A MAN is a Metropolitan Area Network. We will make no jokes at this point, however we might be tempted. A WAN is a Wide Area Network. Since MANs and WANs almost always have to use publicly available commercial facilities, standards for interconnecting become vital. Threee of the most important are these.
ISDN is a set of International Telecommunications standards for digital communications. It has been widely deployed in the public switched telephone network at the switch to switch level but has not won very many adherents among consumers.
SONET and SDH are another set of international standards specifically designed for the very high speeds which fiber optics make possible. The optical standards don’t dictate a particular networking technology. Think of them as the superhighways on which all sorts of vehicles can travel.
ATM has also been standardized in such a fashion that it can be used as the underlying carrier for many other communications techniques.
