How does a pager work?

In the 1990s, before most people had mobile phones, doctors, business people and college students clipped a pager to their belts. It is a small radio receiver with a screen. It can't make calls and it can't reply. It just listens, all day, for messages meant for it.

Look inside. The copper loop antenna runs round the edge of the case and catches the radio signal. The receiver chip tunes to one channel, and a quartz crystal keeps that tuning exact. The decoder chip turns the signal into ones and zeros and checks each message's address. If the address is this pager's, it saves the message in memory, shows it on the LCD and sets off the beeper or the vibration motor: a tiny motor with an off-centre weight that shakes the whole pager.

All of this runs on one AA battery for weeks or months. How? The pager sleeps most of the time and wakes only when its own messages might arrive (see the POCSAG chapter). A 1990s mobile phone lasted days at best.

To page someone in the 1990s, you picked up an ordinary phone and dialled the paging centre. There were two ways to leave a message. You could speak to an operator, who typed your words into a computer. Or, on a numeric service, you tapped your phone number on the keypad, and the centre read the DTMF tones, the beeps each key makes (see TelephoneClear).

The paging terminal put the message in a queue with everyone else's, turned it into a digital code, and sent it to a set of tall transmitter towers. Every tower broadcasts the same signal at the same instant. This is called simulcast. Where two towers overlap, their signals arrive in step, so they add up instead of fighting.

Every pager in the city hears every message. Each one checks the address at the start, and ignores anything that isn't its own. Only the right pager beeps.

A pager is one-way. It never transmits, so the network doesn't know where you are. That's why the whole city gets every message. A mobile phone keeps telling the network where it is, so calls go only to the nearest tower (see MobileClear). Radio waves themselves are covered in WaveClear.

Most pagers of the 1990s spoke a code called POCSAG, worked out by a group at the British Post Office from 1976 and made a world standard in 1981. It is still used today.

Your message is split into characters. Each one becomes a 7-bit number (A is 1000001), sent smallest bit first. The bits are packed into codewords of 32 bits: 1 flag bit, 20 message bits, 10 check bits for fixing errors, and 1 parity bit. The very first codeword is the address: which pager the message is for.

Before any message, the tower sends a preamble of at least 576 bits, 1010…, like clearing its throat. Then come batches: a special sync codeword, then 8 frames of 2 codewords each. Every pager belongs to one frame, set by the last 3 bits of its address. It wakes up for the sync word and its own frame, and sleeps through the other seven. That's the secret of the long battery life.

The bits go out at 512, 1,200 or 2,400 bits per second by frequency shift keying (FSK): the carrier moves up 4.5 kHz for a 0 and down 4.5 kHz for a 1.

Radio is noisy. Lightning, motors, other transmitters and weak signal deep inside a building all add noise. Now and then noise makes the pager read a 1 as a 0, or a 0 as a 1. One wrong bit can turn a C into an A, or 2233 into 2213. In a hospital, that matters.

So every codeword carries 10 check bits, calculated from its 21 information bits using a BCH code (named after Bose, Chaudhuri and Hocquenghem). The pager re-does the calculation. If the answer doesn't match, the mismatch, called the syndrome, points to exactly which bits are wrong. POCSAG's BCH(31,21) code, plus a parity bit, can fix up to 2 wrong bits in each codeword and notice when there are more.

If a codeword has 3 or more errors, the pager can't fix it, but it knows. Real pagers show a warning mark instead of silently showing the wrong message. Burst noise, such as a crackle of static, is the enemy: it can smash many bits in one codeword.

A 1990s mobile phone needed charging every few days. A pager ran for weeks or months on one AA battery. Three reasons:

1. It sleeps. In POCSAG, a pager only needs the sync word and its own frame: 3 codewords out of every 17. So its radio is off about 80% of the time. The clock and screen sip almost nothing. (For how a battery stores energy, see CurrentClear.)

2. It never talks back. Sending radio takes far more power than listening. A phone must keep telling towers where it is. A one-way pager never transmits at all.

3. Strong, low-frequency signals. Paging uses powerful transmitters on tall towers, often around 150 MHz, and every tower sends every page (simulcast). Lower frequencies get through walls and floors better, and a pager only needs to hear, not be heard. So pagers often work in basements and lift lobbies where phones struggle.

That's why many hospitals and fire and emergency crews still use them. In 2017 the NHS in England was reported to use about 130,000 pagers, and a US survey found most hospital doctors were still given one. England told hospitals to phase them out by 2021, but kept some for emergencies such as a Wi-Fi failure.

India's government auctioned paging licences for 27 cities in 1992, and after years of disputes the first licensed service began in 1995. It took off fast. By May 1996 there were reported to be about 2 lakh subscribers, and one 1996 report called India arguably the fastest-growing pager market in the world.

Why? In the 1990s a home landline could take months to get. Mobile phones arrived in 1995 too, but calls cost about ₹16.80 a minute, and you paid even to receive a call. A pager cost a few thousand rupees and a flat monthly rent that the government capped at under $5, about ₹175 then.

Unlike the US and Japan, where people sent numeric codes (143 for "I love you" in the US, 14106 for "aishiteru", I love you, in Japan), India overwhelmingly bought alphanumeric pagers: more than 95% by one 1996 report. You rang an operator, dictated a short message, and read it on the screen. People learned to write short and telegraphic: "PLS CALL", "URGENT", "REACHING 9PM". Then you found the nearest STD/PCO booth to call back.

The boom was short. Subscribers peaked at around 8 lakh in 1998 by one count (some reports say more), and fell as mobile calls got cheaper and SMS arrived. By 2002 only about 5 lakh remained. See MobileClear for what came next, and FaxClear for another machine of that era.