The document provides an overview of Intelligent Network (IN) and Public Land Mobile Network (PLMN) concepts and technologies. (1) IN moves intelligence from exchanges to distributed computer nodes, allowing flexible service development and customization. PLMN provides mobility through cellular infrastructure and mobility management functions. (2) Key IN elements are SCP, SSP, and IP. PLMN elements include MSC, VLR, HLR, AuC, and EIR. Security involves authentication between SIM and AuC using algorithms. Mobility is managed through location updating between MS, VLR, and HLR. (3) Example IN services are freephone, VPN, and number port

1. IN
Intelligent Network
• Basic IN concept & technology
• Some basic IN services

2. Intelligent Network (IN) Concept
The intelligent network concept: intelligence is taken
out of exchanges and placed in computer nodes that
are distributed throughout the network.
Intelligence => access to various databases
This provides the network operator with the means
to develop and control services more efficiently. New
capabilities can be rapidly introduced into the
network. Once introduced, services are easily
customized to meet individual customer's needs.

3. Intelligent Network (IN) Concept
Exchange
STP SCP
SSP
Service Control Point
(a network element containing
the service logic, a database or
register)
Service Switching Point
(enables service triggering in an
exchange)
MAP
INAP
CAP
ISUP
Operator implements service logic (IN Service)

4. IN service subscriber and customer
In a typical IN service scenario, the network operator
or a 3rd party service provider implements the service
for one or several subscribers, after which customers
can use the service.
Service subscriber = company offering the service
(e.g. the 0800 number that anybody can call)
Customers = those who use the service (e.g. those
who call the 0800 number)
Confusion possible:
IN service subscriber  PSTN subscriber

5. Typical call-related IN procedure (1)
SSP
Exchange
SCP
1.
2.
3.
4.
5.
Exchange
1. Call routing proceeds up to Exchange
2. Trigger activated in Basic Call State Model at SSP
3. SSP requests information from SCP (database)
4. SCP provides information
5. Call routing continues (routing to next exchange)
based on information received from SCP

6. SSP
Exchange
SCP
1.
2.
3.
4.
5.
Exchange
2. Trigger activated in Basic Call State Model at SSP
Typical triggers:
Called number (or part of number)
Called user (destination) is busy
Called user does not answer in predefined time
Typical call-related IN procedure (2)

7. SSP
Exchange
1.
2.
3.
5.
Exchange
Example: Number translation in SCP
SSP sends 800 number (0800 1234)
SCP translates into ”real” number which
is used for routing the call
(+358 9 1234567)
4. SCP provides information
SCP
Typical call-related IN procedure (3)
translation
may be
based on
several
variables
4.

8. Destination 1
SCP decides the destination of the call depending on the
calling time or date:
9.00 - 17.00 => Destination 1
17.00 - 9.00 => Destination 2
SCP
Examples of how SCP can affect call (1)
Destination 2
SSP
Exchange
Called number
Time or date

9. Destination 1
SCP decides the destination of the call depending on the
location of calling user:
Calling user in southern Finland => Destination 1
Calling user in northern Finland => Destination 2
SCP
Examples of how SCP can affect call (2)
Destination 2
SSP
Exchange
Called number, Calling number

10. Destination 1
SCP decides the destination of the call depending on the
traffic load in the network:
Traffic load situation 1 => Destination 1
Traffic load situation 2 => Destination 2
SCP
Examples of how SCP can affect call (3)
Destination 2
SSP
Exchange
Called number
Network load

11. Intelligent Peripheral (IP) can (a) send announcements
to the user (usually: calling user) and (b) receive DTMF
digits from the user. IP is not a database; connection to
exchange not via SS7, instead via digital TDM channels.
SCP
Additional IN features (1)
SSP
Exchange Exchange
IP

12. Typical applications:
1) Whenever services need user interaction
2) User authentication
SCP
Additional IN features (2)
SSP
Exchange Exchange
IP

13. SCP
User interaction in IN service
SSP
Exchange Exchange
IP
1.
4.
2.
3.
1. SCP orders IP to select and send announcement
2. IP sends announcement to calling user
3. User replies by giving DTMF number(s) to IP
4. IP sends number information to SCP in a signalling
message
Announcement:
“for this .. press 1,
for that .. press 2”

14. SCP
User authentication (1)
SSP
Exchange Exchange
IP
1.
4.
2.
3.
1. SCP orders IP to select and send announcement
2. IP sends announcement to calling user
3. User gives authentication code (in DTMF form) to IP
4. IP sends authentication code to SCP in a signalling
message
Announcement:
“please press your
PIN code ...”

