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Advanced LTE, HSPA and EPC Cellular Systems Training - Public Seminars by Red Banana Wireless, register today at: http://4G-Seminar.com or contact us directly: [email protected].
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The basic architecture for the LTE system including the EPC is shown in the diagram, with each of
the network elements and key interface functions described in subsequent slides. The combination
of logical functions performed by both the Serving Gateway (S-GW) and Packet Data Network
Gateway (P-GW) provide an access Gateway (a-GW) to support PS services between LTE-UEs
and external packet-data networks, however the P-GW and S-GW functions can exist in the same
physical network element
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Policy and Charging Control functionality encompasses two main functions:
- Flow Based Charging, including charging control and online credit control;
- Policy control (e.g. gating control, QoS control, QoS signalling, etc.).
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The diagram above shows the general architecture for Policy and Charging Control, with the main
decisions being made at the Policy and Charging Rules Function (PCRF)
The key policy control functions are:
- Binding: generation of an association between an SDF and the IP-CAN bearer transporting that
SDF
- Gating control: blocking or allowing of packets belonging to an SDF to pass through to the
desired endpoint
- Event reporting: notification of and reaction to application events to trigger new behaviour in the
user plane and reporting of events related to the resources in the GW(PCEF)
- QoS control: authorisation and enforcement of the maximum QoS that is authorised for an SDF
or an IP-CAN bearer
- IP-CAN bearer establishment: for IP-CANs that support network initiated procedures for IP-CAN
bearer establishment.
The Gz reference point is functionally equivalent to Ga for Legacy PS domain and to Ga or Rf for
Evolved PS domain, and hence is replaced by Ga or Rf within the common charging architecture.
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The purpose of the PCC rule is to:
- Detect a packet belonging to a service data flow.
- The service data flow filters within the PCC rule are used for the selection of
downlink IP CAN bearers.
- The service data flow filters within the PCC rule are used for the enforcement that
uplink IP flows are transported in the correct IP CAN bearer.
- Identify the service the service data flow contributes to.
- Provide applicable charging parameters for a service data flow.
- Provide policy control for a service data flow.
The PCEF shall select a PCC rule for each received packet by evaluating received packets against
service data flow filters of PCC rules in the order of the precedence of the PCC rules. When a
packet matches a service data flow filter, the packet matching process for that packet is
completed, and the PCC rule for that filter shall be applied.
There are two different types of PCC rules as defined in 3GPP TS 23.203:
- Dynamic PCC rules. Dynamically provisioned by the PCRF to the PCEF via the Gx interface.
These PCC rules may be either predefined or dynamically generated in the PCRF. Dynamic PCC
rules can be installed, modified and removed at any time.
- Predefined PCC rules. Preconfigured in the PCEF. Predefined PCC rules can be activated or
deactivated by the PCRF at any time. Predefined PCC rules within the PCEF may be grouped
allowing the PCRF to dynamically activate a set of PCC rules over the Gx reference point.
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The rule name is used to reference a PCC rule in the communication between the PCEF and the
PCRF.
The service identifier is used to identify the service or the service component the service data flow
relates to.
The service flow filter(s) is used to select the traffic for which the rule applies. Wildcarded service
data flow filter(s), for dynamic and predefined PCC rules can be defined.
The gate status indicates whether the service data flow, detected by the service data flow filter(s),
may pass (gate is open) or shall be discarded (gate is closed) in uplink and/or in downlink
direction.
The QoS information includes the QoS class identifier (authorized QoS class for the service data
flow), the Allocation and Retention Priority (ARP) and authorized bitrates for uplink and downlink.
The charging parameters define whether online and offline charging interfaces are used, what is
to be metered in offline charging, on what level the PCEF shall report the usage related to the
rule, etc.
For different PCC rules with overlapping service data flow filter, the precedence of the rule
determines which of these rules is applicable. When a dynamic PCC rule and a predefined PCC
rule have the same precedence, the dynamic PCC rule takes precedence.
PCC rule also includes Application Function record information for enabling charging correlation
between the application and bearer layer if the AF has provided this information via the Rx
interface. For IMS this includes the IMS Charging Identifier (ICID) and flow identifiers.
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The PCRF shall indicate, via the Gx reference point, PCC rules to be applied at the PCEF. This may be
using one of the following procedures:
- PULL procedure (Provisioning solicited by the PCEF): In response to a request for PCC rules being
made by the PCEF, as described in the preceding section, the PCRF shall provision PCC rules in the CC-
Answer; or
- PUSH procedure (Unsolicited provisioning): The PCRF may decide to provision PCC rules without
obtaining a request from the PCEF, e.g. in response to information provided to the PCRF via the Rx
reference point, or in response to an internal trigger within the PCRF. To provision PCC rules without a
request from the PCEF, the PCRF shall include these PCC rules in an RA-Request message. No
CCR/CCA messages are triggered by this RA-Request.
