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SUMMARY

  1. Top of page
  2. SUMMARY
  3. 1 INTRODUCTION
  4. 2 CHARGING-RELATED TERMINOLOGY AND PROCESS INTERACTIONS
  5. 3 ONLINE CHARGING IN 3GPP STANDARDS
  6. 4 STATE OF THE ART IN ONLINE CHARGING RESEARCH
  7. 5 DISCUSSION AND OPEN ISSUES
  8. 6 CONCLUSION
  9. ACKNOWLEDGEMENTS
  10. REFERENCES
  11. Biographies

Modern charging systems routinely apply the user, network, and service-related information while performing online charging. Compared, however, to all the information available to and used in managing the network as a whole, the charging systems only use a limited subset. This work is motivated by the challenge to identify which information is used, and how it is used in online charging-related processes, and also to explore whether it could be utilized ‘better’ or ‘smarter’ to improve future online charging systems functionality. We do not attempt to predict which information will be utilized in such systems and for what purpose, but instead summarize the open issues in view of the emerging trend of exploiting the user, network and service-related information in service provisioning. We focus on the most recent 3GPP standards and relevant research papers, and propose three key aspects of online charging, with respect to information utilization: (a) signaling aspect, (b) inter-domain aspect, and (c) service- and component-based aspect. We present a state-of-the-art review by grouping the works found in the literature based on the aspects they are associated with, and compare them based on the proposed comparison criteria. The discussion presented at the end of the paper indicates three common open issues, namely: (1) lack of common charging information specification and structure; (2) lack of mechanisms for information sharing among stakeholders in the service delivery process; and (3) lack of a common framework for sharing information while protecting user privacy. Copyright © 2012 John Wiley & Sons, Ltd.

1 INTRODUCTION

  1. Top of page
  2. SUMMARY
  3. 1 INTRODUCTION
  4. 2 CHARGING-RELATED TERMINOLOGY AND PROCESS INTERACTIONS
  5. 3 ONLINE CHARGING IN 3GPP STANDARDS
  6. 4 STATE OF THE ART IN ONLINE CHARGING RESEARCH
  7. 5 DISCUSSION AND OPEN ISSUES
  8. 6 CONCLUSION
  9. ACKNOWLEDGEMENTS
  10. REFERENCES
  11. Biographies

Modern telecommunication systems continuously track and regularly access a plethora of information regarding users and services [1]. A subset of this information is, as a rule, used by online charging systems (e.g. available credit, user subscriptions), but there is much more ‘other’ information present in the system that is service-related, contextual, or even implicit, and not necessarily taken into account (e.g. user location, user preferences and service usage habits, service variants, information about other users nearby). This paper is motivated by the challenge to determine to what extent this information is (or could) be used for online charging (i.e. a real-time service cost calculation) in the Next Generation Network as defined by the Third Generation Partnership Project (3GPP) and what are (or could be) the benefits thereof.

We believe that by fully utilizing this ‘other’ information, online charging could be functionally improved. As a simple example, let us consider a mobile smartphone user who has an available credit equivalent to 50 minutes of watching a television program provided by a third-party video-streaming service provider. The user initiates a request for a soccer game recording (which normally lasts 90 minutes). A ‘traditional’ online charging system would allow the initiation of the service, and terminate it after 50 minutes, once the credit runs out. If, however, a ‘smart(er)’ online charging system, having ‘knowledge’ about the maximum of 50 minutes play time, had somehow shared that information with the streaming service provider (assuming prior user's permission to disclose this information), the streaming service provider could have offered a shorter version of the game to the user, say, containing only the most interesting parts, resulting in better user satisfaction.

The contribution of this paper is twofold. First, we review the 3GPP standards, focusing on the 3GPP Online Charging System (Releases 10 and 11) and other related 3GPP architectures, and relevant research work in the area of online charging in 3GPP networks, with respect to the three charging aspects, namely: (a) the signaling aspect; (b) the inter-domain aspect; and (c) the service- and component-based charging aspect. Second, we identify open issues in information utilization and discuss potential improvements for future charging systems.

To the best of our knowledge, no other charging-related survey studies the input information for charging and its utilization. Furthermore, many available online charging-related overview papers are somewhat dated [2-7] and do not cover current achievements in charging. In a more recent comprehensive survey by Kuhne et al. [8], general requirements for future charging systems are identified, but information utilization is not particularly discussed. In this respect, our paper nicely complements their work.

The rest of the paper is structured as follows. In Section 2, charging-related terminology is explained in more detail, including related processes, business relationships, and charging data used. Section 3 elaborates on 3GPP Releases 10 and 11 Online Charging System, including charging requirements and charging information used, and provides a background of charging-related architectures and protocols specified by the Internet Engineering Task Force (IETF) and used in 3GPP. Section 4 explains contributions in the area of online charging of recent research projects, grouped into three aspects listed above. Finally, in Section 5 we discuss the information used for charging in all examined charging systems, and identify open issues and potential improvements in information utilization.

2 CHARGING-RELATED TERMINOLOGY AND PROCESS INTERACTIONS

  1. Top of page
  2. SUMMARY
  3. 1 INTRODUCTION
  4. 2 CHARGING-RELATED TERMINOLOGY AND PROCESS INTERACTIONS
  5. 3 ONLINE CHARGING IN 3GPP STANDARDS
  6. 4 STATE OF THE ART IN ONLINE CHARGING RESEARCH
  7. 5 DISCUSSION AND OPEN ISSUES
  8. 6 CONCLUSION
  9. ACKNOWLEDGEMENTS
  10. REFERENCES
  11. Biographies

The charging-related terminology found in the literature and standards documents is unfortunately not harmonized. For the benefit of the reader and to facilitate understanding of terms to newcomers in the field, we state the terms and definitions adopted for use in this paper and note the differences between terms as used in other sources where applicable. (An experienced reader may skip this section and move on to Section 3.) There are four terms that refer to the most important charging-related processes (Figure 1). Metering refers to a process that collects information about resource usage at a particular network element. Accounting uses metering logs to aggregate information about resource usage from different network elements. Charging refers to a process of calculating a cost, expressed in units acceptable for network management processes, of a given service consumption, by using the accounting information. Within the billing process, the charges are collected, and the service payment procedures are managed towards the party that consumed the service(s).

