<s>
Presence	B-Protocol
service	I-Protocol
is	O
a	O
network	B-General_Concept
service	I-General_Concept
which	O
accepts	O
,	O
stores	O
and	O
distributes	O
presence	B-Protocol
information	I-Protocol
.	O
</s>
<s>
Presence	B-Protocol
service	I-Protocol
may	O
be	O
implemented	O
as	O
a	O
single	O
server	O
or	O
have	O
an	O
internal	O
structure	O
involving	O
multiple	O
servers	O
and	O
proxies	O
.	O
</s>
<s>
There	O
may	O
be	O
complex	O
patterns	O
of	O
redirection	O
and	O
proxying	O
while	O
retaining	O
logical	O
connectivity	O
to	O
a	O
single	O
presence	B-Protocol
service	I-Protocol
.	O
</s>
<s>
Also	O
presence	B-Protocol
service	I-Protocol
may	O
be	O
implemented	O
as	O
direct	O
communication	O
among	O
presentity	B-Protocol
and	O
watchers	B-Protocol
,	O
i.e.	O
</s>
<s>
The	O
Presence	O
Server	O
,	O
that	O
reside	O
in	O
the	O
presentity	B-Protocol
's	O
home	O
network	O
,	O
is	O
able	O
to	O
receive	O
and	O
manage	O
presence	B-Protocol
information	I-Protocol
.	O
</s>
<s>
It	O
is	O
a	O
SIP	B-Application
Application	O
Server	O
and	O
is	O
located	O
using	O
SIP	B-Application
URLs	O
.	O
</s>