15. SCP
User authentication (2)
SSP
Exchange
IP
1.
3.
2.
Display message:
“please press your
PIN code ...”
When connected to the network via a digital
subscriber line, the calling user can be
notified with a digital message (“please press
your PIN code ...”) instead of having to use
the corresponding voice announcement.
1.

16. IN services
A large number of IN services can be implemented by
combining different “building blocks”:
Called number translation (at SCP)
Routing decision based on calling number,
time, date, called user busy, called user
alerting timeout, network load ...
Announcements (from IP) or user notification
(<= ISDN user signalling)
DTMF number reception (at IP) and analysis
(at SCP)
Customised charging (at exchanges)
•
•
•
•
•

17. IN service examples
“Traditional” IN services:
- Freephone / customised charging schemes
- Virtual Privat Network (VPN)
- Number portability
- Televoting
“IN” in mobile networks:
- Mobility management (HLR, VLR = databases)
- Security management (Authentication ...)
- Additional IN services in mobile networks =>
CAMEL (Customised Applications for Mobile
networks Enhanced Logic)

18. Freephone (800) service
User calls 0800 76543. SSP sends this number to SCP
which after number analysis sends back to SSP the
real destination address (09 1234567) and call can be
routed to the destination. Called party is charged.
SSP
Exchange
SCP
1.
2.
3.
4.
5.
Destination
Charging: Destination (service subscriber)
pays the bill

19. Premium rate service
User calls 0200 34343. SSP sends this number to SCP
which after number analysis sends back to SSP the
real destination address (09 676567) and call can be
routed to the destination. Calling party is charged.
SSP
Exchange
SCP
1.
2.
3.
4.
5.
Destination
Charging: Calling user (customer) pays the (usually rather
expensive) bill. Both service subscriber and service provider
or network operator make profit!

20. Virtual private network (VPN) service
A VPN provides corporate customers with a private
number plan within the PSTN. The customer dials a
private (short) number instead of the complete public
number in order to contact another user within the VPN.
User authentication is usually required.
SSP
Exchange
SCP
Destination
IP
User authentication
Number translation: 1212 => 09 1234567
Customised charging

21. Screening of incoming calls
This is an example of an IN service related to the call
destination end. Alert called user only if calling number
is 121212 or 234567, otherwise do something else (e.g.
reject call or redirect call to another destination).
SSP
Exchange
SCP
Called user
Calling number = 121212 or 234567: Accept
All other calling numbers: Reject or redirect
Local exchange of called user

22. VLR
Mobile terminated call (MTC)
By far the most important "IN service" is mobility
management during a mobile terminated call (MTC),
which means finding out under which exchange or
mobile switching center (MSC) a mobile user is
roaming, so that the call can be routed to this
exchange. More about this later.
GMSC
HLR
1.
2.
5.
6.
Serving MSC
3.
4.
7.

23. More about IN and IN services…
The link www.iec.org/online/tutorials/in provides some
examples in Section 10 (AIN Service Creation Examples),
for instance:
Example
of service
creation
template:

24. PLMN
Public Land Mobile Network
(official name for mobile network)
• Circuit-switched (CS) core network
(radio access network is not part of
this course)
• Basic concepts and network elements
• Mobility management in PLMN

25. Cellular concept
A cellular network contains a large number of cells with
a base station (BS) at the center of each cell to which
mobile stations (MS) are connected during a call.
BS
BS
BS
BS
MS
If a connected MS
(MS in call phase)
moves between two
cells, the call is not
dropped.
Instead, the network
performs a handover
(USA: handoff).

26. Mobility concept
A cellular network is divided into location areas (LA),
each containing a certain number of cells.
As long as an idle MS
(idle = switched on)
moves within a location
area, it can be reached
through paging.
If an idle MS moves between
two location areas, it cannot be
reached before it performs
location updating.
Location Area 1
Location Area 3
Location
Area 2

27. Architecture of a mobile network
GSM
BSS
3G
RAN
PS core network
CS core network
GMSC
MSC
VLR
HLR
AuC
EIR
PSTN
Internet
MS

28. Serving MSC
GSM
BSS
3G
RAN
PS core network
CS core network
GMSC
MSC
VLR
HLR
AuC
EIR
PSTN
Internet
The serving mobile switching
center (MSC) is the mobile
counterpart to the local
exchange in the PSTN.
This is the MSC that is currently
serving a mobile user.

