400Gbps PSM4 to 100Gbps Breakout Solution
Overview
400G is now the standard aggregation speed at the network edge and in the data center, but not everything on the other end needs, or can use, the full 400G. Often what's needed is four independent 100G circuits reaching four different places. Our answer is a 400GBASE DR4++ module with four independent 1310nm PSM4 lanes on one MPO/APC connector, each rated to 10 km. A single MPO/APC to 8x LC/UPC breakout cable fans that port into four duplex LC pairs, each terminated at the far end by an ordinary 100G-QSFP28-SL10 module, so one 400G port becomes four independently routable 100G circuits from two part numbers.
Budget calculation
The budget uses the smaller of the two transceivers ratings, since a mixed pair is only as strong as its weaker end. 100G-QSFP28-SL10 publishes 6.3 dB min at 10 km (400G-QSFP-DD-12's 9 dB average is higher and doesn't constrain the design). The breakout's own insertion loss isn't in its SKU description, so 0.75 dB is assumed as a planning figure, plus 0.5 dB for the two mated connections and 3.5 dB of fiber at 10 km (0.35 dB/km): 4.75 dB total loss, 1.55 dB margin at the full 10 km rating.
Bill of materials
Every product in the diagram above, with the quantity each one appears. Open a row for full specifications, pricing and ordering.
- 1x400G-QSFP-DD-124x100G DR4++ QSFP-DD Module (Tx/Rx 4x1310 PSM4, 106.25 Gbps (PAM4) per lane, up to 425 Gbps, Max. 10km over SMF with host FEC, 9 dB (AVG), 6.5dB (OMA), Temp. 0-70C, MPO/APC)
- 4x100G-QSFP28-SL10Double Fiber 100G-LR Single Lambda PAM4 QSFP28 Module (Tx/Rx: 1304.5-1317.5nm, 106.25 Gbps, Max. 10km over SMF with FEC, 6.3 dB, Temp. 0-70C, LC/UPC)
- 1xMPO-8LC-UPC-OS2MPO Female to 8xLC, Type B, OS2 9/125μm, MPO/APC, LC/UPC, LSZH Breakout patch cable, Length 0.5-50 meters
Frequently Asked Questions
How is the 10 km link margin calculated for this breakout?
The weaker transceiver sets the available budget. The 100G-QSFP28-SL10 carries a 6.3 dB budget at 10 km, well under the 400G-QSFP-DD-12's 9 dB average, so the SL10 side is the limiting factor. Planning loss comes to 4.75 dB: 0.75 dB for the breakout cable's own insertion loss, 0.5 dB for the two mated connections, and 3.5 dB of fiber at 10 km (0.35 dB per km). That leaves 1.55 dB of margin at the full 10 km distance.
Why does the far end need an extended reach single lambda module instead of a standard 100GBASE-DR optic?
A standard 100GBASE-DR optic is built to the IEEE 802.3cd 500 m reach target, not 10 km. The 100G-QSFP28-SL10 is EDGE's own extended reach, single lambda 1310 nm part rated to 10 km with a 6.3 dB budget, matching the distance this breakout design targets. Using a standard DR module instead would put it outside its rated reach on the long end of the link.
What FEC settings does this breakout need?
The 100G-QSFP28-SL10 carries built-in RS(544,514) FEC, so host FEC should be turned off on the 100G port it plugs into. On the 400G-QSFP-DD-12 side, each of the four breakout lanes runs on host side FEC rather than FEC built into the module, so the correct per-lane setting depends on how the switch's breakout mode is configured. Check the platform's breakout mode configuration guide for the FEC setting that applies to each 100G lane.
Does the breakout cable's length (0.5 to 50 m) affect the 10 km reach calculation?
No. The MPO-8LC-UPC-OS2 is a short patch and fanout cable, and whatever length is ordered between 0.5 and 50 m, it contributes the same fixed insertion loss used in the budget. The 10 km distance in the link budget is the outside-plant single-mode fiber run beyond the breakout cable, not the cable's own length, so changing the cable length doesn't change the margin calculation.