There are a number of occupations in which the professional has to move around from one job to the next and essentially is almost never available at a wireline telephone. The paging system is designed to receive and store information until the professional is ready to read it. They are most commonly used by home appliance servicemen, office equipment servicemen, doctors, photographers, and in the last few years they have become very popular with teenagers. Different paging systems have varying capabilities. The message received and stored may be as simple as ''Call the paging center to pick up your message'' or it may give the number of the caller or an alphanumeric message, or in some of the more sophisticated systems the caller can leave a voice message. The important difference between paging and other systems of communication is that in paging, there is no need for an immediate response.
In this section, we discuss the design and operation of one of the simpler paging systems currently in use. This is the Post Office Code Standardization Advisory Group (POCSAG) system. This system was introduced in the early 1980s as a standard for the manufacture of pagers for the British Post Office. It can handle up to 2 million addresses per carrier and supports tone only (alert only), numeric, and alphanumeric pagers. There are three speeds at which the POCSAG system transmits its messages; they are 512, 1200, and 2400 bps. These data rates would normally be considered to be slow but this is deliberate because, combined with high transmitter power (hundreds of watts to a few kilowatts), it improves reliability. The message is ''broadcast'' over the entire area of operation and it is supposed to reach the recipients whether they are in a building, on a highway, or in an airplane. Typical carrier frequency of operation of the transmitters is around 150 MHz. Each message is preceded by a CAP code which is a unique 7 or 8 digit code recognizable to only one paging receiver in the geographic area of operation.
18.104.22.168 The Paging Transmitter. Figure 11.11 shows a block diagram of the transmit portion of the paging system. Most of the messages come in over the telephone system. The source of the message can be from a Touch-tone® telephone whose keypad can be used to enter the information to be transmitted. It can be a voice message, in which case a human dispatcher in the paging center has to intervene and key in the appropriate message. The message can also come from a computer with the appropriate software and a modem. Whatever its source or form,
the message goes into an A/D converter. The digital output is used to drive a frequency shift keying encoder in which the digit 0 is assigned a frequency of, say, 1200 Hz and the digit 1 is represented by a tone of frequency 2400 Hz. The message is placed in a queue with other messages. The appropriate CAP code is inserted ahead of each message frame. The dual tone signals may be sent over landlines or wireless systems to a large number of frequency modulated transmitters distributed over a geographic area.
22.214.171.124 Component Circuit Design. The function of the "processor ' is to condition the analog signals coming over the telephone line into the paging center for the A/D converter. The human dispatcher plays the same role. The design of the A/D converter is described in Section 126.96.36.199. The frequency shift keying encoder is a form of modem. Modem circuits are described in Section 9.4.1. The frequency modulated (FM) radio transmitter was the subject of Chapter 4.
188.8.131.52 The Paging Receiver. The block diagram of the basic paging receiver is shown in Figure 11.12 . The paging receiver is typically a small device which can be worn on a waist belt. It is basically an FM receiver with a fixed carrier frequency. It has an internal antenna typical of portable radios. Each receiver has a unique CAP code programmed into it and when a message arrives with the appropriate CAP code, the message is saved in the memory and the controller triggers the alert generator which sends a signal to the alert transducer. All other messages are ignored. The alert may be in the form of a sub-audio vibration, a
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