Bearers Questions

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Bearers Questions

  • Comments 10
There were a couple of good questions on the LTE University forum page recently. I thought it would be good to address those questions. The first question was: are default bearers created on a per UE or a per PDN (Packet Data Network) basis? Default bearers are created on a per PDN basis. So if a UE is connecting to two PDNs it will need to establish two default bearers. The best example that I can come up with for why a subscriber may want to connect to multiple PDNs is if subscriber wants to connect to an IP Multimedia Subsystem (IMS) network and the Internet. The first default bearer will be established during the Attach process when the UE first powers up and the second will be done using Activate default EPS bearer context request procedure. When the second default bearer is established will be dependent on the service and the UE.
 
The second question is, exactly how many TFTs can be used per UE in a P-GW? There are a number of ways to take this question. One way to interpret this question is to ask how many active bearers a UE can have? Another way to interpret this question is to ask what the the number of TFTs that a P-GW can support per UE? Let's look at both interpretations of the question. From the LTE specs, the maximum number of bearers that a UE can establish is 11 (as much as I would love to put a Spinal Tap reference in at this point, I will restrain myself). The maximum of 11 is based on a parameter called the EPS bearer identity size. It is four bits in length with five values reserved. The number of TFTs that will be required for a UE is based on the number of default and dedicated bearers. There will need to be a TFT for every dedicated bearer. That could be a maximum number of TFTs of 10 (assuming one default bearer and 10 dedicated bearers). That seems an unlikely scenario. More than likely there will be a few default bearers and each of those default bearers will also have a dedicated bearer or two, depending on the scenario.
 
I hope these help to understand how the EPS bearers are established. If you have a question, try the LTE University forum.
 
 
  • First of all, Thank you for the very nice explanation.

    >One way to interpret this question is to ask how many active bearers a UE can have? Another way to interpret >this question is to ask what the the number of TFTs that a P-GW can support per UE?

    ...

    >That could be a maximum number of TFTs of 10 (assuming one default bearer and 10 dedicated bearers).

    >More than likely there will be a few default bearers and each of those default bearers will also have a dedicated >bearer or two, depending on the scenario.

    To me it seems the right question for the above answer is more likely to be ...How many TFT's (Bearers) a PDN can support per UE ?  The answer is 10

    Considering there can be multiple (N) PDN associated with a single P-GW and a maximum number of 10 TFTs per PDN, hence N*10 numbers of bearers possible.

    Having said that , The maximum possible number bearer id is restricted up  to 11hence the max linked bearer id (default bearer ID)  or N (max) = 11.

    Maximum number of bearers possible is  11* 10

    Does it make sense?

    I am interested to know the answer for this question. Please reply me at krssagar@oss.com

  • Hello,

    Too many questions today.

    In section 6.5.3.2[UE requested bearer resource allocation procedure] of 24.301, does this apply to a default bearer or dedicated bearer ?

    thanks

    Ashmath

  • hi,

          Is RLC PDU size and grant both same or different.

  • Do Default Bearer Carry and Data?

  • Hi Chris,

    Kindly clarify my below queries,

    1.Can UE establish 11 DRBs per PDN basis?

    2.Consider the case where UE is in 11 DRBs and the 2 SRBs established state and if there is an one to one

       mapping between RBs and Logical channels how can all this 13 bearers get mapped because as per the MAC spec

       there are only 10 Logical channel Identities are possible.

    3.Does the bidirectional bearer need two logical channels at MAC level?

    Thanks in Advance.

  • Can i tell bearer which is default or dedicated bearer by seeing any parameter?

  • Hello Narayan,

    Where are you looking? In the messages themselves, the default bearer will use a different set of GTP and NAS messages than the dedicated. That would be the easiest way to see the difference. Let me know if you have further questions.

    Thanks and take care,

    Chris

  • Hi Chris,

    For UE initiated dedicated bearer activation, how does interaction with PCRF happens? My assumption was that, PDN gateway after receiving Bearer Resource command(BRC) from SGW, should trigger

    PCRF with Credit control request(CCR). Upon successful Credit control answer(CCA), Create bearer request is triggered towards SGW, which in turn goes towards MME, ENB.

    Create Bearer response towards PDN gateway should contain the EPS Bearer ID for the dedicated bearer (apart from Linked EBS bearer id). After receiving Create bearer response at PDN gateway, whether there should be another interaction with PCRF to exchange the received EBS bearer id, is this required at first place? If so, what message should be used for this? Again CCR?

    In TS 23.203 (Figure 7.4), it has been mentioned that Provisional ACK (message number 15) should be sent to PCRF upon completion of IP CAN session signalling (in this case Create bearer response). Whether this should be done only based on event trigger received from PCRF in the earlier credit control answer?

    If this provisional ACK is not requested by PCRF, then is the EPS Bearer ID (created by MME for dedicated bearer) not required for the PCRF?

    Could you please provide your views/comments?

    Thanks,

    kumar

  • Hello Kumar,

    You are correct. My understanding is that the UE initiated dedicated bearer will trigger the P-GW to send a CCR to the PCRF. I can't comment more as I have not seen a UE initiated dedicated bearer used in practice, but that is my understanding.

    Thanks,

    Chris

  • Hi Chris,

             Thanks for sharing ur knowledge with us.

              If u have any idea regarding DYNAMIC RESOURCE ALLOCATION (Scheduler), then plesase share with us.

              I mean how it is going to allocates UL and DL resources to user and control plane packets

              and how it selects the radio bearers whose packets are to be scheduled.

    Thanks,

    Pankaj