Public Switched Telephone Network
<communications> (PSTN, T.70) The collection of interconnected systems
operated by the various telephone companies and administrations (telcos and
PTTs) around the world. Also known as the Plain Old Telephone System (POTS) in
contrast to xDSL and ISDN (not to mention other forms of PANS).
The PSTN started as human-operated analogue circuit switching systems
(plugboards), progressed through electromechanical switches. By now this has
almost completely been made digital, except for the final connection to the
subscriber (the "last mile"): The signal coming out of the phone set is
analogue. It is usually transmitted over a twisted pair cable still as an
analogue signal. At the telco office this analogue signal is usually digitised,
using 8000 samples per second and 8 bits per sample, yielding a 64 kb/s data
stream (DS0). Several such data streams are usually combined into a fatter
stream: in the US 24 channels are combined into a T1, in Europe 31 DS0 channels
are combined into an E1 line. This can later be further combined into larger
chunks for transmission over high-bandwidth core trunks. At the receiving end
the channels are separated, the digital signals are converted back to analogue
and delivered to the received phone.
While all these conversions are inaudible when voice is transmitted over the
phone lines it can make digital communication difficult. Items of interest
include A-law to mu-law conversion (and vice versa) on international calls;
robbed bit signalling in North America (56 kbps <--> 64 kbps); data compression
to save bandwidth on long-haul trunks; signal processing such as echo
suppression and voice signal enhancement such as AT&T TrueVoice.
(2000-07-09)
Nearby terms:
Public-Key Cryptography Standards « public-key
encryption « Public Key Infrastructure « Public
Switched Telephone Network » puff » PUFFT » pull
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