POCSAG


POCSAG is an asynchronous protocol used to transmit data to pagers. The name is an acronym of the Post Office Code Standardisation Advisory Group, the name of the group that developed the code under the chairmanship of the British Post Office that used to operate most telecommunications in Britain before privatization.
Before the development and adoption of the POCSAG code, pagers used one of several proprietary codes such as GOLAY.
In the 1990s new paging codes were developed that offered higher data transmission rates and other advanced features such as European and Network roaming.
The POCSAG code is generally transmitted at one of three data rates; 512, 1200 or 2400 bits per second.
With Super-POCSAG 1200 bits per second, or 2400 bits per second transmission rates are possible. Super-POCSAG has mostly displaced the POCSAG in the developed world but the transition is still in progress.

The History of POCSAG

In 1976 an international group of engineers began to meet to explore the possibility of developing a new code for wide area paging; paging networks covering regions of entire countries. These meetings were successful and in February 1981 the CCIR the forerunner of the ITU-R accepted the code as Radiopaging Code No.1,
The meetings were chaired by R.H.Tridgell and were attended by representatives of British, European, and Japanese pager manufacturers

How it works

The modulation used is FSK with a ±4.5 kHz shift on the carrier. The high frequency represents a 0 and the low frequency a 1.
The ±4.5 kHz frequency shift is used along with a 25 kHz channel spacing, known as "wideband". Some jurisdictions require that all systems move to a "narrowband" configuration, using 12.5 kHz channels and ±2.5 kHz frequency shifts
Often single transmission channels contain blocks of data at more than one of the rates.
Transmission use 32-bit blocks called codewords. Each codeword carries 21 bits of information, 10 bits of error-correcting code, and an even parity bit. Bits 31 through 1 are a binary BCH code. The error-correcting code has a 6-bit Hamming distance: each 31-bit codeword differs from every other codeword in at least 6 bits. Consequently, the code can detect and correct up to 2 errors in a codeword.
The generating polynomial for the BCH code is:
The codewords are either address or data, which is indicated by the first bit transmitted, bit 31. An address codeword contains 18 bits of address, and 2 function bits. Each data codeword carries 20 bits of data.
Codewords are transmitted in batches that consist of a sync codeword, defined in the standard as 0x7CD215D8, followed by 16 payload codewords that are either address or data. Any unused codewords are filled with the idle value of 0x7A89C197.
Although the address is transmitted as 18 bits the actual address is 21-bits long: the remaining three bits are derived from which of the 8 pairs of codewords in the batch the address is sent in. This strategy allows the receiver to turn off for a considerable percentage of the time as it only needs to listen to the pair that applies to it, thus saving a significant amount of battery power.
Before a burst of data there will always be a preamble of at least 576 bits of data containing alternating 1s and 0s, allowing the receiver to synchronize itself to the signal, and is another mechanism that enables the receiver to be turned off for a large percentage of the time.
A message will start with an address codeword followed by a number of data codewords and will continue until another address, a sync, or an idle codeword is sent. When the data bits are extracted they will be in one of two formats.

Message format

There are two message coding formats for the data messages. Numeric messages are sent as 4 bit BCD values, and alphanumeric messages are sent as 7-bit ASCII. The 7-bit ASCII is commonly referred to as 'alpha-paging', and 4-bit BCD is commonly referred to as 'numeric-paging'.

Numeric paging

BCD encoding packs 4 bit BCD symbols 5 to a codeword into bits 30-11.
The most significant nibble is the leftmost of a BCD coded numeric datum.
Values beyond 9 in each nibble are encoded as follows:
BCD messages are space padded with trailing 0xC's to fill the codeword. There is no POCSAG specified restriction on message length, but particular pagers of course have a fixed number of characters in their display.

Text paging

Alphanumeric messages are encoded in 7-bit ASCII characters packed into the 20 bit data area of a message codeword. Since three 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.
What 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. The side benefit of this is a slightly increased error-correcting code reliability for messages that span more than one POCSAG packet.
Within a codeword 7-bit characters are packed from left to right. The LSB of an ASCII character is sent first as per standard ASCII transmission conventions, so viewed as bits inside a codeword the characters are bit reversed.

National implementations

Europe

In the UK, most pager transmissions are in five bands at
The frequency 466.075 MHz was previously used by Hutchison Paging, but the network was shut down in 2000. The frequency is still reserved for paging but is not used.
In Germany, well known transmissions are at
Licensed paging is possible in any other VHF/UHF bands.
In Spain, nationwide service was provided by Telefónica Mensatel but the network was shut down in 2012.
The Swedish pager network marketed as "Minicall" is encoded as POCSAG and broadcast on these frequencies:
The Belgium POCSAG is used for paging over the A.S.T.R.I.D. network:
In Italy, the 26.225-26.935 MHz band and 40.0125-40.0875 MHz may be used for local pagers. These frequencies are often used for on-site hospital paging systems, including voice paging. Use of POCSAG on the 26 MHz and 27 MHz band has been logged by several listeners in Europe, specifically frequencies 26.350 MHz, 26.500 MHz, 26.705 MHz, 26.725 MHz, 26.755 MHz, 27.005 MHz, 27.007 MHz, 27.255 MHz. It appears that US-specification paging systems operating on 27.255 MHz have been sold in Italy and other European countries.
In France, POCSAG is operated by E*Message over the AlphaPage network on the 466 MHz frequency:

United States of America

In addition to the bands listed above, paging may be authorized on any frequency in the land mobile bands authorized under Part 90 of the FCC rules, including frequencies in the 72-76 MHz band as well as the usual 30.56-49.58 MHz, 150.775-162.000 MHz VHF bands and the 450-470 MHz band. In larger metropolitan areas with congested frequency spectrum, paging services will often share the same frequency as land mobile stations, or operate on an adjacent channel. For example, a department store may operate handheld walkie-talkies on 462.7625 MHz while there are high power pager transmitters on 462.7500 MHz and/or 462.7750 MHz in the same city. Or, a restaurant will use 467.7500 MHz to alert customers when their table is ready while a department store nearby uses 467.7500 MHz for their in-store communications. In both of these examples, the department store is forced to use a squelch system such as CTCSS or DCS.
In many areas in the United States, these frequencies are used for land mobile radio communications services in addition to paging. The VHF and UHF frequencies are often used for a mixture of paging and land mobile communications. The VHF low band frequencies are mainly used for local hospital paging and in many areas are completely unused.

Mexico

Australia uses the following frequencies for localised paging, such as in hospitals, hotels and other facilities
Other paging systems for wide-area paging, such as commercial networks are licensed and operate anywhere in the VHF/UHF bands.