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Figure 1. Charging and charging-related processes

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Depending on the payment type agreed, prepaid (also known as prepay) or postpaid (also known as postpay) billing method may be used. These methods are defined as charging options by Kurtansky and Stiller [4]. In prepaid billing, a certain amount of money must be deposited in advance to an account, which will then be spent in accordance with service usage. In postpaid billing, an account is debited as the services are being used, but the payment is performed only after a certain time interval has expired (e.g. a month) by issuing a bill that aggregates the costs of all services that have been used in the given interval.

The works in the literature differentiate between charging models and charging mechanisms. A charging model defines a list of criteria that will be applied in order to calculate a monetary cost of a used service, as well as a (list of) price(s), also known as tariffs, of a defined service unit that will be used in cost calculation, by applying the given criteria. A volume-based charging model uses the amount of data transferred as a criterion, and for example, defines a price of 1$ per MB of transferred data. Other commonly used charging models are the time-based model, which uses the time spent for service usage as a criterion, and the content-based model, which uses the service content of the provided service as a criterion [9]. A survey of the state-of-the-art charging models (also known as pricing schemes) is given by Gizelis and Vergados [10].

A charging mechanism, which may be either offline or online [2], identifies whether the service is charged after, or while being provisioned, respectively.

An offline charging mechanism separates charging and service provisioning in time: charging is requested for the particular service as the service is started, but only accounting and metering processes are initiated. After the service is terminated, charging processes the accounting data, calculates the final service cost, and forwards it to the billing domain.

An online charging mechanism is performed in real time in accordance with service provisioning, requiring accounting and metering to be performed in real time as well. The main advantage of this approach is the ability to control the service cost at each point of the service session. Additionally, this enables introduction of service authorization mechanisms, i.e. granting or denying particular service components. Finally, online charging process can make decisions regarding service termination if certain conditions are met. From now on, if not otherwise noted, the term charging refers to online charging.

2.1 Differences in terminology in standards

The IETF and the 3GPP define the terms metering, accounting, charging, and billing differently. For what is termed charging by 3GPP and in this paper, IETF uses the term rating [11]. IETF RFC 2975 [12] defines other processes in the same way as in this paper. 3GPP defines billing mainly as explained here, but refers to accounting as the process of ‘apportioning charges between the parties involved in service delivery’, TS 21.905 [13]. Furthermore, the term charging is used by 3GPP in a broader sense, as an ‘umbrella term’ for all involved sub-processes [8]. The difference in terminology is explained in more detail in the literature [4-6].

2.2 Roles and stakeholders

Two key roles played by the entities which participate in charging and charging-related processes include a user and a provider, as shown in Figure 2. The user is the entity that uses/receives the given service and is being charged for using it. The provider is the entity that provides the given service. In a general case, the role of the provider may be played by different stakeholders, e.g. a Mobile Network Operator, an Internet Service Provider, or by a third-party service provider. From now on, the term service provider will be used as a general reference to any of these stakeholders.

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Figure 2. Key roles and processes

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The role of the user may be played either by a natural person that receives a service (in that case, it is usually an end user), or by another service provider. The primary service provider (PSP) is a service provider that has a business relationship established with an end user, usually by means of a service-level agreement (SLA) [14], which is responsible for overall provision and control of services to the user (Figure 3). In 3GPP, a network domain under the control of the primary service provider is also known as a home environment [13]. From the point of view of the end user, the PSP is the only stakeholder that is responsible for charging [15]. In a case when a service is provisioned by using resources or services of additional sub-providers, inter-domain charging procedures are needed. Various aspects of the charging responsibility reassignment between the service providers are discussed elsewhere [16-19], including the problems of charging data correlation and charging process coordination.

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Figure 3. Charging from the end user's point of view

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Recent standardization and research efforts focus on policy-based charging systems [20-22]. As defined by Westerinen et al. [23], a policy is ‘(1) a definite goal, course or method of action to guide and determine present and future decisions; (2) a set of rules to administer, manage, and control access to network resources'. By using policy-based systems, it is possible to exchange messages containing aggregated management information (including the charging information) and thus to reduce the amount of signaling.

2.3 Information used in charging

The information contained in the charging data may be grouped into three groups, namely: user profile, charging model, and accounting record, as shown in Figure 4.

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Figure 4. Input and output data used in charging

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The user profile is a collection of information related to the end user, maintained by the PSP. In this paper we consider only a fraction of the user profile, which contains the information used in charging. Such parameters include, for example, the amount of credit that is available for spending, a list of services the end user is allowed to use, potential credit limits for certain services, and references to the used charging models. The user profile is updated during and/or after the charging process, e.g. by modifying counters or deducting the credit spent from the overall available credit.

The charging model is a collection of information containing: (1) the rules that determine how to calculate a service cost and (2) the tariffs used.

The accounting record (generated by the accounting process) is a collection of usage data records (generated by the metering process) aggregated from different network elements. The accounting records are typically used in offline charging for post-processing purposes, and are in that case called charging data records. In a broader sense, they are also used by online charging as input parameters for real-time service cost calculation.

2.4 Interaction between processes, stakeholders, and charging information

Figure 5 illustrates the terminology and relationships between the processes, stakeholders, and data. For the sake of simplicity, it is assumed that no sub-providers are included in the service provisioning.

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Figure 5. Charging-related processes, stakeholders, and data

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Since the billing, charging, accounting, and metering processes are carried out within the PSP's home environment, the PSP has full control over them. The end user, situated in the home environment, has an SLA established with the PSP, which specifies the terms of service use, including the content of the user profile, as well as the charging mechanism, charging model, and the billing method.

Once the service is initiated, the charging process retrieves the user profile and the agreed charging model, uses both of them as input charging data, and starts. Accounting and metering are used accordingly, for generating the accounting records and the usage data records, respectively. The accounting record is then used to calculate the cost of the service at a certain point of the service session, which enables the charging process to allow or deny further service usage. During or after the charging process, the user profile is updated accordingly.

