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 / x25.org¶
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