Abstract: This invention proposes efficient data transmission methods in the mobile cellular network. A specific resource pool is divided into several resource groups to indicate the size level of the data available for transmission. The base station allocates a proper UL grant to the terminal. Based on the allocated UL grant, the terminal compares the granted Transport Block (TB) size with the size of data available for transmission. If the UL grant is large enough, the terminal transmits the data available for transmission in the allocated UL resource, otherwise, re-attempts the random access procedure. In another example, the terminal transmits a BSR message and as much of the data as is available for transmission. The base station may grant additional UL resources if there is remaining data to be transmitted. A timer is used for the terminal to decide whether waiting for the additional UL grant from the base station.