The MDC, SN and CS connector families — duplex connectors small enough to put three or four in the space an LC duplex occupies.
What Are VSFF Connectors?
Very-small-form-factor connectors shrink the duplex optical interface well below what LC allows. CS, SN and MDC each take a different mechanical approach but share the aim: fit two, three or four duplex connections into the faceplate width one LC duplex needs.
All three keep the 1.25 mm ferrule that LC uses, so the optical physics is unchanged — the same polish options, the same insertion loss grades, the same cleaning problem. What changes is the housing, the fibre pitch and the latch mechanism.
What Is Driving Them
Breakout is the first force. A 400G or 800G port that fans out to four or eight duplex destinations needs those connections landed on one module faceplate, and LC simply does not fit. SN and MDC stack the two fibres vertically, which is what allows four duplex per module face where CS, keeping them side by side at a 3.8 mm pitch, fits two.
The second force is what happens at the switch package. Co-packaged optics and XPO bring fibre counts to the ASIC substrate that conventional connectors cannot land in the available area, and the panel-side density has to follow. Work a breakout scheme through the breakout configurator.
The Interoperability Problem
CS, SN and MDC are not intermateable, and there is no adapter that makes them so. Unlike LC, where one design became universal and every vendor built to it, VSFF is three licensed designs competing for the same slot. A site that standardises on one has committed its cords, adapters, cassettes and spares to that choice for the life of the plant.
The practical mitigation is to keep VSFF on the transceiver side of a breakout harness and let the structured plant stay MPO trunk, so a change of family is a change of harness rather than a rebuild. The VSFF comparison article goes through the mechanics of each.
Where You Meet It
On breakout modules for 400G and 800G, in high-density patch designs, and on OSFP-XD and CPO roadmaps. We build harnesses terminated in MDC and SN as well as LC and uniboot.
Testing them is its own small problem. Reference-grade test cords, adapters and inspection tips are all family-specific, so a site running both SN and MDC needs two of everything in the kit before acceptance testing starts rather than after the first panel fails. The ferrule is still 1.25 mm, so the inspection criteria themselves do not change — only the tip that reaches them.
VSFF reference
| Families | CS and SN (Senko), MDC (US Conec) |
|---|---|
| Ferrule | 1.25 mm zirconia — the same as LC |
| CS ferrule pitch | 3.8 mm |
| Density, SN and MDC | about three duplex in one LC duplex footprint |
| Panel density, CS | around 144 duplex ports in 1U |
| Insertion loss | LC-class — 0.25 dB standard grade |
| Polish | UPC and APC both available |
| Multi-fibre VSFF | MMC (US Conec) and SN-MT (Senko), 16 fibres and above |
| Intermateability | none between families |