The LTE supports the PCC(Policy and Charging Control) architecture for QoS control applied to service data flow. In the PCC architecture, the PCRF(Policy and Charging Rules Function) takes the role of a central network component interacting with P-CSCF, SPR(Subscription Profile Repository) and the EPC. The PCRF is always involved in the activities of EPS bearer creation by generating the PCC(Policy and Charging Control) rules and provisioning them to PDN GW(i.e., PCEF). In order to generate PCC rules, the PCRF needs to gather service data flow information from the P-CSCF and possibly the subscription information from the SPR. As the PCC rule shall be applied from the access network to the EPC core with consistency, the PCRF needs to be able to maintain the relations of different sessions over different interfaces across the IP-CAN, EPS bearer and applications, in which the binding mechanism becomes a basic concept.
NOTE In this article, it is assumed that the S5 between the SGW(Serving GW) and PGW(PDN GW) is using GTP, where the PGW becomes the end point of the GTP tunnel.
I. Session binding
The session binding is an association of the AF session information to one and only one IP-CAN session [TS23.203]. Th AF(Application Focus) session is defined as a service level session such as IMS service session that is established by the application level signaling protocol offered by the AF that requires a session-setup with explicit session description before the use of the service[TS29.214].
The IP-CAN session information is obtained by the PCRF during the UE initial attach procedure.
When the UE performs the initial attach to the LTE network, the PGW triggers the CCR-I towards the PCRF. The CCR-I contains the IP-CAN related AVPs such as Framed-IP-Address, IP-CAN-Type, RAT-Type, Called-Station-Id, AN-GW-Address. The following is an example of CCR-I captured over Gx.
The service data flow information is obtained while the service session is set up. Upon UE request for VoLTE call setup, the SIP INVITE is delivered to the P-CSCF(i.e., AF) with an SDP offer. The SDP offer includes media session information such as 5-tuples(source ip address, destination ip address, source port, destination port and protocol), codec information, etc. Upon receiving the SIP INVITE, the P-CSCF interprets the received SDP contents into diameter AVPs and triggers the AAR towards the PCRF. The following shows an example of SDP contents included in the SIP INVITE.
NOTE It would be possible for the P-CSCF to trigger AAR once when the 183 Session In Progress is received(SDP answer) in order to reduce diameter traffic.
The following is an example of diameter AAR over Rx which includes the service data flow information.
Upon receiving the AAR, the PCRF binds the service data flow information received from the P-CSCF to a specific IP-CAN session received from the PGW.
II. Bearer Binding
The bearer binding is the association of the PCC rule and the QoS rule (if applicable) to an IP-CAN bearer within that IP-CAN[TS23.203]. Upon user's request for voice call, the SIP signals will flow through the EPS bearer of QCI=5 and there will be additional EPS bearer newly established to transfer voice traffic. In the same way as described in the previous section, the PCC rules are generated and provisioned to the PGW by the PCRF. The PGW enforces PCC rules accordingly by performing gating control over service data flow.
The PCC rule contains various parameters that can be used to control service data flow such as Flow-Information, Flow-Status, QoS, etc. The following snapshot shows an example of PCC rule contained in the diameter RAR over Gx for voice traffic.
Upon receiving the RAR, the PGW triggers the EPS bearer creation procedure by sending the Create Bearer Request to the MME. The Create Bearer Request contains the TEID for GTP-U created by the PGW and QoS parameters(QCI, MBR, GMBR) as well. In return, the corresponding response from the MME, Create Bearer Response, contains the allocated EBI(EPS Bearer ID) and the TEID for GTP-U created by the eNB. As an end point of the GTP Tunnel, the PGW shall maintain the relations between EPS bearers and PCC rules. The following is an example of Create Bearer Response captured over GTPv2.
The session binding and the bearer binding are basic concepts to understand how different type of sessions are related with each other over different interfaces within the PCC architecture. The IP-CAN session to which the UE is attached is bound with the AF session by the PCRF and the service data flow for that AF session is used to generate the PCC rules. These PCC rules are used to map the specific service data flow to a certain EPS bearer which belongs to that IP-CAN session.
Thanks to this binding relations, one event at one place makes a ripple effect to the other, thereby the LTE core can maintain the consistent status for the same UE. If the UE releases an AF session, the corresponding PCC rules for the service data flow belonging to that AF session are removed and in turn, the EPS bearer for that PCC rules will also be released. If the UE detaches from the network, the IP-CAN session will be released and in turn, all the AF session and EPS bearers belonging to the same IP-CAN will also be released.
NOTE In this article, it is assumed that the S5 between the SGW(Serving GW) and PGW(PDN GW) is using GTP, where the PGW becomes the end point of the GTP tunnel.
