X.25

networkingISP & telecom

Stands for: ITU-T X.25 (packet-switched WAN)

The first widely deployed packet-switching standard - ITU-T's 1976 protocol that ran WANs, ATMs, and card networks for decades.

X.25 was finalized by the CCITT (now ITU-T) in 1976 - older than IPv4 and the OSI model - as a virtual-circuit packet service over the noisy analog links of the day, with heavy per-hop error checking (its link layer, LAPB, is HDLC-based). It carried financial transactions, airline reservations, and early corporate WANs; credit-card networks used it into the 2010s, and aviation still does. Frame Relay is its leaner successor. A genuine ancestor of modern packet networking.

Also known as: x.25, x25, lapb, pad, virtual circuit

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