<s>
In	O
cellular	O
telecommunications	O
,	O
handover	B-Protocol
,	O
or	O
handoff	B-Protocol
,	O
is	O
the	O
process	O
of	O
transferring	O
an	O
ongoing	O
call	O
or	O
data	O
session	O
from	O
one	O
channel	O
connected	O
to	O
the	O
core	B-General_Concept
network	I-General_Concept
to	O
another	O
channel	O
.	O
</s>
<s>
American	O
English	O
uses	O
the	O
term	O
handoff	B-Protocol
,	O
and	O
this	O
is	O
most	O
commonly	O
used	O
within	O
some	O
American	O
organizations	O
such	O
as	O
3GPP2	O
and	O
in	O
American	O
originated	O
technologies	O
such	O
as	O
CDMA2000	O
.	O
</s>
<s>
In	O
British	O
English	O
the	O
term	O
handover	B-Protocol
is	O
more	O
common	O
,	O
and	O
is	O
used	O
within	O
international	O
and	O
European	O
organisations	O
such	O
as	O
ITU-T	O
,	O
IETF	O
,	O
ETSI	O
and	O
3GPP	O
,	O
and	O
standardised	O
within	O
European	O
originated	O
standards	O
such	O
as	O
GSM	O
and	O
UMTS	B-Protocol
.	O
</s>
<s>
The	O
term	O
handover	B-Protocol
is	O
more	O
common	O
in	O
academic	O
research	O
publications	O
and	O
literature	O
,	O
while	O
handoff	B-Protocol
is	O
slightly	O
more	O
common	O
within	O
the	O
IEEE	O
and	O
ANSI	O
organisations	O
.	O
</s>
<s>
In	O
telecommunications	O
there	O
may	O
be	O
different	O
reasons	O
why	O
a	O
handover	B-Protocol
might	O
be	O
conducted	O
:	O
</s>
<s>
in	O
non-CDMA	O
networks	O
when	O
the	O
channel	O
used	O
by	O
the	O
phone	O
becomes	O
interfered	O
by	O
another	O
phone	O
using	O
the	O
same	O
channel	O
in	O
a	O
different	O
cell	O
,	O
the	O
call	O
is	O
transferred	O
to	O
a	O
different	O
channel	O
in	O
the	O
same	O
cell	O
or	O
to	O
a	O
different	O
channel	O
in	O
another	O
cell	O
in	O
order	O
to	O
avoid	O
the	O
interference	O
;	O
</s>
<s>
again	O
in	O
non-CDMA	O
networks	O
when	O
the	O
user	O
behaviour	O
changes	O
,	O
e.g.	O
</s>
<s>
when	O
a	O
fast-travelling	O
user	O
,	O
connected	O
to	O
a	O
large	O
,	O
umbrella-type	O
of	O
cell	O
,	O
stops	O
then	O
the	O
call	O
may	O
be	O
transferred	O
to	O
a	O
smaller	O
macro	O
cell	O
or	O
even	O
to	O
a	O
micro	O
cell	O
in	O
order	O
to	O
free	O
capacity	O
on	O
the	O
umbrella	O
cell	O
for	O
other	O
fast-traveling	O
users	O
and	O
to	O
reduce	O
the	O
potential	O
interference	O
to	O
other	O
cells	O
or	O
users	O
(	O
this	O
works	O
in	O
reverse	O
too	O
,	O
when	O
a	O
user	O
is	O
detected	O
to	O
be	O
moving	O
faster	O
than	O
a	O
certain	O
threshold	O
,	O
the	O
call	O
can	O
be	O
transferred	O
to	O
a	O
larger	O
umbrella-type	O
of	O
cell	O
in	O
order	O
to	O
minimize	O
the	O
frequency	O
of	O
the	O
handovers	B-Protocol
due	O
to	O
this	O
movement	O
)	O
;	O
</s>
<s>
in	O
CDMA	B-Protocol
networks	O
a	O
handover	B-Protocol
(	O
see	O
further	O
down	O
)	O
may	O
be	O
induced	O
in	O
order	O
to	O
