LTE: What does impacts at eNodeB MAC scheduler , if an UE has GBR bearer as default bearer ?

Question

In LTE, when UE is first time attached to a network, it is usually allocated a default non-GBR bearer. Can this bearer be converted to a GBR bearer, without releasing the UE context at eNodeB?

Answer 1

Hi vimal,

RRM will maintain the assigned resource details, say example ENB has 50Mbps of resource in 10mhz Bw. If NW configures default bearer as GBR say example 1mbps of GBR rate then NW can assign only max of 50 GBR default bearer because it has only 50mbps of guaranteed bit rate. Once you assigned GBR bearer with some GBR rate ,ENB should not give that resource to any one else(That is the concept of GBR bearer even if it not utilized by that UE), in case of N-GBR bearer there is no gaurenteed bit rate so NW can admit more than 50 UE(in this case). That is the reason NW always configures N-GBR as default bearers.

Happy Learning :)

Comments

Thanks Jaganathan to share your knowledge on this query. Well explained !!!

Answer 2

UE can not have gbr default bearer. Whenever UE will attach to the core network, it will get non gbr default activate request from core network. After that UE can have dedicated gbr bearer only.

Comments

Hi Veer, I am not completely agree with your answer saying UE can't have GBR as default bearer. In deployment scenario, network does not prefer to establish GBR bearer so that it can use available radio resources (RBs) in the best possible way. Whether network will establish GBR or non-GBR bearer, it depends on subscription of UE. I have tested default bearer (GBR) for an UE in our lab. I have not faced any issue.

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I agree with vimal, because i also tested this scenario. It is not mandatory that default Bearer should be N-GBR, It can be GBR bearer but as i explained GBR bearer should reserve some resource unnecessarily that is the reason network is configuring n-GBR bearer  .

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As per 23.401 3gpp spec , below is mentioned regarding default and dedicated bearer.

A dedicated bearer can either be a GBR or a Non-GBR bearer. A default bearer shall be a Non-GBR bearer.
NOTE 8: A default bearer provides the UE with IP connectivity throughout the lifetime of the PDN connection.
That motivates the restriction of a default bearer to bearer type Non-GBR.

From this my understanding is that default bearer shall be non gbr bearer. Also in HSS when we add new UE then HSS only allow qci 5 to 9 for default bearer.

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Thanks Veer to update me.

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Hi veer,

Ya this is there in spec, but thing is default bearer shall be n-GBR, It is not should be. If you see spec for other mandatory things they will mention it should be. So it is optional , It can be N-gbr or GBR.

Happy Learning :)