Enabling 800-gigabit optics with physical layer ICs

Broadcom recently announced a family of 800-gigabit physical layer (PHY) chips. The device family is the company’s first 800-gigabit ICs with 100-gigabit input-output (I/O) interfaces.

Source: Broadcom

Moving from 50-gigabit to 100-gigabit-based I/O enables a new generation of 800-gigabit modules aligned with the latest switch chips.

“With the switch chip having 100-gigabit I/Os, PHYs are needed with the same interfaces,” says Machhi Khushrow, senior director of marketing, physical layer products division at Broadcom.

Broadcom’s latest 25.6 terabit-per-second (Tbps) Tomahawk 4 switch chip using 100-gigabit I/O was revealed at the same time.

800-gigabit PHY devices

The portfolio comprises three 800-gigabit PHY ICs. All operate at a symbol rate of 53 gigabaud, use 4-level pulse amplitude modulation (PAM-4) and are implemented in a 7nm CMOS process.

Two devices are optical PHYs: the BCM87800 and the BCM87802. These ICs are used within 800-gigabit optical modules such as the QSFP-DD800 and the OSFP form factors. The difference between the two chips is that the BCM87802 includes an integrated driver.

The third PHY - the BCM87360 - is a retimer IC used on line cards. Whether the chip is needed depends on the line card design and signal-integrity requirements; for example, whether the line card is used within a pizza box or part of a chassis-based platform.

Source: Broadcom

“If it is a higher-density card that is relatively small, it may only need 15 per cent of the ports with retimers,” says Khushrow. “If the line card is larger, where things fan out to longer traces, retimers may be needed for all the ports.”

All three 800-gigabit PHYs have eight 100-gigabit transmit and eight receive channels (8:8, as shown in the top diagram).

Applications

The optical devices support several 800-gigabit module designs that use either silicon photonics, directly modulated lasers (DMLs) or externally-modulated lasers (EMLs).

The 800-gigabit PHYs support the DR8 module (8 single-mode fibres, 500m reach), two 400-gigabit DR4 (4 single-mode fibres, 500m) or two FR4 in a module (each 4 wavelengths on a single-mode fibre, 2km) as well as the SR8, a parallel VCSEL-based design with a reach of 100m over parallel multi-mode fibre.

Timescales

Given the availability of these PHYs and that 800-gigabit modules will soon appear, will the development diminish the 400-gigabit market opportinity?

“This is independent of 400-gigabit [module] deployments,” says Khushrow.

The hyperscalers are deploying different architectures. There are hyperscalers that are only now transitioning to 200-gigabit modules while others are transitioning to 400- gigabit. They will all transition to 800 gigabit, he says: “How and when they transition are all at different points.”

Some of the hyperscalers deploying 400-gigabit modules are looking at 800 gigabit, and their deployment plans are maybe two to three years out. “We don’t expect 800 gigabit to cannibalise 400 gigabit, at least not in the near term,” he says.

Broadcom says 800-gigabit modules to ship in the second half of this year. “It all depends on how the switch infrastructure, line cards and optics become available,” says Khushrow.

Next developments

The landscape for high-speed networking in the data centre is changing and optics is moving closer to the switch chip, whether it is on-board optics or co-packaged optics.

“People are looking at both options,” says Khushrow.”It depends on the architecture of the data centre whether they use on-board optics or co-packaged optics.”

Meanwhile, the OIF is working on a 200-gigabit electrical interface standard.

Co-packaged optics is challenging and the technology has its own issues whereas optical transceivers are easier to use and deploy, says Khushrow.

Current industry thinking is that some form of co-packaged optics will be used with the adevnt of next-generation 51.2-terabit switch chips. But even with such capacity switches, pluggables will continue to be used, he says.

There will still be a need for PHYs, whether for pluggables, co-packaged designs or on the linecard.

“We will continue to provide those on our roadmap,” says Khushrow. “It is just a matter of what the form factor will be, whether it will be a packaged part or a die part.”