Author - Mr.Vikram Nair

Designation - Principal Systems Engineer, Aricent

1. INTRODUCTION

Being relatively new, WiMAX technology is still in an evolutionary phase with continuous efforts being undertaken to not only facilitate easy network integration for Service Providers but also provide subscribers with an enhanced experience. Consequently, WiMAX is no longer considered as a standalone technology providing broadband wireless access; instead, with its ability to provide next-generation services, it is fast becoming an access technology for core networks. In fact, Internet Multimedia Subsystem (IMS) is one such area that promises to further the use of WiMAX on the access side for providing next generation services.

Such services necessitate a specialized Quality of Service (QoS) and an equally commensurate charging treatment. And in order to enable a satisfactory subscriber experience, QoS and charging policies determined by core network have to be applied to all network elements including the WiMAX access networks.

This article presents the recent advancements in the area of charging and dynamic QoS management in both the 3GPP and the WiMAX Forum respectively. It also explains the process of integrating such disparate networks in order to ensure end-to-end QoS and charging control.

At the outset, we introduce the concept of Policy Function (PF), its evolution in the 3GPP Forum and how it integrates with the IMS network. We then present the PF as it fits into a WiMAX network. Lastly, we showcase how the PF integrates with the IMS and WiMAX network elements ensuring improved service execution emanating from uniform and consistent end-to-end QoS and charging policies across all network elements.

2. INTRODUCTION TO THE POLICY FUNCTION

The Policy Function coordinates the various network resources to provide requested services to the authorized subscribers. It is responsible for identifying the policy rules for the service that a subscriber may intend using.

These policy rules pertain to access authorization and allocation of the necessary bandwidth and resources in order to provide a service. Such a framework also enables the application layer to operate seamlessly irrespective of the access technology.

From the service deployment perspective it is desirable to deploy services within converged networks in an environment that comprises multiple service applications. Such deployment scenarios are enabled by the application of independent policy rules to track resource usage and setting of priorities across the various applications.

The policy rules can be formulated based on static information, such as the subscription profile, dynamic information, and the available resources. And the combination of such rules, once met for a service request, can trigger a desired action; such as - allowing the service with the requested bandwidth. This type of framework of the policy rules allows the Operators to deploy service logic while optimally utilizing the network resources.

3. POLICY FUNCTION IN 3G (PCC)

The Policy Function was introduced as the Policy Decision Function (PDF) in 3GPP Release 5. Initially, the PDF was limited to determining static charging rules, which were then utilized for postpaid applications. However, today, there are services such as online gaming, which require specialized charging models. However, the Release 5 PDF specification was limited to static charging rules.

3GPP Release 7 evolved the Policy and Charging Control (PCC) architecture for determining and enforcing dynamic QoS and charging policies to all the network infrastructure elements involved in providing a specific requested service. The Policy and Charging Rules Function (PCRF) is the node designated in real-time for the determination of the policy rules. For example, a set of policy rules can be activated to verify access permission, checking and debiting credit balance etc., all in real-time. The PCRF enforces these policy rules through its interaction with 3GPP Release 7 Policy and Charging Enforcement Function (PCEF), which handles the transport plane.

In the preceding illustration:

• Application Function (AF) hosts the service logic and communicates the application level session information to PCRF such as classifiers identifying service flows, on which policy control and differentiated charging is required.
• Online and Offline Charging systems are functions that interface with PCEF for the purpose of online credit control and the collection of offline charging information respectively.

4. PCC SOLUTION FOR IMS

PCC, as introduced in the earlier section, is a policy framework that can be extended to varied technology domains. This section explains the process of integrating the PCC into an IMS network.

IMS networks allow scenarios where a subscriber can access multiple applications via disparate access technologies. PCC ensures a uniform level of end-to-end QoS and facilitates charging data collection from all the involved network elements as per the stipulated charging policies.

The PCRF when deployed in an IMS network determines the policy rules stored locally as well as those originated by the CSCF (equivalent to AF in IMS) in the service plane. It then applies these policies and charging rules to the gateway nodes including access gateway nodes. The gateway nodes, acting as the PCEF, are responsible for enforcing these policy rules.

