Pagers... those ubiquitous "beepers" that always seem to be going off during meetings. Ever wonder how they can make them so small? How do they run for so long on just a penlight cell, when everything else needs big batteries that have to be recharged every night?
The pager's secret is that inside that little case is a simple, yet sophisticated receiver. With the exception of two-way pagers (more on that later), pagers don't have a transmitter. Since it is the transmitter that consumes the lion's share of battery power in portable communications devices, pagers have a real edge when it comes to saving batteries. No transmitter also means that the pager can be made smaller than other communications devices too.
When you call a pager, your incoming call is answered by a paging terminal. You tell the paging terminal (via your telephone keypad or perhaps even a computer) who the message is for, and what the message is. It can be as simple as a phone number or as complex as a text message that is several hundred characters long. The paging terminal then queues your message with others that it needs to send, encodes it, and passes it along to the transmitter network.
The transmitter network can have many zones, or paging areas, either in the same city, or across the country. In the case of nationwide paging, satellite links are commonly used to connect all the zones together. The transmitter then sends the signal out which is received by the particular pager who is supposed to get the message.
So how does the pager know which message is for it? Why doesn't it respond to all of the messages that are sent. The answer lies in the pager coding system. A typical pager consists of a receiver, a decoder, and the user interface (usually a microcontroller with display and keyboard). The decoder has several functions. It decodes the data from the paging signal, correcting errors when possible, it controls the receiver, extending battery life (we'll cover that momentarily), but most important, it decodes the address signals to determine which messages are for it, and which messages are not. It does this by looking at the address information in the data stream.
In digital paging systems, each message consists of two parts: an address and the message. The address is just like the address you would place on a letter. The message is similar to the letter inside. The decoder looks at the address and determines if the paging data to follow is for that particular pager or not. If it is, then the data is decoded and passed on to the microcontroller so that the user can view it. If not, then the pager ignores the data, and waits for the next address.
The paging code consists of blocks of data known as codewords. Some codewords contain address information, while others contain data. Since the decoder knows if a series of codewords are for the particular pager in question, it can decide whether or not to leave the receiver on during those codeword. In other words, if I know that the next batch of codewords are not for me, I can shut the receiver off during that time, and save energy. This is one of the reasons why pagers can last for so long on such a small battery.
The other reason why pagers don't need a big battery (in general) is that they only have a receiver, and don't have a transmitter. This advantage does not apply to two way paging devices. In a two way pager, there is a transmitter inside the unit, which can respond to let the paging system know that it has received the message correctly. If the message is not received correctly, the paging system can send it again until the pager gets it right. Fortunately, even in two way paging, the battery drain is fairly low, since the reply transmitter does not frequently operate. The result is an improvement in reliability, along with added features (such as reply capability), with only a small penalty in battery life.
As paging systems develop, the main improvements are speed and capacity. With higher speed, not only can you add more pagers to a system, but each pager can receive more data without causing a data "traffic jam." For the user, this means new and more flexible services, such as digitized voice messages, portable facsimile, e-mail and other data services. Today's lowly beeper may well become the versatile communications tool of tomorrow!