Do X.25/Frame Relay/ATM/MPLS have their own isolated physical infrastructure? Or are they technologies invented to utilize the then existing infrastructure of the PSTN?

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Short answer: No, and no.

Long answer: At the time X.25, Frame Relay, and ATM were in widespread use, voice networks were still largely "circuit switched," meaning, a dedicated data channel was likely to exist between the PSTN switches terminating each end of a phone call. The PSTN inter-exchange switches themselves handled the circuit-switching.

The technologies you mentioned are all "packet switching" (or cell switching) technologies allowing for over-subscription. This is important when serving data terminals (such as early cash dispensers or finance applications.)

Those packet-switching technologies used some of the same underlying link-layer technologies as voice networks. For example, a DS1/T1 might've carried 24 voice calls or 1.5Mb/s of packet data.

  • What other technologies are analogous to DS1/T1? – Noob_Guy Mar 7 '20 at 10:41

When the telephone networks were digitalized in the 1980s and 90s using ATM and SDH, protocols such as PSTN, X.25, Frame Relay or MPLS became applications running on top of those networks. Dedicated networks were possible but uncommon for wide-range communciation.

  • wait.. PSTN is a protocol?? – Noob_Guy Mar 6 '20 at 14:19
  • Not really (more a stack), but it depends on how you look at it. In any case, telephony runs as an application on top of the digital networks, circuit-switched or packet-switched. – Zac67 Mar 6 '20 at 14:33
  • X.25/Frame Relay/MPLS run on top of ATM?? Aren't they all link layer protocols? – Noob_Guy Mar 7 '20 at 2:52

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