Issue link: https://www.e-digitaleditions.com/i/1327492
42 I ICT TODAY Most 400G transceivers that are shipping today are for SMF. Table 1 maps the alternatives for MMF showing the base rate, the number of wavelengths, and non-return-to-zero (NRZ) requiring no encoding versus PAM4. Across the top is the number of fiber pairs up to 32 fibers (16 pairs). To reiterate, the 32-fiber solution may be dead in the water as there currently are no transceivers to support it. There are already SR8 solutions available in the market and a number of OEMs are working on products, but this is not going to be a general structured cabling solution. It is a nice transceiver to break out and serve 50G NICs today, primarily for hyperscale, and may be a good application to serve 100G NICs when 800G SR8 arrives. The two solutions to focus on now are in the third column, four fiber pairs. The one that is active today and for which there are solutions for 400G is BD4.2. Via a multi-source agreement (MSA), it is really IEEE 400G SR4.2. This is a bidirectional (BiDi) solution with eight fibers transmitting 50G bidirectionally over eight fibers that can also be broken out to four 100G with a breakout cassette or harness. Indications are that BD4.2 is a winner as BiDi is compelling for leaf-spine interconnects because it can be physically broken out. The second solution, 400G-SR4.4, operates over four 25G wavelengths using NRZ modulation. PAM4 has emerged as the preferred modulation signaling choice as it achieves twice as much throughput using half the bandwidth compared to NRZ. Migration Paths and Preserving the Cable Plant One scenario for potential migration starts with a two-fiber cable plant and 10G SR wavelength, and then deploying shortwave wavelength division multiplexing (SWDM) 4 modules as a seamless migration from 10G SR to 40G without changing the cable plant. This SWDM4 solution is available today and can be used with fiber types from OM3 up to OM5. Obviously, OM5 provides the greatest bandwidth and the furthest reach. Another seamless way to get to 40G is to use 40G BiDi optics, 20G each way over two wavelengths. This also preserves the original 10G cable plant. There are now ways to migrate up to 100G with SWDM4, albeit at a shorter reach on OM3, OM4 and OM5. The only change is the 25G line rate. It starts with a 10G line rate for 40G SWDM4, four wavelengths, four transmits and four receives. Each of the four 10G or 25G channels is represented by wavelengths. There is a fairly new BiDi solution which is being presented by at least one vendor as a "dual rate" solution. Channels with this Bidi solution can be configured as 40G or 100G in the switch command line interface (CLI). The only thing that changes from the 40G BiDi solution is that the line rate is 50G. It is 25G encoded by PAM4 up to 50G over two wavelengths. Another example for 40G is SR4 that uses a 12-fiber cable plant (eight fibers live with four left dark), transmitting at 40G on one wavelength at 10G per lane. Again, there are four transmits and four receives. The migration strategy going forward is to increase the lane rate to 25G when moving up to 100G SR4. With this approach, it is not necessary to change the cable plant to get to 100G. In the future, SR4 could extend the life of an SR4 cable plant—transmitting four colors over four wavelengths, 25G per wavelength, but currently no trans- ceiver manufacturers have adopted this approach. Technology (per fiber) 1 fiber pair 2 fiber pairs 4 fiber pairs 8 fiber pairs 16 fiber pairs 25G-λ NRZ 25G-SR 100G-SR4 400G-SR16 50G-λ PAM4 50G-SR 100G-SR2 200G-SR4 400G-SR8 2x50G-λ PAM4 100G-SR1.2 200G-SR2.2 400G-BD4.2 4x25G-λ NRZ 100G-SR1.4 200G-SR2.4 400G-SR4.4 4x50G-λ PAM4 200G-SR1.4 400G-SR2.4 800G-SR4.4 Technology options for 200 & 400 Gb/s links over fewer MMF fiber pairs SRm.n m = # fiber pairs n = # wavelengths TABLE 1: Multimode optical fiber next generation solutions.