Prepaid subscription with roaming

• The initial driving force behind CAMEL was to givebil i h id h bilimobile users with prepaid accounts the ability to

roam to other networks

• Many operators implement their prepaid accounts ona service node with a specialized database for thea service node with a specialized database for theprepaid accounts.

• When a prepaid subscriber places a call, the MSCconsults the service node for a credit check, and the,service node will respond by granting a certainconnection time based on the remaining credit.

• If there is no credit, the service node will tell theMSC to refuse the callMSC to refuse the call.

h d l b d• Such prepaid implementations based on servicenodes or local IN systems do not allow theb ib l ll i h ksubscriber to place calls in other networks,

because there is no standardized interface thatl h SC i i i d k llets the MSC in a visited network consult aservice node in the home network

• CAMEL provides a solution because it lets theCAMEL provides a solution, because it lets the MSC in the visited network consult the SCF in the home networkthe home network.

Prepaid roaming with CAMEL.

Procedure• When the roaming subscriber tries to make an 

outgoing call in a visited network, the VLR first checks for the presence of O CSIchecks for the presence of O‐CSI.

• If O CSI is found the call requires CAMEL treatment• If O‐CSI is found, the call requires CAMEL treatment and the SSF starts the O‐BCSM. 

• The O‐CSI contains a trigger for the prepaid serviceat the Collected Info detection point.at the Collected_ Info detection point.

– The BCSM encounters this detection point after the userhas dialed the destination number, and triggers the

d hprepaid service in the SCF.

• The service logic program for the prepaid service first checks the current credit of theservice first checks the current credit of the subscriber’s prepaid account and instructs the SSF to allow the call to proceed for athe SSF to allow the call to proceed for a limited amount of time. 

– The amount of time that the SCF allows the SSF– The amount of time that the SCF allows the SSF for the call can depend on the visited network location, the destination of the call, and tariff , ,tables for the visited network. 

• The O‐BCSM starts a timer and completes the call Upon• The O‐BCSM starts a timer and completes the call. Uponexpiration of the allowed time, the SSF sends anothertrigger to the SCF.gg

• As a result, the SCF updates the credit of the prepaidaccount and checks again whether the remaining creditis sufficient.– If there is sufficient credit to continue the procedure, steps 4

d 5 dand 5 are repeated.

If h i i ffi i di i h l i Fi• If there is insufficient credit, as in the example in Figure,the SCF clears the call.

GPRS Core Network• The General Packet Radio Service (GPRS) system is used by GSMmobile phones, the most common mobile phone system in the world forcommon mobile phone system in the world, for transmitting IP packets. 

• The GPRS core network is the centralized part of the GPRS system. 

• It also provides support for WCDMA based 3Gnetworksnetworks. 

• The GPRS core network is an integrated part ofThe GPRS core network is an integrated part of the GSM network switching subsystem.

General support functions

• The GPRS core network provides mobility management, session management and transport for Internet Protocolsession management and transport for Internet Protocol packet services in GSM and WCDMA networks. 

• The core network also provides support for other additional functions such as billing and lawful interception.

• Like GSM in general, GPRS module is an open standards driven system The standardization body is the 3GPPdriven system. The standardization body is the 3GPP.

GPRS core structure

GPRS tunnelling protocol (GTP)

• GPRS tunnelling protocol is the defining IP protocol of the GPRS core network. 

• Primarily it is the protocol which allows end users of a GSM t f l t l hil ti i tGSM to move from place to place while continuing to connect to the Internet as if from one location at the Gateway GPRS Support Node (GGSN). 

• It does this by carrying the subscriber's data from the subscriber's current Serving GPRS Support Node (SGSN) to the GGSN which is handling the subscriber's session. 

3 forms of GTP are used by the GPRS core network

• GTP‐U for transfer of user data in separated tunnels for each Packet Data Protocol (PDP) context 

• GTP‐C for control reasons including: g– setup and deletion of PDP contexts

– verification of GSN reachability

– updates; e.g., as subscribers move from one SGSN to another.

• GTP for transfer of charging data from GSNs to the charging functioncharging function.

• GGSNs and SGSNs (collectively known asGGSNs and SGSNs (collectively known as GSNs) listen for GTP‐C messages on UDP port 2123 and for GTP‐U messages on port 21522123 and for GTP U messages on port 2152. 

