<s>
Vertical	B-General_Concept
handover	I-General_Concept
or	O
vertical	B-General_Concept
handoff	I-General_Concept
refers	O
to	O
a	O
network	B-Architecture
node	O
changing	O
the	O
type	O
of	O
connectivity	O
it	O
uses	O
to	O
access	O
a	O
supporting	O
infrastructure	O
,	O
usually	O
to	O
support	O
node	O
mobility	O
.	O
</s>
<s>
For	O
example	O
,	O
a	O
suitably	O
equipped	O
laptop	O
might	O
be	O
able	O
to	O
use	O
both	O
a	O
high	O
speed	O
wireless	B-Architecture
LAN	I-Architecture
and	O
a	O
cellular	O
technology	O
for	O
Internet	O
access	O
.	O
</s>
<s>
Wireless	B-Architecture
LAN	I-Architecture
connections	O
generally	O
provide	O
higher	O
speeds	O
,	O
while	O
cellular	O
technologies	O
generally	O
provide	O
more	O
ubiquitous	O
coverage	O
.	O
</s>
<s>
Thus	O
the	O
laptop	O
user	O
might	O
want	O
to	O
use	O
a	O
wireless	B-Architecture
LAN	I-Architecture
connection	O
whenever	O
one	O
is	O
available	O
,	O
and	O
to	O
'	O
fall	O
over	O
 '	O
to	O
a	O
cellular	O
connection	O
when	O
the	O
wireless	B-Architecture
LAN	I-Architecture
is	O
unavailable	O
.	O
</s>
<s>
Vertical	B-General_Concept
handovers	I-General_Concept
refer	O
to	O
the	O
automatic	O
fallover	O
from	O
one	O
technology	O
to	O
another	O
in	O
order	O
to	O
maintain	O
communication	O
.	O
</s>
<s>
This	O
is	O
different	O
from	O
a	O
'	O
horizontal	O
handover	B-Protocol
 '	O
between	O
different	O
wireless	O
access	O
points	O
that	O
use	O
the	O
same	O
technology	O
in	O
that	O
a	O
vertical	B-General_Concept
handover	I-General_Concept
involves	O
changing	O
the	O
data	B-Protocol
link	I-Protocol
layer	I-Protocol
technology	O
used	O
to	O
access	O
the	O
network	B-Architecture
.	O
</s>
<s>
Vertical	B-General_Concept
handoffs	I-General_Concept
between	O
WLAN	B-Architecture
and	O
UMTS	B-Protocol
(	O
WCDMA	O
)	O
have	O
attracted	O
a	O
great	O
deal	O
of	O
attention	O
in	O
all	O
the	O
research	O
areas	O
of	O
the	O
4G	B-Architecture
wireless	I-Architecture
network	B-Architecture
,	O
due	O
to	O
the	O
benefit	O
of	O
utilizing	O
the	O
higher	O
bandwidth	O
and	O
lower	O
cost	O
of	O
WLAN	B-Architecture
as	O
well	O
as	O
better	O
mobility	O
support	O
and	O
larger	O
coverage	O
of	O
UMTS	B-Protocol
.	O
</s>
<s>
Vertical	B-General_Concept
handovers	I-General_Concept
among	O
a	O
range	O
of	O
wired	O
and	O
wireless	O
access	O
technologies	O
including	O
WiMAX	O
can	O
be	O
achieved	O
using	O
Media	O
independent	O
handover	B-Protocol
which	O
is	O
standardized	O
as	O
IEEE	O
802.21	O
.	O
</s>
<s>
To	O
support	O
vertical	B-General_Concept
handover	I-General_Concept
,	O
a	O
mobile	O
terminal	O
needs	O
to	O
have	O
a	O
dual	O
mode	O
card	O
,	O
for	O
example	O
one	O
that	O
can	O
work	O
under	O
both	O
WLAN	B-Architecture
and	O
UMTS	B-Protocol
frequency	O
bands	O
and	O
modulation	B-Application
schemes	O
.	O
</s>
<s>
For	O
the	O
vertical	B-General_Concept
handover	I-General_Concept
between	O
UMTS	B-Protocol
and	O
WLAN	B-Architecture
,	O
there	O
are	O
two	O
main	O
interworking	O
architecture	O
:	O
tight	B-Application
coupling	I-Application
