Public Switched Telephone Network
public switched telephone network
[¦pəb·lik ‚swicht ′tel·ə‚fōn ‚net‚wərk]Public Switched Telephone Network
(communications)The PSTN started as human-operated analogue circuit switchingsystems (plugboards), progressed through electromechanicalswitches. By now this has almost completely been madedigital, except for the final connection to the subscriber(the "last mile"): The signal coming out of the phone set isanalogue. It is usually transmitted over a twisted pair cable still as an analogue signal. At the telco officethis analogue signal is usually digitised, using 8000 samplesper second and 8 bits per sample, yielding a 64 kb/s datastream (DS0). Several such data streams are usuallycombined into a fatter stream: in the US 24 channels arecombined into a T1, in Europe 31 DS0 channels are combinedinto an E1 line. This can later be further combined intolarger chunks for transmission over high-bandwidth coretrunks. At the receiving end the channels are separated, thedigital signals are converted back to analogue and deliveredto the received phone.
While all these conversions are inaudible when voice istransmitted over the phone lines it can make digitalcommunication difficult. Items of interest include A-law tomu-law conversion (and vice versa) on international calls;robbed bit signalling in North America (56 kbps 64kbps); data compression to save bandwidth on long-haultrunks; signal processing such as echo suppression and voicesignal enhancement such as AT&T TrueVoice.