Thi i i h i hi i l• This communication happens within a single network or may, in the case of international 

i h i i ll i llroaming, happen internationally, typically across a GPRS roaming exchange (GRX).

• The Charging Gateway Function (CGF) listens to GTP messages sent from the GSNs on TCPto GTP messages sent from the GSNs on TCP or UDP port 3386. 

• The core network sends charging information to the CGF, typically including PDP context activation times and the quantity of data which the end user has transferred. 

GPRS support nodes (GSN)

• A GSN is a network node which supports theA GSN is a network node which supports the use of GPRS in the GSM core network. 

• All GSNs should have a Gn interface and h GPRS lli lsupport the GPRS tunnelling protocol. 

• There are two key variants of the GSN, namely Gateway and Serving GPRS Support Node.y g pp

Gateway GPRS Support Node (GGSN)

• The Gateway GPRS Support Node (GGSN) is aThe Gateway GPRS Support Node (GGSN) is a main component of the GPRS network. 

• The GGSN is responsible for the interworking b h GPRS k d lbetween the GPRS network and external packet switched networks, like the Internetd X 25 kand X.25 networks.

Gateway GPRS Support Node (GGSN)• From an external network's point of view, the GGSN is a router to a sub‐network, because the GGSN ‘hides’ the GPRS infrastructure from the external network. 

Wh th GGSN i d t dd d t ifi• When the GGSN receives data addressed to a specific user, it checks if the user is active. 

• If it is, the GGSN forwards the data to the SGSN serving the mobile user, but if the mobile user is inactive, the data is discarded. 

• On the other hand mobile originated packets are ro ted to• On the other hand, mobile‐originated packets are routed to the right network by the GGSN.

Gateway GPRS Support Node (GGSN)

• The GGSN is the anchor point that enables theThe GGSN is the anchor point that enables the mobility of the user terminal in the GPRS/UMTS networks. 

• In essence, it carries out the role in GPRS equivalent to the Home Agent in Mobile IP. 

• It maintains routing necessary to tunnel theIt maintains routing necessary to tunnel the Protocol Data Units (PDUs) to the SGSN that service a particular MS (Mobile Station).

Serving GPRS Support Node (SGSN)

• A Serving GPRS Support Node (SGSN) isA Serving GPRS Support Node (SGSN) is responsible for the delivery of data packets from and to the mobile stations within its geographical g g pservice area. 

• Its tasks include packet routing and transfer, mobility management (attach/detach andmobility management (attach/detach and location management), logical link management, and authentication and charging functions. g g

Serving GPRS Support Node (SGSN)

• The location register of the SGSN storesThe location register of the SGSN stores location information (e.g., current cell, current VLR) and user profiles (e g IMSI address(es)VLR) and user profiles (e.g., IMSI, address(es) used in the packet data network) of all GPRS users registered with this SGSNusers registered with this SGSN.

Common SGSN Functions

• Detunnel GTP packets from the GGSN (downlink)

• Tunnel IP packets toward the GGSN (uplink)• Tunnel IP packets toward the GGSN (uplink)

• Carry out mobility management as Standby mode mobile moves from one Routing Area to another Routing Area

• Billing user data

PDP Context• The packet data protocol (PDP; e.g., IP, X.25, FrameRelay) context is a data structure present on both the Serving GPRS Support Node (SGSN) and theboth the Serving GPRS Support Node (SGSN) and the Gateway GPRS Support Node (GGSN) which contains the subscriber's session information when the b ib h ti isubscriber has an active session. 

• When a mobile wants to use GPRS it must first attach• When a mobile wants to use GPRS, it must first attach and then activate a PDP context. 

• This allocates a PDP context data structure in the SGSN that the subscriber is currently visiting and the GGSN 

i th b ib ' i tserving the subscriber's access point.

PDP Context Contains

• Subscriber's IP addressSubscriber s IP address

• Subscriber's IMSI

S b ib '• Subscriber's – Tunnel Endpoint ID (TEID) at the GGSN

– Tunnel Endpoint ID (TEID) at the SGSN

Interfaces in the GPRS network• Gb Interface between the base station subsystem and the SGSN theGb Interface between the base station subsystem and the SGSN the 

transmission protocol could be Frame Relay or IP. 

• Gn IP Based interface between SGSN and other SGSNs and (internal) GGSNs. DNS also shares this interface. Uses the GTP Protocol. 