reduce	O
the	O
interference	O
to	O
a	O
smaller	O
neighboring	O
cell	O
due	O
to	O
the	O
"	O
near	O
–	O
far	O
"	O
effect	O
even	O
when	O
the	O
phone	O
still	O
has	O
an	O
excellent	O
connection	O
to	O
its	O
current	O
cell	O
.	O
</s>
<s>
The	O
most	O
basic	O
form	O
of	O
handover	B-Protocol
is	O
when	O
a	O
phone	O
call	O
in	O
progress	O
is	O
redirected	O
from	O
its	O
current	O
cell	O
(	O
called	O
source	O
)	O
to	O
a	O
new	O
cell	O
(	O
called	O
target	O
)	O
.	O
</s>
<s>
Such	O
a	O
handover	B-Protocol
,	O
in	O
which	O
the	O
source	O
and	O
the	O
target	O
are	O
different	O
cells	O
(	O
even	O
if	O
they	O
are	O
on	O
the	O
same	O
cell	O
site	O
)	O
is	O
called	O
inter-cell	O
handover	B-Protocol
.	O
</s>
<s>
The	O
purpose	O
of	O
inter-cell	O
handover	B-Protocol
is	O
to	O
maintain	O
the	O
call	O
as	O
the	O
subscriber	O
is	O
moving	O
out	O
of	O
the	O
area	O
covered	O
by	O
the	O
source	O
cell	O
and	O
entering	O
the	O
area	O
of	O
the	O
target	O
cell	O
.	O
</s>
<s>
A	O
special	O
case	O
is	O
possible	O
,	O
in	O
which	O
the	O
source	O
and	O
the	O
target	O
are	O
one	O
and	O
the	O
same	O
cell	O
and	O
only	O
the	O
used	O
channel	O
is	O
changed	O
during	O
the	O
handover	B-Protocol
.	O
</s>
<s>
Such	O
a	O
handover	B-Protocol
,	O
in	O
which	O
the	O
cell	O
is	O
not	O
changed	O
,	O
is	O
called	O
intra-cell	O
handover	B-Protocol
.	O
</s>
<s>
The	O
purpose	O
of	O
intra-cell	O
handover	B-Protocol
is	O
to	O
change	O
one	O
channel	O
,	O
which	O
may	O
be	O
interfered	O
or	O
fading	O
with	O
a	O
new	O
clearer	O
or	O
less	O
fading	O
channel	O
.	O
</s>
<s>
In	O
addition	O
to	O
the	O
above	O
classification	O
of	O
inter-cell	O
and	O
intra-cell	O
classification	O
of	O
handovers	B-Protocol
,	O
they	O
also	O
can	O
be	O
divided	O
into	O
hard	O
and	O
soft	B-Protocol
handovers	I-Protocol
:	O
</s>
<s>
Hard	O
handover	B-Protocol
Is	O
one	O
in	O
which	O
the	O
channel	O
in	O
the	O
source	O
cell	O
is	O
released	O
and	O
only	O
then	O
the	O
channel	O
in	O
the	O
target	O
cell	O
is	O
engaged	O
.	O
</s>
<s>
Thus	O
the	O
connection	O
to	O
the	O
source	O
is	O
broken	O
before	O
or	O
'	O
as	O
 '	O
the	O
connection	O
to	O
the	O
target	O
is	O
made	O
—	O
for	O
this	O
reason	O
such	O
handovers	B-Protocol
are	O
also	O
known	O
as	O
break-before-make	O
.	O
</s>
<s>
Hard	O
handovers	B-Protocol
are	O
intended	O
to	O
be	O
instantaneous	O
in	O
order	O
to	O
minimize	O
the	O
disruption	O
to	O
the	O
call	O
.	O
</s>
<s>
A	O
hard	O
handover	B-Protocol
is	O
perceived	O
by	O
network	O
engineers	O
as	O
an	O
event	O
during	O
the	O
call	O
.	O
</s>
<s>
Soft	B-Protocol
handover	I-Protocol
Is	O
one	O
in	O
which	O
the	O
channel	O
in	O
the	O
source	O
cell	O
is	O
retained	O
and	O
used	O
for	O
a	O
while	O
in	O
