400G buying guide: QSFP-DD vs OSFP, SR8/DR4/FR4/LR4 reach classes, and when to use DAC, AOC or transceivers for the lowest cost per link. By Topnet System engineers, ISO certified since 2008. http://www.topnetsystem.com/400g-transceiver-dac-aoc-complete-guide-how-to-build-every-link-nv652-1.html en