# GSM Anatomy

By Gianpaolo Gasparini

Topic: Computing, AI & Electronics

[Open the interactive Jig](https://ask.jig.com/j/cJdp8lDy4QFE)

## Guide

### 1. A phone call, opened up

GSM is the 2G mobile phone system that first went live in Finland in 1991\. Here are two GSM phones and a mast in between\. Watch them open up so we can follow one word of a call from mouth to ear\.

Drag anywhere to turn the model\. The mast is shrunk to fit the table: a real one can reach phones up to about 35 km away\.

### 2. Sound in: the microphone

Your voice is a wobble of air pressure\. It pushes on the microphone's thin skin, called a diaphragm, which turns each push into a tiny, changing electric voltage\.

That voltage is analogue: a smooth curve, not numbers yet\.

### 3. Sampling: 8,000 snapshots a second

An audio chip measures the voltage 8,000 times every second\. Each measurement, called a sample, becomes a 13\-bit number\. The smooth wave is now a staircase of numbers\.

8,000 samples x 13 bits = 104,000 bits every second\.

### 4. Squeeze, then protect

That is far too many bits for radio\. A speech processor takes 160 samples at a time \(20 milliseconds\) and sends a recipe of your voice instead: 13,000 bits a second, 8 times smaller\. Then it adds spare check bits so mistakes can be fixed later\.

With the check bits added, 13 becomes 22\.8 thousand bits a second\.

### 5. Modulation: bits ride a radio wave

The radio part makes a fast carrier wave and nudges its pitch a little higher or lower for each bit\. This is called GMSK\. The wave never gets taller or shorter, only faster or slower\.

Eight phones share one radio channel: each gets one of eight time slots and only sends in its own short burst\.

### 6. Antenna to mast

The power amplifier boosts the wave and the antenna, hidden inside the phone, launches it as a radio burst\. On the 900 band the phone sends between 890 and 915 megahertz\.

A GSM phone sends at most 2 watts, and usually much less\.

### 7. The mast answers on another channel

The mast's antennas catch the burst and the network passes the call along\. The mast then sends it to the other phone between 935 and 960 megahertz, a different channel from the one phones use to send\.

Sending and receiving sit 45 megahertz apart, so the two directions of a call never bump into each other\.

### 8. Demodulation: from waves back to bits

The second phone's antenna catches the faint wave\. Its receiver amplifies it and reads each small change in pitch back into bits\. Any errors are fixed using the spare check bits\.

Demodulation is modulation in reverse\.

### 9. Sound out: the earpiece

The decoder rebuilds 160 samples for every 20 milliseconds of speech\. A converter turns the numbers back into a smooth voltage, and the earpiece speaker pushes the air\. Hello\!

The whole trip, mouth to ear, takes only a fraction of a second\.