5 Charging Principles

12.153GPPGeneral Packet Radio Service (GPRS)GPRS ChargingRelease 1997TS

5.1 Requirements

1) Every GPRS operator collects and processes their own charging information.

2) GPRS charging shall support anonymous access to the GPRS bearer service.

3) As much as is possible the GPRS charging functions should support open interfaces for possible use in future cellular digital packet based networks.

4) It shall be possible to provide reverse charging as a subscription option. However, reverse charging may not be applicable to certain external data network protocols.

5) Every PDP context shall be assigned a unique identity number for billing purposes. (i.e. the charging id).

6) Data volumes on both the uplink and downlink direction shall be counted separately. The data volumes shall reflect the application data as precisely as possible as delivered by the user.

7) The charging mechanisms shall provide the duration of the PDP context with date and time information.

8) The GPRS operator may define a subset of the charging information specified by GPRS charging standards. This means that it shall be possible to configure the SGSN and GGSN for the CDR information generated.

9) The SGSN and GGSN are not obliged to have non-volatile memory.

This means that a GSN may loose its data when reset. The only permanent information that must be stored in a GSN is the configuration data (e.g. cell/RA definition in SGSN).

5.2 Charging Information

Charging information in the GPRS network is collected for each MS by the SGSNs and GGSNs which are serving that MS. The information that the operator uses to generate an invoice to the subscriber is operator-specific. Billing aspects, e.g., a regular fee for a fixed period, are outside the scope of the present document.

The SGSN collects charging information for each MS related with the radio network usage, while the GGSN collects charging information for each MS related with the external data network usage. Both GSNs also collect charging information on usage of the GPRS network resources.

PTP charging information is collected for the GPRS subscriber.

As a minimum, the SGSN shall collect the following charging information:

1) usage of the radio interface: the charging information shall describe the amount of data transmitted in MO and MT directions categorised with QoS and user protocols;

Charging based on user protocols (PDP context type) for sent/received data volume forms the basis for volume charging. All changes in QoS are recorded separately. This provides post-processing systems, if required, to sort out their charging relevance.

2) usage of the packet data protocol addresses: the charging information shall describe how long the MS has used the packet data protocol addresses;

Duration of PDP context is counted as the time interval from PDP Context activation to PDP Context Deactivation.

3) usage of the general GPRS resources: the charging information shall describe the usage of other GPRS-related resources and the MSs GPRS network activity (e.g., mobility management).

4) location of MS: HPLMN, VPLMN, plus optional higher-accuracy location information;

As a minimum, the GGSN shall collect the following charging information: destination and source: the charging information shall describe the destination and source addresses with a level of accuracy as defined by the GPRS operator;

5) Destination and source: the charging information shall describe the destination and source addresses with a level of accuracy as defined by the GPRS Operator.

Distinction of the data traffic to different source and destination or subnetworks may be performed by using the APN ( Access Point Name).

6) usage of the external data networks: the charging information shall describe the amount of data sent and received to and from the external data network.

External networks can be identified by the APN (access point name). The volume counts can be charged by post-processing as configured.

7) usage of the packet data protocol addresses: the charging information shall describe how long the MS has used the PDP addresses.

8) location of MS: HPLMN, VPLMN, plus optional higher-accuracy location information.

The highest accuracy location information available in GGSN is SGSN address.

5.3 Charging Data Collection Principles

Call data record generation and contents should be flexible and unnecessary redundancy in data should be avoided.

1. There are two main records types (one for the SGSN and one for the GGSN related to PDP contexts). Each PDP context generates its own record. A third record is provided for mobility management in the SGSN. The SGSN may also provide two SMS related records in case of short message delivery.

2. Optional basic location information may be included in the PDP context records.

3. Records shall only include relevant information, i.e. traffic activity since last record.

The criteria for record generation is based on real time needs, information safety (backup) and some specific events, such as expiry of the partial record timer(s), transferred data volume limit(s), inter SGSN routing area update.

