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TUCoPS :: Phreaking Cellular - Misc. :: pagers.txt

All You Need To Know About Pagers




                      All you need to know about : Pagers
                     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

  Pagers are split into two categories, on-site (as used in hospitals) 
where the wearer is alerted to contact the switchboard.  These systems do
not concern phreaks but the frequency bands used are :
160-161 Mhz (with 12.5 kHz channel spacing)
458-459 MHz (with 25 kHz channel spacing)  
  The second type of pager is the wide-area pager, integrated into the
telecommunications network.  Licences are granted to operators, who are
left to generate their own customer base.  Licences are granted to BT,
Racal, Intercity, Infrowave, Digital Mobile Communications, Mercury and
Air Call (amongst others).  These licences are granted by the Department
of Trade and Industry, and each service provider is allocated a frequency in
the 153 MHz band.  A subscribers pager is tuned to the particular network 
frequency.
  To regulate the codes and systems of pagers a group called POCSAG was set
up to standardise paging.  This standard was to allow messages to be sent
at 10 calls/second with a capacity of 1x10^6 pagers with up to four 
alternative addreses per paging unit.
  The recommended bit speed recommended by POCSAG is 512 bit/s with direct
FSK (frequency shift keying) applied to the transmitter with a frequency
deviation of +/- 4.5 kHz (positive deviation = binary 1 and vice versa).
The address of the pager and message (if transmitted) are contained in a
standard 'codeword' format as shown below:

 ______________________________________________________________
| Bit no:  |   1    |  2-19   |  20-21   |   22-31    |   32   |
|__________|________|_________|__________|____________|________|
| Address  | Msg.   | Address | Function | Parity     | Even   |
| codeword:| flag 0 | bits    | bits     | check bits | parity |
|__________|________|_________|__________|____________|________|
| Message  | Msg    | Message            | Parity     | Even   |
| codeword:| flag 1 | bits               | check bits | parity |
|__________|________|____________________|____________|________|

  The codewords are assembled into frames each comprising 2 codewords,
and a transmitted batch is made up from a synchronising codeword plus
eight frames (transmission format) as shown below:

            1 frame = 2 codewords
            --------|  |---------
 ___________________|__|______________________________
| Preamble of   | S |  |  |  |  |  |  |  |  | S |  
| not less than | C |  |  |  |  |  |  |  |  | C |
| 576 bits      |   |  |  |  |  |  |  |  |  |   |
|_______________|___|__|__|__|__|__|__|__|__|___|_____
                |                           |
                |_1 batch (1 sync. codeword_|
                           + 8 frames)

  Each pager has a 24 bit ID code.  The three least significant digits are
not transmitted however they are used  to determine the frame in which the
codeword must appear.  Each pager therefore, only needs to monitor one slot,
reducing power consumption during the other slots.  Bits 20 and 21 select 
which of the four addresses allocated is actually required.  The main 
address is in bits 2-19.  That makes the total number of pagers alloted for
a staggering 2^21.
  Paging requests are accumulated over approximately 2 minutes, and assembled
into frames and batches and transmitted in a burst.  With a message rate of
10/second it is likely that bursts will be transmitted more quickly than 
every two minutes.  This allows sequential operation of transmitters and
consequently a reduction in interference.
  Transmission bursts are preceeded by not less than 576 bits with 101010...
sequence.  The synchronising codeword at the beginning of each batch is:
01111100110100100001010111011000
The codewords in the remainder of the batch can be either address or
message codewords, identified by the first bit (0 for address and 1 for
message).  Messages require, in general, more than one codeword - these 
occupy additional frames immediately following the address codeword.
  If 'bleep only' paging is required it is possible to dial out the control
centre number followed by the decimal equivalent of the pager code on a PSTN
line.  Only the pager code is passed to the control centre, and converted
there into binary.  If message paging is required, both address and
message will be digital and if this is to be conveyed to the control centre
a telex facility is required, either via the telex network or over a modem 
on a PSTN line.
  Pagers use ferrite loop aerials (ie they are directional!).  European wide
systems will also use a POCSAG based systems (at higher bit rates operating
at 466 MHz) or ERMES a 'frequency agile' system on 169.9 MHz.  Apparently
global 'Satellite pagers' are under consideration.
  Pagers may also be used in conjunction with cordless PBX extensions, 
where mobile handsets can make outgoing calls within 200 metres of a
'telepoint'.  These phones are a kind of stop gap between cordless and
cellular (40 duplex channels between 864-868 MHz ; 100 kHz wide), but the 
telepoint cannot accept incoming calls.  The pager makes this a more workable
system.




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