The CWDM8 MSA avoids PAM-4 to fast-track 400G

Another multi-source agreement (MSA) group has been created to speed up the market introduction of 400-gigabit client-side optical interfaces.

The CWDM8 MSA is described by its founding members as a pragmatic approach to provide 400-gigabit modules in time for the emergence of next-generation switches next year. The CWDM8 MSA was announced at the ECOC show held in Gothenburg last week.

Robert BlumThe eight-wavelength coarse wavelength-division multiplexing (CWDM) MSA is being promoted as a low-cost alternative to the IEEE 803.3bs 400 Gigabit Ethernet Task Force’s 400-gigabit eight-wavelength specifications, and less risky than the newly launched 100G Lambda MSA specifications based on four 100-gigabit wavelengths for 400 gigabit.

“The 100G Lambda has merits and we are also part of that MSA,” says Robert Blum, director of strategic marketing and business development at Intel’s silicon photonics product division. “We just feel the time to get to 100-gigabit-per-lambda is really when you get to 800 Gigabit Ethernet.”

Intel is one of the 11 founding companies of the CWDM8 MSA.

Specification

The CWDM8 MSA will develop specifications for 2km and 10km links. The MSA uses wavelengths spaced 20nm apart. As a result, unlike the IEEE’s 400GBASE-FR8 and 400GBASE-LR8 that use the tightly-spaced LAN-WDM wavelength scheme, no temperature control of the lasers is required. “It is just like the CWDM4 but you add four more wavelengths,” says Blum.

The CWDM8 MSA also differs from the IEEE specifications and the 100G Lambda MSA in that it does not use 4-level pulse-amplitude modulation (PAM-4). Instead, 50-gigabit non-return-to-zero (NRZ) signalling is used for each of the eight wavelengths. 

The MSA will use the standard CDAUI-8 8x50-gigabit PAM-4 electrical interface. Accordingly, a retimer chip will be required inside the module to translate the input 50-gigabit PAM electrical signal to 50-gigabit NRZ. According to Intel, several companies are developing such a chip.

When we looked at what is available and how to do an optical interface, there was no good solution that would allow us to meet those timelines

Benefits

Customers are telling Intel that they need 400-gigabit duplex-fibre optical modules early next year and that they want to have them in production by the end of 2018.

“When we looked at what is available and how to do an optical interface, there was no good solution that would allow us to meet those timelines, fit the power budget of the QSFP-DD [module] and be at the cost points required for data centre deployment,” says Blum.

An 8x50-gigabit NRZ approach is seen as a pragmatic solution to meet these requirements.

No PAM-4 physical layer DSP chip is needed since NRZ is used. The link budget is significantly better compared to using PAM-4 modulation. And there is a time-to-market advantage since the technologies used for the CWDM8 are already proven.

We just think it [100-gigabit PAM4] is going to take longer than some people believe 

This is not the case for the emerging 100-gigabit-per-wavelength MSA that uses 50-gigabaud PAM-4. “PAM-4 makes a lot of sense on the electrical side, a low-bandwidth [25 gigabaud], high signal-to-noise ratio link, but it is not the ideal when you have high bandwidth on the optical components [50 gigabaud] and you have a lot of noise,” says Blum.

One-hundred-gigabit-per-wavelength will be needed for the optical path, says Blum, but for 800 Gigabit Ethernet with its eight electrical channels and eight optical ones. “We just think it [100-gigabit PAM4] is going to take longer than some people believe.” Meanwhile, the CWDM8 is the best approach to meet market demand for a 400-gigabit duplex interfaces to support next-generation data centre switches expected next year, says Blum.

The founding members of the CWDM8 MSA include chip and optical component players as well as switch system makers. Unlike the 100G Lambda MSA, no larger-scale data centre operators have joined the MSA.

The members are Accton, Barefoot Networks, Credo Semiconductor, Hisense, Innovium, Intel, MACOM, Mellanox, Neophotonics and Rockley Photonics.