Tencent makes its co-packaged optics move

• Tencent is the first hyperscaler to announce it is deploying a co-packaged optics switch chip

• Tencent will use Broadcom’s Humboldt that combines its 25.6-terabit Tomahawk 4 switch chip with four optical engines, each 3.2 terabit-per-second (Tbps)

Part 2: Broadcom's co-packaged optics 

Tencent will use Broadcom’s Tomahawk 4 switch chip co-packaged with optics for its data centres.

Manish Mehta

“We are now partnered with the hyperscaler to deploy this in a network,” says Manish Mehta, vice president of marketing and operations optical systems division, Broadcom. “This is a huge step for co-packaged optics overall.”

The Chinese hyperscaler will use Broadcom’s 25.6Tbps Tomahawk 4 Humboldt, a hybrid design where half of the chip’s input-output (I/O) is optical and half is the chip’s serialisers-deserialisers (serdes) that connect to pluggable modules on the switch’s front panel.

Four Broadcom 3.2-terabit silicon photonics-based optical engines are co-packaged alongside the Tomahawk 4 chip to implement 12.8Tbps of optical I/O.

Broadcom demonstrated a working version of a Humboldt switch at OFC earlier this year.

Co-packaged optics

Broadcom started its co-packaged optics development work in 2019.

“One of the reasons for our investment in co-packaged optics was that we did see firsthand the ongoing limits of copper interconnect being approached,” says Mehta.

The transmission reach of copper links continues to shrink as the signalling speed has increased from 25 gigabits-per-second (Gbps) non-return to zero (NRZ) to PAM-4 (4-level pulse amplitude modulation) based signalling at 56Gbps, 112Gbps and, in the coming years, 224Gbps. Power consumption is also rising with each speed hike.

Broadcom says data centres now use 1 million optical interconnects, but that much of the connectivity is still copper-based, linking adjacent racks and equipment within the rack.

“Hyperscalers spend ten times more on interconnects than switching silicon,” says Mehta. Given these trends, there needs to be a continual improvement in the power profile, cost and scaled manufacturing of optical interconnect, he says.

In the short term, what is driving interest in co-packaged optics is overcoming the limitations of copper, says Broadcom.

In early 2021, Broadcom detailed at a JP Morgan event its co-packaged optics roadmap. Outlined was the 25.6-terabit Humboldt to be followed by Bailly, a 51.2-terabit all co-packaged optics design using Broadcom’s Tomahawk 5 switch chip which is now sampling. 

Humboldt uses DR4 (4x100-gigabit using 4 fibres) whereas the 51.2-terabit Bailly will add multiplexing-demultiplexing and use the FR4 specification (4x100-gigabit wavelengths per fibre).

Technology and partners

Broadcom’s in-house technology includes lasers (VCSELs and EMLs), mixed-signal expertise (trans-impedance amplifiers and drivers), and silicon photonics, as well as its switch chips.

Broadcom uses a remote laser source for its co-packaged optics design. Placing the laser away from the package (the switch chip and optics) means no cooling is needed.

Broadcom is working with 15 partners to enable its co-packaged optics, highlighting the breadth of expertise required and the design complexity.

A clos architecture in the data centre showing top-of-rack (ToR) and layers of aggregation switches. Source: Broadcom

There are two prominent use cases for the hybrid I/O Humboldt.

One is for top-of-rack switches, where the electrical interfaces support short-reach copper links connecting the servers in a rack, while the optical links connect the top-of-rack box to the next layer of aggregation switching.

The second use is at the aggregation layer, where the electrical I/O connects other switches in the rack while the optical links connect to switch layers above or below the aggregation layer.

“There is a use case for having pluggable ports where you can deploy low-cost direct-attached copper,” says Mehta.

Broadcom says each data centre operator will have their own experience with their manufacturing partners as they deploy co-packaged optics. Tencent has decided to enter the fray with 25.6-terabit switches.

“It is not just Broadcom developing the optical solution; it is also ensuring that our manufacturing partner is ready to scale,” says Mehta.

Ruijie Networks is making the two-rack-unit (2RU) switch platform for Tencent based on Broadcom’s co-packaged optics solution. The co-packaged optics interfaces are routed to 16 MPO connectors while the switch supports 32, 400-gigabit QSFP112 modules.

“It's always important to have your lead partner [Tencent] for any deployment like this, someone you're working closely with to get it to market,” says Mehta. “But there is interest from other customers as well.”

Cost and power benefits

Broadcom says co-packaged optics will lower the optical cost-per-bit by 40 per cent while the system (switch platform) power savings will be 30 per cent.

Humboldt more than halves the power compared to using pluggables. Broadcom’s co-packaged optics consumes 7W for each 800-gigabits of bandwidth, whereas an equivalent 800-gigabit optical module consumes 16-18W.

Its second-generation design will embrace 5nm CMOS rather than 7nm and still more than halve the power: an 800-gigabit pluggable will consume 14-15W, whereas it will be 5.5W for the same co-packaged optics bandwidth.

Broadcom will move to CMOS for its second-generation electrical IC; it uses silicon germanium at present.

Power and operational cost savings are a longer-term benefit for data centre operators, says Broadcom. A more immediate concern is the growing challenge of managing the thermal profile when designing switching systems. “The amount of localised heat generation of these components is making systems quite challenging,” says Mehta.

A co-packaged design eliminates pluggables, making system design easier by improving airflow via the front panel and reducing the power required for optical interconnect.

“They've been telling us this directly,” says Mehta. “It's been a pretty good testimonial to the benefits they can see for system design and co-packaged optics.”

Roadmap

At OFC 2022, Broadcom also showed a mock-up of Bailly, a 51.2 terabit switch chip co-packaged with eight 6.4Tbps optical engines.

Broadcom will offer customers a fully co-packaged optics Tomahawk 5 design but has not given a date.

Since Broadcom has consistently delivered a doubling of switch silicon capacity every 24 months, a 102.4-terabit Tomahawk 6 is scheduled to sample in the second half of 2024.

That timescale suggests it will be too early to use 224Gbps serdes being specified by the OIF. Indeed, Mehta believes 112Gbps serdes will have “a very long life”.

That would require the next-generation 102.2Tbps to integrate 1024, 100Gbps serdes on a die. Or, if that proves too technically challenging, then, for the first time, Broadcom’s switching ASIC may no longer be a monolithic die.

Broadcom’s networking group is focused on high-speed serial electrical interfaces. But the company is encouraged by developments such as the open standard UCIe for package interconnect, which looks at slower, wider parallel electrical interfaces to support chiplets. UCIe promises to benefit co-packaged optics.

Broadcom’s view is that it is still early with many of these design challenges.

“Our goal is to understand when we need to be ready and when we need to be launching our silicon on the optical side,” says Mehta. “That's something we are working towards; it’s still not clear yet.”