<s>
The	O
Project	O
25	O
Inter	O
RF	O
Subsystem	O
Interface	O
(	O
P25	B-Device
ISSI	I-Device
)	O
is	O
a	O
non-proprietary	O
interface	O
that	O
enables	O
RF	O
subsystems	O
(	O
RFSSs	O
)	O
built	O
by	O
different	O
manufacturers	O
to	O
be	O
connected	O
together	O
into	O
wide	O
area	O
networks	O
so	O
that	O
users	O
on	O
different	O
networks	O
can	O
talk	O
with	O
each	O
other	O
.	O
</s>
<s>
The	O
ISSI	O
uses	O
SIP	O
and	O
RTP	B-Protocol
protocols	O
(	O
standardized	O
protocols	O
)	O
to	O
provide	O
the	O
messaging	O
between	O
RFSSs	O
.	O
</s>
<s>
The	O
documentation	O
suite	O
which	O
defines	O
Scope	O
One	O
of	O
the	O
P25	B-Device
ISSI	I-Device
consists	O
of	O
five	O
standards	O
.	O
</s>
<s>
Call	O
control	O
also	O
uses	O
the	O
SIP	O
protocol	O
and	O
describes	O
the	O
messages	O
and	O
procedures	O
necessary	O
for	O
RFSSs	O
to	O
set	O
up	O
and	O
tear	O
down	O
a	O
call	O
,	O
and	O
manage	O
RTP	B-Protocol
resources	O
.	O
</s>
<s>
Transmission	O
control	O
uses	O
the	O
RTP	B-Protocol
protocol	O
and	O
describes	O
the	O
messages	O
and	O
procedures	O
necessary	O
for	O
RFSSs	O
to	O
convey	O
voice	O
information	O
,	O
and	O
manage	O
voice	O
call	O
requests	O
.	O
</s>
