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POCSAG

Recommendation 584
Standard Codes and Formats for
International Radiopaging
Annex 1 - Radiopaging Code No. 1


1 Code and format

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.

POCSAG frames

Fig. 1 Signal Format

1.1 Preamble

Each transmission starts with a preamble to aid the pagers to attain bit synchronization and thus to help them in acquiring word and batch synchronization. The preamble is a pattern of reversals, 101010..., repeated for a period of at least 576 bits i.e., the duration of a batch plus a codeword.

The preamble gives the pager designed opportunity to save battery power. The receiver can be turned on for a few milliseconds and then turned off again for about 1 second if no preamble is detected. When detected, the preamble provides fast bit synchronization.

1.2 Batch Codeword

Codewords are transmitted in batches which comprise 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. Thus each pager is allocated to one of the 8 frames according to the 3 least significant bits (lsb) of its 21 bit identity (see 1.3.2) i.e. 000=frame 0, 111=frame 7, and only examines address codewords in that frame. Therefore each pager's address codewords must be transmitted only in the allocated frame.

This frame structure within a batch not only multiplies the address possibilities of each codeword by 3 but also offers yet another means of battery saving within the pager, since the receiver need only be turned on during the synch codeword and its particular frame. Thus the energy requirement is reduced to about 3/17 of that for constant reception. Further methods of battery saving also are available.

Message codewords for any receiver may be transmitted in any frame but 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. There is at least one address or idle codeword between the end of one message and the address codeword belonging to the next message.

In any batch wherever there is no meaningful codeword to be transmitted, and idle codeword is transmitted.

At a rate of 512 bits each batch is only 1.0625 seconds duration and transmission can then be stopped. Thus, time-slotted multi-transmitter systems can be engineered very conveniently, and such systems might be of the zoned type to achieve reuse of frequency or of the sequential type for lowest cost of transmission.

Priority calling is available with time delays of about 1 second in non-time-slotted systems, and systems catering for any mixture of alert-only and longer messages can be designed and the mixture can be dynamically altered, e.g., to maximize the use of "air-time".

1.3 Types of Codewords

Codewords contain 32 bits which are transmitted with the most significant bit first. The structure of a codeword is illustrated in Figure 2.

 

POCSAG codeword format

Fig. 2 Codeword Format

1.3.1 Synchronization Codeword

The synchronization codeword is shown in Table 1:

Table 1
BIT No 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
BIT 0 1 1 1 1 1 0 0 1 1 0 1 0 0 1 0
BIT No 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32
BIT 0 0 0 1 0 1 0 1 1 1 0 1 1 0 0 0

This codeword has very low correlation with the preamble. Its first 31 bits are a pseudo random sequence which can be generated from a 5-stage feedback shift register.

1.3.2 Address Codewords

The 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 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.

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 Codewords

The 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 Codeword

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 as shown in Table 2:

Table 2
BIT No 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
BIT 0 1 1 1 1 0 1 0 1 0 0 0 1 0 0 1
                                 
BIT No 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32
BIT 1 1 0 0 0 0 0 1 1 0 0 1 0 1 1 1

This idle codeword is equivalent to identity values 2007664 - 2007671. Such a high value will not come within the normal range of identities issued in a system. The sift register which might be used to generate the synchronization codeword could be used for this codeword also.

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 Formats

Although 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 Format

This 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

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 [

Since there are 16 rather than 10 possibilities in this "numeric" format there is considerable scope for the use of coded messages. Typically the coded message might be enclosed by the brackets. The user would carry a printed table of messages together with their corresponding numeric codes.

2.2 Alpha-numeric or general data format

This 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 filled 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

 
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