Stop using 100GBASE-LR4#

Richard Steenbergen – Petabit Scale

• Why: A reference-grade optics talk aimed squarely at the interconnection world (IXP ports, peering cross-connects), which is still standardizing on a 2010 pre-FEC optic that hyperscalers abandoned within one product cycle of CWDM4 (2014). Cost, power, and physics — core PM territory.
• What: A complete guide to 100G client optics generations: Gen 1 LR4 (4×25G NRZ, LAN-WDM grid, four cooled lasers with TECs, no host FEC — raw BER or nothing), Gen 2 CWDM4/4WDM (uncooled lasers, ±6.5nm grid, host RS-FEC worth ~5–6dB margin, roughly half the price, everything else identical), Gen 3 single-lambda (53GBd PAM4, one laser, DSP + mandatory KP4 FEC). The thesis: every LR4 datasheet claim, including reach, has since been beaten by cheaper optics; the interconnection ecosystem has paid the "TEC tax" for a decade out of habit because changing requires coordinating with a remote party.
• How: Physics-driven walkthrough — chromatic dispersion penalty scales with baud², FEC (KR4/KP4 Reed-Solomon) has been baked into every 25G+ SerDes since ~2015 and is idle in the box, and pre-FEC BER is free early-warning telemetry (you can watch a link degrade for weeks before a real error).
• Where applicable: Universal for 100G handoffs; no hardware change needed (FEC is already in the host ASIC). The friction is procedural — coordinating a different optic with peers/IXPs — not technical.