Recommendation ITU-T J.225 defines the fourth-generation of high-speed data-over-cable systems. The fourth-generation transmission systems introduce a number of new features that build upon what was present in previous Recommendations ITU-T J.112, ITU-T J.122, ITU-T J.222.x-series, and ITU‑T J.223.x-series. This Recommendation includes key new features for the physical (PHY) layer and enhancements to the media access control (MAC) layer protocols as well as requirements for upper layer protocols such as Internet protocol (IP), dynamic host configuration protocol (DHCP), etc. The fourth-generation cable modem specifications are incorporated fully in this Recommendation. Informative Supplement 10 to the ITU-T J-series Recommendations contains the correspondence between the data over cable service interface specification (DOCSIS) versions and the ITU-T Recommendations revisions and generations.