400G Ethernet is About to Get a Boost

A new task force with the Institute of Electrical and Electronic Engineers (IEEE) on project 802.3cm was formed in March 2018 to fulfil two new multimode objectives for 400 Gb/s Ethernet operation:

• 400GBASE-SR8 – an 8-pair, 1-wavelength multimode solution supporting reaches of 70/100/100 m over OM3/4/5
• 400GBASE-SR4.2 – a 4-pair, 2-wavelength multimode solution supporting reaches of 70/100/150 m over OM3/4/5

400GBASE-SR8 is the first IEEE fiber interface to use eight pairs of fibers. Its 8-pair fiber interface will have two variants. One will use the 24-fiber MPO, configured as two rows of 12 fibers as shown in Figure 1. The second interface variant will use a single-row MPO-16 as shown in Figure 2.

In comparison, 400GBASE-SR4.2 is the first instance of an IEEE 802.3 solution that employs both multiple pairs of fibers and multiple wavelengths. It will operate over the same type of cabling used to support 40GBASE-SR4, 100GBASE-SR4 and 200GBASE-SR4. This is also the first Ethernet standard to use two short wavelengths to enable doubling of the multimode fiber capacity from 50 Gb/s to 100 Gb/s per fiber. It will do so using bi-directional propagation on each fiber, with each wavelength traveling in opposite directions. As such, each active position at the transceiver is both a transmitter and a receiver.

CLICK TO TWEET: 400G Ethernet is about to get a boost.

The optical lane arrangement is shown in Figure 3. The leftmost four positions are labeled TR because they transmit wavelength λ1 and receive wavelength λ2. Conversely, the rightmost four positions are labeled RT because they receive wavelength λ1 and transmit wavelength λ2. Wavelength λ1 is the traditional one nominally set to 850 nm. The second wavelength, λ2, is set nominally at 910 nm, and can be supported to longer reaches over OM5 multimode fiber, which was designed to optimize short wavelength multiplexing. Thus, the reach over OM5 is 50% longer than that over OM4, and more than double the reach over OM3.

The optical specifications for 400GBASE-SR4.2 are aligned with 100G-BiDi, the solution introduced earlier in 2018 by Cisco as a follow-on to their 40G-BiDi solution. As such, it is anticipated that each fiber pair of the 400GBASE-SR4.2 parallel interface can be broken out to connect to 100G-BiDi ports provided the forward error correction codes are the same.

400GBASE-SR4.2 joins a stable of four other existing transceiver types that also use short wavelength multiplexing. Two of these are the BiDi solutions already mentioned. Two more are the 40G-SWDM4 and 100G-SWDM4 multisource agreements (MSAs) that employ four wavelengths. Each type enjoys longer reach over OM5, as shown in Figure 4, owing to OM5’s SWDM optimization that ensures essentially equal performance for all wavelengths from 850 nm to 953 nm.

Wavelength division multiplexing has long been a staple of singlemode transmission, continuing to this day. Now the use of short wavelength multiplexing has become an important addition to add capacity to multimode fiber that is being employed to reduce the number of fiber pairs. And while using two wavelengths will soon serve to cut the number of pairs in half between 400GBASE-SR8 and 400GBASE-SR4.2, using two more wavelengths opens possibilities for single-pair operation at 200 Gb/s and for four-pair operation at 800 Gb/s, all without requiring an increase in lane rate or a reduction in reach. As with today’s wavelength multiplexed solutions, OM5 will continue to offer improved support as this future unfurls.