Difference between ISDN and public switched telephone networks (PSTN)?

[LM741]

hi all.

stuggling to determine the difference between ISDN and public switched telephone networks (PSTN). I know what each one is individually i just don't know how they relate to each other... Does ISDN use PSTN?
What i undersatnd is that PSTNs have only transmitted voice in the past but now because of ISDN other types of data are also transmitted over the telephone network. So is ISDN some network that works on top of PSTN?

thanks

 

[Evo]

ISDN, whether BRI or PRI leave the premise and are routed to an ISDN Node.they are then carried as SDS (Switched digital service). A true ISDN connection ISDN to ISDN would not utilize the PSTN. However, non ISDN calls can be made from an ISDN line to a pots line and the egress can cross the PSTN.