parallel	O
with	O
the	O
channel	O
in	O
the	O
target	O
cell	O
.	O
</s>
<s>
In	O
this	O
case	O
the	O
connection	O
to	O
the	O
target	O
is	O
established	O
before	O
the	O
connection	O
to	O
the	O
source	O
is	O
broken	O
,	O
hence	O
this	O
handover	B-Protocol
is	O
called	O
make-before-break	O
.	O
</s>
<s>
For	O
this	O
reason	O
the	O
soft	B-Protocol
handover	I-Protocol
is	O
perceived	O
by	O
network	O
engineers	O
as	O
a	O
state	O
of	O
the	O
call	O
,	O
rather	O
than	O
a	O
brief	O
event	O
.	O
</s>
<s>
Soft	B-Protocol
handovers	I-Protocol
may	O
involve	O
using	O
connections	O
to	O
more	O
than	O
two	O
cells	O
:	O
connections	O
to	O
three	O
,	O
four	O
or	O
more	O
cells	O
can	O
be	O
maintained	O
by	O
one	O
phone	O
at	O
the	O
same	O
time	O
.	O
</s>
<s>
When	O
a	O
call	O
is	O
in	O
a	O
state	O
of	O
soft	B-Protocol
handover	I-Protocol
,	O
the	O
signal	O
of	O
the	O
best	O
of	O
all	O
used	O
channels	O
can	O
be	O
used	O
for	O
the	O
call	O
at	O
a	O
given	O
moment	O
or	O
all	O
the	O
signals	O
can	O
be	O
combined	O
to	O
produce	O
a	O
clearer	O
copy	O
of	O
the	O
signal	O
.	O
</s>
<s>
The	O
latter	O
is	O
more	O
advantageous	O
,	O
and	O
when	O
such	O
combining	O
is	O
performed	O
both	O
in	O
the	O
downlink	O
(	O
forward	O
link	O
)	O
and	O
the	O
uplink	B-Application
(	O
reverse	O
link	O
)	O
the	O
handover	B-Protocol
is	O
termed	O
as	O
softer	O
.	O
</s>
<s>
Softer	O
handovers	B-Protocol
are	O
possible	O
when	O
the	O
cells	O
involved	O
in	O
the	O
handovers	B-Protocol
have	O
a	O
single	O
cell	O
site	O
.	O
</s>
<s>
Handover	B-Protocol
can	O
also	O
be	O
classified	O
on	O
the	O
basis	O
of	O
handover	B-Protocol
techniques	O
used	O
.	O
</s>
<s>
An	O
advantage	O
of	O
the	O
hard	O
handover	B-Protocol
is	O
that	O
at	O
any	O
moment	O
in	O
time	O
one	O
call	O
uses	O
only	O
one	O
channel	O
.	O
</s>
<s>
The	O
hard	O
handover	B-Protocol
event	O
is	O
indeed	O
very	O
short	O
and	O
usually	O
is	O
not	O
perceptible	O
by	O
the	O
user	O
.	O
</s>
<s>
In	O
the	O
old	O
analog	B-Protocol
systems	O
it	O
could	O
be	O
heard	O
as	O
a	O
click	O
or	O
a	O
very	O
short	O
beep	O
;	O
in	O
digital	O
systems	O
it	O
is	O
unnoticeable	O
.	O
</s>
<s>
Another	O
advantage	O
of	O
the	O
hard	O
handover	B-Protocol
is	O
that	O
the	O
phone	O
's	O
hardware	O
does	O
not	O
need	O
to	O
be	O
capable	O
of	O
receiving	O
two	O
or	O
more	O
channels	O
in	O
parallel	O
,	O
which	O
makes	O
it	O
cheaper	O
and	O
simpler	O
.	O
</s>
<s>
A	O
disadvantage	O
is	O
that	O
if	O
a	O
handover	B-Protocol
fails	O
the	O
call	O
may	O
be	O
temporarily	O
disrupted	O
or	O
even	O
terminated	O
abnormally	O
.	O
</s>
<s>
Technologies	O
which	O
use	O
hard	O
handovers	B-Protocol
,	O
usually	O
have	O
procedures	O
which	O
can	O
re-establish	O