• Gp IP based interface between internal SGSN and external GGSNs. Between the SGSN and the external GGSN, there is the border gateway (which is essentially a firewall). Also uses the GTP Protocol. 

• Ga The interface servers the CDRs (accounting records) which are written in the GSN and sent to the charging gateway (CG). This interface uses a GTP‐based protocol, with modifications that supports p , ppCDRs (Called GTP' or GTP prime). 

Interfaces in the GPRS networkG I f b h SGSN d h HLR M i• Gr Interface between the SGSN and the HLR. Messages going through this interface uses the MAP3 protocol. 

• Gd Interface between the SGSN and the SMS Gateway. Can use MAP1, MAP2 or MAP3.

• Gs Interface between the SGSN and the MSC (VLR). Uses the BSSAP+ protocol This interface allows paging and stationBSSAP+ protocol. This interface allows paging and station availability when it performs data transfer. When the station is attached to the GPRS network, the SGSN keeps track of which routing area (RA) the station is attached to. An RA is a part of a larger location area (LA). When a station is paged this information is used to conserve network resources. When the station performs pa PDP context, the SGSN has the exact BTS the station is using.

Interfaces in the GPRS network

• Gi IP based interface between the GGSN and a publicGi IP based interface between the GGSN and a public data network (PDN) either directly to the Internet or through a WAP gateway.

• Ge The interface between the SGSN and the SCP; uses the CAP protocol.

CAMEL architecture

HLR gsmSCFHome Network

CAPMAP

MSSGSN

gprsSSF

Visiting NetworkInterrogating Network

28Home/Interrogating/Visiting Network

Functional Architecture of GPRS/IN InterworkingInterworking

The functional entities involved in a GPRS IN system arethe following:

• HLR: The HLR stores for subscribers requiring CAMELsupport the information relevant to the currentsupport the information relevant to the currentsubscribed GPRS CAMEL Subscription Information (GPRS‐CSI).

• SGSN: The SGSN monitors on request the GPRS eventsand informs the gprsSSF of these events during servicegp gprocessing, enabling the gprsSSF to control the executionof the GPRS session or individual PDP contexts in theSGSN.

29

SGSN.

Functional Architecture of GPRS/IN InterworkingInterworking

• gsmSCF: Functional entity (FE) that containsthe CAMEL service logic. It interfaces with the

SSFgprsSSF.

SSF FE th t i t f th SGSN t th• gprsSSF: FE that interfaces the SGSN to thegsmSCF.

MAP/CAP protocol stack

• CAP has the same underlying protocol stack as MAP.

• Ge interface is first defined in 3G, didn’t appear in 2.5G.

MAP MAP CAP CAP

TCAP

SCCP

MTP3

TCAP

SCCP

MTP3

TCAP

SCCP

MTP3

TCAP

SCCP

MTP3MTP3

MTP2

L1

MTP3

MTP2

L1

MTP3

MTP2

L1

MTP3

MTP2

L1

31

L1 L1

SGSN HLRGr gprsSSF gsmSCFGe

Overall CAMEL ArchitecturegsmSCF

GPRS

CAP Interface

DialogueHandler

Internal CAP Interface

gprsSSF

Internal Interface

GTP

32

SGSN GGSNMobileStationGTP

GPRS‐CSI• The GPRS‐CSI consists of the CAMEL subscriber data for aGPRS IN subscriber, such as gsmSCF Address, Service Key,Default GPRS handling Trigger Detection Point (TDP) listDefault GPRS handling, Trigger Detection Point (TDP) list,CAMEL capability handling, and CSI state.

• When processing GPRS request for subscribers requiringIN support, the SGSN receives a GPRS‐CSI from the HLR,i di ti th SGSN t t i t ti f thindicating the SGSN to request instructions from thegprsSSF.

• According to the TDP list, the gprsSSF sets DetectionPoints (DPs) which will invoke GPRS IN service when

33

( )detected.

GPRS CAMEL DPs & Information Flows

• GPRS events may be made visible to the gsmSCF.

• The DPs are the points in association at whichthese events are detected.

• Seven DPs are defined in the CAMEL3Seven DPs are defined in the CAMEL3specifications, four for GPRS session, others forPDP Context.

34

Co te t.

Detection Points (DPs)

• GPRS events maybe made visible to gsmSCF. DPs arepoints at which these events are detected.

