How does a pager work?

Before mobiles, a beep on your belt meant someone needed you. A pager is a pocket receiver with a loop antenna, a crystal-tuned receiver, a decoder chip, a small LCD, a buzz motor and one AA battery. It can't transmit, so it can't reply: it just waits for messages with its own address.

Read how it works: How does a pager work? · The history of the pager · More boxes on Glassbox

Chapters in this interactive model

1. Inside a pager: A pocket radio that only listens. It waits for its own address, then beeps, buzzes and shows a message.
2. One message, a whole city: You phone a paging centre. Tall towers across the city shout your message, and only one pager listens.
3. Turning words into radio: A page travels as a strict digital code called POCSAG: 32-bit codewords, in frames, sent as two tones.
4. Noise in, message out: Radio noise flips some bits. Ten check bits in every codeword let the pager find and fix them.
5. Months on one AA battery: A pager sleeps most of the time, never transmits, and hears signals deep inside buildings.
6. India's pager years: For a few years in the late 1990s, a beeping belt pager was how India's cities stayed in touch on the move.