Transmission happens over a public, shared medium (air), subscribers are mobile with no fixed connection, and they constantly switch base stations — so the network must identify, track, and bill a moving target it can never physically pin down.

The core difficulties:

• Public medium: the air interface is open — anyone in range can receive the signal, so confidentiality cannot be assumed from physical isolation (unlike a cable)
• Mobility: subscribers move, and connections are not fixed to a port or line
• Changing base stations: a user roams across many base stations and transmission resources

The mobile network's answers:

• Unique identification of a subscriber (so a connection can be attributed)
• Unique identification of a device
• Tracking of the user via Location Identifiers (LAC/TAC) across base stations
• A mobile anchor: the home-network provider handles billing and authentication via the HLR

The hard question: how do we keep TCP sessions and encryption alive while moving fast between base stations by car, bus, or train? Every security mechanism in GSM/LTE must survive that mobility — which is exactly why mobile security is its own discipline.

Go deeper:

• Cellular network (Wikipedia) — the cells-and-handover model that creates the moving target: frequency reuse, mobility management, handoff.