GSM is used in over 170 countries to cover Over 400 GSM network operators,Over 550 million people were subscribers to GSM.GSM as an OSI related model.
GSM uses Gaussian Minimum Shift Keying (GMSK).GSM 900 uses 50 MHz in two 25 MHz bands for up and down links.

GSM functions in three bottom layers of the OSI model :

Physical
Data-Link
Layer 3

Goals of GSM:

It improved spectrum efficiency

It support International roaming

Low-cost mobile sets and base stations

High-quality speech

Compatibility with ISDN and other telephone company services.

Support for new services

GSM Architectures is three broad parts

1. Subcribers carries MOBILE STATION
2. BSS controls
3. Network Subsystems

Acronyms in GSM:

MS : Mobile Station
BTS : Base Transceiver Station
BSC : Base Station Controller
MSC : Mobile-services Switching Center
VLR : Visitor Location Register
HLR : Home Location Register
AUC : Authentication center
EIR : Equipment Identity Register