= GSM =

GSM or the global system for mobile communications is a standard developed to
standardize cellphones and their communication and infrastructure.

GSM handsets derive the baudrate from very very accurate clocks on the base
station.

== Bands ==

GSM occupies 

* 850 (USA)
* 900 (europe)
* 1800 (europe)
* 1900 (USA)

*NOTE* GSM-900 uses 880-914mhz
The 33cm ham band is 902-928mhz. Therefore, for 902-914 we can transmit legally
as a eurpean tower.
This band is also the ISM (industrial scientific medical) band. Most devices on
this band are low power and low utilisation.

== IMSI ==

International Mobile Subscriber Identity is a way for devices to id themselves
on a GSM network. An IMSI is a secret key that is stored on the SIM card.

=== IMSI catcher ===

A spoofed GSM tower. In GSM the base station (tower) picks all settings
including Tx power, Rx gain, encryption, etc

When the tower tells the handset to disable encryption (A5/0), and handset
_should_ tell user about this. Hoever every manufacturer disables this