Default Bearer, Dedicated Bearer... What exactly is bearer ?

While trying to get a better understanding of VoLTE and IMS , I came across bearers stuff which I thought might be useful to share with you guys. Before we talk about what default and dedicated bearer is, let's understand what bearer itself is :
 
Bearer is just a virtual concept. It defines how the UE data is treated when it travels across the network. Network might treat some data in a special way and treat others normally. Some flow of data might be provided guaranteed bit rate while other may face low transfer.  In short, bearer is a set of network parameter that defines data specific treatment   e.g. Person A will always get at least 256 Kbps download speed on his LTE phone while for person B there is no guaranteed bit rate and might face extremely bad download speed at times

Now let me talk briefly about Dedicated and Default bearer one by one.


Default Bearer in LTE

When LTE UE attaches to the network for the first time, it will be assigned default bearer which remains as long as UE is attached. Default bearer  is best effort service. Each default bearer comes with an IP address. UE can have additional default bearers as well. Each default bearer will have a separate IP address. QCI 5 to 9 (Non- GBR) can be assigned to default bearer. 



Dedicated Bearer

To put it simple, dedicated bearers provides dedicated tunnel to one or more specific traffic (i.e. VoIP, video etc). Dedicated bearer acts as an additional bearer on top of default bearer. It does not require separate IP address due to the fact that only additional default bearer needs an IP address and therefore dedicated bearer is always linked to one of the default bearer established previously. Dedicated bearer can be GBR or non-GBR (whereas default bearer can only be non-GBR). For services like VoLTE we need to provide better user experience and this is where Dedicated bearer would come handy. Dedicated bearer uses Traffic flow templates (TFT) to give special treatment to specific services

Example

Usually LTE networks with VoLTE implementations has two default and one dedicated bearer

Default bearer 1: Used for signaling messages (sip signaling) related to IMS network. It uses qci 5
Dedicated bearer: Used for VoLTE VoIP traffic. It uses qci 1 and is linked to default bearer 1
Default bearer 2: Used for all other smartphone traffic (video, chat, email, browser etc)

Why dont we use dedicated bearer for ims signaling as well ?
IMS network is separate network from normal internet. And it comes with its own APN. Therefore we need separate default bearer for IMS network. This also helps in separating IMS traffic from normal internet traffic as well

How it is decided to send voip traffic (for example) on dedicated bearer?
Both UE and eNB has TFT which has rules for certain services. For example, incase of VoLTE voip traffic, the rule are defined on the basis of protocol number , destination network ip network etc

How is dedicated bearer linked to default bearer?
The value of "Linked EPS bearer identity" defined in setup info of dedicated bearer is used to link dedicated bearer to default bearer

Please also visit Quality of Service (QoS) in LTE to get more better understanding on bearers and QoS



51 comments:

  1. This comment has been removed by a blog administrator.

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    1. Hi ManuMamm, Thanks for your comments. I understand what you mean but actually its upto how you define Quality of service. I will try to modify so there is no confusion then

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  2. This comment has been removed by the author.

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    1. Hi,
      Its not possible. Since bearers are associated with Traffic flow templates (TFT). TFT is a packet filter and it decides which packets should be sent on particular bearer. For VoIP traffic, all packets will follow same source/ destination ip addresses or port numbers, so they can be sent on only single bearer
      Br,
      Adnan

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  3. As per my understanding we can have totally 11 bearers and out of which only 3 Default bearers w.r.t to 24.301 TS

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    1. We can have a maximum of 11 Bearers or a combination of Default & Dedicated bearer where total number of bearers should not exceed 11.

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  4. How is radio bearer mapped to default/dedicated bearer?
    will there be a separate radio bearer for each default/dedicated bearer?

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    1. Yes there will be separate radio bearer for each default / dedicated bearer. DRBs are mapped via LCID (Logical channel id) at MAC layer. Each LCID defines a radio tunnel to provide QoS to user at radio level.

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  5. If there are multiple PDN connections, will there be multiple Radio bearers with one-to-one mapping?

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    1. Multiple PDN connections means multiple default bearers. Multiple default bearers means multiple radio bearers. So correct, there is separate radio tunnel identified by LCID value for each default bearer

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    2. Does it mean the same radio channel is divided into multiple logical channels?

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  6. What will be the difference if operator decides to use dedicated bearer with QCI5 for SIP signalling or default bearer with QCI5.Why is it that default bearer is always preffered for SIP signalling?Is traffic separation is the only reason, I mean why cant we use dedicated GBR bearer bearer for this more often?

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    1. Well separate default bearer is preferred for SIP signalling because in addition to separating IMS traffic we also want to give SIP signalling priority over other traffic and you know QCI 5 has the highest priority check the QCI table here http://4g-lte-world.blogspot.jp/2013/01/quality-of-service-qos-in-lte.html Hopefully you get more idea

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  7. Hi, we can use dedicated bearer with QCI 5 for SIP signalling i suppose, in case we have same APN for IMS as well as for internet traffic. What you suggest

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  8. Hi Sagar,
    Yes in that case you can use, there is no restriction but again that wouldn't be the standard way

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  10. Hi,

    Since there are up to 9 qci standardized values that a bearer can have, and 10-255 are qci values that an operator can define, how and at which point/network element will the bearer have qci with such operator defined values?

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    1. Hi,
      I think the range is from 128-254
      Operator specific QCI will BE applied on radio bearer, s1 bearer and s5/s8 bearer

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    2. Where do you find the reference information for Operator Specific QCI in the 3GPP documentation ?
      regards

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  11. Very good explanation!! Thanks Basir

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  12. This comment has been removed by the author.