3 ONLINE CHARGING IN 3GPP STANDARDS

  1. Top of page
  2. SUMMARY
  3. 1 INTRODUCTION
  4. 2 CHARGING-RELATED TERMINOLOGY AND PROCESS INTERACTIONS
  5. 3 ONLINE CHARGING IN 3GPP STANDARDS
  6. 4 STATE OF THE ART IN ONLINE CHARGING RESEARCH
  7. 5 DISCUSSION AND OPEN ISSUES
  8. 6 CONCLUSION
  9. ACKNOWLEDGEMENTS
  10. REFERENCES
  11. Biographies

Online charging is covered by a number of 3GPP documents. The 3GPP Technical Specification (TS) 32.240 [24] serves as an umbrella document for charging in 3GPP (both offline and online), and contains the list and the structure of other 3GPP charging specifications. The specifications are categorized into two sets: the first set covers specifications for different charging-related 3GPP architectures, and the second set covers charging parameters, syntax descriptions, and interactions within the network.

The specifications are grouped into stable releases. This paper is mostly based on 3GPP Release 10, but it also explains selected features covered by Release 11. Release 11 (Stage 3) has been completed in September 2012 (functional freeze date).

The end user's home network operator (i.e. the PSP) is defined by the 3GPP as a central entity that is responsible for charging the end user. If the PSP has roaming agreements with other network operators, the end user is able to use their network infrastructure while roaming. In this case, additional inter-domain charging procedures take place. Additionally, the end user may consume services provided by third-party content and application providers.

The general charging requirements and principles [25] for the 3GPP architecture are listed in TS 22.115 [26]. Important requirements for end users are summarized as follows:

  • The end users must be aware of all charges that are related to them. This includes the ability of informing the end users of the charges that are about to happen, and enabling the end users to accept or reject the service, regarding the calculated amount of charge.
  • There must exist an ability to charge separately each media component within a single session, and to perform charging according to the network resources used.
  • There must exist an ability to charge the end users depending on their location and presence, i.e. the context the service is being consumed in.
  • An end user must be able to use the same charging model when roaming, the same way as if the user were in the home environment.

A high-level view of 3GPP online charging is given in Figure 6. Online charging procedures may be used at three different architectural levels, namely the bearer, the subsystem, and the service levels. Each level provides different charging-related functionality. At the bearer level, charging is able to control network resources used for the service delivery (in both the circuit switched and the packet switched domain). At the subsystem level, i.e. the level where the IP Multimedia Subsystem (IMS) is situated [27], charging process controls service sessions and is able to allow/deny session initiation as a whole. At the service level, specific services may be charged (e.g. a Multimedia Service or MMS) as well as particular service content (e.g. movies or music).

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Figure 6. Operating levels of online charging in 3GPP

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An online charging system (OCS), defined in TS 32.296 [28], is a functional architecture that provides support for all three levels of online charging in 3GPP. The OCS also serves the Policy and Charging Control (PCC) architecture [21], a framework that logically connects processes at the subsystem level with the processes at the bearer level. The OCS and the charging-related functionalities of the PCC architecture are elaborated in more detail later in the paper.

3.1 Background in IETF standards

The generic AAA architecture, specified in RFC 2903 [29], represents a framework that incorporates user authentication, service authorization, and accounting procedures, aiming to be used in an Internet environment. Published in 2000, the architecture is still used (partially or entirely) as a basis in building many state-of-the-art charging architectures, including the 3GPP OCS. Telecom-specific requirements, important for the 3GPP OCS, adopt two key ideas from the AAA architecture: (1) accounting (and consequently, charging) is carried out at a central point in the network; and (2) accounting (and consequently, charging) is incorporated within the service authorization procedures.

A more detailed view on accounting procedures in IETF can be found in RFC 3334 [20], where policy-based accounting is specified for the generic AAA architecture. The document defines a three-layer reference model incorporating the layers of metering, accounting, and charging policies. However, only accounting policies are considered in more detail, and are in this case used to instruct a remote network entity on how to collect accounting data. The concept of policies is accepted in 3GPP for network resource management (carried out by the PCC architecture) and for service authorization procedures (carried out by the OCS).

The main signaling protocol used in 3GPP OCS for transmission of AAA information is the Diameter protocol, specified in RFC 3688 [30], a successor of the RADIUS protocol, specified in RFC 2865 [31]. The Diameter Base Protocol [30] contains basic Diameter messages for generic AAA functionality, as well as message content specification, stored in data structures called attribute–value pairs (AVP). All other AAA procedures, which are specific for a particular service or a network function that uses Diameter, can be defined by Diameter applications. A notable application is the Diameter Credit-Control Application, specified in RFC 4006 [11], used by the 3GPP OCS for cost control during the service session; and there are also several 3GPP interface-specific applications used for signaling purposes between 3GPP functions.

Credit control is a mechanism used for real-time interaction between the OCS and the service provider, to control and/or monitor all charges related to the service usage. The mechanism can be used for both event-based and session-based services in online charging scenarios. Figure 7 depicts a simple credit-control scenario between a service provider and a charging system for a single service session as an example. The initial Diameter credit-control message Start is sent by the service provider at the service session start time. The message is sent to the OCS, containing, among other parameters, the number of requested service units r1, the service identifier s1, and the user identifier u1. The OCS then grants a certain number of service units g1, by sending the Diameter authentication message Auth. After receiving the Auth message, the service provider can deliver the granted number of service units to the user. Once the granted units are spent, the Diameter Interim message is sent, requesting r2 new units. The Interim message also contains the number of used units b1, where b1 ≤ g1, and the service and user identifiers. A new number of service units g2 is granted, allowing the service provider to continue with the service provisioning. At the end of the session, a final Diameter Stop message is sent. The Stop message contains the number of used units b2, where b2 ≤ g2, and no further service units are requested.

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Figure 7. Credit control example

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The credit-control mechanism takes place between the function in the end user's PSP's home environment that provides the given service (usually a gateway node providing a network access service), and the PSP's OCS. Considering the charging-related information stored in the end user's User Profile (particularly the available credit information), the OCS grants or denies service usage by using the credit-control mechanism.

Future Diameter extensions are being developed within the IETF Diameter Maintenance and Extensions (DIME) work group. The overview of their work is available on the DIME WG website [32].

3.2 3GPP Online charging system

The OCS [28], illustrated in Figure 8, stands as a central function for online charging within the 3GPP. It consists of four functional elements: an Online Charging Function (OCF), an Account Balance Management Function (ABMF), a Rating Function (RF), and a Charging Gateway Function (CGF). The OCS has standardized interfaces to other functions/nodes across the three levels of charging, e.g. the Mobile Switching Center (MSC), the Serving GPRS Support Node (SGSN), IMS Application Servers (AS), and PCC functions, as well as the Recharging Server. A summary of OCS functions and the interaction between the OCS and the PCC functions is given next. (For a comprehensive tutorial on online charging at the subsystem level, the reader is referred to Kuhrie et al. [2].)