the	O
connection	O
to	O
the	O
source	O
cell	O
if	O
the	O
connection	O
to	O
the	O
target	O
cell	O
cannot	O
be	O
made	O
.	O
</s>
<s>
One	O
advantage	O
of	O
the	O
soft	B-Protocol
handovers	I-Protocol
is	O
that	O
the	O
connection	O
to	O
the	O
source	O
cell	O
is	O
broken	O
only	O
when	O
a	O
reliable	O
connection	O
to	O
the	O
target	O
cell	O
has	O
been	O
established	O
and	O
therefore	O
the	O
chances	O
that	O
the	O
call	O
will	O
be	O
terminated	O
abnormally	O
due	O
to	O
failed	O
handovers	B-Protocol
are	O
lower	O
.	O
</s>
<s>
Thus	O
the	O
reliability	O
of	O
the	O
connection	O
becomes	O
higher	O
when	O
the	O
call	O
is	O
in	O
a	O
soft	B-Protocol
handover	I-Protocol
.	O
</s>
<s>
Because	O
in	O
a	O
cellular	O
network	O
the	O
majority	O
of	O
the	O
handovers	B-Protocol
occur	O
in	O
places	O
of	O
poor	O
coverage	O
,	O
where	O
calls	O
would	O
frequently	O
become	O
unreliable	O
when	O
their	O
channel	O
is	O
interfered	O
or	O
fading	O
,	O
soft	B-Protocol
handovers	I-Protocol
bring	O
a	O
significant	O
improvement	O
to	O
the	O
reliability	O
of	O
the	O
calls	O
in	O
these	O
places	O
by	O
making	O
the	O
interference	O
or	O
the	O
fading	O
in	O
a	O
single	O
channel	O
not	O
critical	O
.	O
</s>
<s>
Another	O
price	O
to	O
pay	O
for	O
soft	B-Protocol
handovers	I-Protocol
is	O
use	O
of	O
several	O
channels	O
in	O
the	O
network	O
to	O
support	O
just	O
a	O
single	O
call	O
.	O
</s>
<s>
By	O
adjusting	O
the	O
duration	O
of	O
soft	B-Protocol
handovers	I-Protocol
and	O
the	O
size	O
of	O
the	O
areas	O
in	O
which	O
they	O
occur	O
,	O
the	O
network	O
engineers	O
can	O
balance	O
the	O
benefit	O
of	O
extra	O
call	O
reliability	O
against	O
the	O
price	O
of	O
reduced	O
capacity	O
.	O
</s>
<s>
While	O
theoretically	O
speaking	O
soft	B-Protocol
handovers	I-Protocol
are	O
possible	O
in	O
any	O
technology	O
,	O
analog	B-Protocol
or	O
digital	O
,	O
the	O
cost	O
of	O
implementing	O
them	O
for	O
analog	B-Protocol
technologies	O
is	O
prohibitively	O
high	O
and	O
none	O
of	O
the	O
technologies	O
that	O
were	O
commercially	O
successful	O
in	O
the	O
past	O
(	O
e.g.	O
</s>
<s>
AMPS	O
,	O
TACS	B-Protocol
,	O
NMT	O
,	O
etc	O
.	O
)	O
</s>
<s>
Of	O
the	O
digital	O
technologies	O
,	O
those	O
based	O
on	O
FDMA	O
also	O
face	O
a	O
higher	O
cost	O
for	O
the	O
phones	O
(	O
due	O
to	O
the	O
need	O
to	O
have	O
multiple	O
parallel	O
radio-frequency	O
modules	O
)	O
and	O
those	O
based	O
on	O
TDMA	O
or	O
a	O
combination	O
of	O
TDMA/FDMA	O
,	O
in	O
principle	O
,	O
allow	O
not	O
so	O
expensive	O
implementation	O
of	O
soft	B-Protocol
handovers	I-Protocol
.	O
</s>
<s>
On	O
the	O
other	O
hand	O
,	O
all	O
CDMA	B-Protocol
based	O
technologies	O
,	O
2G	O
and	O
3G	B-Architecture
(	O
third-generation	O
