In short, it's downgrade. Since 3G, there is mandatory mutual authentication [except for emergency calls] with integrity protected management traffic [except f

In short, it's downgrade. Since 3G, there is mandatory mutual authentication [except for emergency calls] with integrity protected management traffic [except for emergency]. The problem is, identification and key agreement require prior set-up of the lower network layers. If a cell station indicates an error during the early stages (i.e., before mutual authentication) of connection establishment in 4G, the smartphone falls back to earlier generations.