9.2 Billing Principles of WCDMA PS Domain

9.2.1  WCDMA PS Domain Billing System Architecture

Billing system of PS domain consists of three parts: SGSN/GGSN billing module, CG and billing center. There is a standard Ga interface with GTP’ protocol between SGSN/GGSN and CG. Between CG and billing center, bill data are transported with FTP/FTAM file transporting protocol.
The functions of each part are described as follows:
SGSN and GGSN: Generating billing data.
l   Collecting the billing data of SGSN and GGSN.
l   Saving for a long time and executing some pre-processing work, such as integrating and sorting.
l   Transporting the collected billing data to the billing center.
Billing System (billing center) : Processing billing data and generating final bill.
Note: In Ga interface, the billing data is called CDR. CDR that is Call Detail Record.

9.2.2  Billing Data Generated by GSN

1. Billing data

There are 5 types of CDRs generated in 3G billing:
l   M-CDR: Generated by SGSN, used for recording billing information of mobility management for mobile phone. It can be configured whether to generate this bill
or not, not required normally.
l   S-SMO-CDR: Generated by SGSN, used for recording billing information of SM origination.
l   S-SMT-CDR: Generated by SGSN, used for recording billing information of SM termination.
l   S-CDR: Generated by SGSN.
l   G-CDR: Generated by GGSN.
As for the same PDP course, there are 2 types of CDRs generated, S-CDR and G-CDR. Billing center usually calculates the final charge according to G-CDR, while S-CDR is mainly used for statistics.
CDR generated in SGSN and GGSN mainly records the following information:
l   Radio resource utilization
l   Duration
l   GPRS resource utilization
l   Originator and terminator
l   External data network utilization
l   Mobile terminal location.

2. Integration of partial CDRs

One PDP context may be corresponding to multiple partial CDRs, which are generated because of:
l   Data amount limitation
l   Time limitation
l   Changed billing condition (such as tariff change).
Because in the bill finally sent to subscriber, there is only one bill in one PDP context, all partial CDRs in one PDP context must be integrated. Integration of partial CDRs is carried out in 2 steps: The integration in the first step is carried out by CGF. This can reduce the requirement of bandwidth between CGF and billing center and alleviate the processing operation of billing center. For this reason or that, the integration in this step may be incomplete. The second step is carried out by billing center, where those CDRs that are not integrated completely in CGF will be integrated, resulting in final CDRs.
As for each PDP context, GGSN generates one exclusive C-ID. According to C-ID+GGSN address, it is possible to know whether two partial CDRs belong to the same PDP context.
As for G-CDR, all partial CDRs with the same C-ID+GGSN address must be integrated.
As for S-CDR, all partial CDRs with the same C-ID+GGSN address and +SGSN address must be integrated.

9.2.3  CGF

CGF offers a mechanism to send the billing information generated by GGSN and SGSN to designated billing center. CGF is an abstract functional concept, and CG is a specific implementation of CGF. The specific implementation of CG differs with manufacturers.
CGF must offer the following functions:
l   Collecting CDR from SGSN and GGSN
l   Providing long-time reliable storage of CDR
l   Transporting CDR to billing center.
In addition, to reduce the transmission amount between CGF and billing center, CGF should provide some integration function of partial CDRs to minimize the amount of CDRs transmitted to billing center, thus lightening the bandwidth requirement for the billing center.
CGF must offer high degree of reliability and redundancy. One GSN should be corresponding to multiple CGFs in different levels of priority. When CGF in higher priority can not process the communication with GSN, GSN will redirect it to CGF in lower priority.

9.2.4  Billing Center

Billing center has the following main functions:
l   Collecting the CDRs on CGF
l   Figuring out charge according to bill
l   Fully Integrating CDRs
l   Proofreading bills.

9.2.5  GTP' Protocol

GTP' protocol is a communication protocol between GSN and CGF and between different CGFs. It is an application layer protocol. The bottom protocol stack adopts UDP/TCP and IP.

The major functions of GTP' protocol are as follows:
l   Transporting CDRs from GSN to CGF
l   Redirecting CDRs to another CGF
l   Detecting the communication failure between CGF and GSN
l   Recommending another available CGF to GSN in case of failure in one CGF
l   Preventing duplicate CDRs.

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