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).
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.
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 4×200-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.
Nokia shares its vision for cost-reduced coherent optics
Nokia explains why coherent optics will be key for high-speed short-reach links and shares some of its R&D activities. The latest in a series of articles addressing what next for coherent.
Part 3: Reducing cost, size and power
Coherent optics will play a key role in the network evolution of the telecom and webscale players.
The modules will be used for ever-shorter links to enable future cloud services delivered over 5G and fixed-access networks.
The first uses will be to link data centres and support traffic growth at the network edge.
This will be followed with coherent optics being used within the data centre, once traffic growth requires solutions that 4-level pulse-amplitude modulation (PAM4) direct-detect optics can no longer address.
“If you look at PAM4 up to 100 gigabit for long reach and extended reach optics – distances below 80km – it does not scale to higher data rates,” says Marc Bohn, part of product management for Nokia’s optical subsystem group. ”It only scales if you use 100-gigabit in parallel.”
However, to enable short-reach coherent optics, its cost, size and power consumption will need to be reduced significantly. Semiconductor packaging techniques will need to be embraced as will a new generation of coherent digital signal processors (DSPs).
Capacity growth
The adoption of network-edge and on-premise cloud technologies are fueling capacity growth, says Tod Sizer, smart optical fabric & devices research lab leader at Nokia Bell Labs.
Nokia says capacity growth is at 50 per cent per annum and is even faster within the data centre; for every gigabyte entering a data centre, ten gigabytes are transported within the data centre.
“All of this is driving huge amounts of growth in optical capacity at shorter distances,” says Sizer. “To meet that [demand], we need to have coherent solutions to take over where PAM-4 stops.”
Sizer oversees 130 engineers whose research interests include silicon photonics, coherent components and coherent algorithms.
Applications
As well as data centre interconnect, coherent optics will be used for 5G, access and cable networks; markets also highlighted by Infinera and Acacia Communications.
Nokia says the first driver is data centre interconnect.
The large-scale data centre operators triggered the market for 80-120km coherent pluggables with the 400ZR specification for data centre interconnect.
“Right now, with the different architectures in data centres, these guys are saying 80-120km may be an overshoot, maybe we need something for shorter distances to be more efficient,” says Bohn. “Certainly, coherent can tackle that and that is what we are preparing for because there is no alternative, only coherent can cover that space.”
5G is also driving the need for greater bandwidth.
“Traditionally a whole load of processing has been done at the remote ratio head but increasingly, for cost and performance reasons, people are looking at pulling the processing back into the data centre,” says Sizer.
Another traffic driver is how each cellular antenna has three sectors and can use multiple frequency bands.
“Some research we are looking at requires 400 gigabits and above,” says Sizer. “If you want to do a full [mobile] front haul for a massive MIMO (multiple input, multiple output) array, for example.”
Challenges
Several challenges need to be overcome before coherent modules are used widely for shorter-reach links.
To reduce coherent module cost, the optics and DSP need to be co-packaged, borrowing techniques developed by the chip industry.
“Optical and electrical should be brought close together,” says Bohn. “[They should be] co-designed and co-packaged, and the ideal candidate for that is to combine silicon photonics and the DSP.”
The aim is to turn complex designs into a system-on-chip. “Both [the DSP and silicon photonics] are CMOS and you can apply 2D and 3D [die] stacking multi-chip module techniques,” says Bohn, who contrasts it with the custom and manual manufacturing techniques used today.
The coherent DSP also needs to be much simpler than the high-end DSPs used for long-distance optical transport.
For example, the dispersion compensation, which accounts for a significant portion of the chip’s circuitry, is less demanding for shorter links. The forward-error-correction scheme used can also be relaxed as can the bit precision of the analogue-to-digital and digital-to-analogue converters.
Nokia can co-design the silicon photonics and the DSP following its acquisition of Elenion. Nokia is also exploiting Elenion’s packaging know-how and the partnerships it has developed.
Inside the data centre
Nokia highlights two reasons why coherent will eventually be used within the data centre.
The first is the growth in capacity needed inside the data centre. “For the same reason we believe coherent will be the right solution for data centre interconnect and access, the same argument can be made within the data centre,” says Sizer.
A campus data centre is distributed across several buildings and linking them is driving a need for 400-gigabit lanes or more.
This requires a ZR-like solution but for 2km or so rather than 80km.
“It is one of the solutions certainly but that will be driven an awful lot by whether we can make cost-effective solutions to meet the cost targets of the data centre,” says Sizer. That said, there are other ways this can be addressed such as adding fibre.
“Having parallel systems is another area of ongoing research,” says Sizer. “We may need to have unique solutions if traffic grows faster inside the data centre than outside such as spatial-division multiplexing as well as coherent.”
The use of coherent interfaces for networking inside the data centre will take longer.
Bohn points out that 51.2-terabit and 102.4-terabit switches will continue to be served using direct-detect optics but after that, it is unclear because direct-detect optics tops out at 100-gigabits or perhaps 200-gigabits per lane.
“With coherent, it is much easier to get to higher data rates especially over shorter distances,” says Bohn.
Another development benefitting the use of coherent is the next Ethernet standard after 400 Gigabit Ethernet (GbE).
“My research team is looking at that and, in particular, 1.6 Terabit Ethernet (TbE) which is fairly out in the future,” says Sizer. “It will demand a coherent solution, as I expect 800GbE will as well.”
Work to define the next Ethernet standard is starting now and will only be completed in 2025 at the earliest.