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In 10GBASE-R specification, Gearbox is defined to take care the data width adaptation between PCS & PMA. For example, after the 64b66b encoding, the PCS data width will become 66bits.

The gearbox will adapt the 66-bit datawidth to whatever data width that PMA is support.

May i know if we can have a PMA with serializer factor of 66. Does that mean Gearbox is not necassary or not needed in this case?

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Sure, if the width is the same, you won't need a gearbox. But most high performance serializers are going to be a power of two, and 66 is not a power of two, so you're pretty much guaranteed to need a gearbox.

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  • \$\begingroup\$ Yes, i understand that serializer factor of 66 does'nt make sense. Just want to verify my understanding. However, FPGA vendor like altera (Intel) does support 20 & 40 serializer factor. I think mainly is due to 8b10b reason. \$\endgroup\$ – Learner Oct 4 '19 at 7:47
  • \$\begingroup\$ That's a good point, but 5 is a relatively reasonable non power of two factor to handle. Serializers supporting 8b10b usually support 5*2^n widths. 66, on the other hand, is 2*3*11, which needs two different oddball factors, which is not so nice to implement. \$\endgroup\$ – alex.forencich Oct 4 '19 at 7:57

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