Towards 5G Network - Basics of Telecom Online Charging

Basics of Telecom Online Charging

This is the first article in the series of Telecom Charging: Journey towards 5G Network and it shares the basics of Online Charging in a Telecom Network. 

The idea of creating a series of articles on this topic is to make sure we are not discussing all the complexities around 5G Charging in a single article and making it a step-by-step guide for the readers.

Telecom Charging

Now coming to the Basics of Online Charging: Whenever a Customer uses services (e.g. data, voice, video, etc.) over a Telco network, Operators allow service usage by using an Online Charging mechanism for the upfront balance check & reservation. In other words, the Customer cannot use the service if she doesn't have the required balance or credit limit.

Let's see how the Online Charging mechanism works on a high level:-

• Receive charging event information from the network.
• Perform service authorization and credit control.
• Measures the event & performs the required balance reservations.
• Applying balance impacts to the measured event based on rating & pricing configuration.
• Stores the rated events in the database and later billing process picks up these usage events to calculate the bill amount.

(Click to enlarge) Online Charging, Rating, and Billing in a nutshell

The basic idea of Online Charging is to allow access to services only when the Customer has a sufficient balance or credit limit in her account. Charging can be Session-based (like Voice calls or YouTube browsing) or it can be Event-based (like SMS or file transfer over Instant Messaging).

Balance reservations occur from the Customer's account balance during Session-based charging until the session gets terminated, after which the actual balance impact happens & charges for the session are reflected against the customer. 

For Event-based charging, balance check & deduction happens before the usage event is successfully processed in the network. Event-based charging can also happen via balance reservation, followed by a balance deduction after successful service delivery. It is up to the Operators how they want to charge the network events for different kinds of services.

Let's understand the Online Charging mechanism as per the 3GPP standard:-

(Click to enlarge) Online Charging Architecture as per 3GPP

The above figure explains the Online Charging architecture as per the 3GPP standard.

CTF (Charging Trigger Function): This is the node in the network that generates charging triggers whenever a Customer uses services. Let's suppose, a Customer wants to use data sessions over a 4G network. Here, PGW (PDN Gateway) node acts as a CTF and it generates charging triggers (refer to Fig. 3 for more detail).

OCS (Online Charging Server/System): OCS is the Charging Engine and it's an integral part of the Operator's OSS/BSS stack. It receives charging triggers from CTF and performs operations like service authorization, balance check, balance reservations, credit control, balance deductions based on service usage & pricing configuration, generation of rated events in the database, notifications to customer/policy server, etc.

Logically, OCS is divided into OCF (Online Charging Function), ABMF (Account Balance Management Function), and RF (Rating Function). Together, they perform the Online Charging operations as explained in the above para.

Billing System: This is the core system of the Operators' BSS stack. It consumes the rated events stored in the database and adds up the Usage charges against the Customer's bill amount. During Bill Run, charges like monthly recurring charges, one-time charges, cancellation charges, etc. are processed along with usage charges. Other activities like billing time discounts, adjustments, settlements, taxes, etc. are also considered during the Bill Run. Once the Bill is finalized, it becomes ready to accept the Payments against the Invoice.

Now coming to the Basics of Online Charging: Whenever a Customer uses services (e.g. data, voice, video, etc.) over a Telco network, Operators allow service usage by using an Online Charging mechanism for the upfront balance check & reservation. In other words, the Customer cannot use the service if she doesn't have the required balance or credit limit.

As you have understood the basics of different elements of Online Charging from Network & OSS/BSS perspectives, now refer to a simplified version of Online Charging Server in a 4G Network:-

(Click to enlarge) OCS in a 4G Network

OCS is also tightly coupled with PCRF (Policy and Charging Rules Function) so that more control can be levied during online service delivery. The mentioned Charging elements communicate via Diameter interfaces for Credit-control & AAA activities. More details about PCRF, CTF, and OCS integrations can be covered in a separate article.

Please refer Towards 5G Network - Basics of Telecom Offline Charging to read the second article of this series which talks about the basics of Offline Charging.

In the upcoming articles on this series, details about 5G Network Converged Charging will be covered. 
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Glossary: UE (User Equipment), MME (Mobility Management Entity), SGW (Serving Gateway), PGW (PDN Gateway), HSS (Home Subscriber Server), AAA (Authentication, Authorization & Accounting Server), EPC (Evolved Packet Core), PCRF (Policy & Charging Rules Function), CN (Core Network), 3GPP (3rd Generation Partnership Project), BSS (Business Support System), OSS (Operations Support System)

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