ISDN (Integrated Services Digital Network) is a telephone line service that handles voice and data in an integrated digital format. In Japan, it was provided as NTT's INS Net 64 (basic interface, 2 channels with 2B+D) and INS Net 1500 (primary rate interface, 23 channels with 23B+D). When the service launched in 1988, it attracted attention as "the telephone of the digital age."
The difference from PSTN (analog lines) is that ISDN communicates using digital signals. INS Net 64 enabled 2 simultaneous channels (64kbps each) on a single line, offering the advantage of using FAX and telephone simultaneously. It also supported data communication, used for internet access (64kbps, or 128kbps with channel bonding) and video calls. INS Net 1500 was a high-capacity service bundling 23 channels, used to connect with corporate PBX systems for handling numerous simultaneous calls.
ISDN's decline paralleled the spread of broadband. When ADSL (up to 50Mbps) and fiber optic (up to 1Gbps) emerged in the 2000s, ISDN's 128kbps data communication couldn't compete on speed. Voice calls were also replaced by Hikari Denwa and VoIP services, and ISDN subscriptions plummeted. ADSL, the technology that displaced ISDN, has since retired as well: NTT East ended the service in January 2025 and NTT West in January 2026, after closing new sign-ups in July 2023.
NTT ended ISDN data communication (digital communication mode) alongside the PSTN-to-IP migration, which ran in stages from January 2024 and finished in January 2025. Voice calls still work over the IP network, so reading "the end of ISDN" as a shutdown of the line itself is a misunderstanding. Systems that relied on ISDN data communication, such as EDI (Electronic Data Interchange) and payment traffic from POS terminals, did have to move to internet VPNs, dedicated lines, or mobile connections. Because ISDN-based EDI was widespread in the distribution industry, mapping out what was affected and judging when to switch became the hard part of the work. You can review the evolution of telecommunications technology in the history of phone numbers.