4. Change of tariff period (if used) should not cause new CDRs to be sent to avoid peaks in data transfer. Instead such events should close the existing volume counters and open new ones when appropriate traffic is detected. This can be done by having a new record in the same message. It is up to the operator how often the CDRs are transferred from a GSN.

5. Both SSGN and GGSN nodes shall collect information from same chargeable sessions (PDP contexts). A unique reference (Charging ID and GGSN address) is needed to enable connection between information from several records produced from same PDP context.

5.4 Generation of Charging – ID

The concept of serving connections is different in the GSM switching network to that for the GPRS network. Therefore different mechanisms are needed to supply the billing system centres with charging information.

Circuit switched calls can be charged in one MSC (the anchor MSC) where all relevant data is available. That is guaranteed by routing all signalling information though the anchor MSC even if the traffic channel of a call is routed through another MSC due to handover.

In a GPRS network the complete PDP context handling can be switched over from an old SGSN to a new SGSN due to routing area updates with the consequence that charging records will be generated in more than one SGSN. Furthermore different data has to be collected in the SGSNs and GGSNs. So for one PDP context, charging records are needed from both the SGSN and GGSN.

The billing system shall be provided with all relevant information from the network to charge for that one activated PDP context.

During the active PDP context all records which belong to this context could normally be identified by the TID. However

– an MS can activate and deactivate PDP contexts in a very short time interval, and these PDP contexts can have the same TID (only parallel established PDP contexts have different TIDs);

– different SGSNs can be involved in the same PDP context as described above;

– the timing clocks of the GSN elements may not be fully synchronised.

Therefore it is nearly impossible for a billing post-processing system to gather the records of one PDP context only by using the IMSI, NSAPI (TID) and time.

This is solved by assigning a unique Charging-ID number (C-ID) to all records generated for that one PDP context.

The unique C-ID is generated in the GGSN when the PDP context is activated. A C-ID is generated for each activated context, so that each has a unique C-ID. The C-ID shall be transferred from the SGSN to another SGSN (following a routing area update). All PDP CDRs for each activated PDP context generated by each SGSNs and GGSNs shall therefore contain the same unique combination of the C-ID and GGSN address to permit subsequent Charging Gateway / Billing System correlation of the generated CDRs.

The GGSN address together with the C-ID are a unique identification over a long period of time in all GPRS networks.

5.5 Charging for SMS in GPRS

In GPRS the SMS transmission (MO or MT) can be done via SGSN. The SGSN shall provide an S-SMO-CDR when short message is mobile originated and an S-SMT-CDR when it is mobile terminated. In addition, also SMS-IWMSC (MO-SMS) and SMS-GMSC (MT-SMS) may provide SMS related CDRs as described in GSMĀ 12.05.

No active PDP context is required when sending or receiving short messages. If the subscriber has an active PDP context, volume counters of S-CDR are not updated due to short message delivery.

The contents of S-SMO and S-SMT CDRs are presented in tables 8 and 9.

5.6 Charging for Anonymous Access

S-CDRs and G-CDRs are generated by the SGSNs and GGSNs in the case of Anonymous Access, and separately identified in the CDRs.

The external Anonymous Access server is charged by the Operator based on the APN.

5.7 Charging Triggers – CDR Generation

The S-CDR, M-CDR G-CDR, S-SMO-CDR, and S-SMT-CDR are generated by the SGSN and GGSN to collect charging information such that they may be subsequently transferred to the Charging Gateway Function.

5.7.1 Triggers for S-CDR Charging Information Collection

An S-CDR is used to collect charging information related to the packet data information for a GPRS mobile in the SGSN.

An S-CDR shall be opened for each activated PDP context, and record details such as Record Type, Served IMSI, Sequence Number etc. Not all of the charging information to be collected is static, and other charging information is directly dependent on dynamic GPRS usage.

The subsequent sections identify the conditions for adding information to, and closing, the CDR.

