NRZ، PAM4 اور DSP: ری ٹائمرز، گیئر باکسز، LPO، کوہیرنٹ

ہماری ٹیم ترجمے پر کام کر رہی ہے۔ یہ مضمون عارضی طور پر اردو میں دستیاب نہیں ہے، اس لیے اسے انگریزی میں دکھایا جا رہا ہے۔

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.

The electrical side of LPO and DSP modules — C2M compliance points, OIF CEI-112G-Linear, who equalises what per generation: XGMII, XAUI and the AUI family.

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