Preparing for a post-pluggable optical module world

Part 1: OIF: ELSFP, XSR+, and CEI-112G-Linear

The OIF is working on several electrical and optical specifications as the industry looks beyond pluggable optical transceivers.

One initiative is to specify the external laser source used for co-packaged optics, dubbed the External Laser Small Form Factor Pluggable (ELSFP) project. 

Nathan Tracy

Industry interest in co-packaged optics, combining an ASIC and optical chiplets in one package, is growing as it becomes increasingly challenging and costly to route high-speed electrical signals between a high-capacity Ethernet switch chip and the pluggable optics on the platform’s faceplate.

The OIF is also developing 112-gigabit electrical interfaces to address not just co-packaged optics but also near package optics and the interface needs of servers and graphics processor units (GPUs).

Near package optics also surrounds the ASIC with optical chiplets. But unlike co-packaged optics, the ASIC and chiplets are placed on a high-performance substrate located on the host board.

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Broadcom discusses its co-packaged optics plans

If electrical interfaces are becoming an impediment, is co-packaged optics the answer? Broadcom certainly thinks so.

One reason for the growing interest in co-packaged optics is the input-output (I/O) demands of switch chips. If the packet processing capacity of such chips is doubling every two years, their I/O must double too.

Alexis BjörlinRepeatedly doubling the data throughput of a switch chip is a challenge.

Each new generation of switch chip must either double the number of serialiser-deserialiser (serdes) circuits or double their speed.

A higher serdes count - the latest 25.6-terabit switch ICs have 256, 100 gigabit-per-second serdes - requires more silicon area while both approaches - a higher count and higher speed - increase the chip's power consumption.

Faster electrical interfaces also complicate the system design since moving the data between the chip and the optical modules on the switch's front panel becomes more challenging.

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COBO adds co-packaged optics to its agenda

The Consortium of On-Board Optics (COBO) is progressing with its work to create specifications for co-packaged optics.

The decision to address co-packaged optics by an organisation established to promote on-board optics reflects the significant industry interest co-packaged optics has gained in the last year.

So says Brad Booth, director, leading edge architecture pathfinding team in Azure hardware systems and infrastructure at Microsoft.

Source: COBO

The COBO work also complements that of the OIF which has set up its own co-packaged optics framework. 

“We have a different collection of members [to the OIF],” says Booth. “Our members are very strong on optical connectivity and materials whereas the OIF is known for its electrical interface work and module activities like 400ZR.”

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OIF addresses 800-gigabit coherent interfaces

The OIF has started work on 800-gigabit coherent interfaces, a follow-on to its 400-gigabit 400ZR specification work.

Two requirements are being addressed: an 800-gigabit dense wavelength division (DWDM) interface with a 80-120km span for data centre interconnect, and an unamplified single-channel fixed-wavelength 2-10km coherent link for campuses.

The need for 800 gigabit

Tad Hofmeister

“When we hit that 90 per cent mark on 400ZR, we had people stand up and say: ‘We are ready to start 800ZR’,” says Karl Gass, OIF, physical link layer working group – optical vice-chair.

But completing the work has taken time. “The first 90 per cent of a project takes about half the time and the last 10 per cent takes the other half,” says Gass.

So only in mid-2020 did the OIF’s attention turn to the new standard, starting with determining the use cases.

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The compound complexity of co-packaged optics

Part 1: The OIF’s co-packaging initiative

Large-scale data centres consume huge amounts of power; one building on a data centre campus can consume 100MW. But there is a limit as to the overall power that can be supplied.

Jeff Hutchins

The challenge facing data centre operators is that networking, used to link the equipment inside the data centre, continues to consume more and more power.

That means less power remains for the servers; the compute that does the revenue-generating work.

This is forcing a rethink regarding networking and explains the growing interest in co-packaged optics, a technique that effectively adds optical input-output (I/O) to a chip.

Two industry organisations - the OIF and The Consortium for On-Board Optics (COBO) - have each started work to identify the requirements needed for co-packaged optics adoption.

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100-gigabaud optics usher in the era of terabit transmissions

•  NeoPhotonics has unveiled a coherent modulator and receiver that operate at over 100 gigabaud.

Telecom operators are in a continual battle to improve the economics of their optical transport networks to keep pace with the relentless growth of IP traffic.

One approach is to increase the symbol rate used for optical transmission. By operating at a higher baud rate, more data can be carried on an optical wavelength.

Ferris Lipscomb

Alternatively, a higher baud rate allows a simpler modulation scheme to be used, sending the same amount of data over greater distances. That is because the fewer constellation points of the simpler modulation scheme help data recovery at the receiver.

NeoPhotonics has detailed two optical components - a coherent driver-modulator and an intradyne coherent receiver (micro-ICR) - that operate at over 100 gigabaud (GBd). The symbol rate suits 800-gigabit systems and can enable one-terabit transmissions.

NeoPhotonics’ coherent devices were announced to coincide with the ECOC 2020 show.

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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.”

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