• DPs can be armed to notify gsmSCF GPRS event wasd d ll flencountered, and to allow gsmSCF to influence

subsequent handling of GPRS session/PDP Context.

• If DP isn’t armed, processing entity continues processingwithout gsmSCF involvement at the DPwithout gsmSCF involvement at the DP.

35

Detection Points (DPs)Detection Points (DPs) 

• 3 different types of DP:Trigger Detection Point Request (TDP R)– Trigger Detection Point‐Request (TDP‐R)

– Event Detection Point‐Request (EDP‐R)

E t D t ti P i t N tifi ti (EDP N)– Event Detection Point‐Notification (EDP‐N)

36

Detection Points (DPs)

• TDP‐R– Statically armed, initiate a CAMEL control relationship – When GPRS event is encountered, processing is suspended

• EDP R• EDP‐R– Dynamically armed within CAMEL control relationship– When GPRS event is encountered, processing is suspended and gprsSSF waits for inst from gsmSCF

• EDP N• EDP‐N– Dynamically armed within CAMEL control relationship– When GPRS event is encountered, processing is not suspended.

37

GPRS Attach/Detach State Model

• 3 DPs are defined:3 DPs are defined: – DP Attach

– DP Change of Position 

Detached

GPRS session

– DP Detach

Attach request

AD_ExceptionDetach

Attached

User- or network initiateddetach

Attach

Inter SGSNRouteing areaupdate

Intra SGSN Routeingarea update

38

Change of Position GPRS Session

GPRS PDP Context State Model

• 4 DPs are defined:– DP PDP Context

PDP ContextSetup Req

Idle

PDP Context EstDP PDP Context Establishment

– DP PDP Context 

PDP_Context_Setup

C_Exception

PDP Context Est.

PDP ContextSetup Ack.

PDP ContextDisconnection

Establishment Ack

– DP PDP Context Disconnection

PDP_Context_Established

Setup Ack.PDP Context Est. Ack.

User or networkinitiated disc.

R t i d tDisconnection

– DP Change of Position Context Routeing area update

Routeing area update

Change of Position Context

39

Change ofPosition context

Information Flows

• Fourteen information flows (IFS) are specified• Fourteen information flows (IFS) are specifiedfor information exchange between the gsmSCFand the gprsSSFand the gprsSSF.

• These IFs are new CAP messages introduced byCAMEL3.

40

Essential Differences between CAMEL3 GPRS IN and Pre‐SpecificationsSpecifications

• Interworking with GPRS is the new featurei d d b CAMEL3 h i h FEintroduced by CAMEL3, together with new FE,DPs, CAP messages etc.

• Of all these aspects, some are similar withp ,CAMEL2 or CAMEL1, such as GPRS‐CSI and thenew messages. They do not bring substantialg y geffect on existing GSM IN systems.

41

The PDP ContextThe PDP Context

ISP XSame PDP (IP) address and APN

GGSN

PDP Context X1 (APN X, IP address X, QoS1)

N X

Same PDP (IP) address and APN

Terminal SGSN

PDP Context X2 (APN X, IP address X, QoS2)

ISP Y

APN

PDP Context selectionbased on TFT (downstream)Terminal SGSN

GGSN

PDP Context Y (APN Y, IP address Y, QoS)

APN

Y

based on TFT (downstream)

ISP ZPDP Context Z (APN Z, IP address Z, QoS)

APN

Z

42

PDP CONTEXT

•When an MS attaches to the Network, the SGSN creates a Mobility Management context containing information pertaining to e.g., mobility and security for the MS.

•At PDP Context Activation (PDP - Packet Data Protocol), theAt PDP Context Activation (PDP Packet Data Protocol), the SGSN and GGSN create a PDP context, containing information about the session (e.g. IP address, QoS, routing information , etc.),

•Each Subscriber may activate several PDP Contexts towards the same or different GGSNs. •When activated towards the same GGSN, they can use the same or different IP addresses.

43

Control of GPRS Dialogue

A GPRS dialogue is a dialogue between thegprsSSF and the gsmSCF as a result ofgp gopening a new control relationshipbetween the two FEs. A GPRS IN serviceinstance is invoked within a GPRS dialogue.

44

GPRS CAMEL Scenarios

• 2 scenarios for CAMEL control of GPRS2 scenarios for CAMEL control of GPRS– Scenario 1

• Allows CAMEL control of GPRS session and of multiple PDP contexts within 1 GPRS dialogue

– Scenario 2• Allows CAMEL control of a single PDP contextsAllows CAMEL control of a single PDP contexts

• Multiple PDP contexts are controlled via multiple GPRS dialogues.

