<s>
The	O
ISDN	B-Protocol
(	O
Integrated	B-Protocol
Services	I-Protocol
Digital	I-Protocol
Network	I-Protocol
)	O
User	O
Part	O
or	O
ISUP	O
is	O
part	O
of	O
Signaling	O
System	O
No	O
.	O
</s>
<s>
Below	O
is	O
a	O
detailed	O
exchange	O
of	O
ISUP	O
messages	O
involving	O
two	O
ISDN	B-Protocol
telecom	O
switches	O
.	O
</s>
<s>
Release	O
cause	O
codes	O
are	O
used	O
to	O
identify	O
and	O
debug	O
any	O
events	O
occurring	O
in	O
ISDN	B-Protocol
User	I-Protocol
Part	I-Protocol
signaling	O
.	O
</s>
