Saturday, January 19, 2013

LTE call flow beyond PGW

                       Basic LTE Call Flow beyond PGW.

Here are the call flow from PGW to PCRF on Gx interface.

During Initial Attach procedure, PGW check for the APN/Charging Characterstics received in Create session request for whether dynamic Policy(configured in PCRF) or Static policy(configured on PGW) should be applied. Depending upon the configuration, PGW forwards the request to PCRF for providing PCC Rule for the UE attach request.

PCC rules consist of:
  • Rule name - This is to be used to reference a PCC rule in communication between PCEF and the PCRF . 
  • Service data flow filters - shall be used to select the type of traffic that passes through the bearer on which the rule applies.
  • precedence : Determines the precedence of the bearer on which the rule is applied when data is received uplink or downlink for a particular type UDP/TCP etc.  
  • QoS parameters - Includes QoS Class Identifier (QCI), also the Allocation and Retention Priority (ARP) and GBR,MBR for uplink and downlik.
  • Charging type - Defines whether online,offline or both or none charging is applied.
  PGW sends CCR(Init) to PCRF to get the PCC Rule from PCRF.
  PCRF has primarily two entities. PDT (Policy Derivation table) and PCT (Policy and
  Configuration table).
  In PCT, PCRF has number of PCC Rules configured.
  In PDT, PCRF has the selection criteria based upon which PCRF sends the selected PCC Rules in 
  Selection criteria- Can be IMSI,APN name, QCI, MBR/GBR etc. means if CCR request contains 
  the given value of IMSI or QCI and so then particular PDT is applied. 
  For Example: In CCR (Init), PCRF receive IMSI 122334455667001. Based upon this IMSI, PCRF
  has PDT configured in which PCRF applies two PCC Rules let suppose PCC1 and PCC2.
  Upon getting successfull PDT matched PCRF sends successfull CCA (Init) to PGW.
  Now, upon getting of successfull CCA (Init) PGW sends CCR (Update) to confirm the PCRF that  
  the Rule is applied  successfully on the particular bearer (in this case default bearer) and also
  inform the ip address of the UE.Then PCRF acknowlege with CCA(Update) success.

  Now PGW has already applied the Rule(Qos,filters,MBR,GBR etc) received from PCRF to
  the requested bearer, it sends Create Session Response to SGW and then the Default session created
Similar Procedure follows for dedicated bearer also.
  In dedicated bearer creation/updation/deletion, PGW sends CCR Update to PCRF.PCRF 
  acknowledge the request with CCA Update.

Note: CCR (Init) send by PGW only once for a UE session i.e on default bearer creation. 


  1. Hi Amit,

    Nice blog!!!

    I am looking for more details if you have some materials it will be great if you can share.

    There are somethings in call flow diagram like RAR and RAA which I didn't get. Can you pl. explain it?

    1. Hi Sakshi,
      Thanks !!!
      RAR (ReAuth Request) sent by PCRF to PGW(PCEF) upon getting trigger from AF(Application function) part of IMS. This basically happens when there is network initiated bearer creation/deletion/modification. For more detail you can study on Rx interface.

  2. Hi Amit,

    Nice article.

    I want to know a basic information needed in PCT and PDT . It will be very good if you can give some practical example. Lets say user has subscribed to "512kbps internet" , voice and ims. What information is there in PCT and PDT.

    Lets say, at initial attach the UE is just assigned the default beasrer. Which rule is forwarded to P-GW from PCRF in this case.

    Later UE is trying voice call, what happens to the rule sent while creatung the default bearer ?

    Also later, on UE does internet access. What happens in this case ?

    thanks a lot in advance for your time.
    Btw: Do you have any good boooks/reference material, which gives some basic understanding of PCRF rules in PCRF and P-GW. I have tried to read specs, but it is too difficult to understand from beginner.

    1. UE sends a request for Voice it sends a QCI value 1 for voice service. PCRF then selects a PDT based on QCI 1 received in CCR.
      For example:
      PDT Configuration: IF QCI =1 in received then PDT x is selected in PDT x, PCT y ( Rule1) is selected.
      PCT Configuration : If PCT y, Rule 1 is applied and sent in CCA.
      Rule 1 Configuration: QCI 1, GBR 512 kbps, MBR 1 Mbps and charging info ( characterstics & type) etc.

      I hope it clears the process of selection and application of PCC rules.

  3. Hi Amit,

    Sorry for the delay. But your reply helped a lot in understanding the LTE call flow.

    I have another question, regarding the PCRF architecture.

    How does the PCRF store the PCC Rules (based on what index). I mean lets say same QOS is applied for two Applications (mapped to SDF1 and SDF2, both mapped to dedicated bearer id 1. )

    So lets say the App related to SDF#1 is stopped. Then PCRF needs to delete PCC Rule related to SDF#1.

    But the rule is still valid for SDF#2. Any pointers on how this is implemented in PCRF.

    It will be great if you can give some practical example (with just 1 or two rules in PCRF) .

    Also what AVPs will be sent in the Diameter messages esp in Gx interface... Also may be bit about Rx interface ?

    thanks in advance for your time and I appreciate your LTE knowledge.


    1. Sorry for the delay.
      If same QOS is applied for two Applications (mapped to SDF1 and SDF2, both mapped to dedicated bearer id 1.) if App related to SDF#1 is stopped than nothing will be triggered as PCC Rule is for the Bearer id 1 having the Qos value and both the Apps(SDF1,SDF2) are sending the data from the same bearer.

  4. Update bearer/Modify bearer request will be exchanged between MME/SGW with out change in Rule

    1. Thanks for reading the article.
      May you please clarify your doubt/query ? Will be happy to answer.

  5. Hi Amit,
    This is very good information,
    I am stuck in an issue where I wanted to know can a PGW direct request to 2 PCRF's based on the IMSI.


    1. No,
      PGW can select the PCRF based on MCC and MNC only. IMSI based selection is not possible