What is the UE in LTE, and what identity does it carry?
The UE (User Equipment) is any device with an LTE radio — smartphone, tablet, laptop, IoT device — identified by the IMSI stored on the SIM card.
UE essentials:
- LTE jargon says "User Equipment (UE)" instead of "mobile phone" — because the device might be a tablet, laptop, modem, or an IoT sensor
- The subscriber identity is the IMSI (International Mobile Subscriber Identity), stored on the SIM in a 64-bit field — in practice an IMSI is 15 decimal digits (≈50 bits): MCC + MNC + subscriber number
- The IMSI lives on the SIM card (Subscriber Identity Module), not in the device itself
Why the IMSI matters for security:
The IMSI is the key that links a radio transmission to a person's subscription. Anyone who can capture IMSIs can track people — which is exactly what "IMSI catchers" (fake base stations) do. Modern networks try to send the IMSI as rarely as possible, using temporary identifiers instead once the device is registered.
Tip: Device ≠ subscriber. The phone has its own hardware identity (IMEI), while the SIM carries the subscriber identity (IMSI). Swap your SIM into another phone and the network follows the IMSI, not the hardware.
Go deeper:
Fangespielen mit IMSI-Catchern (CCC Camp 2019, de/en) — how rogue base stations harvest IMSIs in practice and how 2G→4G security features change what a catcher can still do; the real-world threat behind this card.
IMSI-catcher (Wikipedia) — the fake-base-station attack itself: why GSM's missing network authentication makes IMSI capture possible, and how 3G/LTE mutual auth (plus 2G-downgrade) changes the game.- International mobile subscriber identity (Wikipedia) — the IMSI itself: its MCC+MNC+MSIN structure, storage on the SIM, and why a TMSI is sent instead to avoid tracking.