Mojtaba HOUSHMAND

Policy, AAA & Charging solutions Technical Manager at dorTECH

All Articles by Mojtaba HOUSHMAND

1. Definition

The AF and the Rx interface between the PCRF and AF are defined in the 3GPP specifications and they are mainly used by the PCRF to connect third-party applications (for example, OTT) and IMS networks to implement PCC-based VoLTE. The section describes only the Embedded PCRF capability in the VoLTE solution.

2. Benefits

Benefits to Carriers:
This feature helps carriers to use PCC-based VoLTE solution to implement IP link–based high-quality voice and video calls on the LTE network.
 

Benefits to End Users:
This feature enables subscribers to make high-quality voice and video calls on LTE networks.

3. Description
 

In the 3GPP specifications, the IMS is defined as a part of the LTE core network. After carriers use the LTE + IMS core network, PCC-based VoLTE solution can be used.
In this solution, the Embedded PCRF connects to the P-CSCF through the Rx interface and to the P-GW through the Gx interface.

Figure 1. VoLTE solution network diagram

4. IMS Common Call

Figure 2. IMS common call process

1. A subscriber attempts to make an IMS common call through E-URAN. The UE sends a request to the PCEF to establish an IP-connectivity access network (IP-CAN) session.
2. The PCEF sends a CCR{Initial} message containing the subscriber ID and access network information to the Policy Charging through the Gx interface.
3. The Policy Charging sends an SLR message containing the subscriber ID and network information reported in the Gx interface to the CBP through the Esy interface.
4. The CBP generates a PCC rule (key parameters: QCI whose value is 5, ARP, APNMBUL, and APNMBDL) based on the subscriber's subscription and network information.
5. The CBP sends an SLA message containing the PCC rule to the Policy Charging through the Esy interface.
6. The Policy Charging sends a CCA{Initial} message containing the PCC rule to the PCEF through the Gx interface.
7. The PCEF creates a default bearer based on the authorization result and installs the PCC rule.
8. The PCEF sends an IP-CAN Establish Response message to the UE.
9. The AF sends an AAR message to the Policy Charging through the Rx interface to establish a dedicated bearer for the voice media stream. The AAR message contains the subscriber ID, AF ID, IMS application layer charging ID, and media information (including the media type and media stream description).
10. The Policy Charging binds the AF session to the corresponding IP-CAN session based on the subscriber IP address in the AAR message.
11. The Policy Charging sends an SLR message containing the received voice media stream to the CBP through the Esy interface.
12. The CBP generates a PCC rule (key parameters: QCI whose value is 1, ARP, GBR, and MBR) for the voice media stream based on the subscriber's subscription information.
13. The CBP sends an SLA message containing the PCC rule to the Policy Charging through the Esy interface.
14. The Policy Charging sends an RAR message containing the PCC rule to the PCEF through the Gx interface.
15. The PCEF searches for the dedicated bearer based on QCI or ARP in the PCC rule. If no dedicated bearer is found, the PCEF creates a dedicated bearer, installs the voice media stream information and PCC rule in the dedicated bearer, and sends an RAA message to the Policy Charging through the Gx interface.
16. The Policy Charging sends an AAA message to the AF through the Rx interface. The dedicated bearer is successfully created for the voice media stream.
17. The PCEF sends a CCR{Update} message to the Policy Charging through the Gx interface, notifying the Policy Charging that the resource is successfully reserved.
18. The Policy Charging sends a CCA{Update} message to the PCEF through the Gx interface.
19. The Policy Charging sends an RAR message to the AF through the Rx interface, indicating that the resource is successfully reserved.
20. The AF sends an RAA message to the Policy Charging through the Rx interface.
21. The AF receives an INVITE 200 message and sends an AAR message to the Policy Charging through the Rx interface to enable bearer gating control.
22. The Policy Charging sends an RAR message to the PCEF through the Gx interface, instructing the PCEF to modify bearer information about the voice media stream.
    • Note: If a subscriber makes a VoLTE voice call after gating control is enabled, the VoLTE voice media stream is transmitted through the dedicated bearer to ensure the QoS.
23. The PCEF updates the voice media stream status and sends an RAA message to the Policy Charging through the Gx interface.
24. The Policy Charging sends an AAA message to the AF through the Rx interface. The voice call is connected.
25. The subscriber releases the call. The AF sends an STR message to the Policy Charging through the Rx interface to terminate the bearer session.
26. The Policy Charging sends an RAR message to the PCEF through the Gx interface, instructing the PCEF to delete the dedicated bearer of the voice media stream. The PCEF deletes the dedicated bearer.
27. The PCEF sends an RAA message to the Policy Charging through the Gx interface.
28. The Policy Charging deletes the Rx session and sends an STA message to the AF through the Rx interface. The process of deleting the dedicated bearer of the voice media stream is complete.

