How many DRB's in LTE? can some one explain their Functions?

Question

Hi All,

It is mentioned in different websites that in LTE max. of 8 DRB can be established per UE. But as per page 652 of 36.331 V13.2.0, the max no. of DRB is 11.
"maxDRB INTEGER ::= 11 -- Maximum number of Data Radio Bearers"

Based on what source are we saying that max. DRB in LTE is 8 and not 11.

It was mentioned in one of the old threads the following:

If EPS bearer ID is = x +4 ;
Then
DRB ID = x;
Logical Channel ID = x + 2

I found the above relationship of DRB ID & EPS Bearer ID to be true for one of the eNB vendors but not for another one.

Example:
EPS ID is 5 and DRB ID is 4 instead of 1. Can somebody comment in this?
NOTE : All these ranges is with respect to DRB establishment

EPS Bearer ID Range is INTEGER (0..15) (But 0-4 is reserved so we use starting from 5 )
DRB ID Range is INTEGER (1..32) (As in LTE max 8 DRB can be established per UE so we use from 1-8)
Logical Channel ID range for DRBs is INTEGER (3..10) ( For for DRB1 we use 3 as logical channed ID)

Thanks

Answer 1

Answer is 8. As per Table B.1-1 of 36.331 following DRB combinations are supported, based on featureGroupIndicators, received in UE capability information message .

Case 1 ( Bit 7 and bit 20 are set 1 ) :

SRB1 and SRB2 for DCCH + 8x AM DRB    OR   SRB1 and SRB2 for DCCH + 5x AM DRB + 3x UM DRB

Case 2 ( BIT 7 is set 0 and BIT 20 is set to 1) :

SRB1 and SRB2 for DCCH + 8x AM DRB

Case 3 ( BIT 7 and BIT 20 are set to any value ie 0/1) :

SRB1 and SRB2 for DCCH + 4x AM DRB

Case 4 ( BIT 7 is set to 1 and BIT 20 is set to any value 0/1) :

SRB1 and SRB2 for DCCH + 4x AM DRB + 1 UM DRB

I hope this should clarify ur doubt.

Comments

Thank You for your earliest Response.

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can u please explain little more about the BIT 7 and BIT 20? why depends on this value only RLC mode is changing?

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Not sure what u are trying to ask, let me explain bit 20 and Bit 7 -
Regardless of what bit number 20 is set to, if bit number 7 is set to ‘1’, UE shall support at least SRB1 and SRB2 for DCCH + 4x AM DRB + 1x UM DRB