Ethernet proudly tows its thirty plus years of legacy along behind it. The IEEE fully embraces this idea and 200G/400G standards are no exception:
While flexibility is desirable, it must be balanced with companies’ desires to leverage their existing investments in devices supporting 200GbE or 400GbE. So, to address these issues and create this inherent flexible architecture, the “MII Extender concept, first introduced as part of the 10 Gigabit Ethernet standard, was adopted. Figure 1 illustrates the 200GMII Extender and 400GMII Extender. These optional extenders consist of extender sublayers (“XS”) that convert the data stream to and from the MII interface with the respective physical AUI interface between them.
I don’t understand this enough, and can’t find enough data to learn what I don’t know. My gut says that 200G/400G standards are uncertain today and having flexibility might seem like a good idea. OTOH, flexiblity or abstraction comes at a cost, sometimes a substantial cost.
Ethernet Alliance | The 200GMII / 400GMII Extenders : https://ethernetalliance.org/blog/2018/03/05/the-200gmii-400gmii-extenders/