5. IMS Emergency Call

The Policy Charging supports IMS emergency calls, for example, calls to 911.

Figure 3. IMS emergency call process

1. A subscriber attempts to make an IMS emergency call through the emergency call APN. The UE sends a request to establish an IP-CAN session.
2. The PCEF sends a CCR{Initial} message containing information such as the access network and emergency call APN to the Policy Charging through the Gx interface.
   • Note: The emergency call APN must be configured in the PCEF to ensure that sessions initialized by the emergency call APN are not restricted by flow control or license and have the highest priority.
   • Note: If the Policy Charging finds a Gx session indicating the emergency call service, the emergency call flag is added to the stored session information. The Policy Charging does not check the subscriber number in sessions matching the emergency call service because the sessions may not contain any subscriber numbers.
3. The Policy Charging generates a PCC rule (key parameters: QCI whose value is 5, ARP, APNMBUL, and APNMBDL) based on the emergency call APN.
   • Note: To establish an IP-CAN session for an emergency call APN, a default global policy must be configured in the PCRF for creating the default emergency call bearer.
   • Note: The CCR{Initial} message sent through the Gx may not contain information such as subscriber IMSI because the subscriber may not belong to the local carrier. In this case, the global default policy configured in the Policy Charging is used for authorization.
4. The Policy Charging sends a CCA{Initial} message containing the PCC rule to the PCEF through the Gx interface.
5. The PCEF creates a default bearer based on the authorization result and installs the PCC rule.
6. The PCEF sends an IP-CAN Establish Response message to UE.
7. Based on the requested destination address, the AF determines that the call is an emergency call and sends an AAR message to the Policy Charging through the Rx interface.
   • Note: In addition to the emergency call AF ID, IMS application layer charging ID, and media information (including the media type and media stream description), the AAR message sent through the Rx interface needs to contain the Service-URN AVP, which is used to indicate an emergency call session.
8. The Policy Charging binds the AF session to the corresponding IP-CAN session based on the subscriber IP address in the AAR message.
   • Note: The IP-CAN session for the emergency call APN allows only IMS emergency calls. When receiving an AAR message without the Service-URN AVP through the Rx interface, the Policy Charging rejects the request and returns error code NAUTHORIZED_NON_EMERGENCY_SESSION.
9. The Policy Charging generates a PCC rule (key parameters: QCI, ARP, GBR, and MBR) for the voice media stream.
   • Note: For the emergency call service, the ARP parameter must be set to the highest priority.
10. The Policy Charging sends an RAR message containing the PCC rule of the voice media stream to the PCEF through the Gx interface. The PCEF searches for the dedicated bearer based on QCI or ARP in the PCC rule. If no dedicated bearer is found, the Policy Charging creates a dedicated bearer and installs the voice media stream information and PCC rule in the dedicated bearer.
11. The PCEF sends an RAA message to the Policy Charging through the Gx interface.
12. The Policy Charging sends an AAA message to the AF through the Rx interface. The dedicated bearer is successfully created for the voice media stream.
13. The PCEF sends a CCR{Update} message to the Policy Charging through the Gx interface, notifying the Policy Charging that the resource is successfully reserved.
14. The Policy Charging sends a CCA{Update} message to the PCEF through the Gx interface.
15. The Policy Charging sends an RAR message to the AF through the Rx interface, indicating that the resource is successfully reserved.
16. The AF sends an RAA message to the Policy Charging through the Rx interface.
17. The AF receives an INVITE 200 message and sends an AAR message to the Policy Charging through the Rx interface to enable bearer gating control.
18. The Policy Charging sends an RAR message to the PCEF through the Gx interface, instructing the PCEF to modify bearer information about the voice media stream.
19. The PCEF updates the voice media stream status and sends an RAA message to the Policy Charging through the Gx interface.
20. The Policy Charging sends an AAA message to the AF through the Rx interface. The voice call is connected.
21. The subscriber terminates the call. The AF sends an STR message to the Policy Charging through the Rx interface to terminate the bearer session.
22. The Policy Charging sends an RAR message to the PCEF through the Gx interface, instructing the PCEF to delete the dedicated bearer of the voice media stream. The PCEF deletes the dedicated bearer.
23. The PCEF sends an RAA message to the Policy Charging through the Gx interface.
24. The Policy Charging deletes the Rx session and sends an STA message to the AF through the Rx interface. The process of deleting the dedicated bearer of the voice media stream is complete.