This document discusses GSM network architecture and components. It describes the key elements like the MSC, HLR, VLR and their functions. It explains cell planning and frequency reuse. It also covers network identities, attaching and roaming processes, call setup, and charging systems like triggered charging for calls and SMS. Compound charging processes for originating calls, voucher refills through IVR are summarized.

1. GSM ARCHITECTURE, GSM
NETWORK IDENTITIES, NETWORK
CASES, CELL PLANNING, AND
COMPOUND CHARGING
Prepared by:
Zorays Khalid (Internee)

2. ERICSSON’S GSM SYSTEM
ARCHITECTURE
• Cellular Matara Ericsson 20 (GSM 900 and
GSM 1800)
• Cellular Mobile System 40 (GSM 1900)
• Table
System P-GSM E-GSM GSM 1800 GSM 1900
BW 25 MHz 35 MHz 75 MHz 60 MHZ
Radio
125 175 375 300
Channels
Wave
length
~33 cm ~33cm ~17cm ~16 cm

3. CAPACITY AND FREQUENCY RE-USE
• Limited frequencies
• Capacity: Number of frequencies in the cell
• Interference is avoided by allocating
different frequencies in adjacent cells and
reallocating these frequencies in a
honeycomb pattern
• The lower frequency re-use distance, the
more capacity is available.

4. GSM TRANSMISSION PROCESS
1.ADC
2.Segmentation
3.Speech Coding
4.Channel Coding
5. Interleaving
6.Ciphering/Encryption
7.Burst Formatting

5. LOGICAL CHANNELS
• Each time slot on the TDMA frame is a
physical channel.
• 8 physical channels per carrier.
• Control Channels
1.Broadcast (connects)
2.Common (request access)
3.Dedicated (set-up)
• Traffic Channels

6. AXE STRUCTURE
• Non- Application Modularity based AXE 105
e.g. BSC node
• Application Modularity based AXE 106
(discussed)

7. SWITCHING SYSTEM
• Mobile services Switching Center/ Visitor
Location Register
• Gateway Mobile services Switching Center
• Home Location Register
• Interworking Location Register*
• Authentication Center and Equipment
Identity Register

8. MSC
• Switching and call routing
• Charging and Service provisioning
• Communication with HLR
• Communication with VLR
• Communication with MSC
• Control of connected BSC’s
• Direct Access to Internet Services

9. VLR
• Temporary storage location for Mobile
Subscription within particular MSC service
area.
• On entering new MSC service area, It
1. Checks data base that MS is listed or not using
IMSI
2. If not listed, it requests HLR to send a copy of MS
subscription
3. HLR does so and updates itself and instructs old
VLR to delete it.
4. VLR stores this information.

10. GMSC
• It enables MSC to interrogate HLR in order
to route a mobile terminating call*.
• Find and interrogate HLR for roaming
number.
• Routes the call according to the
interrogation.

11. HLR
• It is a permanent store for person’s subscription
information e.g. IMSI, MSISDN, subscriber
supplementary services, subscriber location information
(i.e. MSC service area), subscriber authentication
information.
• Subscription database management
• Communication with the MSC’s
• Communication with GMSC’s
• Communication with AUC’s
• Communication with VLR’s/ILR’s

12. CALL SETUP
• Marking the MS as active in the VLR
• The authentication procedure
• Start Ciphering
• Equipment Identification
• Sending the B-subscriber’s number to the
network
• Checking if the subscriber has the service
“Barring of outgoing calls” activated

13. MOBILE INTELLIGENT NETWORK
• Ericsson’s MIN nodes are
• SSF: SSP acts as an interface between the call
control functions of the mobile network and the
service control functions of a Service Control
point.
• SCF: SCP contains the intelligence of the MIN
services.
• SDP
• Manages data which is used by MIN.

