IMS and OCS: Why carriers need a convergent charging solution

Suresh Bhandarkar, NGN IMS ForumRecent reports in the press indicate that for the first time in the US, the number of new prepaid mobile customers is more than the number of new postpaid customers. This paradigm shift in the U.S. mobile market is largely driven by the ongoing economic crisis which is driving customers to find ways to cut down on their mobile phone spending, including switching from postpaid plans to prepaid plans.

These prepaid customers are fundamentally different type of users compared to the traditional demographic from the past. These customers are ready to take advantage of all the advanced services that are available from the carriers. Unfortunately, carrier offerings to customers are driven (or rather limited) by the type of payment method selected by the customers. This difference in offerings based on payment method exists because carriers today maintain separate charging and billing systems for prepaid and postpaid services.

Prepaid services are charged using systems based on Intelligent Network (IN) whereas postpaid services are charged using an offline system wherein a call detail record (CDR) is generated when the service usage is complete and then passed on to a postpaid offline billing system for batch processing. A convergent charging solution eliminates the need to maintain such service-specific charging solutions and enables unified charging for all customers, all services and all payment methods. Using a convergent charging solution, carriers can support multiple payment methods--pre-pay, post-pay, hybrid, etc. It provides many other benefits as well.

From the carrier perspective, by using a convergent charging solution, they can better manage customer experience through service differentiation and reduce time to market for new services. Carriers can also benefit through increased ARPU, reduced operational expenditure, efficient operations and better financial risk management.

Convergent charging is not a new concept. It has been around for many years and is applicable in many convergence contexts--convergence between wireline and wireless services, between voice, data and video services, etc. But the need to implement a convergent charging solution assumes greater urgency when migrating services from legacy IN networks to next generation all-IP networks underpinned by the IP Multimedia Subsystem (IMS) architecture.

Carriers planning on upgrading their networks from 3G to 4G/LTE technology have to act quickly. 3GPP has defined an online charging system (OCS) in the IMS architecture and refined its specifications across multiple releases. The OCS is required for services where in-session charging and control are essential. It provides the charging platform for bearer, sub-system and application layers within IMS. It consists of functional entities such as the adaptation layer, real-time billing engine, policy engine, transaction processing engine and management engine. It provides real-time rating and control of services for different account types and management requirements catering to multiple types of access networks.

The OCS is designed to separate call control from charging control and further separate charging control from charging systems. This enables the OCS system to adapt to any type of networks and services thereby providing access independence; unified account management; and cross-transport mobility at the terminal-level (roaming) and user-level (IMS / SIP).

The OCS uses Diameter Credit Control protocol to interact with the service control layer and supports multiple charging modes for voice, data, video and other value-added services.

Carriers contemplating bringing in an online charging system have a choice of systems from multiple vendors--both traditional billing vendors and traditional IN vendors. Carriers should evaluate products keeping in mind the requirement of real-time processing, high availability, reliability and scalability. Most online charging systems support in-memory databases for faster processing.

When it comes to making a decision about a convergent charging solution, it is much easier during a Greenfield buildout. This is evident from a large number of OCS implementations happening in the emerging markets. Incumbent carriers have the baggage of legacy charging solutions and hence need to do more careful planning to protect their existing investments and to preserve their existing data when migrating from one system to the other.

Regardless of the complexity involved, carriers should not prolong the decision to move towards convergent charging. By implementing OCS, carriers can serve customers in a unified manner, improve customer experience and maximize return on their IMS investments through increased revenue.

Suresh Bhandarkar is the Director of Technology at Tech Mahindra, which is a member of the NGN IMS Forum's Billing/OSS & Security Working Group. He can be reached at: [email protected]