From Wikipedia: "Integrated Services Digital Network (ISDN) is a set of communication standards", and for DSL: "Digital subscriber line is a family of technologies". My question is: Is DSL built on top of ISDN and what are the differences? I am a bit confused, since both describe ways of transmitting data over telephone lines.

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You can't really compare those two technologies.

ISDN is a network protocol encompassing practically the whole OSI model from physical to application layer.

xDSL is a family of physical-layer line protocols running between two interfaces (point-to-point). There are ADSL and VDSL variants that allow the co-existence with POT and ISDN on the same line using a frequency splitter. This is often called DSL over ISDN, but it's actually more DSL besides ISDN because the ISDN (or POTS) doesn't know a thing about DSL.

  • IDSL is ISDN with the signaling/channelizing removed. ("raw" ISDN, if you will)
    – Ricky
    May 10, 2019 at 13:41

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