver shaping	compensate laser non-linearity so the four levels are evenly spaced	tuned per module at production
Monitoring	pre-FEC BER estimate, SNR, eye metrics — the source of CMIS VDM	VDM & FEC metrics
FEC (coherent only)	oFEC / CFEC inside the module; Ethernet modules leave RS-FEC to the host	400ZR: CFEC; OpenZR+: oFEC

Cost: a 400G DSP draws 3–6 W (half the module's budget), adds ~50–100 ns of latency and tens of dollars. It also fixes the module to the applications it was designed for — hence the CMIS application model (CMIS).

Retimed, linear, half-retimed

Architecture	Module contains	Power (400G class)	Host requirement	Status
DSP / fully retimed	ADC, DSP, DAC on both directions	8–12 W	any compliant host — the module presents a clean interface	mainstream 400G/800G
LPO — linear pluggable optics	driver + linear TIA only; no DSP, no CDR	4–6 W	host SerDes must equalise the whole channel including the optics; needs a capable ASIC and tuning per link	AI/HPC short links; LPO MSA (2023+)
LRO / HALO — half-retimed	DSP on the Tx (host→optics) side only	6–8 W	intermediate	emerging compromise
CPO — co-packaged optics	optics on the switch ASIC package; no pluggable	lowest per bit	new switch designs	early deployments

For diagnostics the difference matters: an LPO module has no DSP to report pre-FEC BER or SNR, so the host's FEC counters are the only bit-level view, and link quality depends on the host's equaliser settings as much as on the optics.

Coherent modules in brief

A 400ZR/ZR+ QSFP-DD or OSFP contains a tunable laser (nano-ITLA) used both as transmitter source and as local oscillator, an IQ modulator (four Mach-Zehnder sections, silicon photonics or InP) driven by the DSP's DACs, an integrated coherent receiver (90° hybrid, balanced photodiodes, TIAs) and a DSP that does chromatic dispersion compensation, polarisation demux, carrier recovery and FEC — a former line-card transponder in 15–20 W. Their monitors (OSNR, CD, DGD, Q-factor, Rx signal power) appear in CMIS VDM (Coherent & long haul).

NRZ modules: the classic chain

On 1G–25G modules the chain is short — driver, laser, photodiode, TIA, limiting amplifier, optional CDR — and everything about it is analog. Rate flexibility comes from CDR bypass or rate-select bandwidth switching (Rate select); diagnostics are Rx power and the host's CRC/FEC counters; latency is a few nanoseconds. This simplicity is why 10G and 25G optics remain cheap and why they are still the bulk of what programmers see.

Host-facing consequences

Topic	NRZ module	PAM4 DSP module	LPO
Link-up	immediate	DSP convergence + CMIS data-path init (hundreds of ms to seconds)	host equaliser training
FEC	optional/host	RS-FEC mandatory (host)	RS-FEC mandatory (host)
Breakout	by lane, if optics are parallel	only advertised applications	host-dependent
Power class	1–3	4–7 (QSFP28 LR4) / CMIS 5–8	CMIS 3–5
Interop risk	low	host/module DSP settings, CMIS version	high — host tuning per module type

Bring-up and failures: What the host does, CMIS issues, Speed & rate.

In CodingBox

The identity tells you which chain a module has: NRZ vs PAM4 encoding codes, CDR bits, CMIS applications with their host and media interface IDs, and the power class that betrays a DSP. For CMIS modules the DDM screen adds the DSP's own view — VDM pre-FEC BER and SNR per lane — next to the classic optical monitors.