At the data link and physical layers
The 802.11 wireless Lan function is a set of data link and physical layers. Data link layer or the MAC is responsible for effective link-link data transfer, addressing, error detection and frame validation. Physical layer on the other hand helps in placing bits on the network. Basically, all wireless and WI-FI systems conform use these two layers to format and control data to conform with 802.11 wireless Lan standards