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LTE : UE is having a default bearer and request for a dedicated bearer (Data Call)

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There is a scenario where UE is having a default bearer and request for a dedicated bearer (lets say UE want to trigger a Data Call). As per my understanding:

UE sends Service Request + Bearer resource allocation.
After receiving Initial UE MSG .. the MME initiates activation of the default bearer with the S-GW/P-GW by sending the GTPv2 modify bearer request message to the S-GW.
Once MME receives the Modify Bearer response from SGW. MME will send "Bearer Resource command to SGW >>PWG.
Then PGW will trigger Create bearer request for Dedicated bearer establishment.
My query is why MME send modify bearer request to activate default bearer as already the UE was having default bearer.. Please correct me.
Sambit Routray

posted Sep 5 by Sambit Routray

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1 Answer

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Just thinking can it be handover scenario?
The purpose of the EPS bearer context modification procedure is to modify an EPS bearer context with a specific QoS and TFT.This procedure is initiated by the network, but it may also be initiated as part of the UE requested bearer resource allocation/modification procedure.
The network may also initiate the EPS bearer context modification procedure to update the APN-AMBR of the UE, for instance after an inter-system handover

answer Sep 6 by Sushant Bose
Thanks Sushant for the info. Please also check for the scenario "Mobile-Originated Data Call" in the below document just for reference:
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In LTE the default bearer for the selected APN will always be there, even when the UE goes to idle.
Whatabout 5g ? Which DRB is mapped to default bearer ? I mean which DRB (mapped to which QOS flow and which PDU session)will be acting like default bearer in LTE.

Sorry I haven't digged into all the specs yet. But thought will be get some nice hints here before I try to do that

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