)	O
,	O
have	O
soft	B-Protocol
handovers	I-Protocol
.	O
</s>
<s>
On	O
one	O
hand	O
,	O
this	O
is	O
facilitated	O
by	O
the	O
possibility	O
to	O
design	O
not	O
so	O
expensive	O
phone	O
hardware	O
supporting	O
soft	B-Protocol
handovers	I-Protocol
for	O
CDMA	B-Protocol
and	O
on	O
the	O
other	O
hand	O
,	O
this	O
is	O
necessitated	O
by	O
the	O
fact	O
that	O
without	O
soft	B-Protocol
handovers	I-Protocol
CDMA	B-Protocol
networks	O
may	O
suffer	O
from	O
substantial	O
interference	O
arising	O
due	O
to	O
the	O
so-called	O
near	O
–	O
far	O
effect	O
.	O
</s>
<s>
changing	O
the	O
channel	O
during	O
a	O
hard	O
handover	B-Protocol
is	O
realised	O
by	O
changing	O
the	O
pair	O
of	O
used	O
transmit/receive	O
frequencies	O
.	O
</s>
<s>
For	O
the	O
practical	O
realisation	O
of	O
handovers	B-Protocol
in	O
a	O
cellular	O
network	O
each	O
cell	O
is	O
assigned	O
a	O
list	O
of	O
potential	O
target	O
cells	O
,	O
which	O
can	O
be	O
used	O
for	O
handing	O
over	O
calls	O
from	O
this	O
source	O
cell	O
to	O
them	O
.	O
</s>
<s>
During	O
a	O
call	O
one	O
or	O
more	O
parameters	O
of	O
the	O
signal	O
in	O
the	O
channel	O
in	O
the	O
source	O
cell	O
are	O
monitored	O
and	O
assessed	O
in	O
order	O
to	O
decide	O
when	O
a	O
handover	B-Protocol
may	O
be	O
necessary	O
.	O
</s>
<s>
The	O
downlink	O
(	O
forward	O
link	O
)	O
and/or	O
uplink	B-Application
(	O
reverse	O
link	O
)	O
directions	O
may	O
be	O
monitored	O
.	O
</s>
<s>
The	O
handover	B-Protocol
may	O
be	O
requested	O
by	O
the	O
phone	O
or	O
by	O
the	O
base	B-General_Concept
station	I-General_Concept
(	O
BTS	O
)	O
of	O
its	O
source	O
cell	O
and	O
,	O
in	O
some	O
systems	O
,	O
by	O
a	O
BTS	O
of	O
a	O
neighboring	O
cell	O
.	O
</s>
<s>
In	O
some	O
systems	O
,	O
mainly	O
based	O
on	O
CDMA	B-Protocol
,	O
a	O
target	O
candidate	O
may	O
be	O
selected	O
among	O
the	O
cells	O
which	O
are	O
not	O
in	O
the	O
neighbor	O
list	O
.	O
</s>
<s>
In	O
analog	B-Protocol
systems	O
the	O
parameters	O
used	O
as	O
criteria	O
for	O
requesting	O
a	O
hard	O
handover	B-Protocol
are	O
usually	O
the	O
received	O
signal	O
power	O
and	O
the	O
received	O
signal-to-noise	O
ratio	O
(	O
the	O
latter	O
may	O
be	O
estimated	O
in	O
an	O
analog	B-Protocol
system	O
by	O
inserting	O
additional	O
tones	O
,	O
with	O
frequencies	O
just	O
outside	O
the	O
captured	O
voice-frequency	O
band	O
at	O
the	O
transmitter	O
and	O
assessing	O
the	O
form	O
of	O
these	O
tones	O
at	O
the	O
receiver	O
)	O
.	O
</s>
<s>
In	O
non-CDMA	O
2G	O
digital	O
systems	O
the	O
criteria	O
for	O
requesting	O
hard	O
handover	B-Protocol
may	O
be	O
based	O
on	O
estimates	O
of	O
the	O
received	O
signal	O
power	O
,	O
bit	O
error	O
rate	O
(	O
BER	O
)	O
and	O
block	B-Application
