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.
NTT terminated ISDN data communication (digital communication mode) in 2024 as part of the PSTN-to-IP migration. Voice calls remain available over the IP network, but businesses that depended on ISDN data communication for EDI (Electronic Data Interchange) and POS terminal payment processing need to migrate to internet VPN, dedicated lines, or mobile connections. The impact was particularly significant in the distribution industry, where ISDN-based EDI was widely used, prompting the Ministry of Internal Affairs and the Ministry of Economy to implement migration support measures. You can review the evolution of telecommunications technology in the history of phone numbers.