You have to have security at that level as well. The protocol stack services that maintains the radio links to cell towers are client and servers, other parts

You have to have security at that level as well. The protocol stack services that maintains the radio links to cell towers are client and servers, other parts are between the cell phone and the mobility management node are client and servers. These are services you as a cell phone user don't interact with directly, but still very important as they provide authentication of the user and (ideally and hopefully) authentication of the network, access or indication of your location, and other neccesary services needed to seamless move between cell towers.