Linear, direct-drive, un-retimed, pluggable optics -Too good to be true?

Abstract

reviews the brief history of linear pluggable optics, giving context to its sudden and surprising emergence at OFC 2023

Full text

Linear, direct-drive, un-retimed, pluggable
optics – Too good to be true?
David Piehler
Photonics Society of Chinese Heritage Workshop:
Data Center Technology at the Crossroads
Optical Fiber Communications Conference
San Diego, California
8 March 2023
中華光電學會

Piehler · Linear, direct-drive, un-retimed, pluggable optics –Too good to be true? · PSC Workshop: Data Center Technology at the Crossroads · OFC 2023 · San Diego, California · 8 March 2023 .
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What’s this all about?
DSP
serdes
driver
TIA optics DSP
serdes
driver
TIA
optics
analog
linear
driver
TIA optics analog
linear
driver
TIA
optics
host
DSP
serdes
host
DSP
serdes
host
DSP
serdes
host
DSP
serdes
SWITCH SWITCH
host
electrical
channel
host
electrical
channel
host
electrical
channel
host
electrical
channel
standard high-speed link
linear, direct-drive, un-retimed link
pluggable optical
transceiver
optical fiber
optical fiber

Piehler · Linear, direct-drive, un-retimed, pluggable optics –Too good to be true? · PSC Workshop: Data Center Technology at the Crossroads · OFC 2023 · San Diego, California · 8 March 2023 .
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A brief history of
linear, direct-drive, un-retimed, pluggable optics
•OIF technical meeting –5 November 2020
–Samtec, MACOM propose CEI-112G-LINEAR standardization effort – much debate + a contentious vote to proceed.
•ECOC 2022 (September 2022)
–Word on the street: “It works”
•OIF technical meeting –24 October 2022
–Craig Thompson, NVIDIA, presents “Demonstration and Application of Linear, Non-Retimed Interfaces in Artificial
Intelligence / Machine Learning Systems,” OIF document oif2022.494.02
▪Eight prototype OSFP-800G-2SR4 (VCSEL) optical transceivers built with linear, un-retimed (MACOM) electrical interfaces
▪Tested on NVIDIA pizza-box switch containing NVIDIA NPU with 100G/lane NVIDIA DSP-based serdes
▪1E-6 pre-FEC BER with 7–13 dB serdes-to-module electrical traces, 1–30 m of OM3 (multimode) fiber.
▪All units: power dissipation < 3.8 W @ 55°C case temperature. (Today’s DSP-based OSFP-800G-2SR4 products ~ 14 W)
•Broadcom blog –6 December 2022
–Ram Velaga: [Broadcom Tomahawk 5 switch serdes to support] “direct-drive (linear) pluggable modules”
•Broadcom switches provide unrivaled flexibility for copper and optical connectivity
•OFC –6 March 2023
–1.6 Tb/s+ Intra-DC Networks panel (M4D): Andy Bechtolsheim (Arista) makes linear pluggable optics (LPO)
mainstream.

Piehler · Linear, direct-drive, un-retimed, pluggable optics –Too good to be true? · PSC Workshop: Data Center Technology at the Crossroads · OFC 2023 · San Diego, California · 8 March 2023 .
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Power dissipation in GBASE-SRoptical modules
designation description
optical
modulation
speed
power
dissapation
notes
(Gb/s) (W) (W/[Gb/s]) (pJ/b)
10G-SR 10 GbE SFP+, MMF, 500 m 10G NRZ 10 0.7 0.0700 70.0 kind of analog
25G-SR 25 GbE SFP28, MMF, 100 m 25G NRZ 25 1.25 0.0500 50.0 includes analog CDR
100G-SR4 100 GbE, QSFP28, MMF, 100 m 25G NRZ 100 20.0200 20.0 includes analog CDR
200G-SR4 200 GbE, QSFP56, MMF, 100 m 50G PAM4 200 50.0250 25.0 DSP-based, no gearbox
400G-SR8 400 GbE, QSFP56-DD, MMF, 100 m 50G PAM4 400 12 0.0300 30.0 DSP-based, no gearbox, early version
400G-SR8 400 GbE, QSFP56-DD, MMF, 100 m 50G PAM4 400 80.0200 20.0 DSP-based, no gearbox, recent versions
400G-SR4 400 GbE, QSFP112, MMF, 100 m 100G PAM4 400 70.0175 17.5 DSP-based, no gearbox
800G-SR8 800 GbE, OSFP112, MMF, 100 m 100G PAM4 800 14 0.0175 17.5 DSP-based, no gearbox
800G-SR-linear 800 GbE, OSFP112, MMF, 30 m 100G PAM4 800 40.0050 5.0 linear, direct drive, no-retimer
400G-SR-linear 400 GbE, QSFP112, MMF, 30 m 100G PAM4 400 20.0050 5.0 linear, direct drive, no-retimer
100G-SR-linear 100 GbE, SFP112, MMF, 30 m 100G PAM4 100 0.5 0.0050 5.0 linear, direct drive, no-retimer
efficiency
-1

Piehler · Linear, direct-drive, un-retimed, pluggable optics –Too good to be true? · PSC Workshop: Data Center Technology at the Crossroads · OFC 2023 · San Diego, California · 8 March 2023 .
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CPO-like power reduction without the baggage
In the end, power dissipation (pJ/b) numbers are similar. Trade-off of long
trace length (LPO) vs. external (pluggable) laser source (CPO)
Baggage: Only reason to move from pluggables to CPO is for lower power, right?
From:
Bechtolsheim, OFC-2023 (session M4D)
(lower for VCSEL-based transceivers)
(linear pluggable optics) (co-packaged optics)

Piehler · Linear, direct-drive, un-retimed, pluggable optics –Too good to be true? · PSC Workshop: Data Center Technology at the Crossroads · OFC 2023 · San Diego, California · 8 March 2023 .
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The fine print
•It’s all pretty new –wait for dust to settle.
•Everything made possible by the performance and robustness of next-generation
high-speed (e.g. Broadcom Tomahawk 5 –like) DSP-based serdes.
•How do we specify interoperability? A critical issue.
–Seems very hard to do.
–Ongoing work in OIF CEI-112G-LINEAR
–Inner workings of DSP-based serdes are highly proprietary. No one’s giving up their secret sauce.
–Idea: Create an open-source Atlas of Waveforms –real data from real optics –VCSELs, EMLs, DMLs,
SiPh, others… to set targets for DSP-based signal decoding.
–NVIDIA, with two end-to-end ecosystems (InfiniBand and NVLink) in house, seems to have advantage.
•High-speed DSP-serdes makers are potentially conflicted.
–Broadcom, Marvell… –switching ASIC business vs. optical module ASIC business.