When intermittent, low-volume data transfers are needed, asynchronous modems and
analog dialed telephone lines provide low capacity, on-demand, dedicated switched
connections. Traditional telephony uses a copper cable, called the local loop, to connect the
telephone handset in the subscriber premises to the telephone network. The signal on the
local loop during a call is a continuously varying electronic signal that is a translation of
the subscriber voice.
The local loop is not suitable for direct transport of binary computer data, but a modem can
send computer data through the voice telephone network. The modem modulates the binary
data into an analog signal at the source and, at the destination, demodulates the analog
signal to binary data.
The physical characteristics of the local loop and the connection of the local loop to the
PSTN limit the rate of the signal. The upper limit is around 53 kbps.
For small businesses, the PSTN can be adequate for the exchange of sales ?¬?gures, prices,
routine reports, and e-mail. Using automatic dialup at night or on weekends for large ?¬?le
transfers and data backup can take advantage of lower off-peak tariffs (line charges). Tariffs
are based on the distance between the endpoints, the time of day, and the duration of the call.
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