As described in the previous chapter, the base station explicitly schedules all the transmissions that the mobile carries out on the physical uplink shared channel. If the mobile wishes to transmit on the PUSCH but does not have the resources to do so, then it usually sends a scheduling request on the physical uplink control channel. If it does not have the resources to do that, then it initiates the random access procedure. This can happen in a few different situations, primarily during the establishment of an RRC connection, during a handover or if the mobile has lost timing synchronization with the base station. The base station can also trigger the random access procedure, if it wishes to transmit to the mobile after a loss of timing synchronization.
The procedure begins when the mobile transmits a random access preamble on the physical random access channel (PRACH). This initiates an exchange of messages between the mobile and the base station that has two main variants, non-contention-based and contention-based. As a result of the procedure, the mobile receives three quantities: resources for an uplink transmission on the PUSCH, an initial value for the uplink timing advance and, if it does not already have one, a C-RNTI.
The random access procedure is defined by the same specifications that were used for data transmission and reception. The most important are TS 36.211 , TS 36.213 , TS 36.321  and TS 36.331 .