Paging

     Paging
     Paging systems started off as very-low-power systems giving effective coverage over a limited
     area, say in the several buildings of a major hospital complex. Users carried, usually clipped to
     their belts, a receiver half the size of a paperback book. When they were needed on the
     operating theater a coded call went out from the central transmitter. This operated a buzzer
     in the receiver being called. The user then hurried to the nearest phone and called in to the
     operator to find out where he was needed.

In addition there are a large number of private paging services. A pager may be :

A tone pager , providing a simple beep to alert the user when called ;
A numeric pager , incorporating a small display panel which is able to display telephone numbers or other numeric data ;
A alphanumeric pager , which is able to display both textual and numeric data and so may be used to pass a message to the user .

As technologies advanced it became possible to provide smaller and smaller portable units ,
wider coverage areas and rather more information via the radio path.

     A paging message is broadcast by a network of long-range radio transmitters that are
     operated by the providers of the paging services . The message is first passed to the paging
     service either by calling an operator or by tone signalling . Radio paging customers must
     indicate to the operator which are in the country they wish to be paged. When a call is made,
     a paging message is sent to all paging transmitters in the area in which the person is expected
     to be .

     Pagers use the ITU-R paging code 1 , also known as POCSAG ( Post Office Code
     Standardization Advisory Group ) , which is the internationally agreed standard for radio
     paging . POCSAG has a system capability of        addresses and operates in the frequency
     band 138-174 MHz with a 25 kHz channel spacing . The modulation employed is NRZ FSK
     at 512 bits/s .

     Pagers do not need to receive voice signals ( although special pagers are in fact now available
     which do pass on voice messages ) or to transmit any signals at all back to their base stations
     so the portable units can be relatively small and cheap and with low consumption. They
     need only a narrow frequency band to pass their data signals. This means that many users
     can be given service even if radio frequencies are limited .

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