POCSAG

POCSAG is a protocol of Radiomessagerie. Its name comes from the committee of standardization of the British post office (Post Office Codes Standardization Advisory Group, POCSAG), which ensured the development of it.

POCSAG uses a modulation FSK, with a shift compared to the carrying one ± 4,5 Khz. The high frequency represents one 0, and the low frequency one 1. The original specification envisaged a flow of 512 bits a second, but in practice, one also uses flows of 1200 or 2400 bits a second. Often, the transmission channels contain storage blocks transmitted to different flows.

The data are transmitted in words of 32 bits. Each word can be either an address, or a data. The first transmitted bit (bit n°31) makes it possible to make the distinction between these two types of words. A word of address contains 18 bits of address (bits 30 to 13), and two bits of function (bits 12 and 11). A data word contains to him 20 bits of data (bits 30 to 11). The two types of words then contain 10 bits of error correcting code, which is a Code BCH, normally indicated by BCH (31,21). This code can correct two erroneous bits in each mot. the words are transmitted per blocks which contain a word of synchronization, always equal to 0x7CD215D8, followed by 16 other useful words. The unutilised words carry the special value 0x7A89C197. In practice, the values of the codes synchronization and vacuum are sometimes different.

Although address (also called RIC) is transmitted in the form of 18 bits, the length is in fact of 21 bits: the 3 missing bits are defined by the number of the pair of words in the block used to transmit the address. Thus, the receiver can remain extinct most of the time, and listen to only the pairs of words likely to interest him, which makes it possible to realize savings of substantial batteries.

Before each package a preamble to at least 576 bits is transmitted, alternatively to 1 and 0, which makes it possible the receiver to be synchronized on the signal. This mechanism also allows him the receiver to remain extinct a good part of time.

A message begin with a word from address, followed by a certain number of words of data. The same message continuous until another address, a word of synchronization or a word empties is transmitted. The contents of the message can be expressed in two different formats:

the numerical messages are transmitted in the forms of values BCD on 4 bits;
the alphanumeric messages are transmitted in ASCII on 7 bits.

In the the United Kingdom, the majority of the transmissions are done on 138 MHz and 466 MHz. The Australia uses the 148,3375 MHz in VHF, and 450,375 MHz or 450,325 MHz in UHF.

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