5.7.1.1 Triggers for S-CDR Charging Information Addition

The "List of traffic volumes" attribute of the S-CDR consists of a set of containers which are added when specific trigger conditions are met, and identify the volume count separated for uplink and downlink traffic on encountering that trigger condition.

Table 1: Triggers for S-CDR charging information addition

Trigger Conditions

Description/Behaviour

QoS change

A change in the QoS shallresult in a "List of traffic data volumes" container being added to the CDR.

Tariff time change

On reaching the tariff time change a "List of traffic data volumes" container shall be added to the CDR.

CDR Closure

A list of "List of traffic data volumes" container shall be added to the S-CDR.

5.7.1.2 Triggers for S-CDR Closure

The S-CDR shall be closed on encountering some trigger conditions. The following table identifies which conditions are supported to permit closures of the S-CDR.

Table 2: Triggers for S-CDR closure

Closure Conditions

Description/Behaviour

End of PDP context within the SGSN

Deactivation of the PDP context in the SGSN shall result in the CDR being closed. The trigger condition covers:-

– termination of PDP context,

– SGSN change (inter-SGSN routing area update),

– any abnormal release.

Partial Record Reason

O&M reasons permit the closure of the CDR for internal reasons. The trigger condition covers:-

– data volume limit,

– time (duration) limit,

– maximum number of charging condition changes,

– management intervention.

In the event that the S-CDR is closed and the PDP context remains active, a further S-CDR shall be opened with an incremented Sequence Number.

5.7.2 Triggers for M-CDR Charging Information Collection

An M-CDR is used to collect charging information related to the mobility management of a GPRS mobile in the SGSN.

An M-CDR shall be opened for each GPRS mobile upon GPRS Attach, and record details such as Record Type, Served IMSI, Sequence Number etc. Not all of the charging information to be collected is static, and other charging information is directly dependent on GPRS mobility.

The subsequent sections identify the conditions for adding information to, and closing, the CDR.

5.7.2.1 Triggers for M-CDR Charging Information Addition

The "Change of Location" attribute of the M-CDR consists of a set of containers which are added when specific trigger conditions are met, and identify the timestamped routing area on encountering that trigger condition.

Table 3: Triggers for M-CDR Charging Information Addition

Trigger Conditions

Description/Behaviour

Mobility change

A change in the Routing area shall result in a "Change of Location" container being added to the M-CDR.

5.7.2.2 Triggers for M-CDR Closure

The M-CDR shall be closed on encountering some trigger conditions. The following table identifies which conditions are supported to permit closures of the M-CDR.

Table 4: Triggers for M-CDR closure

Closure Conditions

Description/Behaviour

End of MM context within SGSN

Deactivation of the MM context in the SGSN shall result in the CDR being closed. The trigger condition covers:-

– SGSN change (inter-SGSN routing area update),

– GPRS detach,

– any abnormal release.

Partial Record Reason

O&M reasons permit the closure of the CDR for internal reasons. The trigger condition covers:-

– time (duration) limit,

– maximum number of mobility changes, and

– Management intervention.

In the event that the M-CDR is closed and the GPRS mobile is still known to the SGSN, a further M-CDR shall be opened with an incremented Sequence Number.

5.7.3 Triggers for G-CDR Charging Information Collection

A G-CDR is used to collect charging information related to the packet data information for a GPRS mobile in the GGSN.

A G-CDR shall be opened for each activated PDP context, and record details such as Record Type, Served IMSI, Sequence Number etc. Not all of the charging information to be collected is static, and other charging information is directly dependent on dynamic GPRS usage.

The "List of traffic volumes" attribute of the G-CDR consists of a set of containers which are added following specific trigger conditions, and identify the volume count on encountering that trigger condition. The trigger conditions are as for the S-CDR (see previous section on "Triggers for S-CDR Charging Information Collection") with exception that the SGSN change does not need to close the CDR.