error/erasure	I-Application
rate	I-Application
(	O
BLER	B-Application
)	O
,	O
received	O
quality	O
of	O
speech	O
(	O
RxQual	B-Protocol
)	O
,	O
distance	O
between	O
the	O
phone	O
and	O
the	O
BTS	O
(	O
estimated	O
from	O
the	O
radio	O
signal	O
propagation	O
delay	O
)	O
and	O
others	O
.	O
</s>
<s>
In	O
CDMA	B-Protocol
systems	O
,	O
2G	O
and	O
3G	B-Architecture
,	O
the	O
most	O
common	O
criterion	O
for	O
requesting	O
a	O
handover	B-Protocol
is	O
Ec/Io	O
ratio	O
measured	O
in	O
the	O
pilot	O
channel	O
(	O
CPICH	O
)	O
and/or	O
RSCP	O
.	O
</s>
<s>
In	O
CDMA	B-Protocol
systems	O
,	O
when	O
the	O
phone	O
in	O
soft	O
or	O
softer	O
handover	B-Protocol
is	O
connected	O
to	O
several	O
cells	O
simultaneously	O
,	O
it	O
processes	O
the	O
received	O
in	O
parallel	O
signals	O
using	O
a	O
rake	O
receiver	O
.	O
</s>
<s>
A	O
usual	O
design	O
of	O
a	O
rake	O
receiver	O
in	O
mobile	O
phones	O
includes	O
three	O
or	O
more	O
rake	O
fingers	O
used	O
in	O
soft	B-Protocol
handover	I-Protocol
state	O
for	O
processing	O
signals	O
from	O
as	O
many	O
cells	O
and	O
one	O
additional	O
finger	O
used	O
to	O
search	O
for	O
signals	O
from	O
other	O
cells	O
.	O
</s>
<s>
The	O
set	O
of	O
cells	O
,	O
whose	O
signals	O
are	O
used	O
during	O
a	O
soft	B-Protocol
handover	I-Protocol
,	O
is	O
referred	O
to	O
as	O
the	O
active	O
set	O
.	O
</s>
<s>
The	O
cells	O
in	O
the	O
neighbour	O
list	O
(	O
called	O
in	O
CDMA	B-Protocol
neighbouring	O
set	O
)	O
are	O
checked	O
more	O
frequently	O
than	O
the	O
rest	O
and	O
thus	O
a	O
handover	B-Protocol
with	O
a	O
neighbouring	O
cell	O
is	O
more	O
likely	O
,	O
however	O
a	O
handover	B-Protocol
with	O
others	O
cells	O
outside	O
the	O
neighbor	O
list	O
is	O
also	O
allowed	O
(	O
unlike	O
in	O
GSM	O
,	O
IS-136/DAMPS	O
,	O
AMPS	O
,	O
NMT	O
,	O
etc	O
.	O
</s>
<s>
There	O
are	O
occurrences	O
where	O
a	O
handoff	B-Protocol
is	O
unsuccessful	O
.	O
</s>
<s>
There	O
are	O
also	O
inter-technology	O
handovers	B-Protocol
where	O
a	O
call	O
's	O
connection	O
is	O
transferred	O
from	O
one	O
access	O
technology	O
to	O
another	O
,	O
e.g.	O
</s>
<s>
a	O
call	O
being	O
transferred	O
from	O
GSM	O
to	O
UMTS	B-Protocol
or	O
from	O
CDMA	B-Protocol
IS-95	O
to	O
cdma2000	O
.	O
</s>
<s>
The	O
3GPP	O
UMA/GAN	O
standard	O
enables	O
GSM/UMTS	O
handoff	B-Protocol
to	O
Wi-Fi	O
and	O
vice	O
versa	O
.	O
</s>
<s>
Different	O
systems	O
have	O
different	O
methods	O
for	O
handling	O
and	O
managing	O
handoff	B-Protocol
request	O
.	O
</s>
<s>
Some	O
systems	O
handle	O
handoff	B-Protocol
in	O
same	O
way	O
as	O
they	O
handle	O
new	O
originating	O
call	O
.	O
</s>
<s>
In	O
such	O
system	O
the	O
probability	O
that	O
the	O
handoff	B-Protocol
will	O
not	O
be	O
served	O
is	O
equal	O
to	O