29. VLR
GSM
BSS
3G
RAN
PS core network
CS core network
GMSC
HLR
AuC
EIR
PSTN
Internet
The visitor location register
stores temporary information
on mobile users roaming in a
location area under the
control of the MSC/VLR.
MSC
VLR

30. Gateway MSC
GSM
BSS
3G
RAN
PS core network
CS core network
GMSC
HLR
AuC
EIR
PSTN
Internet
MSC
VLR
The gateway MSC (located in the home
PLMN of a mobile user) is the first contact
point in the mobile network when there is
an incoming call to the mobile user.

31. HLR
GSM
BSS
3G
RAN
PS core network
CS core network
GMSC
HLR
AuC
EIR
PSTN
Internet
The home location register
stores information on mobile
users belonging to this mobile
network (e.g. subscription data
and present VLR under which
the mobile user is roaming).
MSC
VLR

32. AuC
GSM
BSS
3G
RAN
PS core network
CS core network
GMSC
HLR
AuC
EIR
PSTN
Internet
The authentication center safely
stores authentication keys (Ki)
of mobile subscribers belonging
to this mobile network.
MSC
VLR

33. EIR
GSM
BSS
3G
RAN
PS core network
CS core network
GMSC
HLR
AuC
EIR
PSTN
Internet
The equipment identity register
stores information on stolen
handsets (not stolen SIMs).
MSC
VLR

34. SIM
GSM
BSS
3G
RAN
PS core network
CS core network
GMSC
HLR
AuC
EIR
PSTN
Internet
Important mobile user information is stored in the
subscriber identity module within the handset.
MSC
VLR
SIM

35. CS core network
GSM
BSS
3G
RAN
PS core network
CS core network
GMSC
HLR
AuC
EIR
PSTN
Internet
MSC
VLR
The CS core network architecture is
basically the same in 2G (GSM) and 3G
mobile networks.
In North America, IS-MAP signalling is
used instead of GSM-MAP signalling.
Europe: GSM core network
N. America: ANSI-41 core network

36. Basic functions in a mobile network
Session Management (SM)
Call Control (CC)
Mobility Management (MM)
Radio Resource Management (RRM)
MOC, MTC
PDP Context
Random access and channel reservation
Handover management
Ciphering (encryption) over radio interface
IMSI/GPRS Attach (switch on) and Detach (switch off)
Location updating (MS moves to other Location Area)
Authentication
1
2
3
4
Number
refers to
following
slides in the
the slide set
Later lecture

37. Range of functions
GSM
BSS
or
3G
RAN
PS core network
CS core network
RRM
MM
CC
SM

38. Random access in a mobile network
Communication between MS and network is not possible
before going through a procedure called random access.
Random access must consequently be used in:
Network-originated activity
• paging, e.g. for a mobile terminated call (MTC)
MS-originated activity
• IMSI attach, IMSI detatch
• GPRS attach, GPRS detach
• location updating
• mobile originated call (MOC)
• SMS (short message service) message transfer
1

39. Random access in action (GSM)
1. MS sends a short access burst over the Random
Access CHannel (RACH) in uplink using Slotted Aloha (in
case of collision => retransmission after random time)
2. After detecting the access burst, the network returns
an ”immediate assignment” message which includes the
following information:
- allocated physical channel (frequency, time slot) in
which the assigned signalling channel is located
- timing advance (for correct time slot alignment)
3. The MS now sends a message on the dedicated
signalling channel assigned by the network, indicating
the reason for performing random access.
1

40. Multiplexing vs. multiple access
In downlink, multiplexing (e.g. TDM)
In uplink, multiple access (e.g. TDMA)
Multiple access is always associated with random
access. MS requests signalling channel, and network
decides which channel (e.g. time slot) will be used.
Network decides channel…
Network decides channel also in this case

41. 1) PIN code (local authentication of handset
=> local security measure, network is not involved)
2) Authentication (performed by network)
3) Ciphering of information sent over air interface
4) Usage of TMSI (instead of IMSI) over air interface
IMSI = International Mobile Subscriber Identity
(globally unique identity)
TMSI = Temporary Mobile Subscriber Identity
(local and temporary identity)
Security measures in a mobile network

