Forward Error Correction Enables 40km Reach QSFP28 100Gb Pluggable Optical Transceivers

You may have heard of Forward Error Correction (FEC), since it’s used in many forms of data communication. You’ll find it in wireless networks, space communication, undersea fiber optic networks, bar code scanners, and your CD player (if you still have one). We’ll talk more about FEC in a subsequent post, but in this post we’re showing how it enables longer reach in a new pluggable optical transceiver that Cisco released in September, called QSFP100 “ER4-Lite”. It’s a new addition to the Cisco “QSFP100” product family, as it’s in a QSFP28 form factor, and is great for data center interconnects up to 40km reach without optical amplification.

You might also be wondering why we need a “Lite” version, when we already have a 40km IEEE standardized “100GBASE-ER4” transceiver. It turns out the laser power and receiver sensitivity required by the IEEE standard make it more expensive and require the larger CFP form factor. In contrast, QSFP100 ER4-Lite uses components with relaxed specifications and consumes less power, so it can fit in a QSFP28 form factor. This is a much better size for high density data center applications. And it can still support 40km reach as long as the host platforms at both ends encode and decode FEC.

FEC on host platforms is not that new. It’s actually required by IEEE 100GBASE-SR4 and other non-IEEE optical interface standards such as the CWDM4 MSA and the PSM4 MSA. So all Cisco switches and routers with QSFP28 ports have it.

For those who need to link to other systems already in place, the QSFP100 ER4-Lite interoperates with CPAK ER4-Lite, IEEE 100GBASE-ER4, and IEEE 100GBASE-LR4 at reaches identified in the figure below. These don’t use FEC, so make sure to have it turned off at both ends.