As part of the standardization of 100 Gigabit Ethernet, 40 Gigabit Ethernet was also specified by the Higher Speed Study Group( HSSG). 40 Gigabit Ethernet can be operated over optical fiber and over Category 7( Cat 7) copper cable.
The 40GBase-CR4 interface was specified specifically for data transport over short distances of up to 10 meters. The letter "C" stands for Copper, the letter "R" for the interface type with 64B66B coding and the number 4 for four parallel twinaxial cables, alternatively over four lanes via Active Optical Cables( AOC). The transmitted baud rate for each twinaxial cable is 10.3125 GBaud. The 40GBase-CR4 interface has two coaxial cables in addition to the four twinaxial cables. It is in the QSFPtransceiver module.