42. Algorithm Algorithm
The same? If yes,
authentication is successful
SIM
(in handset)
Air
interface
Network (algorithm
running in AuC)
Random number
Challenge
Response
Authentication key Authentication key
RAND
SRESS
Ki Ki
Basic principle of authentication
2
SRESA

43. Algorithm for calculating SRES runs within SIM (user
side) and AuC (network side). The authentication key
(Ki) is stored safely in SIM and AuC, and remains there
during authentication.
The two SRES values are compared in the VLR.
Where does the algorithm run?
2
AuC
SIM
VLR
Air interface
SRESS SRESA
RAND
Ki Ki

44. Using output and one or more inputs, it is in practice
not possible to calculate “backwards” other input(s),
“brute force approach”, “extensive search”
Key length in bits (N) is important (in case of brute
force approach 2N calculation attempts may be needed)
Strength of algorithm is that it is secret => bad idea!
“Security through obscurity”
Better: open algorithm can be tested by engineering
community (security through strong algorithm)
Algorithm considerations
2

45. HLR
MSC
VLR 1
Most recently allocated TMSI and last visited LAI (Location
Area ID) are stored in SIM even after switch-off.
After switch-on, MS monitors LAI. If stored and monitored
LAI values are the same, no location updating is needed.
(Most generic scenario, see van Bosse for details)
MSC
VLR 2
IMSI
LAI 1
TMSI
LAI 1
IMSI
LAI 1
3
(in broadcast messages)
Case study: Location updating (1)
SIM
IMSI
TMSI

46. SIM
MSC
VLR 1
MS has moved from a cell belonging to VLR 1 to another
cell belonging to VLR 2.
MS notices that the LAI values are different => location
update is required!
MSC
VLR 2
LAI 2
HLR
(in broadcast
messages)
3 Location updating (2)
IMSI
LAI 1
IMSI
TMSI
IMSI
LAI 1
TMSI

47. SIM
MSC
VLR 1
MSC
VLR 2
HLR
3 Location updating (3)
IMSI
LAI 1
SIM sends old LAI (i.e., LAI 1) and TMSI to VLR 2.
VLR 2 does not recognize TMSI since there is no TMSI-
IMSI context. Who is this user?
LAI 1, TMSI
No TMSI - IMSI context!
IMSI
TMSI
IMSI
LAI 1
TMSI

48. SIM
MSC
VLR 1
MSC
VLR 2
HLR
3 Location updating (4)
IMSI
LAI 1
However, VLR 2 can contact VLR 1 (address: LAI 1) and
request IMSI.
IMSI is sent to VLR 2. There is now a TMSI-IMSI context.
IMSI
Address: LAI 1
IMSI
TMSI
IMSI
TMSI
IMSI
LAI 1
TMSI

49. SIM
MSC
VLR 1
MSC
VLR 2
HLR
3 Location updating (5)
IMSI
TMSI
Important: HLR must be updated (new LAI). If this is not
done, incoming calls can not be routed to new MSC/VLR.
HLR also requests VLR 1 to remove old user data.
IMSI
TMSI
IMSI
LAI 1
LAI 2
LAI 2
IMSI
LAI 1
TMSI

50. SIM
MSC
VLR 1
MSC
VLR 2
HLR
3 Location updating (6)
IMSI
LAI 2
VLR 2 generates new TMSI and sends this to user. User
stores new LAI and TMSI safely in SIM.
Location updating was successful!
IMSI
LAI 1
TMSI
LAI 2
TMSI
LAI 2
TMSI
IMSI
TMSI
TMSI

51. Trade-off when choosing LA size
Affects signalling load
If LA size is very large (e.g. whole mobile network)
location updating not needed very often
paging load is very heavy
If LA size is very small (e.g. single cell)
small paging load
location updating must be done very often
High paging channel capacity required
+
+
3

52. Role of TMSI
MS Network
Random access
Authentication
Start ciphering
IMSI detach New TMSI
allocated by
network
New TMSI stored in SIM
CC or MM transaction
Uses
TMSI
IMSI is not
sent over air
interface if
not absolutely
necessary!

