Telephone system may be classified as:
Telephone system may be classified as Duplex and symmetrical
A symmetrical connection has the same sized pipe or the same available bandwidth in each direction. While it's commonly assumed that most connections are symmetrical, this is rarely true of fee -based connections. In other words, LANs are usually symmetrical; however, WANs seldom are.
A business telephone system is a multiline telephone system typically used in business environments, encompassing systems ranging from the small key telephone system (KTS) to the large private branch exchange (PBX).
The NPN (new public network) is an emerging communication system that converges the PSTN (public-switched telephone network) and the Internet. It is a packet-switched network that will deliver voice with the same reliability now provided by the PSTN.