14. GSM NETWORK IDENTITIES
• These are the unique numbers used to locate
a mobile subscriber e.g., E.164 numbering
plan is used in PSTN/ISDN network
• Three main types of GSM Network Identities
1.Subscriber Related Identities
2.Equipment Related Identities
3.Location Related Identities

15. SUBSCRIBER RELATED IDENTITIES
• MSISDN
• IMSI
• TMSI

16. TMSI
MSC/VLR allocate
TMSI to IMSI (SIM) at
time of registration.
It helps protect
subscriber identity
Significant with
MSC/VLR
It is changed on
location update
8 digits
Subscriber Identity
confidentiality is
enforced when
subscriber is using
PLMN prevents
tracing of the mobile
IMSI
For identification over
radio path and
through the network.
Network related
subscriber info. for all
signaling in a PLMN.
Ki, A3, A8
VLR,SIM,HLR
MCC+MNC+MSIN
MSISDN
0313 5171 327
CME 20
CC+NDC (PLMN
codes) +SN = 15
dig. Max
CMS 40
NDC -> NPA

17. EQUIPMENT RELATED IDENTITIES
• IMEI
• IMEISV

18. IMEISV
Software version
helps determine
services of MS
and speech
coding
capabilities
IMEI
356384 00 610648
6
TAC + FAC + SNR + spare
bit
Uniquely identifies
MS equipment to
network
For security
procedures
stolen/unauthorize
d access to the
network
EIR functions

19. LOCATION RELATED IDENTITIES
• MSRN
• LAI
• CGI
• BSIC
• LN
• RSZI

20. MSRN
• It is assigned
during
establishment of a
call to a roaming
subscriber
• CC + NDC + SN*
LAI
• Required for routing
• Paging: MSC knows
current LA of MS
• Location updating
• MCC + MNC + LAC*
CGI
• For Identifying
individual cells
within a Location
Area
• 65536 X 65536 = 4.3
billion max. no. of
cell identities in one
operator’s network

21. BSIC
• To distinguish
b/w different
base stations
ending on
same
frequency
• NCC* + BCC*
LN
• Geographical
• For
local/regional
subscription
• CC + NDC +
LSP*
RSZI
• For regional
subscription
• CC + NDC +
ZC

22. TRAFFIC CASES
• MS in Idle mode
• MS in Active mode
1.Mobile Originated
2.Mobile Terminated

23. ATTACHING TO THE NETWORK
A. IMSI attach
On switching MS
1. It scans all RF channels tuning to the strongest
carrier and checks If it is a BCCH and tries
otherwise 2nd strongest carrier to lock with.
2. If BCCH belongs to other network only
“Emergency calls allowed”
3.List stored in a memory to directly find specific
carriers

24. ATTACHING TO THE NETWORK
MS sends IMSI attach message signifying
that it turned from detached to idle.
VLR determines if subscriber already present
If not, it asks for a copy of subscriber info
from HLR
VLR updates MS to idle
MS is informed in an acknowledgement
message

25. ATTACHING TO THE NETWORK
(CONT.)
B. Location Updating, type IMSI attach
In case Location Area has changed while MS
was detached
VLR is updated of this new location during
IMSI attach

26. ROAMING IN THE NETWORK
A. Changing cell within a Location Area
No update b/c MS knows that the new cell
belongs to the same Location Area by listening
to BCCH
B. Location Updating, same MSC/VLR area
In case MS detects a change in LAI by
listening to BCCH* and comparing with the one
stored on the SIM. It requests the network for
update via Location Updating Message

27. ROAMING IN THE NETWORK (CONT.)
MS connects via SDCCH and authentication
is performed
MSC/VLR authenticates that MS belongs to
the network or not and requests the RBS and
MS to release the signaling channel
accordingly

28. ROAMING IN THE NETWORK (CONT.)
C. Location Update, New MSC/VLR
Now if MSC/VLR do not find the MS
registered
HLR is requested to provide a copy of MS
subscription
New VLR is informed of location and status,
meanwhile HLR updates itself
Old VLR is told to remove MS subscription
VLR acknowledges to MS

29. ROAMING IN THE NETWORK (CONT.)
D. Location Updating, type Periodic
BCCH let’s know the refreshing rate
If MS misses such a registration, network
marks it detached preventing needless
paging
MS tries to register until it gets
acknowledgement