I. Session binding
The session binding is an association of the AF session information to one and only one IP-CAN session [TS23.203]. Th AF(Application Focus) session is defined as a service level session such as IMS service session that is established by the application level signaling protocol offered by the AF that requires a session-setup with explicit session description before the use of the service[TS29.214].
The IP-CAN session information is obtained by the PCRF during the UE initial attach procedure.
When the UE performs the initial attach to the LTE network, the PGW triggers the CCR-I towards the PCRF. The CCR-I contains the IP-CAN related AVPs such as Framed-IP-Address, IP-CAN-Type, RAT-Type, Called-Station-Id, AN-GW-Address. The following is an example of CCR-I captured over Gx.
- Framed-IP-Address: The valid routable IPv4 address that is applicable for the IP Flows towards the UE at the PCEF. The PCRF shall use this address to identify the correct IP-CAN session binding[TS29.214].
- Freamed-IPv6-Prefix: A valid full IPv6 address that is applicable to an IP flow or IP flows towards the UE at the PCEF. The PCRF shall use this address to identify the correct IP-CAN session binding[RFC4005,RFC3162].
- IP-CAN-Type: Type of Connectivity Access Network in which the user is connected[TS29.212].
- RAT-Type: The Radio Access Technology that is currently serving the UE[TS29.212].
- Called-Station-Id: APN [TS29.212].
The service data flow information is obtained while the service session is set up. Upon UE request for VoLTE call setup, the SIP INVITE is delivered to the P-CSCF(i.e., AF) with an SDP offer. The SDP offer includes media session information such as 5-tuples(source ip address, destination ip address, source port, destination port and protocol), codec information, etc. Upon receiving the SIP INVITE, the P-CSCF interprets the received SDP contents into diameter AVPs and triggers the AAR towards the PCRF. The following shows an example of SDP contents included in the SIP INVITE.
NOTE It would be possible for the P-CSCF to trigger AAR once when the 183 Session In Progress is received(SDP answer) in order to reduce diameter traffic.
The following is an example of diameter AAR over Rx which includes the service data flow information.
- Media-Sub-Component: contains the requested bitrate and filters for the set of IP flows identified by their common Flow-Identifier[TS29.212].
- Flow-Description: defines a packet filter for an IP flow with the following information[TS29.212].
- Flow-Status: describes whether the IP flow(s) are enabled or disabled[TS29.214].
- Max-Requested-Bandwidth-UL/DL: Indicates the maximum requested bandwidth in bits per second for an uplink/downlink IP flow[TS29.214].
- RS-Bandwidth: indicates the maximum required bandwidth in bits per second for RTCP sender reports within the session component[TS29.214].
- Codec-Data: contain codec related information known at the AF[TS29.214].
Upon receiving the AAR, the PCRF binds the service data flow information received from the P-CSCF to a specific IP-CAN session received from the PGW.
II. Bearer Binding
The bearer binding is the association of the PCC rule and the QoS rule (if applicable) to an IP-CAN bearer within that IP-CAN[TS23.203]. Upon user's request for voice call, the SIP signals will flow through the EPS bearer of QCI=5 and there will be additional EPS bearer newly established to transfer voice traffic. In the same way as described in the previous section, the PCC rules are generated and provisioned to the PGW by the PCRF. The PGW enforces PCC rules accordingly by performing gating control over service data flow.
The PCC rule contains various parameters that can be used to control service data flow such as Flow-Information, Flow-Status, QoS, etc. The following snapshot shows an example of PCC rule contained in the diameter RAR over Gx for voice traffic.
Upon receiving the RAR, the PGW triggers the EPS bearer creation procedure by sending the Create Bearer Request to the MME. The Create Bearer Request contains the TEID for GTP-U created by the PGW and QoS parameters(QCI, MBR, GMBR) as well. In return, the corresponding response from the MME, Create Bearer Response, contains the allocated EBI(EPS Bearer ID) and the TEID for GTP-U created by the eNB. As an end point of the GTP Tunnel, the PGW shall maintain the relations between EPS bearers and PCC rules. The following is an example of Create Bearer Response captured over GTPv2.
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The session binding and the bearer binding are basic concepts to understand how different type of sessions are related with each other over different interfaces within the PCC architecture. The IP-CAN session to which the UE is attached is bound with the AF session by the PCRF and the service data flow for that AF session is used to generate the PCC rules. These PCC rules are used to map the specific service data flow to a certain EPS bearer which belongs to that IP-CAN session.
Thanks to this binding relations, one event at one place makes a ripple effect to the other, thereby the LTE core can maintain the consistent status for the same UE. If the UE releases an AF session, the corresponding PCC rules for the service data flow belonging to that AF session are removed and in turn, the EPS bearer for that PCC rules will also be released. If the UE detaches from the network, the IP-CAN session will be released and in turn, all the AF session and EPS bearers belonging to the same IP-CAN will also be released.
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