Mobile Fundamentals · Cellular Networks · Lesson 3 of 4

2G to 5G

What actually changed with each generation.

Four generations

Cellular networks come in generations, shortened to 2G, 3G, 4G and 5G.

• 2G (from 1991, mostly GSM) made calls digital and brought text messages.
• 3G (from 2001) was the first designed around mobile data.
• 4G LTE (from 2009) brought broadband speeds and made streaming normal.
• 5G (from 2019) adds faster peak speeds, lower lag and room for far more devices.

Phones keep support for older generations as a fallback. If the newer signal is weak or missing, a phone may drop down a generation. Many carriers have shut 3G down, and some have shut 2G, but 2G still runs in many countries.

That fallback is a security problem. On 2G, the phone proves its identity to the network, and the network never proves its identity back. A fake tower can claim to be real and the phone has no way to check. From 3G onward, the standards added mutual authentication, so the phone checks the network too. Many Android phones now let you switch 2G off for this reason.

On the simulated phone, switch the network mode between 5G, 4G LTE, 3G and 2G and compare what changes.

Fake towers

A fake tower is a device that pretends to be a real cell tower. The usual name is IMSI catcher, because the first thing it grabs is the IMSI, the unique subscriber ID stored on a SIM. You'll also hear cell-site simulator, or Stingray, after a well-known model sold to US police.

Phones tend to connect to the strongest tower nearby, so a fake one can pull in every phone around it. Once they connect, the device can log which SIMs are present. That can reveal who was at a protest, in a building or at a border crossing. More capable devices push phones down to 2G, where weak or switched-off encryption can let the operator intercept calls and texts.

Newer networks help, though they aren't perfect. Even on 4G, a phone can be tricked into revealing its IMSI before the secure connection starts. Standalone 5G hides the IMSI with encryption, but a phone that still allows 2G can be downgraded.

Police use these devices, and so can criminals or spies. Groups like the EFF build tools to detect them.