30. DETACHING FROM THE NETWORK
A. IMSI detach
On Switching off, VLR marks the
corresponding IMSI detach
HLR not informed, no acknowledgement sent
to MS
B. Implicit detach
In case of periodic registration failure

31. TRAFFIC CASES: CALL FROM AN MS
MS uses RACH to ask for Signaling channel
BTS assigns a signaling channel
(SDCCH) using AGCH
MS sends a call set-up
request to MSC/VLR via
SDCCH*
MSC/VLR forwards B-number to an
exchange in the PSTN. If answered,
connection is established.
The MSC/VLR instructs
the BTS to allocate an idle
TCH. The RBS and MS
tunes to the TCH

32. CALL SET-UP
1. MSC/VLR knows that the MS is in which LA, it sends a
paging message to the BSC.
2. The BSC distributes the paging message to different BTS in
the desired Location Area using PCH.
3. When MS detects its MSIN it sends a request for signaling
channel using RACH
4. The BSC uses AGCH to inform MS about SDCCH and
SACCH
5. SDCCH and SACCH are used for call set-up. A TCH is
allocated and SDCCH is released.
6. The MS and BTS switches to the TCH frequency and time
slot. During call, SACCH is used to keep call status.
7. FACCH checks for handover*.

33. CALL TO AN MS
• The exact location of the MS called is
unknown
• Paging is done
• MSISDN is analyzed in PSTN (a call to
MS)
• GMSC finds out which HLR does it
belong to….
• Checks if C-number call forwarding is
activated
• HLR requests MSRN from
MSC/VLR…..sent to GMSC

34. CALL TO AN MS
• GMSC analyzes MSRN and routes the call to
the MSC/VLR
• MSC/VLR knows about LA the MS is situated
in and a paging message is sent
• BSC uses PCH to page IMSI or TMSI*
• The rest is the same.

35. CELLS
• Cell planning: Site, equipment and its
configuration
• A cell may be defined as an area of radio
coverage from one BTS*/hexagon
(graphically).
• Two main Types:
1. Omni directional Cell (360
degrees/coverage)
2. Sector Cell (in a given direction only e.g.
120 or 180 degrees/ capacity)

36. CELL PLANNING PROCESS
Start (go C.W.)
Traffic and
Coverage
Analysis
Nominal Cell
Plan
Surveys
System Design
Implementation

37. OVERLAID/UNDERLAID SUBCELLS
• A way to increase traffic capacity without
building new sites
• Overlaid at lower power level (smaller
coverage area)
• Underlaid at a bit higher power level (larger
coverage area comparatively) and thus two
different frequency re-use patterns.

38. CHARGED CALLS CS1+
• Ericsson’s IN protocol supporting all
capabilities of Ericsson INAP 2.1 and ETSI
Core INAP CS1
• Applies to Originating, Terminating, and
Forwarded Calls in the HPLMN and
Terminating Calls in the VPLMN

39. ORIGINATING CHARGED CALL
• I am a Charging System Subscriber*
• I initiate a call
• My Originating IN Category Key in the VLR routes the call to SSF
• SSF collects call data and triggers CCN
• CCN selects SDP and sends call data (MNP/LBC are performed
optionally)
• SDP reserves money from the account and sends the calculated time to
CCN + announc.
• CCN tells SSF to play announcements if requested
• CCN directs SSF to setup the call and monitors it based on calculated call
time.
• Call lasts longer than the time sent to SSF ….. CCN is notified
• CCN requests another reservation from SDP

40. ORIGINATING CALL
• SDP makes new charging analysis and deducts the amount previously
reserved from the account*
• SDP reserves money for next period and forwards a new call time to CCN
• CCN passes the new call time on to SSF
• If my account balance reaches account empty limit
• In case the call lasts longer, a notification is sent to CCN
• CCN requests SDP to reserve another amount
• SDP makes a new charging analysis and updates the account
• Since no enough funds the SDP calculates call time to CCN together with
an indication
• A call cutoff warning announcement is played after a fixed time
• This time between call cutoff warning and call cutoff is excluded from new
call time and passed on to SSF

41. ORIGINATING CALL
• SSF notifies CCN that the time sent has expired
• CCN send remaining 30 seconds to SSF
• SSF tells CCN that these 30 seconds have expired
• CCN tells SSF to play call cutoff announcement and to disconnect
• SSF notifies CCN about call disconnect
• CCN sends a final report to SDP where final charging is done
• SDP also rates the total call and sends the report to CCN
• CCN tells a call release to SSF.

