POCSAG
POCSAG is a standard used to transmit data to pagers. The name comes form Post Office Code Standardisation Advisory Group, this being the British Post Office which used to run nearly all telecommunications in Britain before privatisation.
The modulation used is FSK with +- 4.5 kHz shift on the carrier. The high frequency represents a 0 and the low frequency a 1. The original specification was for a rate of 512 bits per second, but rates of 1200 and 2400 bits per second are also used. Often single transmission channels contain blocks of data at more than one of the rates.
Data is transmitted in 32bit codewords. Each code word can be either an address or data, which is indicated by the first bit transmitted, bit 31. An address codeword 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 codewords then have 10 bits of ECC that is a BCH code normally referred to as BCH(31,21) and has the ability to correct 2 bit errors in each codeword. Codewords 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 codewords are filled with the idle value of 0x7A89C197. In practice other values are sometimes used to indicate sync and idle.
Although the address (also refered to as a RIC) is transmitted as 18 bits the actual length is 21 bits 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 the a burst of data there will always be a preamble of at least 576 bits of data containing alternating 1's and 0's, allowing the receiver to synchronise up to the signal, and is another mechanism that enables the receiver to be turned off for a large parcentage 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. Numeric messages are sent as 4 bit BCD values, and alphanumeric messages are sent as 7 bit ASCII
In the UK most pager transmissions are in three bands at 138MHz, 153MHz and 466MHz