and	O
loose	B-Application
coupling	I-Application
.	O
</s>
<s>
The	O
tight	B-Application
coupling	I-Application
scheme	O
,	O
which	O
3GPP	O
adopted	O
,	O
introduces	O
two	O
more	O
elements	O
:	O
WAG	O
(	O
Wireless	O
Access	O
Gateway	O
)	O
and	O
PDG	O
(	O
Packet	O
Data	O
Gateway	O
)	O
.	O
</s>
<s>
So	O
the	O
data	O
transfers	O
from	O
WLAN	B-Architecture
AP	O
to	O
a	O
Corresponding	O
Node	O
on	O
the	O
internet	O
must	O
go	O
through	O
the	O
Core	O
Network	B-Architecture
of	O
UMTS	B-Protocol
.	O
</s>
<s>
Loose	B-Application
coupling	I-Application
is	O
more	O
used	O
when	O
the	O
WLAN	B-Architecture
is	O
not	O
operated	O
by	O
cellular	O
operator	O
but	O
any	O
private	O
user	O
.	O
</s>
<s>
So	O
the	O
data	O
transmitted	O
through	O
WLAN	B-Architecture
will	O
not	O
go	O
through	O
Cellular	O
Networks	O
.	O
</s>
<s>
In	O
traditional	O
handovers	B-Protocol
,	O
such	O
as	O
a	O
handover	B-Protocol
between	O
cellular	O
networks	O
,	O
the	O
handover	B-Protocol
decision	O
is	O
based	O
mainly	O
on	O
RSS	O
(	O
Received	O
Signal	O
Strength	O
)	O
in	O
the	O
border	O
region	O
of	O
two	O
cells	O
,	O
and	O
may	O
also	O
be	O
based	O
on	O
call	O
drop	O
rate	O
,	O
etc	O
.	O
</s>
<s>
In	O
vertical	B-General_Concept
handover	I-General_Concept
,	O
the	O
situation	O
is	O
more	O
complex	O
.	O
</s>
<s>
Two	O
different	O
kinds	O
of	O
wireless	O
networks	O
normally	O
have	O
incomparable	O
signal	O
strength	O
metrics	O
,	O
for	O
example	O
,	O
WLAN	B-Architecture
compared	O
to	O
UMTS	B-Protocol
.	O
</s>
<s>
In	O
,	O
WLAN	B-Architecture
and	O
UMTS	B-Protocol
networks	O
both	O
cover	O
an	O
area	O
at	O
the	O
same	O
time	O
.	O
</s>
<s>
The	O
handover	B-Protocol
metrics	O
in	O
this	O
situation	O
should	O
include	O
RSS	O
,	O
user	O
preference	O
,	O
network	B-Architecture
conditions	O
,	O
application	O
types	O
,	O
cost	O
etc	O
.	O
</s>
<s>
Based	O
on	O
the	O
handover	B-Protocol
metrics	O
mentioned	O
above	O
,	O
the	O
decision	O
about	O
how	O
and	O
when	O
to	O
switch	O
the	O
interface	O
to	O
which	O
network	B-Architecture
will	O
be	O
made	O
.	O
</s>
<s>
while	O
some	O
others	O
,	O
using	O
fuzzy	O
logic	O
,	O
neuron	O
network	B-Architecture
or	O
MADM	O
methods	O
to	O
solve	O
the	O
problem	O
.	O
</s>
<s>
When	O
a	O
mobile	O
station	O
transfers	O
a	O
user	O
's	O
session	O
from	O
one	O
network	B-Architecture
to	O
another	O
,	O
the	O
IP	B-Protocol
address	O
will	O
change	O
.	O
</s>
<s>
The	O
Mobility	O
Management	O
problem	O
can	O
be	O
solved	O
in	O
different	O
layers	O
,	O
such	O
as	O
the	O
Application	O
Layer	O
,	O
Transport	O
Layer	O
,	O
IP	B-Protocol
Layer	O
,	O
etc	O
.	O
</s>
<s>
The	O
most	O
common	O
method	O
is	O
to	O
use	O
SIP	B-Application
(	O
Session	B-Application
Initiation	I-Application
Protocol	I-Application
)	O
and	O
Mobile	B-Protocol
IP	I-Protocol
.	O
</s>
<s>
The	O
handover	B-Protocol
procedure	O
specifies	O
the	O
control	O
signalling	O
used	O
to	O
perform	O
the	O
handover	B-Protocol
and	O
is	O
invoked	O
by	O
the	O
handover	B-Protocol
decision	O
algorithm	O
.	O
</s>
