1. GSM

1.1. How GSM works

How GSM Works (No Fluff Version)

If you don’t understand the flow, you don’t understand GSM. So here’s the real mechanism:

1. Your Phone Converts Voice to Digital Data

When you speak:

  • The microphone converts sound → electrical signal.

  • The phone converts that signal → digital bits.

  • The bits are compressed to reduce bandwidth usage.

No magic. Just encoding.

2. SIM Card Identifies You

Your SIM (Subscriber Identity Module) contains:

  • IMSI (your unique subscriber ID)

  • Authentication keys

Without a SIM, the network doesn’t know who you are. GSM is subscriber-based, not phone-based.

3. Phone Connects to the Nearest Base Station

Your phone communicates with a nearby Base Transceiver Station (BTS) using radio waves.

Each geographic area is divided into cells. That’s why it’s called a cellular network.

The BTS:

  • Receives your signal

  • Forwards it to the network core

4. The Core Network Routes the Call

Inside the GSM core:

  • MSC (Mobile Switching Center) – routes your call

  • HLR (Home Location Register) – stores your subscriber data

  • VLR (Visitor Location Register) – tracks where you are

  • AuC (Authentication Center) – verifies you’re legitimate

If you call someone:

  • MSC checks if they are on the same network or another network

  • Routes the call accordingly

5. Time Division Multiple Access (TDMA)

Here’s where GSM becomes clever.

GSM uses TDMA:

  • One frequency channel

  • Divided into time slots

  • Multiple users share it by transmitting in different time slots

Instead of giving each person a full frequency, GSM slices time into pieces.

Efficient. Cheap. Scalable.

GSM Network Architecture (Simplified)

  1. Mobile Station (MS) → Your phone + SIM

  2. Base Station Subsystem (BSS) → BTS + BSC

  3. Network Switching Subsystem (NSS) → MSC + databases

  4. Operation Support System (OSS) → Network monitoring

Data in GSM

Originally designed for voice.
Data came later:

  • GPRS – packet-based data (2.5G)

  • EDGE – faster data (2.75G)

But GSM is slow by modern standards.

Why GSM Was Dominant

  • SIM-based flexibility

  • International roaming support

  • Strong encryption (for its time)

  • Standardized globally

Compare that to early CDMA systems that were fragmented.

The Brutal Truth

GSM is old.
It’s 2G technology.

Modern networks use:

  • 3G (UMTS)

  • 4G (LTE)

  • 5G

But GSM still exists in many countries (including parts of Africa) for:

  • Basic calls

  • SMS

  • IoT devices

  • Backup networks