|THE POCSAG RECOMMENDATION|
This specification describes a standard code format suitable for large capacity, wide area radiopaging systems. The purposes are:
Note 1: It is expected that if a standard code is widely adopted a rationalization of components particularly the decoder, will result and this, together with a larger market, should encourage mass production with a consequent reduction of costs.
Note 2: During the design process it became evident that little practical benefit would be derived from the adoption of a code with a significantly lower capacity. This code is entirely suitable for city wide paging or lower capacity systems.
Note 3: This worst case probability would be expected from a pager fully using the error correction potential in a weak field strength and subject to Gaussian noise such that a bit error rate of about 1 in 10 results.
A transmission consists of a preamble followed by batches of complete codewords, each batch beginning with a synchronization codeword (SC). The format of the signals is illustrated in Fig 1. Transmission ceases when there are no further calls.
Fig. 1 Signal Format
Codewords are transmitted in batches each of which comprises a synchronization codeword followed by 8 frames each containing 2 codewords. The frames are numbered 0 to 7 and the pager population is similarly divided into 8 groups. Each pager is allocated to one of the 8 frames according to the 3 least significant bits (lsb) of its 21 bit identity (see 3.3.2) (e.g., 000=frame 0, 111=frame 7) and will only examine address codewords in that frame. Therefore each pager's address codewords must only be transmitted in the frame that is allocated to those codewords.
Message codewords for any receiver may be transmitted in any frame but will follow, directly, the associated address codeword. A message may consist of any number of codewords transmitted consecutively and may embrace one or more batches but the synchronization codeword must not be displaced by message codewords. Message termination is indicated by the next address codeword or idle codeword. In any frame an idle codeword will be transmitted whenever there is no address codeword or message codeword to be transmitted.
Codewords contain 32 bits which are transmitted with the most significant bit first. The structure of a codeword is illustrated in Figure 2.
Fig. 2 Codeword Format
The synchronization codeword is shown in Table 1:
Bits 2-19 are address bits corresponding to the 18 most significant bits of a 21 bit identity assigned to the pager. The three least significant bits are not transmitted but serve to define the frame in which the address codeword must be transmitted (see 3.2). Hence the total number of identities is 221 (over 2 million).
Bits 20 and 21 are the two function bits which are used to select the required address from the four assigned to the pager. Hence the total number of addresses is 223 (over 8 million).
Bits 22 to 31 are the parity check bits (see 1.4) and the final bit (bit 32) is chosen to give even parity.
Note: Means to multiply the address capacity by tens or even thousands of times this figure are known without disturbing pagers conforming to the above. POCSAG has decided not to standardize such means until the need to do so is felt.
The structure of a message codeword is shown in Fig 2. A message codeword always starts with a 1 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 will be 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 will be transmitted, starting with the first appropriate to the first free 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 3.4 below.
In 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:
|4-bit Combination||Displayed Character|
|Bit No: 4 3 2 1|
|0 0 0 0||0|
|0 0 0 1||1|
|0 0 1 0||2|
|0 0 1 1||3|
|0 1 0 0||4|
|0 1 0 1||5|
|0 1 1 0||6|
|0 1 1 1||7|
|1 0 0 0||8|
|1 0 0 1||9|
|1 0 1 0||Spare|
|1 0 1 1||U (urgency indicator)|
|1 1 0 0||Space|
|1 1 0 1||Hyphen|
|1 1 1 0||]|
|1 1 1 1||[|
The page address which introduces a message (or segment of a message) using this format has its function bits set to 11.
The ISO 7-bit encoded character set, as shown in Table 2, is used in this format. As for the other format, bit order starting with bit No 1 of each character, and character reading order are preserved in transmission. The complete message shall be 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 shall be filled with appropriate non-printing characters such as "end of message", "end of text", Null, etc. No character except Null shall be incomplete.
|0||1||0||1||5||TC (ENQ)||TC (NAK)||%||5||E||U||e||u|
|0||1||1||0||6||TC (ACK)||TC (SYN)||&||6||F||V||f||v|
|1||0||0||0||12||FE (FF)||IS (FS)||,||<||L||l|
|1||1||0||1||13||FE (CR)||IS (GS)||-||=||M||m|
Note 4: The redundancy within a codeword allows for the correction of up to 2 random bit errors reception. The pager designer has the option of how much correction is actually provided within this capability.
Note 5: It will be very convenient if a paging control center can warn the caller when the message being input approaches the storage limit for the pager for which the message is intended.
(End of the POCSAG Recommendation)
|END OF SECTION|
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