Broadband is a general term for always-on internet connections that carry large volumes of data. There is no single speed threshold that defines it - the figure varies by country and by program, and Japan has no statutory definition either. In practice the word is used in contrast to dial-up over a phone line (56kbps at best), covering always-on high-speed links such as fiber optic, cable TV (CATV), and mobile (4G/5G) lines. ADSL, once the mainstay, was a form of broadband too, but NTT East ended the service on 31 January 2025 and NTT West on 31 January 2026.
Broadband uptake in Japan accelerated after the 2001 "e-Japan Strategy," in which the government set a goal of becoming the world's most advanced IT nation by 2005 and pushed the build-out of communications infrastructure. Yahoo! BB broke the price of ADSL, nationwide fiber deployment followed, and by the 2020s the normal picture was a choice between fiber and cable TV in urban areas, with mobile lines covering places fiber does not reach.
Broadband changed telephony at its root. Practical VoIP became possible over high-speed lines, and video calls, cloud PBX, and telework turned into everyday business assumptions. The voice-only public switched telephone network (PSTN) itself was migrated as well: NTT East and West moved it to an IP network in stages from January 2024 and completed the switch by January 2025. Voice, video, and data now travel over the same IP network.
Two points trip people up at contract time. First, a headline figure such as "up to 1Gbps" is the ceiling of the specification (best effort), not a promise of the speed you will get. Second, in apartment buildings the fiber may stop at the building entrance and the last stretch to each unit may run over telephone wiring (the VDSL method); in that case moving to a higher plan does not raise the speed. A further pitfall is a power outage. A traditional subscriber line is powered from the phone line itself, so it kept working during blackouts, whereas a phone carried over a broadband line cannot place or receive calls once the ONU or router loses power. Do not let a fixed-line handset be your only means of contact in an outage.
The regional gap - the digital divide - also remains a challenge. In mountainous areas and on remote islands beyond the reach of fiber, satellite internet (such as Starlink) and mobile networks serve as alternatives. See VoIP basics for more on the relationship between broadband and the telephone.