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  13. Hello,
    Can you tell me how does the UE know whether it is scheduled on the dedicated bearer or the defaults one? Is there any indication of it in DCI? From my undertanding it cannot be the TFT because there is one TFTs for each dedicated bearer.

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    1. Lets consider one example.
      There is some Uplink data in UE buffer related to VoIP (VoLTE rtp traffic). So now assuming the dedicated bearer (DRB) is already established previously and there is TFT defined in UE. So UE checks the ip data and finds that this data should flow on one specific DRB defined. DRB mapping at MAC layer is done through LCID to provide unique QoS to specific bearer

      Sorry if it is still not clear..

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    2. So now its mac layer responsibility how to schedule this to obtain required QoS

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  14. hi Adnan
    could you tell me in which 3GPP TS I can find the source of this information about the VoLTE have to have 2 default and one dedicated bearer please

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    1. Sorry for late reply
      http://www.gsma.com/newsroom/wp-content/uploads/2013/04/IR.92-v7.0.pdf

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  15. What is the value of GBR in case of dedicate bearer?

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    1. The GBR or MBR can be configured on the PCRF per PCC Rule. You can refer to TS 29.212.

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  16. Establishment of dedicated bearer or the default bearer is the reservation of some capacity in the core and the radio network??
    if it is the reservation of the capacity then how much it is for default bearer ?
    and don't you think the default bearer is the wastage of the capacity?

    please reply me .. Thanks in advance

    Regards
    Ashwani

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    1. Hi, its quite hard to explain default bearer in terms of how much resources it will utilize..
      I think its important to have one default bearer available to UE all the time since our smartphones are always connected and they send/receive lot of data in the background..

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  17. If we look at the QCI table then for QCI 8 and QCI 9 packet delay and packet loss is same, then what is the difference between them?

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  18. is there any relationship b/w dedicated bearer and secondary PDP?

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    1. I am not sure if there is any relationship during PS handover scenario .. will try to check it..

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  20. As way of comparison, the LTE Attach/Default EPS Bearer will be equivalent to the UMTS Attach and then doing a Primary PDP Context establishment procedure. The Secondary PDP Context Activation is similar to the Dedicated EPS Bearer Setup procedure. So, there is a variation of a theme here, but not a significant difference.

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  21. Hi Adnan,

    Please send me some link where I can have under standing of EPS attach,default and dedicated bearer.

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  22. This comment has been removed by the author.

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  23. Hi Adnan,

    Great Blog!

    I'm interested what is the trigger for Dedicated Bearer activation? Lets say we have UE, which is attached, and it has a Default Bearer. Now we start using VoIP application, and we have two additional traffic types, the first SIP signalling, and the second RTP voice. Will the UE recognize QoS requirements from VoIP application, map them to QCI and send request to MME/SGW/PGW for new bearers, or core network will recognize new type of traffic, and send request to UE?
    In which case UE can initiate bearer modification?

    Thank you in advance!

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  24. Hi Adnan

    Is it possible to have 2 default bearers towards the same PDN( PGW)? One for say IMS signalling and the other for other signallings that require different QCI.
    Thanks

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  25. Hi,

    As evident,if i understood correctly, an user(UE) can have maximum of 11 Bearers(default+dedicated) and Each default bearer will have a separate IP address. It would be great if anyone can clarify with some real-time example.
    I am bit confused here, does the above facts means an user can access only 11 websites at a time.(obviouslu which should not be there).

    Thanks in Advance.

    Regards,
    Rajesh

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  26. Very good points with exemples..

    thanks,
    Vipin

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  27. HI ... new to this group ... can any tell that if I need to have only 3 dedicated bearers per UE , 4th dedicated bearer will not happen , where will I set this policy/rule for UEs

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  28. suppose I have 3 APNs (MMS, Internet, VOLTE) configured in phone, So there will be 3 PDNs and hence 3 default bearers ?
    Is that correct no. of PDNs configured are same as no. of default bearers ?

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  29. Hi
    How MMS wotk in 4G ?
    as per my understanding,
    In the HSS you have the default APN for let's say Internet, you have as well defined the MMS APN.
    When you are attached to the LTE, you will be provided the APN Internet, so you will have a default bearer linked to the APN Internet.
    Then If you want to do MMS, You will have to go through the APN Internet ((I think) because it is not possible to use MMS APN, unless your MMS APN is defined as default somewhere else in an other HSS, in an other domain (like IMS) but I don't think it is the case.
    You can only have one default bearer, because you are not able to have to default bearer at the same time, unless using an other network so a an other HSS for example the one in IMS network for VoLTE
    An other case is that you can Imagine that the default APN Internet in the HSS is replaced by the APN MMS when the service MSS is asked, but this action supposed to detached from LTE an reattach with this APN MMS, so not sure it is what we have
    In 2G/3G it is different, because once you are attached, you are not supposed to have APN connexion, so you have the choice. as long as these APNs are defined in HLR.
    In 4G you are blocked with the default define in the HSS which is normally not the MMS one,
    so How MMS is working in 4G ?

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  30. What will happen to QCI values in case traffic from LTE is offloaded to Wifi?

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  31. It really is incredibly fantastic and informative website. Good to discover your site Very well article! I’m simply in love with it. IP Phones in Pakistan

    ReplyDelete
  32. As a good habit, always provide full forms when first time a short form is used

    ReplyDelete
  33. can anyone please clarify difference between default and dedicated bearer , like any parameters changes or any scenario's difference

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