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Figure 8. The 3GPP Online Charging System and selected functions that use it (Release 11). Sy interface is not included in Release 10

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The ABMF contains information about the end user's credit as well as the counters associated with the end user. A counter is an aggregation of units of service usage (or monetary units), which may be related to the end user contractual terms with the PSP (e.g. the number of free-of-charge voice call minutes per month). As the service is being used, the value of counters associated with the service is updated accordingly. By using counters, it is possible to establish an end-user-specific loyalty program providing such benefits as service price discounts and bonuses. The ABMF also has an interface to the Recharging Server, which enables buying more credits.

The RF contains the prices of all available services. It provides the OCF with information about the price of a certain service unit considering the charging model used, or, with information about the price of a given service session, considering the price of the service unit, charging model used, and the number of service units consumed. Additionally, the RF maintains the end-user context information, which in this architecture is defined as a list of currently active services per end user. By using the context information, the RF is able to perform a correlation process, i.e. a process in which service prices applied to the end user may be modified depending on other active services in progress.

The OCF is a central OCS function that interacts with other functions in the network and performs online charging. Charging may be performed in an event-based manner (i.e. only once), or in a session-based manner (i.e. continuously during the session), depending on the service. The OCF is connected with the ABMF and the RF by using Diameter-based Rc and Re reference points, respectively. These interfaces are used for retrieval and/or update of the end user's available credit, service prices, possible counters, etc.

As mentioned earlier, OCS supports all three levels of online charging: the bearer, the subsystem, and the service levels. The bearer-level procedures are responsible for establishment, modification, and termination of IP flows used for service delivery, TS 23.125 [33]. At this level, it is possible not only to allow/deny service usage by using the OCS functionalities, but also to reserve network resources, in order to achieve a certain level of Quality of Service (QoS) for each IP flow within the service session, according to the end user's subscription profile.

To support these mechanisms, the 3GPP defines the PCC architecture, TS 23.203 [21], which provides a framework for mapping session-related data in the signaling layer to the network-related data in the connectivity layer, i.e. the QoS data and the charging data. The session-related data are received from the Application Function, situated at the subsystem level. Each media flow is given a policy for QoS assurance and charging, called a PCC rule. The Policy and Charging Rules Function (PCRF) makes policy decisions and creates the PCC rules. When creating PCC rules, a decision is made on how a certain media flow is treated in the packet switched network, depending on the user-specific information (e.g. allowed QoS), retrieved from the Subscription Profile Repository (SPR). The Policy and Charging Enforcement Function (PCEF) enforces the policies by creating, modifying, and deleting IP flows the given service consists of and assures the required QoS for each flow. Starting with Release 8, additional functions are introduced in the PCC architecture to support mobility and roaming. These functions are well explained by Balbas et al. [34], and the need for such support has been previously discussed by Kueh and Wilson [35].

The PCC architecture has an interface to the OCS by using the Gy reference point (between the PCEF and the OCF), and, as of Release 11, the Sy reference point (between the PCRF and the OCS).

The Gy reference point implements the IETF's Credit-Control application [11] to allow online charging of each IP flow used during the service session, as explained in Section 3.1.

The Sy reference point is used to utilize the counters that are maintained at the OCS, the status of which may influence the policy decision making process at the PCRF. By using Sy, PCRF is able to access the information about the end user's spending limits for a certain service, stored at the OCS. If a limit threshold is reached, the PCRF is able to (for example) adjust the QoS. Note that such an interface has been previously proposed by Grgic et al. [36]. A possible evolution path for the PCC architecture with respect to fixed-mobile convergence has been discussed by Ouellette et al. [37].

3.3 Charging information used in 3GPP OCS

Information used in 3GPP online charging may be grouped into two groups: (1) information provided by network functions to the OCS when requesting online charging, including the service level, the subsystem level, and the bearer level functions; and (2) the information already stored at the OCS and used in online charging.

The first group of information is shown in Table 1. Instead of listing which information is provided by which network function, each row in the table aggregates the given information, depending on the stakeholder the information is related to. In a general case, the following stakeholders may be included in service delivery (and accordingly in charging): an end user, an end user's home environment (i.e. the PSP), a visited network (if an end user is in roaming), and a third-party service provider (if an end user has access to its service(s)). The data mostly contain information regarding identities (for identifying, for example, a network, a user terminal, and a requested service), requested service units (measured as network resources or a content provided), requested QoS, location, and presence information.

Table 1. List of selected charging information (3GPP TS 22.115)
Charging information provided by an end userEnd user identity
Home environment identity
Terminal Identity
Resource requested
QoS parameters
Identifier for service requested
Charging information provided by a home environment or a roaming networkServing network identity
Universal Time for specific events during the charging session
Quantity of data transferred both to and from the user
Resource allocated to the user
QoS provided to the user
Location of an end user
Unique charging information identity
Presence information
Charging information provided by a third-party service providerThird-party identity
Universal Time for specific events during the charging session
Type of service
Type of content

The second group of information is stored at functions that belong to the OCS, and have already been mentioned in the previous section. The most important information includes:

  • the end user's available credit;
  • the counters related to end user's service usage history, allowed number of service units, etc.;
  • a list of active services per end user; and
  • a list of service prices.

The first three items represent information that is related to the end user: available credit is a part of the User Profile; certain counters may have values depending on the end user's subscription; and the list of active services represents a part of the context in which the end user consumes the given service. However, the information about the services being charged is reduced to the information about the service prices. Additionally, the OCS itself does not provide a specification of inter-domain charging procedures, which come out of the information stored in SLA agreements established between providers. Finally, OCS has many interfaces to other network functions which provide the OCS with relevant charging information, as well as initiate charging requests (especially for per-flow charging). Consequently, there emerges a problem of increasing amount of signaling generated by charging.

In the next section we present the state-of-the-art research related to the 3GPP online charging open issues.

