Glossary:
PSTN
PSTN is a public switched telephone network. PSTN is the public part of a telephone system based on copper wires carrying analog voice data. This does not included private telephone systems which are managed by their own PBX. PSTN contrasts to newer telephone networks such as ISDN and FDDI which are based on digital technologies.
Recently the public telephone infrastructure is almost entirely digital in technology except for the final link from the local telephone exchange to the user.