The "List of traffic volumes" attribute of the G-CDR consists of a set of containers which are added following specific trigger conditions, and identify the volume count on encountering that trigger condition. The trigger conditions are as for the S-CDR (see previous section on "Triggers for S-CDR Charging Information Collection") with the following exceptions:

– an SGSN change does not need to close the CDR;

– an inter-PLMN SGSN change causes the closure of a partial record.

In the event that the G-CDR is closed and the PDP context remains active, a further G-CDR is opened with an incremented Sequence Number.

5.8 Example charging scenarios

This clause contains a number of example scenarios illustrating the purpose and practical usage of the various types of records defined in the previous subclauses. These examples are by no means exhaustive.

For the purpose of these examples the following assumptions have been made:

– the CDR records are sent to a CGF;

– the generation of all of the CDR record types has been enabled.

The following conventions have been used for the figures contained within this subclause:

1) Network connections and signalling transactions are illustrated by means of solid lines and referenced by number e.g. (1).

2) Operation & Maintenance actions, such as the transfer of call records, are represented by means of dotted lines and referenced by letter e.g. (A).

5.8.1 GPRS Mobile to PDN Context

Figure 5 illustrates a simple outgoing GPRS context from a PLMN GPRS subscriber "A" to a mainframe "B" via a PDN (1).

The respective PDP context is activated in the SGSN and GGSN and PDP PDUs are routed in MO and MT direction. The SGSN shall create a S-CDR and the GGSN shall create a G-CDR for subscriber "A".

The records generated are subsequently transferred to the CGF (A). The CGF transfers the CDRs to the BS.

Figure 5: GPRS Mobile to PDN Context

5.8.2 GPRS Mobile to Mobile Context

Figure 6 illustrates a simple GPRS mobile to mobile context within the same HPLMN.

The respective A-party related PDP context is activated in the SGSN-A and the GGSN (1).

After the location of subscriber "B" is determined, the B party related PDP context is activated (2) in the SGSN-B and the GGSN and PDP PDUs are routed in MO and MT direction. The SGSN-A shall create an S-CDR and the GGSN shall create a G-CDR for subscriber A, the SGSN-B shall create a S-CDR and the GGSN shall create a G-CDR for subscriber "B".

If subscriber "A" and subscriber "B" use the same GGSN, both G-CDRs are produced at that GGSN.

If session leg (2) requires a PDP context activation the respective PDP records will contain a network initiated PDP context activation-flag.

The records generated are subsequently transferred to the CGF (A). The CGF transfers the CDRs to the BS.

Figure 6: GPRS Mobile to Mobile Context

5.8.3 PDN to GPRS Mobile Context

Figure 7 illustrates a simple incoming GPRS context from a mainframe "A" to GPRS mobile subscriber "B" via a PDN (1). After the location of subscriber "B" is determined, the PDP context is activated (2).

The GGSN receiving the PDUs shall generate a G-CDR whereas the SGSN currently serving subscriber "B" creates an S-CDR. These records contain a flag that the PDP context is activated due to network request.

The records generated are subsequently transferred to the CGF (A). The CGF transfers the CDRs to the BS.

Figure 7: PDN to GPRS Mobile Context

5.8.4 GPRS Mobile to PDN Context while roaming, GGSN in HPLMN

Figure 8 illustrates an outgoing GPRS context from a roaming GPRS mobile subscriber "A" to mainframe "B" via Boarder Gateway, inter PLMN backbone and GGSN of the HPLMN (1).

The respective a-party related PDP context is activated in the SGSN and GGSN and PDUs are routed in MO and MT direction. The SGSN shall create an S-CDR (VPLMN) and a G-CDR is generated at the used GGSN (HPLMN) for subscriber "A". From the GGSN the packets are sent via the PDN to the mainframe "B".

The records generated in the HPLMN and the VPLMN are subsequently transferred to the CGFs (A). The CGFs transfer the CDRs to the BS. (B)

Later on the records created in the VPLMN are transferred from the ADC/BC to the ADC/BC of the HPLMN via TAP procedure (C).

Figure 8: GPRS Mobile to PDN Context whilst roaming via BG