PCI

PCI was a parallel bus of 16 bit wide. PCI express change to use serial bus design. The protocol insert additional bits to achieve error for data inyegrity thus the data rate is less than the transfer capability.  The effective data rate improves from generation to generation by varying the amii pi unit of these overhead bits  

PCIe support full rate transfer bidirectionally. The standard support multi-lanes transfer from 1x to 32x.