What is GSM and what are its key features?

GSM (Global System for Mobile Communications) is a 2G digital cellular standard developed in Europe. Key features include: TDMA (Time Division Multiple Access) for channel access, GMSK modulation, 200 kHz channel bandwidth, support for voice and SMS, SIM card-based authentication, and international roaming capability. GSM operates in various frequency bands (900 MHz, 1800 MHz, 850 MHz, 1900 MHz). It introduced the concept of separating user identity (SIM) from the device, enabling easy phone switching while keeping the same number.