Paging System basics
This page describes paging system basics. It mentions digital protocol used for pagers and describes digital paging receiver.
Paging is a one way communication system. The paging system broadcasts signal or messages to individual carrying small battery powered devices referred as pagers. This is very important way of communicating to employees and/or customers. Let us see how paging system works. Pager usually will have LCD display or buzzer which is used to indicate that some one has paged either using displayed message or audible tone. Paging system operates in VHF/UHF frequency bands. The system helps one locate individual within the radius of about 30 miles.
Fig.1 Paging System
As shown in the fig-1, to contact the person having pager, one need to dial telephone number assigned to that person. This call is received by the paging company. The telephone company responds caller to enter the number which paged person should call. After the number is entered caller will press the pound sign to indicate end of the number entry. The calling party then will hang up. The paging system records the number and transmits it to the serial binary coded form which modulates the RF carrier of the radio transmitter. Before modulating the signal it need to be packed as per the pager protocol described below.
Fig.1 Pager protocol
Fig-2 depicts digital protocol used by the pagers. As shown it consists of preamble, sync packet, address packet and data to be carried.
All the pagers will be assigned unique code. All the paging receivers receive the paged message and will recognize only the message whose code matches to its assigned code. This code is referred as "cap code". This cap code is composed of sequence of numbers or combination of letters as well as numbers.
There are two common protocols followed by the digital paging systems viz. POCSAG and FLEX. POCSAG is the short form of Post Office Code Standard Advisory Group. It has two level modulation scheme such as ASK/PSK. It transmits data at the 2.4Kbps rate in 25KHz bandwidth channel in 150/470/900 MHz frequencies. FLEX paging protocol is developed by MOTOROLA. It uses 2 or 4 level modulation schemes such as ASK/FSK. It provides data rate in the range from 1.6-6.4Kbps.
Fig.1 Paging Receiver
Typical pager will have LCD display with 4 lines with 20 characters in each line. They will have buzzer which will beep or vibrate when message is received.
A typical FLEX paging receiver is shown in the fig-3. It consists of LNA, mixer, LO, ADC in the front end portion. LO frequency is set to the same value as of received frequency. The serial data after ADC is sent to the DSP chip. The chip recognizes the data message based on processing of the packet as per protocol defined. The same will be displayed on the screen.
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