X.25 is a packet switched technology that falls within the domain of PSPSDN (Packet Switched Public Data Network). Contrary to the circuit-switched telephony network, it is based on packet switching technology. It was the first major vendor-independent / interoperable standard for such networks.

Interestingly, despite X.25 being packet switched, it has the notion of establishing "calls" over it, and packets are exchanged between the two ends after a conenction setup. Think in terms of of TCP having a connection abstraction over the packet switched IP network.

The advantages of X.25 compared to using modems in the PSTN / telephony network were mainly:
  • the modems were used only on the last mile to the next digital switch / central office, and not end-to-end, rendering higher line quality and hence higher data rates than possible over long/bad analog lines
  • billing was usually done based on traffic, not on duration of a call/session

FIXME: more background information. Diagrams.

Community X.25 networks

Telebahn /

BAN AI Systems

X.31 - X.25 over ISDN

The ITU T-REC X.31 specifies two modes of using X.25 over ISDN:
  • using an ISDN B-channel
  • using the ISDN D-channel
    • this provided up to 9600 bps and was used heavily in Germany e.g. for burgler alarm systems or banking card acceptance terminals in shops, restaurants and the like

Updated by laforge 7 months ago · 2 revisions

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