<s>
Network	O
switching	O
subsystem	O
(	O
NSS	O
)	O
(	O
or	O
GSM	O
core	B-General_Concept
network	I-General_Concept
)	O
is	O
the	O
component	O
of	O
a	O
GSM	O
system	O
that	O
carries	O
out	O
call	O
out	O
and	O
mobility	O
management	O
functions	O
for	O
mobile	O
phones	O
roaming	O
on	O
the	O
network	O
of	O
base	O
stations	O
.	O
</s>
<s>
The	O
NSS	O
originally	O
consisted	O
of	O
the	O
circuit-switched	B-Protocol
core	B-General_Concept
network	I-General_Concept
,	O
used	O
for	O
traditional	O
GSM	O
services	O
such	O
as	O
voice	O
calls	O
,	O
SMS	B-Device
,	O
and	O
circuit	B-Protocol
switched	I-Protocol
data	O
calls	O
.	O
</s>
<s>
It	O
was	O
extended	O
with	O
an	O
overlay	O
architecture	O
to	O
provide	O
packet-switched	O
data	O
services	O
known	O
as	O
the	O
GPRS	O
core	B-General_Concept
network	I-General_Concept
.	O
</s>
<s>
This	O
allows	O
mobile	O
phones	O
to	O
have	O
access	O
to	O
services	O
such	O
as	O
WAP	B-Protocol
,	O
MMS	O
and	O
the	O
Internet	O
.	O
</s>
<s>
The	O
mobile	B-Device
switching	I-Device
center	I-Device
(	O
MSC	O
)	O
is	O
the	O
primary	O
service	O
delivery	O
node	O
for	O
GSM/CDMA	O
,	O
responsible	O
for	O
routing	O
voice	O
calls	O
and	O
SMS	B-Device
as	O
well	O
as	O
other	O
services	O
(	O
such	O
as	O
conference	O
calls	O
,	O
FAX	O
,	O
and	O
circuit-switched	B-Protocol
data	O
)	O
.	O
</s>
<s>
Only	O
at	O
the	O
MSC	O
is	O
this	O
re-coded	O
into	O
an	O
"	O
analogue	O
"	O
signal	O
(	O
although	O
actually	O
this	O
will	O
almost	O
certainly	O
mean	O
sound	O
is	O
encoded	O
digitally	O
as	O
a	O
pulse-code	B-Algorithm
modulation	I-Algorithm
(	O
PCM	B-Algorithm
)	O
signal	O
in	O
a	O
64-kbit/s	O
timeslot	O
,	O
known	O
as	O
a	O
DS0	O
in	O
America	O
)	O
.	O
</s>
<s>
The	O
anchor	O
MSC	O
is	O
the	O
MSC	O
from	O
which	O
a	O
handover	B-Protocol
has	O
been	O
initiated	O
.	O
</s>
<s>
The	O
target	O
MSC	O
is	O
the	O
MSC	O
toward	O
which	O
a	O
handover	B-Protocol
should	O
take	O
place	O
.	O
</s>
<s>
A	O
mobile	B-Device
switching	I-Device
center	I-Device
server	O
is	O
a	O
part	O
of	O
the	O
redesigned	O
MSC	O
concept	O
starting	O
from	O
3GPP	O
Release	O
4	O
.	O
</s>
<s>
The	O
mobile	B-Device
switching	I-Device
center	I-Device
server	O
is	O
a	O
soft-switch	O
variant	O
(	O
therefore	O
it	O
may	O
be	O
referred	O
to	O
as	O
mobile	O
soft	O
switch	O
,	O
MSS	O
)	O
of	O
the	O
mobile	B-Device
switching	I-Device
center	I-Device
,	O
which	O
provides	O
circuit-switched	B-Protocol
calling	O
mobility	O
management	O
,	O
and	O
GSM	O
services	O
to	O
the	O
mobile	O
phones	O
roaming	O
within	O
the	O
area	O
that	O
it	O