• Scenario 1 and scenario 2 are mutually exclusive.

45

• Scenario 2 is easily implemented based on existingGSM IN t b h PDP t t ld bGSM IN system, because each PDP context could behandled similar to a call.

• For the first scenario, a GPRS dialogue alwaysconsists of one GPRS session State Model andoptionally of additional multiple GPRS PDP Contextoptionally of additional multiple GPRS PDP ContextState Models related to the GPRS session StateModel.

h d l d– The various PDP Context State Models are treatedindependently of each other.

• The GSM IN system, which only supports thesequential execution of a single service logic, cannotsatisfy this scenario

46

satisfy this scenario.

GPRS CAMEL Scenario 1

• Involves Attach/Detach state model• Uses less resource (TCAP dialogue)• More complicated when implementation

SGSN / SSP SCPGPRS Dialogue

A/DSM

PDP#1

Information flow related tothe Session

PDP#2SM

PDP#1SM Information flow related to

PDP Context #1

Information flow related to

47

SMPDP Context #2

GPRS CAMEL Scenario 2

• Doesn’t involves Attach/Detach state model

• Uses more resource (TCAP dialogue)

• Simpler when implementationSGSN / SSP SCPGPRS Dialogue #1

PDP#1SM Information flow related to

PDP context #1

PDP#2SM Information flow related to

PDP context #2

48

GPRS Dialogue #2

GPRS‐CSIGPRS CSI

• GPRS‐CSI contains following information– gsmSCF Address

– Service Key

– Default GPRS Handling

– TDP List

– CAMEL Capability Handling

CSI state– CSI state

– Notification flag

– gsmSCF address list for CSI

49

gsmSCF address list for CSI

PDP Context Activation in 2 5GPDP Context Activation in 2.5G

• C1 :DP for CAMEL_GPRS_PDP_Context_Establishment

• C2 :DP for CAMEL_GPRS_PDP_Context_Establishment_Acknowledgement

2G GGSN2G SGSNBSSMS 2G-GGSN

1. Activate PDP Context Request

2G-SGSNBSSMS

C1

4. Create PDP Context Request

2. Security Functions

3. Invoke Trace

4. Create PDP Context Response

7. BSS Packet Flow Context Procedures

C2

50

9. Activate PDP Context AcceptC2

PDP Context Activation in 3GPDP Context Activation in 3G

• C1 :DP for CAMEL_GPRS_PDP_Context_Establishment.

• C2 :DP for CAMEL_GPRS_PDP_Context_Establishment_Acknowledgement

3G GGSN3G SGSNUTRANMS 3G-GGSN

1. Activate PDP Context Request

3G-SGSNUTRANMS

C1

4. Create PDP Context Response

4. Create PDP Context Request

5. Radio Access Bearer Setup

6. Invoke Trace

8. Update PDP Context Response

8. Update PDP Context Request

51 9. Activate PDP Context AcceptC2

p p

SGSN Routing Update

• A GPRS IN customer may roam to a new service area(within the same SGSN or belonging to a new SGSN)(within the same SGSN or belonging to a new SGSN)while involved in a GPRS dialogue , which is calledSGSN Routing Area Update.SGSN Routing Area Update.

• When the roaming occurs within a SGSN it is calledWhen the roaming occurs within a SGSN, it is calledIntra‐SGSN Routing Area Update, which would makeno effect on IN service execution because the gprsSSFis implemented on SGSN; otherwise it is an Inter‐SGSNRouting Area Update

52

SGSN Routing Update• The old attached GPRS session would be detached from the old SGSN and attached to the new SGSN once a GPRS 

SGSN dcustomer roams to a new SGSN, and • the activated PDP contexts would be handed over to the new SGSN and reestablished there at the same timenew SGSN and reestablished there at the same time. 

• The gsmSCF should be informed about the transitionThe gsmSCF should be informed about the transition using defined DPs and CAP messages if the customer is currently involved in a GPRS IN service instance. – The gsmSCF is responsible for continuing the service while making the transition transparent to the customer. 

