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E1/BRI
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SIP/VOIP
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T1/PRI
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Analog/POTS
EXPLANATION
Analog or Plain Old Telephone Service (POTS) are your
standard (essentially) one line in per call telephone lines. These were
fairly inefficient for larger organizations due to the sheer volume of
calls needed and the number of physical lines that would be needed to
accomplish many concurrent calls, for example a call center needing the
ability to work with 45 concurrent calls would need essentially 45
Analog lines (although, later uses of the lines could incorporate some
usage of more than one call per physical line), which is both costly and
takes up a large volume of real-estate (space).
In order to make
multiple concurrent calls across one physical medium, the US
telecommunications industry began to use T1 or PRI lines - which could
support up to 23 concurrent calls over one "line" (or circuit). So, for
that same business to make approximately 45 concurrent calls, only *2*
circuits would be required.
As demands increased yet again and
higher-speed backbone connections became available, VoIP (Voice over
Internet Protocol) or SIP trunks have become the de-facto standard for
high-usage / high-volume (and even low-volume) environments. The
concurrent call limit per SIP or VoIP trunk is essentially limitless,
bound *in-theory* only by the amount of bandwidth available to the
destination/origin.
BRI circuits are essentially the same (or
extremely similar) to the North American/US T1/PRI circuits, however,
they are only found overseas/in European countries and NOT in the US.
*This
question and the answers, including this explanation, are a
simplification of many PSTN and Telecommunications terms and
technologies. Additional details and information can be found by
searching Wikipedia, as well as other scholarly resources,
for the aforementioned terms,
SOURCE
https://en.wikipedia.org/wiki/Public_switched_telephone_network
