10GBASE-R using 64b/66b encoding scheme (3% overhead) 10GBASE-X using 8b/10b encoding scheme (20% overhead)
10GBASE-R is achieved with single xcvr lane 10GBASE-X is achieved with 4 XAUI xcvr lanes & 2 RXAUI lanes.
Looking at the advantages 10GBASE-R having today, is 10GBASE-X is still being used in today Ethernet world? Or it is just a legacy protocol?