PS CHARGING – PCC, OFFLINE & ONLINE CHARGING

The main elements introduced in the PCC Architecture are:
1). OCS (Online Charging system): Allows operators to charge subscribers in real time, based on their service usage.
2).PCEF (Policy and Charging Rule Enforcement Function): The GGSN will play the PCEF role and enforce the Polices sent from PCRF, perform online charging to OCS by using Gy interface.
3). PCRF (Policy and Charging Rule Function): Manages Services, and QoS policies. PCC rules are stored in PCRF and sent to PCEF through Gx interface for enforcement.
4).SPR (Subscription Profile Repository): The SPR contains all subscriber/subscription related information needed for subscription-based policies and bearer level charging rules by the PCRF.

PREPAID,POSTPAID, ONLINE & OFFLINE CHARGING -THE CONCEPTS.

1). Prepaid & Postpaid: From a billing perspective.
Prepaid: you pay first, when you consume, deduct amount from your credit.
Postpaid: you consume first, then pay the bill.
2). Online & Offline Charging: From a technical perspective.
Online Charging: accumulate your usage and rate it in real time, deduct from your account or record your bill.
Offline Charging: collect your CDR and post process your CDR to get the accumulative usage, then deduct from your account or record your bill.

From the above, we can see that there is no obvious relationship between Prepaid & Postpaid and Online Charging &Offline Charging.
However, the general requirement of Prepaid services is to perform charging in real time to avoid charging leakage.Therefore online charging is suitable for prepaid services.

In the PCC architecture:

1). GGSN connect to PCRF via Gx interface. Used to dispatch policy and charging rules to GGSN.

2). GGSN connect to OCS via Gy interface.  Used for Online Charging.

The Gx and Gy interfaces are all based on Diameter interface, by using DCCA (Diameter Credit-Control Application) protocol. 

3). The GGSN connect to CG via Gz interface to send the G-CDR, actually the Gz interface is same as Ga interface before 3GPP R7. Used for Offline Charging.

CALL FLOW IN SUMMARY - FOCUS ON PCC:

• The GGSN receives a PDP context activation request from the MS/UE.
• The GGSN sends a CCR message to instruct the PCRF to create PDP. The message carries the context ID (Bearer-Identifier) and user information such as the IP address, IMSI, and QoS information about the UE. In the message, the value of CC-Request-Type AVP is INITIAL_REQUEST and that of Bearer-Operation AVP is ESTABLISHMENT.
• The PCRF queries the subscription information about the UE and make decisions and generates policy rules. The PCRF sends the policy rules to the GGSN. The policy rules includes information such as QoS and charging information (Online or Offline Charging).    
•      The GGSN receives the policy rules and enforces the policy. If the charging information indicates online charging. Then GGSN generate CCR message to OCS. If the charging information indicate offline charging, then the GGSN will generate CDR directly, and send to CG. Will not talk to OCS anymore.
•   The OCS sends the CCA response to the GGSN, indicating the quota allocates to the user. Before the quota exhaust, the GGSN will generate CCR update message to OCS to update the quota.
•  The browsing service continues until the subscriber has no more Volume/ Credit for Prepaid or when their usage limit is hit for Postpaid.

NOTE:

The online & offline charging information can be configured in GGSN based on Global Setting or APN. Therefore,without the PCRF, the GGSN  can also perform online charging.

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