| slovo | definícia |
public switched telephone network
(foldoc) | Public Switched Telephone Network
Plain Old Telephone System
PSTN
(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)
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