adio resource allocation (e.g. with time slots reservation, which is possible in mobile networks that use TDMA in combination with other multiple access technologies). Additionally, WiFi operates in unlicensed frequency bands (2.4 GHz, GHz, and lately 60 GHz), which are not reserved (as spectrum bands for mobile networks), SO they can become congested when many users are using WiFi networks that are close to each other and overlap in the coverage area (e.g. in dense residential areas or in enterprises). Similar to Ethernet, IEEE defines the WiFi standard on the lowest two layers of the protocol stack, that is, the physical layer (i.e. layer 1) and the data-link layer (i.e. layer 2). However, after being frozen as the main standard IEEE 802.11, additional developments of WiFi are given as amendments, noted as additional letters added to the standard. There are different amendments on protocol layer 1 and layer 2. The initial IEEE 802.11 standard provided bitrates up to 2 Mbit/s; it was followed by IEEE 802. 11b with up to 5.4 QoS and Giga Speed WiFi Table 5.4 WiFi maximum and bitrates. Theoretical Throughput maximum (assuming 70% Frequency WiFi standard bitrate efficient layer 2) bands IEEE 802.11 2 Mbit/s 1.4 Mbit/s 2.4 GHz IEEE 802.11b 11 Mbit/s 7.7 Mbit/s 2.4 GHz IEEE 802.11a 54 Mbit/s 37.8 Mbit/s 5 GHz IEEE 802.11g 54 Mbit/s 37.8 Mbit/s 2.4 GHz IEEE 802.11n 600 Mbit/s 420 Mbit/s 2.4 and 5 GHz IEEE 802.11ac 6.9 Gbit/s 4.8 Gbit/s 5 GHz IEEE 802.11ad 6.9 Gbit/s 4.8 Gbit/s 60 GHz IEEE 802.11ay 40 Gbit/s 28 Gbit/s 60 GHz 11 Mbit/s (in 2.4 GHz), and further continued in the 2000s with IEEE 802.11g (2.4GHz) and IEEE 802.11a (5 GHz), which provided bitrates up to 54 Mbit/s (Table 5.4). Regard- ing the physical layer amendments, the development of WiFi continued with IEEE 802.11n, which provides up to 600 Mbit/s (in both 2.4 and 5 GHz), and IEEE 802.11ac with up to 6.9 Gbit/s (operating in 5 GHz band). WiFi further extends into the 60 GHz unlicensed spectrum with IEEE 802.11ad, and further with IEEE 802.11ay, which is 