MME

If you had to point at one node and call it the controller of an LTE network, it would be the MME. It never touches your user data — not a single packet of voice or video passes through it — but almost nothing happens without it. It terminates NAS signalling (the direct conversation between your phone and the core), runs authentication against the HSS, sets up and tears down bearers, tracks which tracking area you're in, and decides when and how to page you for an incoming call.

Because it's pure control plane, the MME is dimensioned by signalling load, not traffic volume — attaches, handovers, tracking-area updates, paging. That distinction trips people up: a network can have plenty of data-plane headroom and still buckle under a signalling storm, which is exactly what happens when a popular app hammers keep-alives or a software bug triggers mass re-attaches. MMEs are usually deployed in pools so several share the load and one failing doesn't strand a region. The 5G core splits this single node's jobs across the AMF and SMF.