3. GSM Over view – Few Points to Remember
The GSM network can be divided into three broad parts
GSM
Mobile
station
BSC
GSM MSC
HLR , VLR,
EIR, AuC
GSM
- Mobile Station (carried by the subscriber)
- Base Station Subsystem controls the radio link with the Mobile Station
- Mobile services Switching Center (MSC), performs the switching of calls between
the mobile users, and between mobile and fixed network users.
The mobile station (MS) consists of :
- Mobile equipment (the terminal) , identified by the International
Mobile Equipment Identity (IMEI)
GSM
- A smart card called the Subscriber Identity Module (SIM). The SIM
card contains the International Mobile Subscriber Identity (IMSI) used
to identify the subscriber to the system, a secret key for authentication
and some other information.
BSC
BSC
BTS
CELL
GSM
BSC
The IMEI and the IMSI are independent, thereby allowing personal
mobility. The SIM card may be protected against unauthorized use by a
password or personal identity number.
The Base Station Subsystem is composed of two parts :
- Base Transceiver Station (BTS) : Houses the radio transceivers that define
a cell and handles the radio-link protocols with the Mobile Station
- Base Station Controller (BSC) : Manages the radio resources for one or
more BTSs. It handles radio-channel setup, frequency hopping, and
handovers, The BSC is the connection between the mobile station and the
Mobile service Switching Center (MSC).
Mobile services Switching Center (MSC) : Acts like a normal switching node of the PSTN or ISDN, and additionally provides all the functionality needed to
handle a mobile subscriber, such as registration, authentication, location updating, handovers, and call routing to a roaming subscriber.
4. GSM Over view – Few Points to remember
The Home Location Register (HLR) and Visitor Location Register (VLR), together with the MSC, provide the call routing and
roaming capabilities of GSM
The Equipment Identity Register (EIR) is a database that contains a list of all valid mobile equipment on the network.
The Authentication Center (AuC) is a protected database that stores a copy of the secret key stored in each subscriber's SIM card,
which is used for authentication and encryption over the radio channel
Updating Locations are required when moving between location areas, and mobile stations are paged in the cells of their current
location area
The directory number dialed to reach a mobile subscriber is called the Mobile Subscriber ISDN (MSISDN), this number includes a
country code and a National Destination Code that identifies the subscriber's operator. The last few digits of the remaining
subscriber number identify the subscriber's HLR within the home PLMN.
GSM Services are categorized into Teleservices ( Basic Telephony, Facsimile group, Emergency calls, Telex, Short Message Services,
Fax mail and Voice mail ) and Supplementary Services ( Call Forwarding - when the called mobile is busy (CFB), unreachable
(CFNRc) or if there is no reply (CFNRy). Call forwarding can also be applied unconditionally (CFU). Call Barring – Barring all
Outgoing, Outgoing International etc..Call hold - Puts an active call on hold, Call Waiting, Advice of Charge, AoC - Provides the user
with an online charge information, Multiparty service. Possibility of establishing a multiparty conversation.
Closed User Group, CUG. - Corresponds to a group of users with limited possibilities of calling (only the people of the group and
certain numbers).
Calling Line Identification Presentation, CLIP. It supplies the called user with the ISDN of the calling user.
Calling Line Identification Restriction, CLIR. It enables the calling user to restrict the presentation.
5. Rating - Overview
GSM
MSC
Polls the CDR file from the
GSM MSC, converts the file
from ASN.1 format to
common Text format and
sends to the Billing Server.
Mobile
Switching
Center
Billing
Mobile
station
Server
Database
Mediation
ASN.1 file
GSM
MSC
The RATING module is
responsible for the calculation of
charges for the different type of
Call Detail Records (CDR)
generated in the GSM (Global
System of Mobile) switch. The
records can be for an voice call,
SMS, Data services etc.
Receives the call from
a mobile or a landline
and transfers the call
to a mobile or to a
different switch. It
generates the CDR
having the information
of the call made or
received by a mobile
subscriber.
Mobile
Station
Device
CIF (Common Internal
Format) is different for
different switches and the
information available in
those are also not uniform.
Text CDR file
Rating
Receives the file from the
mediation device, Process it
for calculating the charges of
the calls and stores the
charging information in the
Billing server database. The
billing process that
generates he bills against the
customers picks this data.
Engine
After calculating the charges
of the calls, it generates the
summary information
required by the BILLING
module that generates the
bills for the customer.
6. CDR Structure
20204162444839042024080312
024080312
0011 31624000070 2041601187885 20080829123123009920000000
31624702386
20
204162444839042
000000
31624702386
S.P. IMSI
Y
15
Dialed digits
Y
24
4
Printed dialed digits
Y
24
Service type
Y
2
6
Service code
Y
4
MSC address
Y
15
8
Network Code + LAC + Cell ID
Y
13
Filler
N
1
10
Start timestamp
Y
14
Call duration
Y
6
12
Data volume
Y
6
Call reference
Y
15
14
Seq no
N
2
15
009920
2
13
20080829123123
2
11
2041601187885
Y
9
31624000070
Record type
7
11
Length (bytes)
5
00
Mandatory
3
024080312
Description
1
024080312
No.
Filler
N
41
16
S.P. MSISDN
N
20
17
Filler
N
8
18
Premium charge
N
10
19
Premium number class event type
Premium number class
N
1
N
10
20
7. CDRs – How it looks
Record Number: This is a sequence number generated in the switch for each record.
Calling IMSI: This is the International Mobile Subscriber Identity code that is unique in the world and which is embedded in the SIM
(Subscriber Identity Module) card. If the calling party is a landline, then this number is NULL.
Calling Number: This is the mobile number or a landline number that has originated the call.
