SR (Scheduling request) is use to notify the eNB that there is data to transmit at the UE RLC/PDCP. So eNB grant some minimum resources to UE.
With these resources UE start transmitting UL data, where MAC header consist of MAC CE , indicating BSR. By reading BSR then eNB keep giving grant to UE as long as there is BSR.
So in short, BSR cannot be sent in uplink unless UE have some UL grant. And to get UE UL grant, UE has to send SR.
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