The Public Switched Telephone Network
(PSTN) was originally designed to carry analog voice signals across
telephone lines. A technology called Integrated Services Digital
Network (ISDN) was created to convert analog signals into digital
signals to allow data transfer rates faster than PSTN.
An advantage of ISDN over PSTN is
speed. The fastest connection a modem can establish using a PSTN
analog line is 56 Kbps. Data is converted by the modem from the PC's
digital signals to analog signals, then sent across the wires to a
remote network, where the data is again converted from analog signals
to digital signals. ISDN enables digital signals to travel over
regular telephone lines in its digital form; transmitting data in half
the time of analog modems. An ISDN BRI line can carry data at 128 Kbps, and ISDN BRI
lines can be aggregated to create an ISDN PRI line to carry 1.472 Mbps(T1) or 1.920 Mbps(E1).
Another advantage of ISDN over PSTN is the
ability to be connected to the network "all the time"
without tying up the analog telephone line, which is especially useful
for telecommuters.
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