4 STATE OF THE ART IN ONLINE CHARGING RESEARCH

  1. Top of page
  2. SUMMARY
  3. 1 INTRODUCTION
  4. 2 CHARGING-RELATED TERMINOLOGY AND PROCESS INTERACTIONS
  5. 3 ONLINE CHARGING IN 3GPP STANDARDS
  6. 4 STATE OF THE ART IN ONLINE CHARGING RESEARCH
  7. 5 DISCUSSION AND OPEN ISSUES
  8. 6 CONCLUSION
  9. ACKNOWLEDGEMENTS
  10. REFERENCES
  11. Biographies

This section provides an overview of the state of the art in online charging. Starting from the 3GPP three levels of charging (Figure 6), we categorize the relevant research work into three aspects (Figure 9): (1) signaling aspect; (2) inter-domain aspect; and (3) service- and component-based aspect. Each aspect is presented as an aggregation of work related to one of the 3GPP charging levels. First, the signaling aspect aggregates research work that may be used as an extension to existing bearer-level signaling procedures. Next, the inter-domain aspect broadens the view of the service session charging at the subsystem level in respect to inter-domain problems and known limitations. Finally, the service- and component-based aspect considers charging of composed services, as a (possible) extension of service-level 3GPP charging procedures. In each aspect we define a different set of comparison parameters and apply them to compare the research work. Note that charging functionalities elaborated in this section, especially their functions and interfaces, do not necessarily match the 3GPP OCS architecture, nor are they designed for it. However, the key ideas used in those systems are discussed in light of their possible use in 3GPP.

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Figure 9. Mapping of the 3GPP charging levels to research aspects

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4.1 Signaling aspect

The signaling aspect addresses research work related to charging signaling procedures, which are used to authorize the network resources and thereby allow or deny service usage. An example of such a procedure is the Credit-Control application [11].

The following parameters are used for the comparison:

  • Amount of signaling exchanged—Depending on whether the research is focused on adding new features to signaling interfaces or optimizing the existing features, the amount of necessary signaling may be increased or decreased, respectively. The parameter is normalized to the standard amount of signaling.
  • Premature service session termination probability—This parameter compares existing procedures with respect to their ability to react on time to charging events that may result in terminating the service session, e.g. if the end user's credit is depleted and no additional service provisioning is allowed, and to prevent such termination, if possible. The parameter is normalized given the standard session termination probability.
  • Multi-service support—This parameter assesses how a certain signaling procedure enhances (if at all) charging of multiple services used in parallel by the same end user. The possible parameter values are low, medium, and high.
  • 3GPP compatibility—This parameter assesses to which extent the proposed signaling procedure is compatible with the existing 3GPP charging architecture. The possible parameter values are low, medium, and high.

The problem of charging the end user's multiple parallel services is addressed by Kurtansky and Stiller [38], proposing a concept of service bundles. Service bundles are defined as a list of services that may be used by an end user. Instead of running the credit-control procedures separately for each media flow in each service session in progress, it is done per bundle by using a Time Interval Calculation Algorithm (TICA). Depending on the input parameters, TICA calculates a time interval within which an end user can consume any combination of services in the bundle. When the time interval elapses, another interval is calculated. In Kurtansky et al. [39] algorithm improvements are proposed, including statistical prediction of resource consumption, learning from the past methods, and service classification. The evaluation of the algorithm in Kurtansky et al. [40] showed the reduction of credit-check messages, compared to traditional credit-check procedures, with minimal charging fraud detected. As a next step of this work, Oumina and Ranc [41] deal with incorporating service bundles into the 3GPP OCS. They discuss when to initiate time-interval adaptation, e.g. in the case of a service bundle change, or in the case of a change in a price of a service used within the bundle. Finally, Oumina and Ranc [42] propose a modification of an OCS's Rating Function in order to support applications using multiple (third-party) services. The idea is to be able to calculate a service price depending on a combination of other services used throughout the application. To do so, they propose a service identifier (used when requesting the Rating Function to determine the service price) to consist of two parts: a fixed part (which defines in an unique way the service in the OCS), and a variable part (which refers to the application that uses the service for which charging has been requested).

Lin et al. [43] deal with multiple parallel services consumed by an end user, in a situation when an additional new service is initiated. A Prepaid Credit Reclaim (PCR) mechanism is proposed, enabling credit redistribution between the services that are already in progress and those that are about to be established. This approach also generates additional signaling, but enables all services to share equally the available credit, instead of (possibly) rejecting new services due to insufficient credit, while at the same time the services already established (potentially) have a surplus. This approach is opposite from the one proposed later, by Yang et al. [44], where the probability of terminating ongoing service sessions is reduced by introducing a mechanism that rejects new session requests, when the end user's credit is below a certain threshold.

Sou et al. [45] propose the Recharge Threshold-based Credit Reservation (RTCR) mechanism, enabling the charging system to recharge an end user's account by sending a message to the credit recharge server. An analytical model calculates the optimal credit threshold, which, when reached, triggers the charging system to request credit recharge. The key challenge in this approach is how to determine the right threshold. If the threshold is too small, credit units may be depleted before the credit has been recharged, resulting in forced service termination. If the threshold is too large, the end user will receive recharge messages too frequently, resulting in heavy network traffic. Sou et al. [46] deal with charging signaling optimization for mid-session charging events, e.g. when a change of QoS occurs. As opposed to the ‘traditional’ systems, where previously allocated but unused credits are freed and credits for the modified session are requested anew, the authors propose a Threshold-based Scheme for Credit Re-authorization (TSCR) procedure, enabling the charging system to omit the re-authorization procedure if the number of remaining credit units is sufficient to accommodate the new reservation, and thereby reduce the amount of signaling.

Figure 10 shows comparison results of the explained charging signaling enhancements given the identified criteria. RTCR and PCR introduce charging functionality that results in a reduced probability of a premature session termination. However, the amount of signaling they produce is increased compared to the standard signaling. A particular load on the network is being put by the PCR mechanism, due to the credit reclaiming procedure for every parallel session in progress. Both TICA and TSCR reduce the overall signaling amount, but due to the increased inaccuracy of credit information [46], TSCR poses a potential threat to session termination, increasing the session termination probability.

image

Figure 10. Comparison results of the signaling enhancements given the identified criteria

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We assess the 3GPP compatibility for TSCR and RTCR as high, since the proposed functionalities are able to use existing 3GPP infrastructure without any changes in signaling interfaces. TICA's and PCR's compatibility are assessed as medium, due to the proposed modifications that are needed in the network. When comparing the multi-service support, TSCR and RTCR deal only with the single service scenario, without any signaling optimization proposals when multiple services are initiated, and are consequently assessed as medium. On the other hand, PCR and TICA base their solutions on the signaling optimization techniques in the case of multiple services being concurrently charged, and are marked as high.