serves	O
.	O
</s>
<s>
MSS	O
and	O
media	O
gateway	O
(	O
MGW	O
)	O
makes	O
it	O
possible	O
to	O
cross-connect	O
circuit-switched	B-Protocol
calls	O
switched	O
by	O
using	O
IP	O
,	O
ATM	O
AAL2	O
as	O
well	O
as	O
TDM	B-Protocol
.	O
</s>
<s>
The	O
term	O
Circuit	B-Protocol
switching	I-Protocol
(	O
CS	O
)	O
used	O
here	O
originates	O
from	O
traditional	O
telecommunications	O
systems	O
.	O
</s>
<s>
However	O
,	O
modern	O
MSS	O
and	O
MGW	O
devices	O
mostly	O
use	O
generic	O
Internet	O
technologies	O
and	O
form	O
next-generation	B-Protocol
telecommunication	I-Protocol
networks	I-Protocol
.	O
</s>
<s>
MSS	O
software	O
may	O
run	O
on	O
generic	O
computers	O
or	O
virtual	B-Architecture
machines	I-Architecture
in	O
cloud	B-Architecture
environment	O
.	O
</s>
<s>
The	O
UMTS	O
terrestrial	O
radio	O
access	O
network	O
(	O
UTRAN	O
)	O
which	O
handles	O
the	O
radio	O
communication	O
with	O
3G	B-Architecture
mobile	I-Architecture
phones	O
.	O
</s>
<s>
Other	O
MSCs	O
for	O
procedures	O
such	O
as	O
hand	B-Protocol
over	I-Protocol
.	O
</s>
<s>
Arranging	O
handovers	B-Protocol
from	O
BSC	O
to	O
BSC	O
.	O
</s>
<s>
Carrying	O
out	O
handovers	B-Protocol
from	O
this	O
MSC	O
to	O
another	O
.	O
</s>
<s>
The	O
home	O
location	O
register	O
(	O
HLR	O
)	O
is	O
a	O
central	O
database	O
that	O
contains	O
details	O
of	O
each	O
mobile	O
phone	O
subscriber	O
that	O
is	O
authorized	O
to	O
use	O
the	O
GSM	O
core	B-General_Concept
network	I-General_Concept
.	O
</s>
<s>
Each	O
SIM	O
has	O
a	O
unique	O
identifier	O
called	O
an	O
IMSI	O
which	O
is	O
the	O
primary	B-Application
key	I-Application
to	O
each	O
HLR	O
record	O
.	O
</s>
<s>
The	O
primary	O
MSISDN	O
is	O
the	O
number	O
used	O
for	O
making	O
and	O
receiving	O
voice	O
calls	O
and	O
SMS	B-Device
,	O
but	O
it	O
is	O
possible	O
for	O
a	O
SIM	O
to	O
have	O
other	O
secondary	O
MSISDNs	O
associated	O
with	O
it	O
for	O
fax	O
and	O
data	O
calls	O
.	O
</s>
<s>
Each	O
MSISDN	O
is	O
also	O
a	O
unique	B-Application
key	I-Application
to	O
the	O
HLR	O
record	O
.	O
</s>
<s>
The	O
HLR	O
is	O
a	O
system	O
which	O
directly	O
receives	O
and	O
processes	O
MAP	B-Protocol
transactions	O
and	O
messages	O
from	O
elements	O
in	O
the	O
GSM	O
network	O
,	O
for	O
example	O
,	O
the	O
location	O
update	O
messages	O
received	O
as	O
mobile	O
phones	O
roam	O
around	O
.	O
</s>
<s>
The	O
authentication	O
center	O
(	O
AuC	O
)	O
is	O
a	O
function	O
to	O
authenticate	O
each	O
SIM	O
card	O
that	O
attempts	O
to	O
connect	O
to	O
the	O
gsm	O
core	B-General_Concept
network	I-General_Concept
(	O
typically	O
when	O
the	O