42. SMS IN TRIGGERED CHARGING
• An SMS is sent based on OICK and the destination address routes it to
the SSF
• The SSF collects data about the call and triggers CCN to check the
account
• The data collected in step 2, is sent in a first interrogation to where the
service data scenario of the account is checked and a a preliminary
charging analysis takes place
• No reservation of money is performed in this step
• SDP sends the result of the interrogation to CCN
• CCN sends a successful result to the SSF
• The MSC delivers the SMS to the SMS-C
• The delivery of the SMS is confirmed by the SMS-C
• The MSC/SSF triggers CCN to deduct the fee of the SMS
• CCN sends a first interrogation to SDP where the service scenario of the
account is checked

43. SMS IN CS+1
• And a preliminary charging analysis takes place
• SDP reserves money from the account and sends a successful result
together with other call data to CCN
• CCN sends a final report to the SDP
• SDP rates the total cost, deducts money from the account and sends a
final report result to CCN
• CCN sends a release operation with a successful deduct code to the
MSC/SSF

44. VOUCHER REFILL THROUGH IVR
• A refill call is initiated by me
• The call is routed to Interactive Voice Response. The IVR check if the
calling party no. is complete
• The IVR requests account information from AIR (Account Information and
Refill System)
• AIR interrogates Account Finder (AF) to get the SDP IP address
• AF returns the SDP IP address
• AIR uses the SDP IP address to request account and subscriber data
information from SDP
• SDP checks if any account updates are necessary and sends the result of
account information request back to AIR
• AIR sends the requested information to IVR e.g. preferred language

45. VOUCHER REFILL THROUGH IVR
• The IVR plays a standard welcome announcement and a menu
announcement/. The subscriber selects the menu option voucher refill and
enters the voucher activation number
• The entered activation code and the mobile number of the subscriber is
sent to AIR for verification
• AOR requests account information from SDP
• SDP sends the result of account information request back to AIR. AIR
verifies that the subscriber exists and is not barred from refill
• AIR sends the entered voucher activation code to the Voucher Server
(VS) for verification
• When the VS has verified the voucher activation code and has reserved
the voucher, it returns a response to AIR

46. VOUCHER REFILL THROUGH IVR
• AIR receives a response from the VS indicating if the verification was
successful or not. It was successful, so AIR sends a refill request to SDP
• The account balance increased in SDP database for account
• CDR processing is used and has caused an SMS/GPRS barring.
Unbarring of SMS and GPRS in the HLR is supported when a certain
account threshold has been reached. In this case through a refill
• SDP sends the result of the refill back to AIR
• The refill was successful so AIR requests the VS to set the voucher in
used state
• The VS responds with the result back to AIR
• AIR sends a response to the IVR including the account balance and an
indication if the refill was successful or not

47. VOUCHER REFILL THROUGH IVR
• A CDR including the refill data is generated
• The IVR uses the voice prompt to notify the subscriber of the result
• The subscriber releases the call

48. USSD BALANCE ENQUIRY
• I originate a USSD message with the USSD service code corresponding to
enquiry
• The MSC forwards the message to the HLR
• The HLR analyses the USSD service code and forwards the message to AIR
• AIR interrogates AF to get the SDP IP address
• AF returns the SDP IP address
• AIR uses the returned SDP IP address to request account and subscriber data
information from SDP
• SDP sends the requested account information to AIR
• AIR reformats the response into a USSD text string and send to the HLR. The
response is successful, so the appropriate successful message is sent,
otherwise a failure response with the reason for failure would have been sent
• The HLR forwards the response to the MSC and the response is displayed on
the subscriber’s handset