1. Overview
GSM (Global System for Mobile Communications) is the most widely deployed 2G standard. Launched commercially in 1991, it underpins voice, SMS, and early data services across Europe, Asia, Africa, and the Americas.
2. GSM Architecture
GSM is divided into three subsystems: the Mobile Station (MS), the Base Station Subsystem (BSS), and the Network Switching Subsystem (NSS).
3. Radio Interface (Um)
GSM uses TDMA (Time Division Multiple Access). Each frequency carrier is divided into 8 time slots. Each call occupies one time slot per frame (200 kHz carrier, 270.8 kbps).
| Parameter | Value |
|---|---|
| Carrier spacing | 200 kHz |
| Frame duration | 4.615 ms |
| Slots per frame | 8 |
| Modulation | GMSK |
| Speech codec | RPE-LTP (13 kbps) |
4. GPRS & EDGE (2.5G / 2.75G)
GPRS added packet-switched data to GSM. Multiple time slots can be combined for one user, giving theoretical speeds up to 171.2 kbps.
EDGE (Enhanced Data rates for GSM Evolution) upgraded the modulation from GMSK to 8-PSK, tripling the data throughput to ~384 kbps.
| Technology | Max Speed | Modulation |
|---|---|---|
| CSD (2G) | 9.6 kbps | GMSK |
| GPRS (2.5G) | 171 kbps | GMSK |
| EDGE (2.75G) | 384 kbps | 8-PSK |
5. Security (A3/A5/A8)
GSM introduced three security algorithms:
- A3 ā Authentication algorithm. Runs on the SIM to prove identity to the network.
- A5 ā Encryption algorithm. Encrypts voice/data over the air interface (A5/1 in Europe, A5/2 export).
- A8 ā Key generation algorithm. Derives the session key Kc from the secret key Ki.