phone	O
is	O
powered	O
on	O
)	O
.	O
</s>
<s>
An	O
encryption	O
key	O
is	O
also	O
generated	O
that	O
is	O
subsequently	O
used	O
to	O
encrypt	O
all	O
wireless	O
communications	O
(	O
voice	O
,	O
SMS	B-Device
,	O
etc	O
.	O
)	O
</s>
<s>
between	O
the	O
mobile	O
phone	O
and	O
the	O
GSM	O
core	B-General_Concept
network	I-General_Concept
.	O
</s>
<s>
When	O
a	O
particular	O
IMSI	O
requests	O
access	O
to	O
the	O
GSM	O
core	B-General_Concept
network	I-General_Concept
,	O
the	O
MSC	O
sends	O
the	O
RAND	O
part	O
of	O
the	O
triplet	O
to	O
the	O
SIM	O
.	O
</s>
<s>
Therefore	O
,	O
a	O
new	O
set	O
of	O
security	O
methods	O
was	O
designed	O
for	O
3G	B-Architecture
phones	I-Architecture
.	O
</s>
<s>
The	O
Visitor	O
Location	O
Register	O
(	O
VLR	O
)	O
is	O
a	O
database	O
of	O
the	O
MSs	O
(	O
Mobile	O
stations	O
)	O
that	O
have	O
roamed	O
into	O
the	O
jurisdiction	O
of	O
the	O
Mobile	B-Device
Switching	I-Device
Center	I-Device
(	O
MSC	O
)	O
which	O
it	O
serves	O
.	O
</s>
<s>
Each	O
main	O
base	B-General_Concept
transceiver	I-General_Concept
station	I-General_Concept
in	O
the	O
network	O
is	O
served	O
by	O
exactly	O
one	O
VLR	O
(	O
one	O
BTS	B-General_Concept
may	O
be	O
served	O
by	O
many	O
MSCs	O
in	O
case	O
of	O
MSC	O
in	O
pool	O
)	O
,	O
hence	O
a	O
subscriber	O
cannot	O
be	O
present	O
in	O
more	O
than	O
one	O
VLR	O
at	O
a	O
time	O
.	O
</s>
<s>
Therefore	O
,	O
in	O
the	O
delivery	O
of	O
the	O
basic	O
components	O
of	O
the	O
network	O
switching	O
subsystem	O
(	O
core	B-General_Concept
network	I-General_Concept
)	O
is	O
often	O
already	O
present	O
EIR	O
with	O
basic	O
functionality	O
,	O
which	O
includes	O
a	O
‘	O
whitelisted’	O
response	O
to	O
all	O
CheckIMEI	O
and	O
the	O
ability	O
to	O
fill	O
IMEI	O
blacklist	O
,	O
which	O
will	O
be	O
given	O
a	O
‘	O
blacklisted’	O
response	O
.	O
</s>
<s>
Sending	O
SMS	B-Device
notifications	O
to	O
subscribers	O
in	O
separate	O
scenarios	O
.	O
</s>
<s>
To	O
do	O
this	O
,	O
there	O
are	O
blocked	O
all	O
services	O
,	O
except	O
the	O
following	O
:	O
calls	O
to	O
a	O
specific	O
service	O
number	O
,	O
sending	O
SMS	B-Device
to	O
a	O
specific	O
service	O
number	O
,	O
and	O
all	O
Internet	O
traffic	O
is	O
redirected	O
to	O
a	O
specific	O
landing	O
page	O
.	O
</s>
<s>
This	O
is	O
achieved	O
by	O
the	O
fact	O
that	O
EIR	O
can	O
send	O
commands	O
to	O
several	O
MNO	O
systems	O
(	O
HLR	O
,	O
PCRF	B-Protocol
,	O
SMSC	O
,	O
etc	O
.	O
</s>
<s>
Connected	O
more	O
or	O
less	O
directly	O
to	O
the	O
GSM	O
core	B-General_Concept
network	I-General_Concept
are	O
many	O
other	O
functions	O
.	O
</s>