The following work deals with functional improvements of the 3GPP OCS functions, which are not necessarily related to service authorization, and therefore not included in the above comparison. For purposes of completeness, they are described next.

Grgic et al. [47] extend standard Rx and Gx interfaces within the PCC architecture with additional Diameter AVPs, in order to support advanced service adaptation mechanisms for complex multimedia services. The adaptation mechanisms, introduced by Skorin-Kapov et al. [48], specify and utilize a data structure called the Media Degradation Path (MDP), which enables agreement of several service configurations for a single service session, any of which may be used during the servicesession, enabling the service, by selecting the adequate configuration, to adapt to different network conditions. In Grgic et al. [36], modifications of the standard OCS are proposed and implemented in a laboratory prototype, to support the model presented in Grgic et al. [47]. Finally, in Grgic et al. [49], knowledge about end user's service subscriptions (and the agreed service prices) is included in a process of finding optimal service configuration, by using the proposed interface situated between the PCRF and the OCS (as of 3GPP Release 11, also known as the Sy reference point).

Albaladejo et al. [50] notice that the 3GPP specifications (deliberately) omit the mapping procedure between the session data and the PCC rules at the PCRF, and propose two possible strategies of filling the PCC rules with the session information, followed by performance measurements and comparison of the two approaches.

Shengyao [51] presents a design of the IMS Gateway Function, enabling the call-session control functions to initiate/modify/terminate session-based online charging with the OCS.

4.2 Inter-domain aspect

This section studies work related to inter-domain charging, explaining the issues that emerge from limitations posed by business agreements among stakeholders. The research works are mutually independent and presented in a chronological order. The following parameters are used for comparison:

  • ability to dynamically change business agreements—assesses possibilities to create and/or modify business agreements among stakeholders depending on the current situation and interests, including change of charging model, service tariff, etc.;
  • privacy assurance mechanisms between stakeholders—analyzes available mechanisms in each research approach that assure the end user's privacy while disseminating user-related data in the network;
  • 3GPP compatibility—assesses to what extent a certain work is compatible with the 3GPP subsystem-level and inter-domain charging procedures. The possible values are, as before, low, medium, and high.

Koutsopulou et al. [17-19] propose an architecture in which Charging, Accounting, and Billing (CAB) functions are deployed within a network of a trusted third party. By using a CAB gateway, charging data are collected from the entities that participate in service delivery, but belong to different administrative domains. By using open application programming interfaces (APIs), the service providers can define their accounting and charging policies. This approach is not compatible with the 3GPP network operator-centric model, where all charging-related functions (from the point of view of an end user) are performed by the PSP. Other models, e.g. content provider-centric model and content aggregator-centric model, may be found in the literature [16].

Bormann et al. [52, 53] propose a context-aware charging and billing mechanism, as a part of the project entitled Local Mobile Services (LOMS). They developed a system that enables providers of local mobile services to offer their services to end users by using another network operator's infrastructure. These services are often context-aware (e.g. aware of a user's location), thus enabling service providers to offer and utilize charging models that take the given context into consideration.

Within the Ambient Networks Project [54], Huitema et al. [55] have created the architecture that supports negotiation mechanisms in accounting, charging, and billing, called the Compensation architecture. The key idea is to allow automated and dynamic negotiation about relationships between parties known or unknown to each other, and to realize the negotiated agreement in near real time. Without a need to establish business agreements, the architecture enables negotiation of (for example) tariffs and time of payment between the parties. A charging system to be used in this environment also is defined.

In a novel approach proposed by Murata et al. [56], an open network is described as one in which network resources can be deployed not only by existing operators but also by other interested parties: companies, universities, etc. The idea is to create an open-type mobile business environment as similarly as it has already been achieved on the Internet. They propose an Open Heterogeneous Mobile Network (OHMN) consisting of five functional layers. The layers are mapped to the existing layers standardized by the TISPAN NGN. Although it is believed the OHMN will open the mobile market, the proposed architecture is still in an experimental phase, and the charging models to be used in such a network are yet to be determined.

Simultaneously with the growing number of stakeholders and new business relationships, the market is flooded with numerous charging models and available tariffs offered to end users. Cheboldaeff [57] explains known approaches in organizing the charging models, such as service buckets and discounts usage, and highlights charging-related issues when using these approaches.

In the approach proposed by Tran and Tuffin [58], they list the key requirements for pricing schemes (i.e. charging models) in inter-domain charging, and discuss the important characteristics an ‘ideal’ inter-domain pricing scheme must have.

When comparing the elaborated approaches given their ability to support dynamic change of business agreements, only the OHMN and the Compensation architecture (which is designed for that purpose) would support such scenarios. The motivation for introducing a five-layer environment in OHMN is to allow and promote the introduction of new business models for the benefit of users. Other approaches assume the existence of a fixed business model (e.g. a third-party charging provider in CAB, small provider–large provider relationship in LOMS, or offering groups of services by using predefined buckets in Cheboldaeff [57]). Next, if considering the aspect of privacy assurance, CAB enables exchanging CDRs between the network operator and the CAB gateway without specifying any mechanism that controls end user's (potentially) private information contained within the CDRs. In LOMS, it is assumed that users allow information about their context (e.g. preferences or location) to be used freely. However, Compensation allows end users to negotiate which information they wish to share with the service provider. Finally, no architecture provides full compatibility with the 3GPP specifications. CAB enables connecting with the IMS via an Open Service Architecture, but does not provide any signaling scenarios. LOMS uses Parlay X API for interconnecting with other networks. Finally, Compensation does not consider mapping to 3GPP at the current stage of development.

4.3 Service- and component-based aspect

This section addresses service-level charging aspects of complex multimedia services. Such services, called composed services [59], usually consist of several elementary services, i.e. service components, often provided by different service providers. Charging of such services requires coordinated charging procedures for each service component included, and therefore makes the service price determination, as well as the charging data coordination/correlation, complex and difficult to achieve. According to Ghys and Vaaraniemi [59], service component types are defined as follows:

  • media components—audio, video, data, etc.;
  • value-added service components, e.g. a call-forwarding service is an upgrade to standard call service and can be treated as a service component;
  • business-model-based components—includes different service providers in service delivery, each providing a different aspect of a service (e.g. access service or network service);
  • network-based components—in the case of roaming users, a service is divided into network components depending on the borders between the operators' administrative domains;
  • content-based components, such as music songs or traffic information messages.