5. THE POLICY FUNCTION IN WiMAX

The WiMAX Forum has specified a framework for Service Management and QoS. Service Flow Agent (SFA) and Service Flow Manager (SFM) are the entities that act as policy decision and enforcement points respectively for Service Management and QoS. The following figure depicts the policy framework for WiMAX networks:

In the above illustration:

• The SFM Entity: Is responsible for the creation, admission, activation, modification, and deletion of 802.16 service flows. It consists of an Admission Control (AC) function and the associated local resource information.
• The Policy Functions (PFs): Reside in both home and the visited network, comprising their respective databases. The databases include general policy rules as well as application-dependant policy rules.
• The AAA Server: Holds the subscriber's QoS profile and the associated policy rules per subscriber
• Service Flow Authorization (SFA): Is responsible for evaluating any service request against the subscriber's QoS profile

6. IMS AND WIMAX INTERWORKING

This section briefly describes the inter-working aspects that are defined between IMS and WiMAX networks. However, the inter-working of PF procedures across IMS and WiMAX network is not yet completely addressed by the standardization bodies. The progress made so far regarding PF procedure inter-working has been presented later in this article.

IMS can be broadly termed as a session control entity managing sessions that could originate or terminate at different access networks.

WiMAX however, is yet another access networks technology that could be used to reach an IMS network. The efforts at the WiMAX Forum in evolving the architecture for WiMAX is limited to the point where a WiMAX subscriber is provided with connectivity to the IP network over the air. This IP connectivity enables the WiMAX subscribers to reach the IMS core in order to avail services hosted at the IMS.

This section subsequently presents the manner in which IMS-WiMAX inter-working is perceived.

IMS may be located in the private network of a Network Service Operator (NSP) or at a third-party Service Provider. IMS could also be connected either directly to the Service Provider's backbone network, or connected via a specific gateway or even via the Internet.

NOTE: For detailed definition and function of Access Service Network (ASN) and Connectivity Service Network (CSN) refer to WiMAX forum specifications.

To provide the similar level of subscriber experience for IMS-enabled services over a WiMAX access network, it is important to inter-work the policy and charging policies dynamically formulated at IMS core with the WiMAX network elements.

7. THE PCC SOLUTION FOR WIMAX

As of now, the inter-working defined between the IMS core with a WiMAX access network does not address the integration and inter-working aspects of the policy function and charging rules between the two. The convergence of charging and policy functions between IMS and WiMAX is underway in the WiMAX Forum. The following diagram depicts an architectural view of how IMS and WiMAX are converging from the PCC perspective

In the above illustration:

• The PCRF entity replaces the PF entity of the WiMAX network. Refer to figure "The Policy Framework in WiMAX" for PF entity in WiMAX network.
• The PCC Inter-working Function (PCC-IWF) is an inter-working function between PCRF and WiMAX network elements and hides the details of the WiMAX network from PCRF. It connects to the PCRF through Gx reference point and to the ASN through R3 reference point.
• The PCEF function resides in the ASN and acts as the enforcement point of PCC rules and charging in the ASN, which it terminates over the R3 reference point. PCEF collaborates with the ASN functions such as SFA, accounting client and SFM in order to enforce policies.

Optionally, the PCEF function can also be part of CSN interfacing with "PCC-IWF" where it enforces specific QoS and charging policies in addition to PCEF function that resides in the ASN.

For roaming scenarios, the PCRF node can be deployed in the subscriber's home network. The PCC-IWF node will then interface with policy function (PF) in visitor network to enforce the necessary QoS and charging policies.

8. CONCLUSION

The Policy and Charging Control (PCC) is a recent advancement in 3GPP and is a step towards evolving a universal solution for dynamic QoS and charging management. The PCC framework is applicable to IMS based networks as well.

Using WiMAX as an access technology for IMS-based 4G networks is increasingly gaining attention these days. The reason behind this is the evolution of WiMAX technology, which enables a subscriber to acquire connectivity to IP networks. Extending this connectivity to the IMS core enables the subscribers to avail IMS-enabled services.
However, to maintain similar subscriber experience for IMS-enabled services over WiMAX, the charging and QoS policies as determined dynamically at the IMS core needs to be communicated to the WiMAX network elements where they can be suitably enforced.

The PCC-based framework, being evolved in WiMAX forum is a step towards inter-working IMS and WiMAX access network for the purpose of enforcing QoS and charging policies uniformly across all network elements involved in service execution.

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