53. Mobile network identifiers (1)
SN
CC
MSISDN
CC = Country Code (1-3 digits)
NDC = National Destination Code (1-3 digits)
SN = Subscriber Number
NDC
=
Globally
unique
number
E.164 numbering
format
Mobile station ISDN (MSISDN) numbers are based on the
ITU-T E.164 numbering plan and can therefore be used for
routing a circuit-switched call.
When the calling (PSTN or PLMN) user dials an MSISDN
number, the call is routed to the gateway MSC (GMSC)
located in the home network of the called (mobile) user.

54. Mobile network identifiers (2)
TN
CC
MSRN
CC = Country Code (1-3 digits)
NDC = National Destination Code (1-3 digits)
TN = Temporary Number
NDC
=
Temporarily
allocated
number
E.164 numbering
format
Mobile station roaming numbers (MSRN) are also based
on the ITU-T E.164 numbering plan and can therefore be
used for routing a circuit-switched call.
The MSRN is selected by the MSC/VLR serving the called
(mobile) user, sent to the GMSC, and used for routing the
call from the GMSC to the serving MSC.

55. Mobile network identifiers (3)
MSIN
MCC
IMSI
MCC = Mobile Country Code (3 digits)
MNC = Mobile Network Code (2 digits)
MSIN = Mobile Subscriber Identity Number
(10 digits)
MNC
= E.212 numbering
format
The international mobile station identity (IMSI) is based
on the ITU-T E.212 numbering plan and cannot be used
for routing a circuit-switched call (exchanges or switching
centers do not understand such numbers).
The IMSI is stored in the HLR and SIM of the mobile user.
Globally
unique
number

56. Mobile network identifiers (4)
LAC
MCC
LAI
MCC = Mobile Country Code (3 digits)
MNC = Mobile Network Code (2 digits)
LAC = Location Area Code (10 digits)
MNC
= E.212 numbering
format
The location area identity (LAI) points to a location area
belonging to a certain MSC/VLR. This identity must be
stored in the HLR so that mobile terminated calls can be
routed to the correct serving MSC/VLR.
Globally
unique
number
IMEI ≈ ”Serial number of handset” (not SIM)

57. 4 Case study: Mobile terminated call (1)
VLR
1. Using the MSISDN number (dialled by the calling
user located in the PSTN or the PLMN of another
operator) and standard SS7/ISUP signalling, the
call is routed to the GMSC in the home network of
the called mobile user.
GMSC
HLR
1.
2.
4.
5.
Serving MSC
3.
4.
6.
(see van Bosse for details)

58. 4 Mobile terminated call (2)
VLR
GMSC
HLR
1.
2.
4.
5.
Serving MSC
3.
4.
6.
2. The GMSC contacts the HLR of the called mobile
user. The SS7/MAP signalling message contains
the MSISDN number which points to the mobile
user record (containing IMSI, LAI where user is
roaming, etc.) in the HLR database.

59. 4 Mobile terminated call (3)
VLR
GMSC
HLR
1.
2.
4.
5.
Serving MSC
3.
4.
6.
3. Using global title translation (GTT), the HLR
translates the IMSI and LAI information into the
signalling point code of the serving MSC/VLR.
The HLR sends SS7/MAP request “Provide roaming
number” (i.e. MSRN) to the VLR.

60. 4 Mobile terminated call (4)
VLR
GMSC
HLR
1.
2.
4.
5.
Serving MSC
3.
4.
6.
4. The VLR selects a temporary MSRN. Note that
there must be binding between MSRN and IMSI in
the VLR.
The VLR sends the MSRN to the GMSC (using
SS7/MAP signalling).
MSRN  IMSI

61. 4 Mobile terminated call (5)
VLR
GMSC
HLR
1.
2.
4.
5.
Serving MSC
3.
4.
6.
5. Using the MSRN number and standard SS7/ISUP
signalling, the call is routed to the serving MSC.
Although not shown in the figure, there may be
intermediate switching centers (serving MSC/VLR
may be located at the other end of the world).

62. 4 Mobile terminated call (6)
VLR
GMSC
HLR
1.
2.
4.
5.
Serving MSC
3.
4.
6.
6. MSC/VLR starts paging within the location area
(LA) in which the called mobile user is located,
using TMSI for identification. Only the mobile user
with the corresponding TMSI responds to the
paging via the random access channel (RACH).
MSRN  IMSI
IMSI  TMSI