Standard & transmitter
GBd
–
–
nm
nm
dBm
dB
dB
dB
nm
Channel
m
dB/km
–
dB
–
dB
nm
nm
ps/nm²·km
MHz·km
ps
MHz
Receiver & spec allocations
dBm
dBm
dB
dB
m
Summary
Link budget — worst lane
Per-lane dispersion — λ₀ corners
Energy per bit — DSP vs LPO vs CPO
W
Gb/s
–
What this means
Budget waterfall (dBm, OMAouter)
Margin vs length
Chromatic dispersion D(λ) — λ₀ corner band
Method: IEEE 802.3-style dB budgeting. OMA↔Pavg via
OMA = Pavg + 10·log₁₀(2(er−1)/(er+1)) and ER penalty 10·log₁₀((er+1)/(er−1)) (802.3 Cl. 52/121;
Agrawal §4.7). Fiber dispersion D(λ) = S₀/4·(λ − λ₀⁴/λ³) (Corning PI-1424 / ITU-T G.652), evaluated at the
worst λ₀ corner per lane; CD penalty 5·log₁₀(1+(4·B·L·D·σλ)²) is Agrawal's broad-source model
(§5.4) — conservative for narrow-linewidth EMLs, indicative for chirp-dominated DMLs. MMF reach uses the
IEEE 10GbE link model (AV02-2485EN): BWmodal = EMB/L, BWcd = 0.187·10⁶/(L·σλ·|D|),
composite 10–90% response by sum-of-squares (C₁ = 480, C₂ = 329 ns·MHz) and Gaussian ISI penalty
−10·log₁₀(erf(0.9062·T/Tc)) — an NRZ-era model, only indicative for PAM4 with DSP equalization.
TDECQ is a measured transmitter conformance metric (802.3 Cl. 121.8.5, captured through a worst-case
channel on a reference equalizer): this tool budgets against the spec's TDECQ/SECQ allocation — above 1.4 dB
it debits the budget dB-for-dB, per the 802.3 spec-table structure — and does not claim to predict it. In
"stressed" sensitivity mode only dispersion/ISI in excess of the spec channel is subtracted, since
the stressed sensitivity already embeds worst-case-compliant TX and channel impairments.
Where did all the loss budget go at 100G and 200G per lane?
Three compounding taxes. Receiver sensitivity degrades with noise bandwidth
(~10·log₁₀ of the baud), so doubling the symbol rate costs ~3 dB. PAM4 then splits the eye into three,
giving up another 10·log₁₀(3) = 4.77 dB of optical eye versus NRZ at the same OMA. TX power can't rise to
compensate — Class 1 eye safety and silicon-photonics insertion loss cap the launch. KP4 RS(544,514) FEC
claws back a few dB by tolerating pre-FEC BER of 2.4×10⁻⁴, but the net result is why an 800G-DR8 channel
gets 3.0 dB where gigabit-era links enjoyed 7+ — and why every tenth of a dB of connector loss now shows
up in a compliance review.