For each request from the PCEF or upon the unsolicited provision the PCRF shall provision zero or more
PCC rules. The PCRF may perform an operation on a single PCC rule by one of the following means:
- To activate or deactivate a PCC rule that is predefined at the PCEF, the PCRF shall provision a
reference to this PCC rule within a Charging-Rule-Name AVP and indicate the required action by
choosing either the Charging-Rule-Install AVP or the Charging-Rule-Remove AVP.
- To install or modify a PCRF-provisioned PCC rule, the PCRF shall provision a corresponding Charging-
Rule-Definition AVP within a Charging-Rule-Install AVP.
- To remove a PCC rule which has previously been provisioned by the PCRF, the PCRF shall provision
the name of this PCC rule as value of a Charging-Rule-Name AVP within a Charging-Rule-Remove AVP.
- If, for certain accesses, the PCRF performs the bearer binding, the PCRF may move previously installed
or activated PCC rules from one IP CAN bearer to another IP CAN bearer.
As an alternative to providing a single PCC rule, the PCRF may provide a Charging-Rule-Base-Name
AVP within a Charging-Rule-Install AVP or the Charging-Rule-Remove AVP as a reference to a group of
PCC rules predefined at the PCEF. With a Charging-Rule-Install AVP, a predefined group of PCC rules is
activated. With a Charging-Rule-Remove AVP, a predefined group of PCC rules is deactivated.
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The PCRF accepts input for PCC decision-making from the PCEF, the BBERF if present, the SPR
and if the Application Function (AF) is involved, from the AF, as well as the PCRF may use its own
pre-defined information. These different nodes should provide as much information as possible to
the PCRF. Depending on the particular scenario all the information may not be available or is
already provided to the PCRF.
An example of an Application Function is the IMS Proxy Call Session Control Function (P-CSCF)
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1. The AF provides service information to the PCRF at a set-up of a new AF session or at a
modification of an existing AF session.
2. The GW receives a request for bearer establishment or modification.
3. The GW determines that the PCC interaction is required and sends the Policy and Charging
Rules request to the PCRF. The PCRF correlates the application and bearer sessions with the
binding information (e.g. user's IP address) and, if available, the PCRF also retrieves the saved
application service information in order to make the authorization and policy decision.
4. The PCRF may fetch the subscription related information from the SPR, if the PCRF does not
have it and the SPR replies with the subscription related information, and the PCRF stores this.
5. The PCRF makes the authorization and policy decision and sends the decision(s) to the GW.
6. In this scenario, the PCEF interacts with the Online Charging System to receive request the
service to be authorised (based SDF credit).
7. The GW can now enforce the decision and acknowledge the Bearer Establishment/Modification
request.
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1.BBERF initiates Gateway Control Session Establishment procedure if involved
2.GW(PCEF) receives request for IP-CAN Bearer establishment and assigns IP address for UE
3.PCEF determines that the PCC authorization is required, requests the authorization of allowed
service(s) and PCC Rules information. The PCEF may also include charging ID information.
4.If the PCRF does not have the subscriber's subscription related information, it sends a request to
the SPR in order to receive the information related to the IP-CAN session.
5.PCRF stores the subscription related information containing the information about the allowed
service(s) and PCC Rules information.
6. PCRF makes the authorization and policy decision.
7. PCRF sends the decision(s) , including the chosen IP-CAN bearer establishment mode, to the
PCEF which enforces the decision.
8. If online charging is applicable, and at least one PCC rule was activated, the PCEF activates the
online charging session, providing relevant input information for the OCS decision.
9.The OCS provides the possible credit information to the PCEF and may provide re-authorisation
triggers for each of the credits.
10.If at least one PCC rule was successfully activated and and credit was not denied by the OCS,
the GW(PCEF) acknowledges the IP-CAN Bearer Establishment Request.
11.The GW may initiate the establishment of additional IP-CAN bearers..
12. If the PCRF in step 7 requested an acknowledgement based on PCC rule operations, the
GW(PCEF) sends the IP-CAN Session Establishment Acknowledgement to the PCRF in order to
inform the PCRF of the activated PCC rules result.
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In the roaming case, when the users connectivity is via the home network, connectivity via the S9
interface allows transactions between PCRFs between the home and visited networks. This allows
PCC rules to be modified locally in the visited network if required and also prevents the PCRF of the
home network directly controlling the GW in the visited network
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In the roaming case where the users connectivity breaks out to the external PDN via the visited
network, QoS is controlled at both the SGW and PGW by the PCRF in the visited network. Once
again however, the V-PCRF will interact with the H-PCRF to obtain PCC Rules etc.
