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Tencent lines up to deploy Broadcom's co-packaged optical switches

A faster 51.2Tbit/sec co-packaged switch is also in the works

Broadcom will deploy its 25.6Tbit/sec Humbolt co-packaged optical (CPO) switches in China-based cloud provider Tencent's datacenters in a bid to accelerate adoption of the emerging network tech.

The switches are based on a design announced in early 2021 that melds traditional switching with co-packaged optics. The tech effectively shifts the optics and digital signal processor from the pluggable and packages them alongside the switch ASIC.

By reducing the number of pluggable modules, Broadcom says it can significantly reduce power consumption as the fiber-optic cables can be connected directly to the switch.

"The distance that you can interconnect these notes is getting shorter. As 100G PAM4 really starts to proliferate through the datacenter and 200G after that the reach severely gets limited, and this is where we see this huge opportunity for optical connectivity," Rajiv Pancholy, director of hyperscale strategy and products, told The Register.

In the case of Broadcom's first Humboldt switches, the actual switch logic is handled by a 25.6Tbit/sec Tomahawk 4 ASIC connected to set of four 3.2Tbit/sec CPO interfaces that break out to 16 multi-fiber push-on (MPO) connectors at the front of the switches 2U chassis. This means pluggable modules aren't required for half of the available switching capacity.

The remaining 12.8Tbit/sec of switching capacity is handled electrically and divvied up between 32 400Gbit/sec-capable QSFP112 ports.

Broadcom executives told The Register this provides flexibility for deployments in traditional networks, and will allow Tencent to drop these switches into their network using their existing pluggable modules. "I can still use some cheap pluggables if I want to keep that down on the cost curve," Pancholy said.

CPO offers other advantages, they explain, namely lower capex and opex, because MPO connectors don't require a pluggable module, which, depending on its bandwidth, could run a few hundred dollars each. What's more, Broadcom says its hybrid CPO ASIC consume just half the power of a standard pluggable module per port.

"The spend on optics right now in the datacenter is huge. It's maybe 10x the spend on like switch silicon," Pancholy said. "Today an 800G optical module is going to run about 16W, that's with a 7nm DSP… for that same 800Gbit/sec of bandwidth we're at 7W. For 51.2T it will be 5.5 watts."

However, the design does introduce some complexity. While the optics have been co-packaged, that laser has not, with each of the four CPO interfaces getting its own user-serviceable laser module which supplies constant light to the system.

The switch also inherits some of the limitations of the Tomahawk 4 ASIC, namely 100G serializer/deserializers, which limit the per-port bandwidth to 800Gbit/sec when using eight fiber pairs.

Broadcom says a 51.2Tbit/sec full CPO switch is in the works, which will likely be based on the company's brand new Tomahawk 5 ASIC announced last week. However, it remains to be seen whether the faster Humboldt switches will use 200G SerDes for 1Tbit/sec-plus port speeds.

"When you go to 51.2Tbit/sec… the ability to fan out and interconnect at distances we think you’re going to need, it's only geared toward optics," Pancholy said.

The switches are slated for deployment in Tencent's cloud sometime next year, but will be on display at the China International Optoelectronic Exposition in Shenzhen in September. ®

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