Call setup to/from a GSM subscriber

Technology for executives, Moscow - 13 September 2004Author: Hans Myhre

How does the network know where the subscribers are?

The radio network consists of a large number of BTSs. Each of these is given an identity.

The BTSs are grouped in Location Areas, which also are given an identity.

Each MSC/VLR (Mobile Services Switching Center/Visitor Location Register) serves the BTSs in an number of Location Areas

The GSM phones reports to the network (VLR) when it moves from a BTS in one Location Area to a BTS in another Location Area.

How does the network know where ths subscribers are?

VLR always knows in which Location Area the GSM subscriber is located in at the moment

HLR always knows in which MSC/VLR the GSM subscriber is just now

The GSM subscriber’s telephone number tells the network to which HLR the actual GSM subscriber belongs.

Location update HLRA

MSC/VLR1

Location Area 1 info1

2 3

4

Mobile A

1) Mobile A is staying in Location Area 1. MSC/VLR1 has reported this to Mobile A’s HLR.

2),3) Mobile A moves to Location Area 2, and identifies a new Location Area info (LA2). It reports its arrival to MSC/VLR2.

4) MSC/VLR2 informs HLRA, and receives sets of Authentication Data for Mobile A.

MSC/VLR2

Location Area1

Location Area2

Mobile A

1

Call to a mobile telephone

HLRB

MSC/VLR

Calling9212345

Where is 9212345?

9212345 is in MSC/VLRB

MSC/VLRB1

2 3

4

5

1) The call is routed through the telephone network to the MSC closest to the called GSM subscriber (”Gateway” MSC)

2), 3)Gateway MSC checks with HLRB ”Where is the GSM subscriber?”

4) The call is established to the actual MSC/VLR (Visiting MSC) either directly or through the fixed or international telephone network

5) The request for mobile 9212345 is transmitted over all BTSs in the actual Location Area of the called GSM subscriber. Mobile B recognises its own identity, and ringing is generated.

Mobile B 9212345

Physical connection which must be remain connected during the whole call

Gateway MSC (GMSC)

Visiting MSC

Call to a GSM phone which has activated Unconditional Call Forward

HLRB

MSC/VLR

9212345

Where is 9212345?

9212345 shall be forwarded to number xxxxxxx

MSC/VLRB1

2 30

0) The GSM subscriber has activated the service ”Forward all calls to another number (Unconditional call Forward)”.

3) When calling GSM subscriber 9212345, the HLRB request the call to be forwarded to the desired number.

4) ”Gateway MSC” ensures call forwarding of the call, e.g. to the GSM subscriber’s Voice Mailbox.

4

0

VMS

Mobile B 9212345Gateway MSC

(GMSC)

Calling9212345

Call from a GSM phone to a fixed phone

HLRA

MSC/VLRA

Calling2212345

Request for authentication data

Authentication data

1

2 3

4

Mobile A

1) Via the radio path and the base station network a call request for Fixed subscriber B 2212345 is sent to the MSC/VLRA.

2-3) MSC/VLRA collects authentication data from HLRA (if such data has not been collected earlier) .

4) The connection is established via the closest telephone switch in the fixed telephone network, and are treated further as a normal connection in the national/international fixed telephone network.

4 Subscriber B 2212345

Call from a GSM phone to another GSM phone

HLRA

MSC/VLRA

Calling9212345

Authentication data

MSC/VLRB1

2 3

6 7

1) Via the radio path and the base station network a call request for GSM subscriber B 9212345 is sent from Mobile A to MSC/VLRA.

2-3) MSC/VLRA collects authentication data from HLRA (if such data has not been collected earlier) . Normally a bulk of such data is transferred, thus avoiding repeating authentication requests to HLRA.

4-7) MSC/VLRA requests HLRB of the actual location of GSM subscriber B, and the call is establised as described earlier.

HLRB

Mobile AMobile B 9212345

Where are 9212345?

9212345 is in MSC/VLRB

4

5

NOTE:

The four MSCs/HLRs can be everything from one integrated node to four network elements being in four different countries belonging to four different mobile operators, everything dependent of the identity and location of the two GSM mobiles.

Request for authentication data

Roaming- Call setup

- Accounting

Example: Call from Norwegian GSM mobile visiting Thailand, and calling a Malayan GSM mobile who also are visiting Thailand

Mobile A Mobile B

+6016***

HLRA

MSC/VLRA

Calling+6016**

Request of authentication data

Authentication data

1

2 3

MSC/VLRB

MSC/VLR

HLRB

Where is +6016**?+6016** is in MSC/VLRB

5

47

6

Norway

Malaysia

Thailand

National/ internationaltelephone network

+6016**

Physical connection which must be established during the whole call

Note:

Mobile A and B can either be in same or different mobile networks in Thailand.

A pays for international connection Thailand – Malaysia

B pays for international connection Malaysia - Thailand

Example: Call from Norwegian GSM mobile visiting Thailand, and calling a Malayan GSM mobile who also are visiting Thailand.

Call data flow

TnM

DiGi

DTAC

Billing

system

Mobile A (TnM)

Mobile B (DiGi)

Note:

The responsibility for paying DTAC (Thailand) is the GSM operators (TnM and DiGi)!

TnM and DiGi has the responsibility to collect money from their GSM subscribers

Clearing house X

Billing

system

Billing

system

MSC/VLR

Clearing house Roameo

MO CDR $

MT CDR $

MT TAP $ MO TAP $

MT TAP $

MO TAP $

MO TAP $

Roaming and accounting

GSM Association has defined standard format on call/billing data which shall be transferred between the operators

Eariler: All operators transferred data to all => chaos! Now: All operators utilises Data Clearing Houses

– Largest actors: MACH, Roameo, EDS– Single point of contact for data float between operators– Doesn ot change the responsibilities between the operators! (Acts as

the extended arm of the operator…)

Data Clearing House services:– Can convert data formats (local format to TAP-format, TAP2 to TAP3…)– Checks invoices and call records– Fraud control– Etc

Revenue Assurance - an exampe

Control

Switch configuration

EDR errors

CommissionsData accuracy

Tariff accuracy

Inventory management

Interconnect reconciliations

Fraud management

Prepaid controls Mediation rules

Tariff settingsInvoice

production

Least cost routing

Credit controlPayments

Acess Network

Revenue Assurance- example: SMS, TnM

HLR

MSC/TSC

SMSC

IN

EPPBilling

GatewayVAS-Rating

Geneva

NA S212 (CABS)

2

1

3 5 6 7

12 11

13

49

10

8

Production/traffic collections Charging Invoicing