|
POCSAG is an asynchronous protocol used to transmit data to pagers. 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, which uses higher speeds and a four level modulation method. GOLAY, TONE and ERMES 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 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.〔http://hem.passagen.se/communication/pocsag.html〕 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 (for example, the U.S. Federal Communications Commission (FCC) has mandated this transition be completed prior to 2013.〔http://transition.fcc.gov/pshs/public-safety-spectrum/narrowbanding-faq.html〕) 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 normally referred to as BCH(31,21) and has the ability to correct 2 bit 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「POCSAG」の詳細全文を読む スポンサード リンク
|