Scheduling Request

Scheduling Request -
It is used to request the uplink resources. Suppose there is UL data from upper layer and UE does not have any resources for UL data transmission. So there should be some mechanism to inform the eNodeB like i have some data so please assign some resources.

There are two ways to request resources from eNodeB -
1. Using Rach procedure.
2. Scheduling Request.

Scheduling request is fast as compare to RACH as RACH make use of common resources whereas SR procedure make use of dedicated resources.

eNodeB configure the UE for SR resources, it not always mandatory for eNodeB to configure the SR resources, so in case there is no SR resources then there is only one option for resource request i.e. Rach procedure.

SR request can only be sent if UE is uplink time aligned means "TimeAlignmentTimer" is running.
If there is UL data but TimeAlignmentTimer has been expired then UE make use of RACH procedure.

SR parameter configuration - 

Logical channel ID and Logical Channel group

Logical channel ID and Logical Channel group.
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To understand this, we need to understand little about bearer (SRB/DRB). A Radio bearer is a logical connection between the UE & eNodeB.
Except SRB 0, all RB's make use of dedicated resources assigned by eNodeB.
SRB 0 is used when there is no RRC connection exist.

==> In LTE system, there exists three SRB i.e. SRB0, SRB1 and SRB2 and corresponding logical channel identities are 0, 1 and 2 respectively. These identities are fixed as per specification.

==> In LTE system, there exists at max 8 DRB and corresponding logical channel identities are 3 to 10 respectively.

DRB & EPS bearer has one to one mapping, so it means there can at max 8 EPS bearer. An EPS bearer is logical connection between UE & P-GW.

==>  Logical channel ID = DRB ID + 2 as 0, 1 and 2 identity is used for SRB.

                                       

Logical channel Group - There are maximum 4 logical channel group and each logical channel belongs to some logical channel group.
Logical channel ID are aggregated in a logical channel group based on some parameters like similar QoS.
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What is BSR and how it work ?

 What is BSR and how it work ?
BSR is buffer status report. As we know that DL/UL resource allocation is controlled by eNodeB, so for the planning for resource allocation it is necessary for eNodeB to get to know about the pending data from different UE.

Direction of BSR is from UE to eNodeB, there are three different type of BSR as follows-
1. Long BSR.
2. Short BSR.
3. Truncated BSR.
4. Regular BSR.
5. Periodic BSR.

To understand  the difference between Long, short and trucated BSR, it is necessary to know the concept of logical channel and logical channel group.