800G MSA defines PSM8 while eyeing 400G’s progress

A key current issue regarding data centres is forecasting the uptake of 400-gigabit optics.

If a rapid uptake of 400-gigabit optics occurs, it will also benefit the transition to 800-gigabit modules. But if the uptake of 400-gigabit optics is slower, some hyperscalers could defer and wait for 800-gigabit pluggables instead.

So says Maxim Kuschnerov, a spokesperson for the 800G Pluggable MSA (multi-source agreement).

Maxim Kuschnerov

The 800G MSA has issued its first 800-gigabit pluggable specification.

Dubbed the PSM8, the design uses the same components as 400-gigabit optics, doubling capacity in the same QSFP-DD pluggable form factor.

“Four-hundred-gigabit modules hitting volume is crucially important because the 800-gigabit specification leverages 400-gigabit components,” says Kuschnerov. “The more 400-gigabit is delayed, it impacts everything that comes after.”

PSM8

The PSM8 is an eight-channel parallel single-mode (PSM) fibre design, each fibre carrying 100 gigabits of data.

The 100m-reach PSM8 version 1.0 specification was published in August, less than a year after the 800G MSA was announced. 

The 800G Pluggable MSA is developing two other 800-gigabit specifications based on 200-gigabit electrical and optical lanes.

One is a 500m four-fibre 800-gigabit implementation, each fibre a 200-gigabit channel. This is an 800-gigabit equivalent of the existing 400-gigabit IEEE DR4 standard.

The second design is a single-fibre four-channel coarse wavelength-division multiplexing (CWDM) with a 2km reach, effectively an 800-gigabit CWDM4.

Specifications

The 800G MSA chose to tackle a parallel single-mode fibre design because the components needed already exist. In turn, a competing initiative, the IEEE’s 100-gigabit-per-lane multi-mode fibre approach, will have a lesser reach.

“The IEEE has an activity for 100-gigabit per lane for multi-mode but the reach is 50m,” says Kuschnerov. “How much market will you get with a limited-reach objective?”

In contrast, the 100m reach of the PSM8 better serves applications in the data centre and offers a path for single-mode fibre which, long-term, will provide general data centre connectivity, argues Kuschnerov, whether parallel fibre or a CWDM approach.

Investment will also be needed to advance multi-mode optics to achieve 100 gigabits whereas PSM8 will use 50 gigabaud optics already used by 400-gigabit modules.

Kuschnerov stresses that the PSM8 is not a repackaging of two IEEE 400-gigabit DR4s designs. The PSM8 uses more relaxed specifications to reduce cost; a possibility given PSM8’s 100m reach compared to the DR4’s 500m.

“We have relaxed various specifications to enable more choice,” says Kuschnerov. For example, externally modulated lasers (EMLs), directly modulated lasers (DMLs) and silicon photonics-based designs can all be used.

The transmitter power has also been reduced by 2.5dB compared to the DR4, while the extinction ratio of the modulator is 1.5dB less.

The need for an 800-gigabit in a QSFP-800DD form factor is to serve emerging 25.6-terabit Ethernet switches. Using 400-gigabit optics, a 2-rack-unit-high (2RU) switch is needed whereas a 1RU switch platform is possible using 800-gigabit pluggables.

“The big data centre players all have different plans and their own roadmaps,” says Kuschnerov. “From our observation of the industry, the upgrading speed for 400 gigabit and 800 gigabit is slower than what was expected a year ago.”

First samples of the PSM8 module are expected in the second half of 2021 with volume production in 2023.

800-gigabit PSM4 and CWDM4

The members of the MSA have already undertaking pre-development work on the two other specifications that use 200-gigabit-per-lane optics: the 800-gigabit PSM4 and the CWDM4.

“It was a lot of work discussing the feasibility of 200-gigabit-per-lane,” says Kuschnerov. There is much experimental work to be done regarding the choice of modulation format and forward error correction (FEC) scheme which will need to be incorporated in future 4-level pulse-amplitude modulation (PAM-4) digital signal processors.

“We are progressing, the key is low power and low latency which is crucial here,” says Kushnerov. A tradeoff will be needed in the chosen FEC scheme ensuring sufficient coding gain while minimising its contribution to the overall latency.

As for the modulation scheme, while different PAM schemes are possible, PAM-4 already looks like the front runner, says Kuschnerov.

Working 200-gigabit optics was demonstrated at CIOE in Shenzhen. Source: 800G Pluggable MSA

The 800G Pluggable MSA is at the proof-of-concept stage, with a demonstration of working 200-gigabit-per-lane optics at the recent CIOE show held in Shenzhen, China. “Some of the components used are not just prototypes but are designed for this use case although we are not there yet with an end-to-end product.”

The designs will require 200-gigabit electrical and optical lanes. The OIF has just started work on 200-gigabit electrical interfaces and will likely only be completed in 2025. Achieving the required power consumption will also be a challenge.

Catalyst

Since the embrace of 200-gigabit-per-lane technology by the 800G Pluggable MSA just over a year ago, other initiatives are embracing the rate.

The IEEE has started its ‘Beyond 400G’ initiative that is defining the next Ethernet specification and both 800-gigabit and 1.6 terabit optics are under consideration. As has the OIF with its next-generation 224-gigabit electrical interface. 

“These activities will enable a 200-gigabit ecosystem,” says Kuschnerov. “Our focus is on 800-gigabit but it is having a much wider impact beyond 4x200-gigabit, it is impacting 1.6 terabits and impacting serdes (serialisers/ deserialisers).”

The 800G Pluggable MSA is doing its small part but what is needed is the development of an end-to-end 200-gigabit ecosystem, he says: “This is a challenging undertaking.”

The 800G Pluggable MSA now has 40 members including hyperscalers, switch makers, systems vendors, and component and module makers.