by Chris Woodford. Last updated: October 10, 2016.
Beep beep beep! Beep beep beep! Before the days when cellphones started interrupting our lives, there was another form of instant messaging system called paging. Mobile phones have made pagers less popular than they were in the mid-1990s, but paging (or radio paging, to give it its full name) is still a vitally important form of sending messages—especially for doctors and emergency workers. Let's take a look at how it works!
Photo: A modern Motorola Bravo pager. This one has a two-line alphanumeric LCD display and scrolls messages from right to left or up and down, as you prefer. It can either receive pages silently and discreetly, or it can signal a new message by vibrating or beeping. It runs off a small battery that goes in the slot at the bottom.
What is radio paging?
Broadcasting (sending instant information to hundreds, thousands, or millions of people at a time) is the big idea that made radio and television so powerful. But what if you want to send a message to only one person? You can certainly send very personal messages over the radio: think how people get their favorite DJs to read "happy birthday" messages out on air. That's instant communication and it works fine if the recipient of the message happens to be listening—but it's a bit hit and miss. And it can be irritating for the other two million listeners who have to "tune out" this irrelevant information that has nothing to do with them!
A pager is a small personal radio receiver that you carry around in your pocket. You have a personal code number or phone number and anyone who wants to send a message to you dials or quotes that number with the message they want to send. That puts them through to a switchboard at a central message broadcasting office, where a person (or more likely a machine) immediately sends out the message with thousands of others, just like a normal radio broadcast, using a network of radio transmitting antennas. All the pagers in people's pockets are constantly picking up all the messages being sent out, but your personal pager ignores any messages that don't contain your personal code. Once a message comes through with your code, your pager buzzes or beeps and displays the message with the date and time.
What is a page?
A page is simply the message or alert sent to or received by a pager and encoded in a burst of radio waves. Back in the 1950s, the first pagers used a crude system called two-tone, in which each pager was designed to respond to a page consisting of two unique sound tones (beeps) sent in rapid succession. If someone paged you, you heard your pager beep twice and you knew someone wanted to talk to you, but not who or why, so you then had to telephone your office or home to find out more (or, in some systems, plug in an earpiece to receive a voice message). Systems like this could be used to contact a maximum of 870 different pagers, which was fine in those days, when the technology was still very new. By the 1970s, pagers had been improved to use a system called five-six tone, so they responded to a unique code carried in a series of five or six higher-pitched radio frequency tones, with each one lasting 33 milliseconds and an entire page taking about 200 milliseconds (a fifth of a second). Systems like this could support between 100,000 and a million pagers.
Photo: Three views of a pocket pager patented in 1960 by Donald R. Jones of Motorola. In this system, pages were delivered in two parts. First, you received a short alert tone from a loudspeaker (19, green). That signaled you to flick a receiver switch (24, purple), and hook a headphone up to a receiver socket (21, brown), whereupon you could listen to a short voice message from your caller relayed via an operator. The other components I've highlighted are a radio antenna (37, light blue), selective frequency switch (red 40), and battery (35, yellow). Artwork from US Patent #2,924,705: Pocket type radio receiver construction by Donald Jones, Motorola, courtesy of US Patent and Trademark Office
Pagers that receive actual messages (rather than simply beeping to alert you) work in a different way, typically using one of two standard page formats (or protocols) called POCSAG or FLEX™. POCSAG (named for the UK's Post Office Code Standardisation Advisory Group, which invented it) can support up to two million pagers and arranges them into a number of different groups that are either "sleeping" (in a battery-saving mode) or "woken up" (ready to receive messages). To send a message to a particular pager with POCSAG, the system first transmits an initial "wakeup" message to activate every pager in the same group and then sends data to them in chunks called frames, with each pager extracting only the messages specifically addressed to it. FLEX™, developed by Motorola, is a newer and much higher-speed paging system that does away with "wakeup" messages by getting pagers to scan for pages at predefined intervals. This makes better use of the airwaves and reduces battery consumption (which means pagers can be made much smaller). According to Motorola, FLEX™ can support over five billion pagers.