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Placing a Call
The functional model of the PSTN is simple, and although the technologies have
changed over the years, the telephone system behaves the same way it did when it
was initially comprised of electromechanical switches in the 1930s. In the simplest
of terms, a telephone call is a completed circuit. The telephone system is a large
collection of looped circuits between subscriber locations (homes, offices, and so on)
and a local central switching office. These looped circuits are called local loops.
The PSTN is based on DC voltage. Each local loop has a test tone of .001 watts (1
milliwatt) running through it. The maximum line voltage permitted across the PSTN
is 2.2 +/- VDC (Volt's Direct Current); this is the baseline for measuring signal gain
and loss across the network. Gain and loss is measured in decibels (dB).
NOTE
The formula for calculating gain or loss is dB = 10 log 10 P0/P1, where P0 = output
(transmit power) and P1 = input (received power). 10 log 10 is also known as the
common logarithm (base 10). The base 10 logarithm indicates how many times 10
must be multiplied by itself to yield a given number, for example, 10 log 10 1000 =
10 x 10 x 10 = 3.
A call is placed by taking the telephone "off hook," which breaks the test tone circuit
and places the line in "short" state. This short is acknowledged by the central office
(CO) switch, which listens for a series of pulses (variations in the applied voltage)
that tell the telephone switch the subscriber number you want to connect to, and the
connection is made. The original PSTN used a rotary dial, which sent a series of
voltage pulses down the line to represent numbers. Today, tones are used to
represent different numbers. In the U.S., subscriber numbers are assigned using
the North American Numbering Plan (NANP). The NANP defines a 10-digit subscriber
number for establishing calls within the PSTN in North America. The first three digits
identify the numbering plan area (NPA), which is the geographical area where the
call is being placed. The second three digits identify the local dial exchange, and the
last four digits identify the subscriber ID.
When the switch has the subscriber ID, it establishes a dedicated circuit between
the two parties, who then have use of the circuit's transmission bandwidth for the
duration of the call. When the call is completed, the bandwidth will be reallocated for
another call.