blocking	O
probability	O
of	O
new	O
originating	O
call	O
.	O
</s>
<s>
So	O
in	O
order	O
to	O
avoid	O
this	O
abrupt	O
termination	O
of	O
ongoing	O
call	O
handoff	B-Protocol
request	O
should	O
be	O
given	O
priority	O
to	O
new	O
call	O
this	O
is	O
called	O
as	O
handoff	B-Protocol
prioritization	O
.	O
</s>
<s>
Guard	O
Channel	O
Concept	O
In	O
this	O
technique	O
,	O
a	O
fraction	O
of	O
the	O
total	O
available	O
channels	O
in	O
a	O
cell	O
is	O
reserved	O
exclusively	O
for	O
handoff	B-Protocol
request	O
from	O
ongoing	O
calls	O
which	O
may	O
be	O
handed	O
off	O
into	O
the	O
cell	O
.	O
</s>
<s>
Queuing	O
Queuing	O
of	O
handoffs	B-Protocol
is	O
possible	O
because	O
there	O
is	O
a	O
finite	O
time	O
interval	O
between	O
the	O
time	O
the	O
received	O
signal	O
level	O
drops	O
below	O
handoff	B-Protocol
threshold	O
and	O
the	O
time	O
the	O
call	O
is	O
terminated	O
due	O
to	O
insufficient	O
signal	O
level	O
.	O
</s>
<s>
If	O
during	O
ongoing	O
call	O
mobile	O
unit	O
moves	O
from	O
one	O
cellular	O
system	O
to	O
a	O
different	O
cellular	O
system	O
which	O
is	O
controlled	O
by	O
different	O
MTSO	O
,	O
a	O
handoff	B-Protocol
procedure	O
which	O
is	O
used	O
to	O
avoid	O
dropping	O
of	O
call	O
is	O
referred	O
as	O
Inter	O
System	O
Handoff	B-Protocol
.	O
</s>
<s>
An	O
MTSO	O
engages	O
in	O
this	O
handoff	B-Protocol
system	O
.	O
</s>
<s>
When	O
a	O
mobile	O
signal	O
becomes	O
weak	O
in	O
a	O
given	O
cell	O
and	O
MTSO	O
can	O
not	O
find	O
other	O
cell	O
within	O
its	O
system	O
to	O
which	O
it	O
can	O
transfer	O
the	O
call	O
then	O
it	O
uses	O
Inter	O
system	O
handoff	B-Protocol
.	O
</s>
<s>
Before	O
implementation	O
of	O
Inter	O
System	O
Handoff	B-Protocol
MTSO	O
compatibility	O
must	O
be	O
checked	O
and	O
in	O
Inter	O
System	O
Handoff	B-Protocol
local	O
call	O
may	O
become	O
long-distance	O
call	O
.	O
</s>
<s>
If	O
during	O
ongoing	O
call	O
mobile	O
unit	O
moves	O
from	O
one	O
cellular	O
system	O
to	O
adjacent	O
cellular	O
system	O
which	O
is	O
controlled	O
by	O
same	O
MTSO	O
,	O
a	O
handoff	B-Protocol
procedure	O
which	O
is	O
used	O
to	O
avoid	O
dropping	O
of	O
call	O
is	O
referred	O
as	O
Intra	O
System	O
Handoff	B-Protocol
.	O
</s>
<s>
An	O
MTSO	O
engages	O
in	O
this	O
handoff	B-Protocol
system	O
.	O
</s>
<s>
When	O
a	O
mobile	O
signal	O
becomes	O
weak	O
in	O
a	O
given	O
cell	O
and	O
MTSO	O
finds	O
other	O
cell	O
within	O
its	O
system	O
to	O
which	O
it	O
can	O
transfer	O
the	O
call	O
then	O
it	O
uses	O
Intra	O
system	O
handoff	B-Protocol
.	O
</s>
<s>
In	O
Intra	O
System	O
Handoff	B-Protocol
local	O
calls	O
always	O
remain	O
local	O
call	O
only	O
since	O
after	O
handoff	B-Protocol
also	O
the	O
call	O
is	O
handled	O
by	O
same	O
MTSO	O
.	O
</s>
