among the sibs the common time alignment timer is the timer used in the pcell when the ue does not receive u2018 dedicated time alignment timer u2019 or u2018 scell time alignment timer u2019 form the enb this timer is used in the random access procedure for the ue to transition from idle state to active stat this is described in detail later if the sib 2 is received at step 611 the ue stores the u2018 common time alignment timer u2019 and transmits a random access preamble to the cell at step 613 the random access preamble is a special signal transmitted from the ue to the cell for uplink access or synchronization if the random access preamble is received successfully the enb transmits a random access response rar message at step 615 the rar includes timing advance ta command as the information on how much bring forward back the uplink timing based on the ue 39 s uplink timing here the time when the random access preamble has been transmitted the ue starts the tat for the cell here pcell 603 upon receipt of the rar at step 617 it is assumed that the uplink synchronization between the cell and the ue is acquired while the tat is running and thus the cell may perform resource allocation to the ue for uplink data transmission the tat expiry time corresponds to the time when the u2018 common timer alignment timer u2019 received at step 611 expires by taking notice that the ue supports carrier aggregation.