53

SGSN Routing UpdateI CAMEL3 th ld GPRS di l b t th SCF d th ld• In CAMEL3 the old GPRS dialogue between the gsmSCF and the old gprsSSF (on the old SGSN) should be closed, at the same time a new GPRS dialogue between the gsmSCF and the new gprsSSF (on the 

SGSN) ld b dnew SGSN) would be opened.

• This is different from the GSM IN system where the CAMEL relationship would be maintained all through the service execution due to Anchor MSC which would be the same during a call even if adue to Anchor MSC which would be the same during a call even if a roaming occurs. 

• Though the GPRS dialogue is not the same in case of interSGSNRouting area update, the service is not disturbed from the point of 

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g p , pview of the GPRS customer involved. 

Charging• The difference between the GPRS IN system and the GSM• The difference between the GPRS IN system and the GSM IN system is that charging may be on data volume in the GPRS IN system. 

• Adding new charging function to the gsmSCF is required while maintaining the conventional service executionwhile maintaining the conventional service execution mechanism. 

R l i h i i l PDP i• Real‐time charging on simultaneous PDP contexts is virtually the most difficult problem regarding charging. 

• In particular, when the customer whose account is not obeyed to overdraw  (a PPS customer) has more than one activated PDP contexts at one time the gsmSCF should

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activated PDP contexts at one time, the gsmSCF should always ensure the account balance positive while providing duration/volume as much as possible to the customer. 

Charging

• The basic way for charging is a “request/assign”process, that is,

• prior to the actual data transfer a new activated PDP• prior to the actual data transfer, a new activated PDPcontext must ask for some duration/volume from thegsmSCF which then assigns fixed duration/volume(called a unit) to the PDP context according to related(called a unit) to the PDP context according to relatedaccount balance and the customer’s status, such as thenumber of activated PDP contexts for him.

• When the assigned threshold is reached, the PDPcontext must apply again which invokes anothercontext must apply again, which invokes another“request/assign” process.

• The PDP context must return the remainingd ti / l t th t hil fi i h d

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duration/volume to the account while finished.

CAMEL procedures for charging activities

Procedure ssf > scf scf >ssfProcedure ssf -> scf scf ->ssf

ApplyChargingGPRS -

l hApplyChargingReportGPRS -

InitialDPGPRS -

C G SContinueGPRS -

EventReportGPRS -

G SRequestReportGPRSEvent -

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CAMEL PREPAID SERVICE

• The Prepaid Service (PPS) permits the GSM/UMTS serviceprovider to collect subscriber charges before thetelephony resources are actually utilized.

• The caller's mobile charges are debited from thesubscriber's account in realtime.

• On originating of a call, the service provider may choosei f h b ib f h i i i ll d ito inform the subscriber of their remaining allocated time

prior to the call proceeding.

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CAMEL PREPAID SERVICE

• A Service Control Point (SCP) acts as a centralizeddatabase within a network and performs IN service calldatabase within a network, and performs IN service callprocessing function.

• The SCP call handling uses the TCAP protocol service thatprovides reliable transaction associations between SCPsand SSPs.

h SS i h SC i h CA L i• The SSP is the MSC in the CAMEL services.

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CAMEL PREPAID SERVICE

A typical PPS normal call flow are as follows

1. The mobile station (MS), calling party, places a call using PPS Service.

2. The MSC/SSP triggers on the calling party MSISDN, and queries the SCP. 

3. The SCP retrieves the subscriber's profile (using the Calling Party MSISDN) applies appropriate tariffsCalling Party MSISDN), applies appropriate tariffs, through tariff screening of the Called Party Number and converts the account balance into maximum call duration

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duration.

CAMEL PREPAID SERVICE

4 The SCP requests the MSC/SSP to arm events4 The SCP requests the MSC/SSP to arm eventsand to continue the call as dialed.

The SCP sends the Prepaid Call Tariff informationThe SCP sends the Prepaid Call Tariff informationto the SSP to indicate the subscribers currentaccount balance and the call's flat andaccount balance, and the call s flat andvariable rate.

CAMEL PREPAID SERVICE5. The MSC/SSP receives an answer indication, and notifies

the SCP with an event response. The SCP begins changingfor the call by decrementing the subscriber's SCP balanceinformation at each debit interval. For each debit interval

d i dan event record is generated.

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Service Call flow

CAMEL PREPAID SERVICE

• There is a need for MSC/SSP to send Terminating Information to the SCP the TCAP message flowInformation to the SCP, the TCAP message flow

Service Message Flow

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