ADSL (Asymmetric Digital Subscriber Line) provided high-speed internet over existing copper telephone lines. "Asymmetric" means different download and upload speeds: up to 50Mbps down and 5Mbps up.
Launched in Japan in 2000, Yahoo! BB's entry at 2,280 yen/month drove explosive adoption. Hundreds of times faster than 56kbps dial-up, ADSL brought "always-on" internet to households and accelerated Japan's broadband revolution.
ADSL's weakness was distance-dependent speed degradation from NTT exchanges. Beyond 4km, speeds often became impractical. Sharing copper lines with telephone service also made it susceptible to electromagnetic interference.
As fiber optic lines spread, ADSL declined. NTT East ended Flets ADSL in January 2023; NTT West in March 2024. The 24-year service ended, but its role in Japan's internet adoption was immense. See telephone evolution for the technology timeline.