What Was ISDN?

Original Article ↗ Hacker News Discussion ↗

Once billed as a digital upgrade to analog phone lines, ISDN is remembered here as both a short‑lived bridge to broadband and a surprisingly capable workhorse. Commenters recall its near‑instant call setup, low latency and channel bonding that made 64–128 kbps feel transformative for web browsing, gaming, remote work and broadcast audio, even as per‑minute tariffs, complex provisioning and the rapid rise of DSL and cable limited its consumer appeal. The thread also highlights how ISDN underpinned early ISP infrastructure, spawned creative billing and signaling hacks, and exposed the telecoms’ misreading of an Internet future they expected to control end‑to‑end.

Home and Personal Use

  • Many recall ISDN as a huge quality‑of‑life jump from 14.4–56k dial‑up: instant connect, lower latency, simultaneous phone + internet, and optional 2×64k bonding to ~128 kbps.
  • Especially valued in late‑90s/early‑00s transition periods (before DSL/cable were available) in Europe, US, South Africa, Australia, Japan, and elsewhere.
  • Common theme: relieved family conflicts over “hogging the phoneline,” and enabled early always‑on setups, remote access, Napster, and online gaming.

Performance and Latency

  • Users stress that latency and snappy connect times mattered more than raw throughput; web pages and games felt “instant” for the era.
  • Some note that moving to early DSL sometimes worsened latency (interleaving vs fast‑path debates).
  • There’s disagreement over bandwidth details: some lines were full 64 kbps per B‑channel; others were limited by signaling/bit‑robbing to 56 kbps or 112 kbps when bonded.

Business, ISP, and Broadcast Use

  • ISDN was central to dial‑up ISP scaling: PRIs feeding modem banks, remote site connectivity, small corporate WANs, and Centrex‑style managed PBXs.
  • Widely used for radio and TV contribution links and outside broadcasts because of predictable bitrate and latency, even after IP existed.
  • Some enterprises used PRIs as their “T1/E1” voice/data backbone, including early videoconferencing.

Pricing, Tariffs, and Regional Variation

  • Experiences range from “massively subsidized and cheap” to “10× the price of dial‑up and metered by the minute.”
  • Tariff quirks (e.g., unmetered local/“intercom” or “voice” calls) enabled tricks like flat‑rate Internet over Centrex or Data over Voice.

Technical Characteristics

  • Common points: BRI = 2×64k B‑channels + 16k D‑channel; PRI = 23B+1D (T1) or 30B+1D (E1).
  • Symmetric bandwidth and exposed signaling (D‑channel) enabled advanced call routing and multi‑number setups at home.
  • Discussion of line voltages, backup power, interaction with pair gain, load coils, and xDSL over former ISDN pairs.

Hacks and Creative Uses

  • D‑channel and call‑setup abuse for “free” low‑volume data or messaging.
  • Centrex/DoV to dodge per‑minute data rates.
  • Alarm circuits and special provisioning to get better analog or early DSL performance.
  • Scripts to juggle two B‑channels for quasi‑24/7 access and seamless redial.

Limitations and Decline

  • Criticisms: high cost, metered billing, limited speeds, and telcos’ circuit‑switched mindset that missed packet‑switched Internet’s trajectory.
  • Generally displaced by DSL, cable, and later fiber, though some niche voice/broadcast uses persist.
  • Several note nostalgia for openness (multiple ISPs over same line) and reliability, even as modern links are far faster.