Print this chapterPrint this chapter

Networking

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