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PRI technology is a bit
simpler than BRI technology. The wiring is not multipoint, and there
is only the straight connection between the channel service
unit/data service unit (CSU/DSU) and the PRI interface (see the
Figure).
A regular ISDN "S"
interface can have several endpoint devices with different
capabilities occupying the same bus (the "S" bus). When
the switch can communicate with multiple devices, it is typically
referred to as "multipoint." Unfortunately, this calls for
complexity in terms of service profile identifiers (SPIDs) and
endpoint identifiers (EIDs), both in ISDN device setup and call
processing.
As a result, there is arbitration at
Layers 1 and 2, so multiple devices can access the network without
collisions or interruptions between devices that need to share the
ISDN network. Because there are not multiple devices, PRI does not
require this arbitration.
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