In order to compare existing research work in this field, we define the following comparison criteria:

  • charging support for different component types—analyzes which component type(s) is (are) supported by each work;
  • tariff adaptation ability—analyzes the ability of the charging system to adapt the tariff to the combination of included components; and
  • 3GPP compatibility—analyzes whether the given work is compatible with service-level charging mechanisms in 3GPP.

One of the first research works in the field has been written by Van Le et al. [60], who propose mechanisms of outsourcing the accounting and charging processes, i.e. having a specialized service provider in the network responsible only for these functions. In such an environment, the proposed accounting and charging architecture would support the service session composition across multiple domains.

Next, service tariff determination in QoS-enabled networks (e.g. a Differentiated Services-based network) is addressed by Wang and Schulzrinne [61]. They propose a model for dynamic definition of tariffs, which includes different parameters in tariff calculation, such as service priority, service usage rate, and current network load. The idea is to enable service providers to perform service adaptation to current network conditions by raising prices while congestion is in place.

Jennings and Malone [62] specify a two-phase rating process (i.e. a service price determination process) for composed services. In addition to summarizing prices of each component (phase 1), the approach takes into consideration the context in which each component is used, i.e. the tariff for each component is modified depending on the combination of other components that are used within the same service (phase 2). Next, they develop an Accounting Logic Generator that automates the generation, deployment, and application of charging schemes for composed IMS services [63].

Huang et al. [64, 65] address the problem of specifying the scheduling and load-balancing algorithms for processing online charging requests depending on the service type, service priority, etc. The algorithm application results in response times to charging requests for different services.

Van Le et al. [66, 67] expand the standard OCS functionality to support charging for composed services. The included support consists of three elements: (1) the service composition information model used by the OCS; (2) functional components of the OCS to support composed services; and (3) the interaction between the functional components.

Faria and Nogueira [68, 69] take into consideration the context information present in collaborative service agreements between the involved service providers when charging composite services. This information enables, for example, allowing certain discounts in tariffs or promotive periods. A mechanism to support inter-domain contextual information dissemination towards charging systems is also proposed.

When comparing the presented works regarding charging support for different component types, only OCS extension work [66, 67] supports media and content-based components. Works [60, 68, 69] include support for business-model-based components, while the two-phase rating [62] supports a combination of value-added and network-based components.

Next, in the Diffserv approach [61] the tariff is adapted based on network congestion status, while the outsourcing mechanism [60] uses charging records, but the tariff definition remains unclear. Only the two-phase rating [62] and the context utilization approach [68, 69] achieve a high level of tariff adaptation abilities based on the components included.

Currently, there is no support in standards for charging of composed services at the 3GPP OCS. Other than Van Le et al. [66, 67], which propose an approach that enables the support, only the two-phase rating process could be compatible (although certain adaptations on the Rating Function are needed) with the 3GPP. Other approaches do not consider 3GPP-specific procedures.

5 DISCUSSION AND OPEN ISSUES

  1. Top of page
  2. SUMMARY
  3. 1 INTRODUCTION
  4. 2 CHARGING-RELATED TERMINOLOGY AND PROCESS INTERACTIONS
  5. 3 ONLINE CHARGING IN 3GPP STANDARDS
  6. 4 STATE OF THE ART IN ONLINE CHARGING RESEARCH
  7. 5 DISCUSSION AND OPEN ISSUES
  8. 6 CONCLUSION
  9. ACKNOWLEDGEMENTS
  10. REFERENCES
  11. Biographies

In this section, some open issues in utilization of charging-related information are identified and discussed.

5.1 Charging information used in the signaling aspect

In the signaling aspect, main issues include the end user's credit management, e.g. the ability to know when to recharge the user's credit to avoid premature service termination, and decisions on how to optimize credit (re)distribution among different parallel services, or to reject a new service initiation considering the available credit. Table 2 lists the information used in each approach. RTCR sets and modifies the level of the credit depletion threshold, enabling a dynamic mechanism of credit recharge. PCR and TSCR redistribute the allocated credit for each service, depending on the occurrence of new service(s). The difference is that PCR uses information about credit of all an end user's active services, instead of TSCR's single service. TICA maps available credit to a minimal time interval, within which all services in the bundle may be used without performing the credit recheck. Online charging that uses MDP considers the potential user's service subscriptions while tariffing the service and deducing the available credit.

Table 2. A summary of the approaches using end user credit information in the signaling aspect
ApproachKey information
RTCRCredit depletion threshold
PCRAllocated credit per service, for all services used
TICAMinimal time interval
TSCRAllocated credit per service
MDPPotential user subscription(s) that may determine how to use credit

5.2 Charging information used in the inter-domain aspect

The goal in the inter-domain aspect is to unify and simplify charging information that is exchanged between stakeholders, to achieve having fewer compatibility issues, easier establishment of business agreements, and easier privacy management. Table 3 lists the information used in works described in Section 4.2. CAB uses standard charging information, such as CDRs or tariff policies, but accesses them over the Application Programming Interface (API), thus having control over the information exchanged. LOMS uses user context for building context-aware services and defines service templates, which are then used by small service providers to build applications. For example, such a template may define tariff discount rules to be used in charging. Next, the most important information in the Compensation architecture is the result of negotiation between parties, which is then used as an input parameter to define all charging-related rules. Finally, the concept of service buckets uses rules that define which service belongs to which bucket (and thus how they should be charged).

Table 3. Information used in inter-domain charging approaches
ApproachKey information
CABCDR, tariff policies, CAB API
LOMSService templates, discount rules, context
CompensationService negotiation results
Service bucketsService-to-bucket mapping rules

5.3 Charging information used in the service- and component-based aspects

Charging information used in the service- and component-based aspects mostly consists of a cross-referenced knowledge about service components and their prices with respect to other components included in the service, or other contextual data. Table 4 lists information used in the elaborated approaches. The tariff calculation model in the Diffserv network uses three types of available charges (i.e. tariffs): holding charge (used when network resources are reserved for the service that does not currently generate any traffic); usage charge (used in normal network conditions); and congestion charge (used in case the network is congested). Two-phase rating uses a charging scheme, i.e. a set of rules defining which tariff to apply in which phase and to which combination of components. The approach utilizing context stored in service agreements defines a new 3GPP function, called context broker, to collect and monitor the contextual data.

