Transceiver Form Factor Guide

A BOM line rarely says more than "10G SFP" or "100G QSFP28". The two modules that arrive can look identical from the front of the rack and still behave completely differently: a 40G QSFP+ and a 100G QSFP28 share the same cage width, an SFP+ and an SFP28 share the same latch, and an OSFP will not enter a QSFP-DD port at all. The port does not care what the label says. It cares about the cage, the lane count and the firmware it can talk to.

This guide sets out the transceiver form factors APEX ships from 1.25G to 800G, what each one physically plugs into, and the checks that decide the part number before a single unit is ordered.

Form Factors from 1.25G to 800G

The ladder below is the one used across access, metro, DCI and AI fabric links. Lane count and port type, not the headline rate, are what make two form factors interchangeable or not.

How to Read a Port Before You Order

Most wrong-form-factor orders come from reading a datasheet rather than the hardware. Six checks settle it:

1. Measure the cage. OSFP is deeper and taller than QSFP-DD, with a heatspreader on the module body. It cannot be forced into a QSFP-DD port, and the reverse is equally true.
2. Count the host lanes. A 400G QSFP-DD port runs eight lanes at 50G; a QSFP28 port runs four at 25G. The module and the port must agree on lane speed before link training starts.
3. Check the coding the platform expects. Many switches verify EEPROM contents, not only the optical signal, so the module has to be coded for the host platform.
4. Plan the thermal budget. A panel of 800G modules dissipates far more than the same number of 100G ports. If the rack was designed around short-reach optics, cooling becomes the limit before the optics do.
5. Fix the reach from the measured route, not from the price list. A long-reach part on a short span links with an attenuator and costs more in power and spares.
6. Confirm the direction of backward compatibility. A QSFP28 module works in a QSFP-DD port; a QSFP-DD module in a QSFP28 port does not give you 400G.

Four Mistakes That Return in Every Project

The same four assumptions appear when a link fails at commissioning:

• Treating QSFP+ and QSFP28 as one part. Same cage, different lane speed. A 40G module will not deliver 100G because the port is a 100G port.
• Assuming OSFP and QSFP-DD are interchangeable. Both reach 800G, and both need a matching cage and cable assembly. They are not the same footprint.
• Expecting SFP28 to run wherever an SFP+ fits. Mechanical fit is not rate support; the port has to accept 25G as well.
• Mixing reach classes across a span. Both ends of a link use the same reach code. The receiver class is part of the design, not a spare part that can be substituted.

What APEX Ships in Each Form Factor

APEX covers the whole ladder, so a project can be specified from one place instead of patched together from several. The 1.25G to 2.5G SFP range covers LX, EX, ZX, CWDM, DWDM and BiDi options for access and industrial links. The 25G SFP28 range adds 25G in 10km, 15km, 40km and 80km variants with CWDM, DWDM and single-fiber BiDi versions. Above that, the 100G QSFP28 range and the 400G QSFP-DD and OSFP range cover short parallel runs, CWDM4 and PSM4 links, and amplified ZR and ZR+ spans, while the 800G OSFP and QSFP-DD range serves AI fabric uplinks from 50m to 450km.

Every unit carries DDM/DOM diagnostics, so case temperature, optical power and supply voltage can be read from the host. Modules are coded for the platform in use, and a project mixing short parallel links, 10G legacy ports and long amplified spans can be quoted and shipped as one order with matching spares at each site.

Frequently Asked Questions

Can I use a QSFP28 module in a QSFP-DD port?

Yes, in most platforms. QSFP-DD ports accept QSFP and QSFP28 modules, and the port negotiates down to the 100G or 40G lane structure. Confirm the host supports the lower rate on that port before you plan on it.

Will a 25G SFP28 module work in a 10G SFP+ port?

Only if the port is rate-flexible and supports 25G. Mechanical fit is not the deciding factor. On fixed 10G ports the SFP28 module stays dark, and a 10G module in the same cage is the correct choice.

Which form factor should I use for 800G AI cluster links?

OSFP dominates 800G switch ports, QSFP-DD appears on platforms that prioritise backward compatibility with 400G and 100G. Match the switch port, then choose reach: DR and FR for racks and halls, ZR and ZR+ where the fabric crosses metro distances.

Can CFP2-DCO and QSFP-DD ZR+ serve the same route?

Both are coherent and both carry 400G. CFP2-DCO suits line cards that already have CFP2 ports and long unamplified or amplified spans, while ZR+ fits QSFP-DD switch ports. The route and the host hardware decide, not the rate.

Next Step

Send the port list per platform with the measured distance for each span. We will confirm the form factor, lane speed, reach code and coding each link needs, quote the mix together, and ship a coded sample ahead of the volume order so the first rack is not also the first test.