s available for a longer time than the DIFS, the station can transmit without the backoff timer. If the radio channel is busy and another station wishes to transmit, it must use the backoff timer, which is the product of a random number and the time of the time slot (ST) (Figure 2.9). ST (SlotTime) traffic Station A traffic Station B traffic Station C traffic Station D traffic Station E Figure 2.9. Backoff mechanism Wi-Fi Integration to the 4G Mobile Network This timer avoids the collision, which occurs only when two stations have drawn the same random number. At startup, the random number is chosen in the contention window between 0 and 15. At each collision, the contention window is doubled until it reaches the maximum value 1023. The radio channel is declared inaccessible after N access attempts, N being a parameter of the transmitter. For a station, the consumption of this timer stops when the radio resource has been allocated. It resumes when the resource becomes free after the DIFS timer. 2.2.4. Clear channel assessment Clear channel assessment (CCA) is determined at the physical level or at the logical level. At the physical level, the station is based on the detection of energy or the carrier in the radio channel. At the logical level, the station uses the Duration field of the MAC header. Logical level detection solves the problem of hidden stations. If two stations A and B are separated by an obstacle, these two stations being connected to the same access point, each station cannot detect a transmission from the other station. The frames coming from the access point and containing the Duration field provide each station with an indication of the occupancy time of the radio channel. The Duration field of the RTS frame contains the occupancy time of the radio channel. It is equal to the sum of the duration of three SIFS intervals, CTS and ACK control frames and a traffic frame (Figure 2.10). The Duration field of the CTS frame contains an update of the occupancy time of the radio channel. It is equal to t