
To address power consumption issues, the OIF is pursuing two parallel tracks: using 56 Gigabit non-return-to-zero (NRZ) signalling and 4-level pulse amplitude modulation (PAM-4) which encodes two bits per symbol such that a 28 Gbaud signalling rate can be used. The 56 Gig NRZ uses simpler signalling but must deal with the higher associated loss, while PAM-4 does not suffer the same loss as it is similar to existing CEI-28 channels used today but requires a more complex design.
More online…
http://www.oiforum.com/oif-shows-56g-electrical-interfaces-cfp2-aco/