|The maximum transfer speed
is not 33,6 kbps or 56 kbps. As the xDSL technology (Digital Subscriber
Line) shows, transfer rates up to 9 mbps with ADSL (Asymmetric DSL) and
even 52 mbps with VDSL (Very High Data Rate DSL) are possible. The technology
behind is easy to understand: the telephone companies have defined a range
of 3,3 kHz in which voice, faxes and data are transferred. So it is possible
to send 35'000 bits per second through one telephone line. Theoretically
a copper wire can transfer signals in a bandwidth up to several megahertz
but only for short distances. The maximum distance between the two connection
points is about five to six kilometers. At least in Switzerland this should
be no problem as the telephone net is on a very high level.
In the 60s Bellcore developed a new method: DSL (Digital Subscriber Line). With multiplexing several digital channels could be used parallel. The maximum transfer speed was limited to 160 kbps. There were two data channels with 64 kbps and a control channel with 32 kbps. This technology has been adapted worldwide and is called now ISDN.
In the 70s HDSL (High Data Rate DSL) was developed on the base of DSL.
HDSL provides 24 channels with a capacity of 64 kbps. The total capacity
is 1,5 mbps. In the US this permanent line is called T1. In Europe 30 channels
are available under the name of E1 and in Switzerland you can rent an ISDN
multiplex line. Because of the high cost, this connection type is only
for business users.