: : PagerDaemon - многоканальный декодер сигналов POCSAG, текущая версия 2002.1019 : : | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
1.3.2 Address CodewordsThe structure of an address codeword is illustrated in Fig 2. Bit 1 (the flag bit) of an address codeword is always a zero. This distinguishes it from a message codeword. Bits 2-19 are address bits corresponding to the 18 most significant bits of a 21 bit identity assigned to the pager. For information regarding the least significant bit see 1.2. Bits 22 to 31 are the parity check bits (see 1.4) and the final bit (bit 32) is chosen to give even parity. Several methods of increasing the identity capacity yet further by many times have been suggested. These methods can be compatible and introduced later without disturbing the pagers already on the system.However, the 2 million identities provided for should be sufficient for both national and international systems for many years. 1.3.3 Message CodewordsThe structure of a message codeword is shown in Fig 2. A message codeword always starts with a 1 (the flag bit) and the whole message always follows directly after the address codeword. The framing rules of the code format do not apply to the message and message codewords continue until terminated by the transmission of the next address codeword or idle codeword. Each message displaces at least one address codeword or idle codeword and the displaced address codewords are delayed and transmitted in the next available appropriate frame. Although message codewords may continue into the next batch, the normal batch structure is maintained, i.e., the batch will consist of 16 codewords, preceded by a synchronization codeword. At the conclusion of a message any waiting address-codewords are transmitted, starting with the first appropriate to the first free frame or half frame. Message codewords have 20 message bits, viz bit 2 to bit 21 inclusive and these are followed by the parity check bits obtained according to the procedure outlined in 1.4 below. 1.3.4 Idle CodewordIn the absence of an address codeword or message codeword, an idle codeword is transmitted. The idle codeword is a valid address codeword, which must not be allocated to pagers and has the following structure as shown in Table 2: Table 2
1.4 Codeword Generation (31 : 21 BCH + Parity)Each codeword has 21 information bits, which correspond to the coefficients of a polynomial having terms from x30 down to x10. This polynomial is divided, modulo-2, by the generating polynomial x10+x9+x8+x6+x5+x3+1. The check bits correspond to the coefficients of the terms from x9 to x0 in the remainder polynomial found at the completion of this division. The complete block, consisting of the information bits followed by the check bits, corresponds to the coefficients of a polynomial which is integrally divisible in modulo-2 fashion by the generating polynomial. To the 31 bits of the block is added one additional bit to provide an even bit parity check of the whole codeword. The Hamming distance of these codewords is 6. The known error control deciding algorithms are: For "hard decision" decoding:
For "soft decision" decoding:
As can be seen, there is ample scope for design ingenuity and innovation within the above possibilities. 2 Message FormatsAlthough in principle, any message format can be inserted into message codewords, the following formats are regarded as standard. Adherence to these standards will enable a greater measure of interworking to be possible. The formats are not mixed within any one message. 2.1 "Numeric-only" Message FormatThis format is provided for the transmission of messages which may be represented solely in decimal numerals together with spaces, hyphens, opening and closing brackets, an urgency symbol "U" and one other symbol. There are 4 bits per character in this format and its use will save air-time compared to the other format. The address which introduces a message (or segment of a message) using this format has its function bits set to 00. The character-set used for the message is as shown in Table 3 which is based on Binary Coded Decimal (BCD). The bits of each character are transmitted in numerical order starting with bit No 1. Characters are transmitted in the same order as they are to be read and are packed 5 per message codeword. Any unwanted part of the codeword of the message is filled with space characters. Table 3 "Numeric-only" Character set
2.2 Alpha-numeric or general data formatThis format can be used for the transmission of messages requiring a greater range of characters than that provided within the "numeric-only" format but it may also be used to replace the latter when circumstances make this essential or desirable. There are 7 bits per character in this format. The page address which introduces a message (or segment of a message) using this format has its function bits set to 11. The CCITT Alphabet No 5 (7 bits per character) is used in this format. As in the case of the "numeric-only" format, bit order starting with bit No 1 of each character, and character reading order are preserved in transmission. The complete message is partitioned into contiguous 20 bit blocks for the purpose of filling consecutive message codewords. Thus a character may be split between one message codeword and the next. Any unwanted part of the last codeword of the message is fulled with appropriate non-printing characters such as "End of Message", "End of Text", Null, etc. All characters, except Null, are complete. CCITT Alphabet No 5 (or ASCII) is accepted internationally for data exchange. Provision for the transmission of this alphabet in its entirety will have significant impact on future development in message paging. END OF RECOMMENDATION 584
|