Calling IMEI: This is the International Mobile Equipment Identity that is an unique number for the handset.
Called IMSI: This is the International Mobile Subscriber Identity code of the called party. If the called party is a landline number, then this field
is NULL.
Called Number: This is the called number.
Called IMEI: This is the International Mobile Equipment Identity of the called party.
Called Number TON: This is the called number Type Of Number .The values indicates whether the called number is a STD, ISD or a local
number
Partial Output Record Number: This states whether the call is a complete call or it is a partial call. In case of complete call in ERICSSON switch,
the value is 0, else it is a running number starting from 1.
Last Partial Output Number: This states whether the call is the last call for the long duration call set or not. For calls that are partial, the value
is 0 and for the last record, the value is 1.
No of supplementary service record: This says the supplementary service record number if one supplementary call contains more than one
record.
SS Code: This is a two-digit code for the type of supplementary service used. These codes are GSM defined.
8. CDRs – How it looks
Dialed Digits: This is the digit dialed by the calling party. This number can be same as the called number or different depending on the switch.
Some switch like ERICSSON doesn’t give the value of this field.
LAC: This is the Location Area Code. In GSM, the area are divided into hexagonal LAC and Cells. The LAC and CELL are grouped into ZONES
Cell ID: This is a sub division of the LAC. One LAC may contain one or multiple Cells.
MSC ID: This is an id generated for the GSM MSC.
Out Circuit Group: This is id of the physical link between switches meant for outgoing calls.
Out Circuit ID: This is a subdivision of the Circuit Group. One circuit group may contain one or multiple circuit Ids. Some switches like
ERICSSON does not provide this field.
In Circuit Group: This is id of the physical link between switches meant for incoming calls.
In Circuit ID: Same as out circuit id. *
Basic Service Type: This is the GSM defined basic service like Telephony, Data etc. The value for this field for telephony is “00”.
Basic Service Code: This is the sub division of the Basic Service. Speech service comes under Telephony and the value is “11”.
Start Time: This is the call start time with time stamp.
Duration: This is the duration of the call.
MSRN: This is the Mobile Subscriber Roaming Number that is generated for each Mobile Terminating Call. This field identifies whether the
called person was roaming at the time of the call. This field contains a value only in the RCF (Roaming Call Forwarding) CDR.
9. Rating - Basic Modules
Take long duration
records from the long
duration table, club
them and put the
clubbed record in the
transaction table
CLUBBING
Take a block of READY TO PROCESS records,
calculate the charges of the calls, group them
by customer contract and update in the
history table as well as billing table for bill
processing.
Take the next block. Continue.
Input CDR
file from
MD
LOADING
Billing
RATING
Database
Take the file from the specific
directory, read records, do some
validation and store into the
database transaction table
MATCHING
Find out the MTC corresponding to
an MOC and vice versa, update the
MOC record with the LAC, CELL of
the MTC and vice versa and update
the status of the record to “ready to
process” .
10. Charging of CDRs
CDR TYPE
PRODUCT
WHEN GENERATED
USE
A1
MOC
This type of CDR is generated when a Mobile
Station (MS) originates a call from his mobile.
The call may be terminated to another mobile
or a landline phone.
This is a very important CDR for the mobile operator because a lot of revenue
comes from these CDR. The call is charged for Airtime and DOT depending on
where the call has terminated. Same type of CDR comes whether the call is a
local, STD or ISD call.
A2
RCF
This is a ticket generated in the GSM MSC for
all incoming calls meant for its own subscriber.
The CDR is generated irrespective of whether
the called party is roaming or not.
This is a very important CDR for identifying a roaming call and charging the
call. When the called person is roaming, the Local Airtime part of the call is
charged by this CDR and the Roaming part comes in the TAP (Transfer
Account Protocol) file.
A3
CF
This is the ticket generated in the GSM MSC
when the called party forwards the incoming
calls to some other number. The forwarded
number may be another mobile number, a
landline number or a voice mail number.
This CDR is used for charging the forwarding party for the forwarded leg of
the call. The charging for this call is usage based, meaning the charges are
directly dependent on the time for which the call was forwarded.
A4
MTC
This is generated when a mobile subscriber
receives a call. The call may be originated by
another mobile or a landline.
This is also a very important CDR for the Billing system. In the networks where
the incoming is given free, the charging of this call will not be that important,
but the main importance lies in finding the termination zone of the call which
is used for charging the corresponding Mobile Originating Call.
A5
SMSO
This is generated when a mobile subscriber
originates a SMS from his mobile.
This CDR is used for charging the mobile for the SMS that he sends from his
mobile. Generally a fixed charge is taken for one SMSO.
A7
SMSI
This is generated when a mobile subscriber
receives an SMS from any other mobile
subscriber.
This CDR is used for charging the mobile for the SMS that he receives in his
mobile. Generally a fixed charge is taken for one SMSI.
A9
SS
A0
Transit
This CDR is generated for each invocation of
This CDR is used for charging the mobile subscriber for the invocation of the
the supplementary services used by the mobile supplementary services. Some of the supplementary services are CLIP, CLIR,
subscriber.
CH, CW etc.
This CDR is generated when a call goes out of a This CDR is not used for any Rating purpose.
mobile network or comes into a mobile
network from any other network.
11. Call Scenarios ( Sample Cases )
Normal Outgoing Calls
• Mobile to Mobile ( Same Service provider )
• Mobile to Mobile ( Different Service provider )
• Mobile to PSTN
Roaming Outgoing Calls
• Mobile to Visiting Location’s Mobile number
• Mobile to Home Location Number
• Mobile to PSTN
Roaming Incoming Calls
• From Visiting Location’s Mobile number
• From Home Location Number
• From PSTN