Table 4. Information used in service- and component-based charging approaches
ApproachKey information
DiffservHolding charge, usage charge, congestion charge
Two-phase ratingCharging scheme
OCS extensionsStandard 3GPP information
SLA contextInter-domain context stored in context broker

5.4 Open issues in charging information utilization

Our analysis shows that the 3GPP OCS uses a limited set of information for online charging, the most important of which are: the end user's available credit; the charging model and the corresponding service price(s); the counters that can represent end users' spending limits; and a list of active services per end user. The current research work builds upon this information by introducing, for example, different thresholds (credit, counter, etc.), knowledge stored in SLAs, and means to modify service prices according to other currently used services. However, along with the development of charging systems and more demanding requirements on charging, there emerges a need for accessing additional information, such as service usage statistics, device capabilities, and detailed information about active service configurations. We do not attempt to predict which of the information will be included in future charging systems and for what purpose, but instead we summarize open issues that(will) occur when utilization of such information becomes inevitable. We identify the three most important open issues, as follows.

First, this information is already available in the network, but it is stored at different functional entities, and it is not conveniently structured for use in charging systems. Each of the reviewed research approaches presented in this paper uses only a segment of the overall available information, and assumes that it can be reached from the charging system. In the future, researchers will have to tackle the problem of overall charging information specification and structuring.

Second, even if the information were structured, it is still unclear how to deliver the information to participants in service delivery and consequently in charging (PSP, third party providers, etc.) and how it would interfere with advanced service-related mechanisms, such as service adaptation. Therefore, what is needed is the specification of signaling scenarios for charging information exchange.

Third, the user privacy protection rules (other than the generic ones provided in SLA agreements) while exchanging the information between stakeholders are currently not specified in an open and/or standardized way. It is likely that the future context-based services will require access to (some of) the user's personal information, and that charging systems will have to find a way to exchange such information without compromising the user's privacy.

6 CONCLUSION

  1. Top of page
  2. SUMMARY
  3. 1 INTRODUCTION
  4. 2 CHARGING-RELATED TERMINOLOGY AND PROCESS INTERACTIONS
  5. 3 ONLINE CHARGING IN 3GPP STANDARDS
  6. 4 STATE OF THE ART IN ONLINE CHARGING RESEARCH
  7. 5 DISCUSSION AND OPEN ISSUES
  8. 6 CONCLUSION
  9. ACKNOWLEDGEMENTS
  10. REFERENCES
  11. Biographies

What we presented in this paper clearly shows that there is a trend of including additional information in the state-of-the-art online charging systems. This information includes richer descriptions of users, services and network properties. Taking ‘user context’ into account may become a crucial input for making advanced charging-related decisions. Knowledge about the service (e.g. service composition or its adaptation capabilities) is also important when calculating tariffs and/or performing service authorization. There is also a need for charging information simplification and unification in dynamically changing business environments. The trend noted above creates several research challenges with respect to accessing and utilizing the relevant information in future online charging systems. The most important issues, as identified in this paper, are the lack of information specification and structuring, information sharing issues, and user privacy issues.

ACKNOWLEDGEMENTS

  1. Top of page
  2. SUMMARY
  3. 1 INTRODUCTION
  4. 2 CHARGING-RELATED TERMINOLOGY AND PROCESS INTERACTIONS
  5. 3 ONLINE CHARGING IN 3GPP STANDARDS
  6. 4 STATE OF THE ART IN ONLINE CHARGING RESEARCH
  7. 5 DISCUSSION AND OPEN ISSUES
  8. 6 CONCLUSION
  9. ACKNOWLEDGEMENTS
  10. REFERENCES
  11. Biographies

This work was carried out within the research project 036-0362027-1639 'Content Delivery and Mobility of Users and Services in New Generation Networks', supported by the Ministry of Science, Education and Sports of the Republic of Croatia.

REFERENCES

  1. Top of page
  2. SUMMARY
  3. 1 INTRODUCTION
  4. 2 CHARGING-RELATED TERMINOLOGY AND PROCESS INTERACTIONS
  5. 3 ONLINE CHARGING IN 3GPP STANDARDS
  6. 4 STATE OF THE ART IN ONLINE CHARGING RESEARCH
  7. 5 DISCUSSION AND OPEN ISSUES
  8. 6 CONCLUSION
  9. ACKNOWLEDGEMENTS
  10. REFERENCES
  11. Biographies
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Biographies

  1. Top of page
  2. SUMMARY
  3. 1 INTRODUCTION
  4. 2 CHARGING-RELATED TERMINOLOGY AND PROCESS INTERACTIONS
  5. 3 ONLINE CHARGING IN 3GPP STANDARDS
  6. 4 STATE OF THE ART IN ONLINE CHARGING RESEARCH
  7. 5 DISCUSSION AND OPEN ISSUES
  8. 6 CONCLUSION
  9. ACKNOWLEDGEMENTS
  10. REFERENCES
  11. Biographies
  • Tomislav Grgic received his Dipl-Ing degree in electrical engineering from the University of Zagreb, Faculty of Electrical Engineering and Computing, in 2006. He is employed as a research assistant at the Department of Telecommunications of the same Faculty. As a graduate student, he is working on his PhD thesis entitled ‘Online charging for services in communication network based on user-related context’. His research interests include online charging issues in a multi-provider environment for adaptive multimedia services, specification of a user-related charging context, and its utilization in online charging.

  • Maja Matijasevic is a Professor in the Faculty of Electrical Engineering and Computing at the University of Zagreb (FER), Croatia, and the leader of the Networked Media research group within the FER's Department of Telecommunications. Her research interests include networked multimedia and quality of service in IP-based next-generation networks, with particular focus on session negotiation, adaptation, and mobility. She is a principal researcher in a Croatian national research project and a research program, and she has led research projects in collaboration with industry. She has over 80 journal and conference publications, and she has co-authored several books and book chapters. She has served as a TPC member, TPC (co)chair, and reviewer in international conferences, and as a guest editor in several journal special issues. She received her Dipl-Ing, MSc and PhD degrees in electrical engineering from the University of Zagreb, and the MSc in Computer Engineering from the University of Louisiana at Lafayette, LA, USA. She is a senior member of IEEE and a member of ACM.