POCSAG
Encyclopedia
Pager
A pager is a simple personal telecommunications device for short messages. A one-way numeric pager can only receive a message consisting of a few digits, typically a phone number that the user is then requested to call...
s. The name comes from Post Office Code Standardization Advisory Group, this being the British Post Office which used to run nearly all telecommunications in Britain before privatization.
The main alternative to POCSAG is FLEX
FLEX (protocol)
FLEX is a communications protocol developed by Motorola and used in many pagers. FLEX provides one-way communication only , but a related protocol called ReFLEX provides two-way messaging....
, which uses higher speeds and a four level modulation method. GOLAY, TONE and ERMES
ERMES
ERMES is a pan-European radio paging system. Radio paging services depend on the allocation and availability of appropriate frequencies in order to transmit and receive between fixed-base stations and radio paging receivers respectively...
are previous paging protocols that have been used in the past.
The family of POCSAG protocols can operate at three speeds, 512 bits per second (the original specification speed) is the base standard.
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 related Flex Synchronous protocol achieves speeds of 1600 bits per second, 3200 bits per second and 6400 bits per second.
How it works
The modulationModulation
In electronics and telecommunications, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted...
used is FSK
Frequency-shift keying
Frequency-shift keying is a frequency modulation scheme in which digital information is transmitted through discrete frequency changes of a carrier wave. The simplest FSK is binary FSK . BFSK uses a pair of discrete frequencies to transmit binary information. With this scheme, the "1" is called...
with a (+/-) 4.5 kHz shift on the carrier.
The high frequency represents a 0 and the low frequency a 1.
Often single transmission channels contain blocks of data at more than one of the rates.
Data is transmitted in 32bit 'Pre-amble' code words that are used to synchronise the data transmissions. Each code word can be either an address or data, which is indicated by the first bit transmitted, bit 31. An address code word then contains 18 bits of address (bit 30 through to 13), and 2 function bits (12 & 11).
A data codeword has 20 bits of data (bit 30 through to 11). Both code words then have 10 bits of ECC that is a BCH code
BCH code
In coding theory the BCH codes form a class of parameterised error-correcting codes which have been the subject of much academic attention in the last fifty years. BCH codes were invented in 1959 by Hocquenghem, and independently in 1960 by Bose and Ray-Chaudhuri...
normally referred to as BCH
BCH code
In coding theory the BCH codes form a class of parameterised error-correcting codes which have been the subject of much academic attention in the last fifty years. BCH codes were invented in 1959 by Hocquenghem, and independently in 1960 by Bose and Ray-Chaudhuri...
(31,21) and has the ability to correct 2 bit
Bit
A bit is the basic unit of information in computing and telecommunications; it is the amount of information stored by a digital device or other physical system that exists in one of two possible distinct states...
errors in each codeword.
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.
Code words are transmitted in batches that consist of a sync codeword, defined in the standard as 0x7CD215D8, followed by 16 others containing the data. Any unused code words are filled with the idle value of 0x7A89C197. In practice other values are sometimes used to indicate sync and idle.
Although the address (also referred to as a RIC - Radio Identity Code) is transmitted as 18 bits the actual length is 21 bits the remaining three bits are derived from which of the 8 pairs of code words 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 code words 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 defined for the information content part of messages. Numeric messages are sent as 4 bit BCDBinary-coded decimal
In computing and electronic systems, binary-coded decimal is a digital encoding method for numbers using decimal notation, with each decimal digit represented by its own binary sequence. In BCD, a numeral is usually represented by four bits which, in general, represent the decimal range 0 through 9...
values, and alphanumeric messages are sent as 7 bit ASCII
ASCII
The American Standard Code for Information Interchange is a character-encoding scheme based on the ordering of the English alphabet. ASCII codes represent text in computers, communications equipment, and other devices that use text...
. 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 (bits 30,29,28,27) is the leftmost (or most significant) of a BCD coded numeric datum.
Values beyond 9 in each nibble (i.e. 0xA through 0xF) are encoded as follows:
- 0xA Reserved (possibly used for address extension)
- 0xB Character U (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. 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 (bits 30-11). 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 ECC reliability for messages that span more than one POCSAG packet.
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 to distinguish them from numeric messages.
National implementations
EuropeIn the UK
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...
most pager transmissions are in three bands at
- 138 MHz
- 153 to 153.5 MHz
- 454 MHz
The frequency 466.075 MHz was previously used by Hutchinson paging, but the network was shut down in 2000. The frequency is still reserved for paging but is not currently used.
Asia-Pacific
Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
uses the following frequencies for localised paging, such as in Hospitals, Hotels and other facilities
- 148.3375 MHz (VHF)
- 450.375MHz (UHF)
- 450.325MHz (UHF)
Other paging systems for wide-area paging, such as commercial networks are licenced and operate anywhere in the VHF/UHF bands.