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Pager Pocsag

  

	The following summary describes the coding used on POCSAG pager

signals and may be of interest to those curious about what those ear-splitting

beeps and buzzes mean and how they encode data.   This summary is

based on a very old text of the standard from my files; the current

text of the POCSAG standard is available as CCIR Radiopaging Format 1.

 

	Note that some current POCSAG signals (so called Super-POCSAG)

transmit paging at 1200 or 2400 baud instead of the 512 baud I refer to

here, but use essentially a similar coding standard. 

 

	The interested USA reader is reminded that willfully intercepting

other than tone only paging is a violation of the ECPA with similar

penalties and criminal status to willfully intercepting cellular phone calls.

 

	The interested reader is advised that at least two of Universal

Shortwave's RTTY reading devices (the M8000 and the new C-400) are

capable of reading at least the older 512 baud version of POCSAG paging,

so commercial devices for this purpose are currently being sold in

the USA.

 

	And finally, much alphanumeric paging - particularly that installed

some time ago, uses a proprietary Motorola encoding format called

GOLAY which is quite different from POCSAG.  The two can be told apart

by their baud rates - GOLAY is 600 baud.

 

 

		POCSAG

 

 

	First POCSAG stands for Post Office Code Standarization Advisory

Group.   Post office in this context is the British Post Office

which used to be the supplier of all telecommunications services in

England before privatization.

 

	POCSAG as defined in the standard, (original POCSAG) is 512 bits

per second direct FSK (not AFSK) of the carrier wave with +- 4.5 khz shift

(less deviation than that is used in some US systems).  Data is

NRZ coded with the higher frequency representing 0 (space) and the

lower one representing 1 (mark).

 

	The basic unit of data in a POCSAG message is the codeword which

is always a 32 bit long entity.  The most significant bit of a codeword

is transmitted first followed immediately by the next most significant

bit and so forth.  The data is NRZ, so that mark and space values (plus

and minus voltages) as sampled on the output of the receiver

discriminator at a 512 hz rate corrospond directly to bits in the

codeword starting with the MSB.   (Note that the audio output circuitry

following the discriminator in a typical voice scanner may considerably

distort this square wave pattern of bits, so it is best to take the

signal directly off the discriminator before the audio filtering).

 

	The first (msb) bit of every POCSAG codeword (bit 31) indicates 

whether the codeword is an address codeword (pager address) (bit 31 = 0)

or a message codeword (bit 31 = 1).  The two codeword types have

have different internal structure.

 

	Message codewords (bit 31 = 1) use the 20 bits starting at bit

30 (bit 30-11) as message data.  Address codewords (bit 31 = 0) use 18

bits starting at bit 30 as address (bits 30-13) and bits 12 and 11 as

function bits which indicate the type and format of the page.  Bits 10 through

1 of both types of codewords are the bits of a BCH (31,21) block ECC

code computed over the first 31 bits of the codeword, and bit 0

of both codeword types is an even parity bit.

 

	The BCH ECC code used provides a 6 bit hamming distance between

all valid codewords in the possible set (that is every valid 32 bit

codeword differs from ever other one in at least 6 bits).  This makes

one or two bit error correction of codewords possible, and provides

a robust error detection capability (very low chance of false pages).

The generating polynomial for the (31,21) BCH code is x**10 + x**9

+ x**8 + x**6 + x**5 + x**3 + 1.

 

	Codewords are transmitted in groups of 16 (called batches), and

each batch is preceeded by a special 17th codeword which contains a

fixed frame synchronization pattern.  At least as of the date of the

spec I have, this sync magic word was 0x7CD215D8. 

 

	Batches of codewords in a transmission are preceeded by a start

of transmission preamble of reversals (10101010101 pattern) which must

be at least 576 bits long.  Thus a transmission (paging burst) consists

of carrier turnon during which it is modulated with 512 baud reversals

(the preamble pattern) followed by at least 576/512 seconds worth of

actual preamble, and then a sync codeword (0x7CD215D8), followed by 16

data/address codewords, another sync codeword, 16 more data/address

codewords and so forth until the traffic is completely transmitted.  As

far I am aware there is no specified postamble.  I beleive that all 16

of the last codewords of a transmission are always sent before the

carrier is shut off, and if there is no message to be sent in them the

idle codeword (0x7A89C197) is sent.  Later versions of the standard may

have modified this however. 

