The dual-density version of the Quad Small Form-factor Pluggable (QFSP) Multi-Source Agreement (MSA) module standard looks set to be the rule for new line-cards, following a raft of demonstrations and alpha module tests in 2018. The hidden message of the QSFP-DD craze is not the physical characteristics of the module itself, but what it says about the turn to advanced signaling methods. Half a decade ago, QSFP28’s arrival heralded the coming dominance of 25Gbps electrical channels using NRZ signaling. Now, QSFP-DD, and its larger cousin, the octal SFP or OSFP, use PAM4 signaling. This most often uses 50Gbps channels, but QSFP-DD is flexible enough to use eight-channel interfaces in a variety of implementations: 25G x 8 for 200Gbit Ethernet, 50G x 8 for 400Gbit Ethernet, and perhaps some early 100G x 8 for a pre-standard 800Gbit Ethernet.