Buffer Status Reporting:-
The mobile transmits Buffer status report (BSR) MAC control elements to tell the base station
about how much data it has available for transmission. There are three types of buffer status
report, of which the most important is the regular BSR. A mobile sends this in three situations:
if data become ready for transmission when the transmit buffers were previously empty, or if
data become ready for transmission on a logical channel with a higher priority than the buffers
were previously storing, or if a timer expires while data are waiting for transmission. The
mobile expects the base station to reply with a scheduling grant.
If the mobile wishes to send a regular BSR, but does not have the PUSCH resources on which
to do so, then it instead sends the base station a scheduling request on the PUCCH. (In fact a
scheduling request is always triggered in this way, by an inability to send a regular BSR.) If,
however, the mobile is in RRC_IDLE or has lost timing synchronization with the base station,
then it has no PUCCH resources either. In that situation, it runs the random access procedure
instead.
There are two other types of buffer status report. The mobile transmits periodic BSRs at
regular intervals during data transmission on the PUSCH and padding BSRs if it has enough
spare room during a normal PUSCH transmission.

Timing Advance Commands:-
After initializing a mobile’s timing advance using the random access procedure, the base
station updates it using MAC control elements known as Timing advance commands. Each
command adjusts the timing advance by an amount ranging from −496T s to +512T s , with a
resolution of 16T s [3]. This corresponds to a change of −2.4 to +2.5 km in the distance between
the mobile and the base station, with a resolution of 80 m.
The mobile expects to receive timing advance commands from the base station at regu-
lar intervals. The maximum permitted interval is a quantity known as timeAlignmentTimer,
which can take a value from 500 to 10 240 subframes (0.5 to 10.24 s) or can be infinite if
the cell size is small [4]. If the time elapsed since the previous timing advance command
exceeds this value, then the mobile concludes that it has lost timing synchronization with the
base station. In response, it releases all its PUSCH and PUCCH resources. Any subsequent attempt to transmit will trigger the random access procedure, through which the mobile can recover its timing
synchronization.