ording to the power received from each serving cell of the PLMN network, the mobile selects the best cell. The power measurement is calculated from Cell Specific Reference Signals (CRS). The previously calculated value Ncell also determines the mapping of the Cell-Specific RS physical signal to the Resource Element (RE). The RSRP (Reference Signal Received Power) measurement is the average power measured on the first antenna port. The RSRQ (Reference Signal Received Quality) power measurement compares the power received from the cell (RSRP) to the total NG-RAN and 5G-NR power of the received signal (RSSI- Reference Signal Strength Indicator). The RSSI corresponds to the power of the signal received from the cell, the noise and interference present is added by averaging the measurement on N block resources. Cell re-selection is also based on the PLMN priority list stored in the USIM module. The mobile selects a (new) serving cell by comparing the measurement made at the physical level (L1) between all of the cells that belong to the list of neighbor cells. The re-selection threshold used for comparison and the list of neighbor cell are carried in an SIB message transmitted by the serving eNB entity. As long as the RSRP power measurement of the serving cell minus an offset is greater than or equal to the power measured at the level of the neighboring cell, the mobile remains on the serving cell. The re-selection criteria are defined for each type of radio access (4G intra and inter frequencies, 3G and 2G). 3.1.2. Random access procedure The random access procedure is initialized by the mobile and it is required in the following cases: - when establishing the connection to the eNB entity; - when changing the cell during the session (handover); - when updating Timing Advance (TA); - when re-establishing the connection to the eNB entity. The random access procedure is said to be with contention when the mobile randomly chooses the resources (PRACH channel, preamble). A contention may occur for radio connection establi