PCC: An IMS IP-AN interaction layer for policy control

As described in section 1.1, the interaction between the service control overlay network and the IP-AN enables to achieve different objectives among them, policy control. This section presents a 3GPP normalized solution for policy control, called Policy and Charging Control (PCC) [14]. PCC deals with policy control and charging. In the following, only policy control aspects are detailed. Work on PCC has begun from Release 5. In few words, PCC enables to authorize the service and to calculate the corresponding policy rules to be enforced on the IP-AN level. These rules are related mainly to the QoS authorized for the service on the IP-AN, and the service-related data flows authorized to pass through the IP-AN.

PCC introduces four groups of functions (figure 1.2), whose definitions are tied to their locations:

the AF (Application Function) which is in the service control layer, the PCRF (Policy and Charging Rules Function) which is in an independent node containing the main policy control functions, the PCEF (Policy and Charging Enforcement Function) and the BBERF (Bearer Binding and Event

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CHAPTER 1. ARE CURRENT MOBILE NETWORKS READY FOR THE NEW ECOSYSTEM?

Reporting Function) which are in the IP-AN. The PCRF has standardized interfaces towards the AF, PCEF and BBERF. In this way, the PCRF can be shared between different IP-ANs and different service control layers (IMS or others).

1.2.3.1 AF (Application Function)

The AF is located in the service control layer, within the nodes located on the session signalling path. The AF is specific to the service control layer. For IMS, it is within the P-CSCF; otherwise it can be within a streaming server or an FTP server.

The AF identifies the data flows (applications) related to the service and maintains the map- ping between the session identifier and the data flow descriptors (source/destination IP addresses and source/destination transport ports).

As shown in figure 1.3, the AF translates the service description, contained in the session signalling, into a standardized format (Service Information (SI)). Then, it sends the SI to the PCRF to ask for its authorization and for the formulation of policy rules towards the IP-AN (more information is in section 2.1).

When the service control layer is the IMS, the session identifier is the SIP dialog identifier, the service is generally described through the Session Description Protocol (SDP) (see section A.2), and the Data flow descriptors are deduced from the SDP.

P-CSCF

first IP router

IP-AN node1

Service Information (SI)

(IP) policy rules -> (IP-AN) policy rules : >authorized (IP-AN) QoS/ data flow

>flow descriptors, >gate status...

CN

(IP) policy rules

Policy control function

AF

Session signalling (service description, e.g. SDP)

PCEF

SI -> (IP) policy rules: >authorized (IP) QoS/ data flow, >flow descriptors, >gate status PCRF MN IP access network IP-AN node

(IP) policy rules -> (IP-AN) policy rules : >authorized (IP-AN) QoS/ data flow

>flow descriptors, >gate status...

BBERF

Session id <–> data flow descriptors e.g. Dialog <-> e.g. SDP

Authorization (advanced information) Authorization (IP-AN capabilities)

Figure 1.3: Policy control functions in the IP-AN//PCC//IMS model

1.2.3.2 PCRF (Policy and Charging Rules Function)

The PCRF includes the main functions of the PCC. It is located within a node independent of the service control layer and the IP-AN.

1.2. IP-AN//PCC//IMS MODEL 21

As shown in figure 1.3, the PCRF receives the Service Information (SI) from the AF. It may authorize the service based on advanced information. It also calculates for the SI, the (IP)

policy rulesand sends them to the PCEF and BBERF, located in the IP-AN.

The (IP) policy rules are calculated for the IP level, to ensure their independence from IP-ANs specificities. They include (more information is in section 2.1):

• The data flow descriptors identifying the data flows, generated by the applications which are parts of the service.

• The authorized (IP) QoS corresponding to the QoS authorized for the service on the IP level.

• The events configuration identifying the events that shall be reported by the IP-AN to the PCRF.

• The gates status indicating for each data flow whether the related traffic is authorized or not to pass through the IP-AN.

1.2.3.3 PCEF (Policy and Charging Enforcement Function) and BBERF (Bearer Binding and Event Reporting Function)

The PCEF is located in the IP-AN, in the first IP router. Its role is to translate the (IP) policy rules, received from the PCRF, into (IP-AN) policy rules understandable by the IP-AN, specif- ically the bearer level (figure 1.3).

The (IP-AN) policy rules include the same elements as the (IP) policy rules, except for the

authorized (IP) QoSthat is converted into authorized (IP-AN) QoS.

The PCEF enforces/applies the (IP-AN) policy rules on the IP-AN, that includes the bearers supporting the data flows. The PCEF possibly enforces the (IP) policy rules. Indeed, the

authorized (IP) QoS may be needed to perform local IP scheduling, in addition to the IP-AN

scheduling performed based on the authorized (IP-AN) QoS.

This PCEF role was defined in Release 5 and was valid until Release 7. It supposes two elements: (1) the first IP router is the bearer endpoint and (2) the bearer endpoint does not change for a given MN whatever be its mobility. Based on these two elements, a single enforcement point (PCEF) within the IP-AN, located in the first IP router, is an appropriate and optimized choice. For some IP-AN cases, the two stated elements are not fulfilled: the first IP router is not the bearer endpoint but just represents the IP layer, and the bearer endpoint may change during MN mobility. There is thus a need for a second enforcement function (BBERF) within an IP-AN node ending the bearer, and a procedure sending the policy rules from the source bearer endpoint to the target bearer endpoint during MN mobility.

For the stated IP-AN cases, the PCEF in the first IP router is used to enforce the (IP) policy

ruleson the IP layer and the BBERF is used to enforce the (IP-AN) policy rules on the bearer

level, and to handle the change of the bearer endpoint during MN mobility.

As it will be shown in the next sections, the need for the BBERF was identified only from Release 8 since, before that Release, 3GPP has been working only on UMTS as IP-AN (for UMTS the first IP router i.e. the GGSN is the bearer endpoint). In Release 8, when defining EPS IP-AN, the BBERF has been introduced since the first IP router i.e. the P-GW may be not the bearer endpoint (see section 1.3).

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CHAPTER 1. ARE CURRENT MOBILE NETWORKS READY FOR THE NEW ECOSYSTEM?