 

	In order to save on battery power and not require that a pager

receive all the bits of an entire transmission (allowing the receiver

to be turned off most of the time, even when a message is being transmitted

on the channel) a convention for addressing has been incorperated which

splits the pager population into 8 groups.  Members of each group

only pay attention to the two address code words following the synch

codeword of a block that corrospond to their group.  This means that

as far as addressing is concerned, the 16 codewords in a batch are

divided into 8 frames of two codewords apiece and any given pager

pays attention only to the two in the frame to which it assigned.

 

	A message to a pager consists of an address codeword in the

proper two codeword frame within the batch to match the recipients frame

assignment (based on the low three bits of the recipient's 21 bit

effective address), and between 0 and n of the immediately following

code words which contain the message text.  A message is terminated by

either another address code word or an idle codeword.  Idle codewords

have the special hex value of 0x7A89C197.  A message with a long text

may potentially spill over between two or more 17 codeword batches. 

 

	Space in a batch between the end of a message in a transmission and

either the end of the batch or the start of the next message (which of

course can only start in the two correct two codeword frame assigned to

the recipient) is filled with idle codewords.  A long message which

spills between two or more batches does not disrupt the batch structure

(sync codeword and 16 data/address code words - sync code word and 

16 data/address codewords and so forth) so it is possible for a pager

not being addressed to predict when to next listen for its address and

only turn on it's receiver then.

 

	The early standard text I have available to me specifies a 21 bit

address format for a pager (I beleive this has been extended since) 

with the upper 18 bits of a pager's address mapping into bits 

30-13 of the address codeword and the lower 3 bits specifiying which

codewords within a 17 codeword batch to look at for possible messages.

The address space is further subdivided into 4 different message classes

as specified by the function bits (bits 12 and 11 of a codeword).  These

address classes corrospond to different message types (for example

bits 12 and 11 both zero indicate a numeric message encoded in 4 bit BCD,

whilst bits 12 and 11 both set to 1 indicate an alpha message encoded

in 7 bit ASCII).  It was apparently envisioned that a given pager could

have different addresses for different message types.

 

	

	There are two message coding formats defined for the text of messages,

BCD and 7 bit ASCII.   BCD encoding packs 4 bit BCD symbols 5 to a codeword

into bits 30-11.  The most significant nibble (bits 30,29,28,27) is the

leftmost (or most significant) of a BCD coded numeric datum.  Values beyond 9

in each nibble (ie 0xA through 0xF) are encoded as follows:

 

	0xA	Reserved (probably used for something now - address extension ?)

	0xB	Character U (for urgency)

	0xC	" ", Space (blank)

	0xD	"-", Hyphen (or dash)

	0xE	")", Left bracket

	0xF	"(", Right bracket	

 

	BCD messages are space padded with trailing 0xC's to fill the codeword.

As far as I know there is no POCSAG specified restriction on length but

particular pagers of course have a fixed number of characters in their

display.

	

	Alphanumeric messages are encoded in 7 bit ASCII characters packed

into the 20 bit data area of a message codeword (bits 30-11).  Since four

seven bit characters are 21 rather than 20 bits and the designers of the

standard did not want to waste transmission time, they chose to pack the

first 20 bits of an ASCII message into the first code word, the next 

20 bits of a message into the next codeword and so forth.  This means

that a 7 bit ASCII character of a message that falls on a boundary can

and will be split between two code words, and that the alignment of character

boundaries in a particular alpha message code word depends on which code

word it is of a message.   Within a codeword 7 bit characters are packed

from left to right (MSB to LSB).   The LSB of an ASCII character is sent

first (is the MSB in the codeword) as per standard ASCII transmission

conventions, so viewed as bits inside a codeword the characters are

bit reversed.

 

	ASCII messages are terminated with ETX, or EOT (my documentation

on this is vague) and the remainder of the last message codeword is

padded to the right with zeros (which looks like some number of NULL

characters with the last one possibly partial (less than 7 bits)). 

 

	The documentation I have does not specify how long a ASCII

message may be, but I suspect that subsequent standards have probably

addressed the issue and perhaps defined a higher level message protocol

for partitioning messages into pieces.   The POCSAG standard clearly

does seem to allow for the notion of encoding messages as mixed

strings of 7 bit alpha encoded text and 4 bit BCD numerics, and it

is at least possible that some pagers and paging systems use this

to reduce message transmission time (probably by using some character

other than ETX to delimit a partial ASCII message fragment).



 

Brett Miller N7OLQ                  brett_miller@ccm.hf.intel.com

Intel Corp.

American Fork, UT                   DoD#1461

 

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file: /Techref/pager/pocsag.txt, 10KB, , updated: 1999/9/3 11:41, local time: 2020/7/9 13:10,
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