Written by Ray Nering, Product Manager, Transceiver Modules Group, Cisco

On July 9, 2018, the 400GE BiDi MSA Group announced its formation. The founding members consist of Alibaba, Broadcom, Cisco, Corning Incorporated, Foxconn Interconnect Technology, InnoLight Technology, Inphi Corporation and Sumitomo Electric. The MSA is focused on promoting interoperable 400GE optical specifications that utilize multi-mode fiber (MMF) for applications up to 100m link distances. The MSA leverages BiDi technology that has been widely deployed by Cisco and others for 40GE and 100GE applications over duplex MMF.

What is BiDi technology?

BiDi technology transmits and receives optical signals over a single fiber. In most BiDi applications, the wavelengths of the signals are different. The designs leverage the difference in wavelength to reduce cross talk between the Tx and Rx. In the case of duplex transceivers such as Cisco's QSFP-40G-SR-BD, each fiber is both transmitting and receiving data as shown in the diagram below.

Why use BiDi technology?

In the cases of 40GE and 100GE, it allows an upgrade strategy for customers that are migrating network equipment from 10GE to 40GE and then to 100GE, while re-using the duplex MMF they originally deployed for 10GE. In contrast, the IEEE specifications for 40GBASE-SR4 and 100GBASE-SR4 require four pairs of fibers. 40GE and 100GE BiDi save the customer money by not requiring the expensive task of deploying new fiber.

Since BiDi technology has been widely deployed, the vendor familiarity, infrastructure and knowledge base is widely understood. As the market migrates to higher speeds such as 400GE, leveraging prior generation technology should reduce time to market.

With the arrival of 400GE applications, customers will finally have no choice but to start using MMF transceivers that require more than duplex fiber. 400GE ports can be used to aggregate traffic from 100GE ports, still leveraging the duplex MMF from 100GE BiDi modules to 400GE modules in a breakout configuration as shown in the diagram below.

Even though duplex MMF probably will not make the transition to 400GE, the 400GE BiDi module is still an improvement over the current IEEE standard of 400GBASE-SR16 which requires 16 pairs of fiber, saving the cost of 4x the number of MMFs.

The IEEE has recognized that the 400GBASE-SR16 standard has not been broadly adopted by the industry and recently formed a task force to develop a specification for 400GE over MMF using BiDi. The IEEE taskforce (802.3cm) made a baseline proposal that is aligned with the 400GE BiDi MSA.Typically the IEEE process takes 12 to 18 months to ratify a new standard. In the meantime, the industry has already aligned behind the 400GE BiDi MSA.

The MSA expects to release a specification before the end of Sept 2018.