What specification transformed two-way cable TV networks into broadband networks?

The transformation of two-way cable TV networks into broadband networks is primarily attributed to the Data Over Cable Service Interface Specification, commonly known as DOCSIS. This specification was developed to enable high-speed data transfer over existing coaxial cable infrastructure, which allowed cable operators to provide internet services alongside traditional cable television.

DOCSIS facilitates the transmission of data by defining how data packets are sent and received over coaxial cables, allowing for different types of data services, such as voice, video, and internet access. The introduction of DOCSIS significantly increased the bandwidth available to users, enabling much faster internet speeds compared to earlier technologies, thus effectively converting cable systems into robust broadband networks.

Other choices, such as the Internet Engineering Task Force (IETF) and Asynchronous Transfer Mode (ATM), relate to networking and data transfer methods, but do not directly pertain to the transformation of cable networks into broadband as DOCSIS does. Similarly, Digital Video Broadcasting (DVB) focuses on digital television broadcasting rather than broadband internet services. Therefore, the impact of DOCSIS on cable network functionality and growth makes it the correct answer in this context.