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The Figure shows the sequence of events that occur during the establishment of a BRI call.
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When
the call is initiated, the D channel comes up. The
called number is sent to the local ISDN switch.
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The
local switch uses the Signaling System 7 (SS7)
signaling protocols to set up a path and pass the
called number to the terminating ISDN switch.
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The
far-end switch brings up the D channel to the
destination. The D channel is used for call setup,
signaling, and call termination, which are the
call-control functions.
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When
the terminating end answers, the B channel is
connected end to end. A B channel carries the
conversation or data. Both B channels can be used
simultaneously to the same or different
destinations.
The
maximum length of most ISDN local loops in North
America is about 18,000 feet (5.5 kilometers [km]),
using standard plain old telephone service (POTS)
wiring. ISDN is a local-loop technology. After the
ISDN switch processes the call, the communication uses
the SS7 infrastructure to reach its destination.
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