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THE POCSAG RECOMMENDATION

Specification for a standard Code Format for use in Wide Area Radiopaging Systems


Contents:

Introduction
Principal Characteristics
PART 1 Code
PART 2 Message Format

1 Introduction

This specification describes a standard code format suitable for large capacity, wide area radiopaging systems. The purposes are:

2 Principal Characteristics

The principal characteristics of the code format are:

The derivation of many of these can be found in the Final Report of the Studies of the British Post Office Code Standardisation Advisory Group.

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.

3 Code 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

3.1 Preamble

Each transmission starts with a preamble to permit the pagers to attain bit synchronization and to prepare them to acquire word 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.

3.2 Batch Codeword

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.

3.3 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

3.3.1 Synchronization Codeword

The synchronization codeword is shown in 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

3.3.2 Address Codewords

The structure of an address codeword is illustrated in Fig 2. Bit 2 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. 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.

3.3.3 Message Codewords

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.

3.3.4 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:

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

3.4 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. (See Note 4).

4 Message Formats

Although in principle, any message can be inserted into message codewords, the following formats are regarded as standard. Adherence to these standards will enable the system purposes set out in paragraph 1 to be met. The formats shall not be mixed within any one message.

4.1 "Numeric-only" Message Format

This format can only be used 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 pager address which introduces a message (or segment of a message) using this format shall have its function bits set to 00. The character-set used for the message shall be as shown in Table 1 which is based on Binary Coded Decimal (BCD). The bits of each character are transmitted in numerical order starting with bit No 1. Characters shall be transmitted in the same order as they are to be read and are packed 5 per message codeword. Any unwanted part of the last codeword of the message is filled with space characters.

Table 1 "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 [

4.2 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 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.

Table 2 The basic code table of the ISO 7-bit character set (extracted from ISO 646)

Bits
b1.......b7
b7 0 0 0 0 1 1 1 1
b6 0 0 1 1 0 0 1 1
b5 0 1 0 1 0 1 0 1
b4 b3 b2 b1   0 1 2 3 4 5 6 7
0 0 0 0 0 NUL TC (DLE) SP 0   P   p
0 0 0 1 1 TC (SOH) DC ! 1 A Q a q
0 0 1 0 2 TC (STX) DEC " 2 B R b r
0 0 1 1 3 TC (ETX) DC # 3 C T c s
0 1 0 0 4 TC (EOT) DC $ 4 D S d t
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
0 1 1 1 7 BEL TC (ETB) ' 7 G W g w
1 0 0 0 8 FE (BS) CAN ( 8 H X h x
1 0 0 1 9 FE (HT) EM ) 9 I Y i y
1 0 1 0 10 FE (LF) SUB * : J Z j z
1 0 1 1 11 FE (VT) ESC + ; K   k  
1 0 0 0 12 FE (FF) IS (FS) , < L   l  
1 1 0 1 13 FE (CR) IS (GS) - = M   m  
1 1 1 0 14 SO IS (RS) . > N ^ n -
1 1 1 1 15 SI IS (US) / ? O - o DEL

4.3 Minimum Pager Storage Capacity

Usually for message display and re-display purposes storage will be provided in non-printing pagers. In those cases the minimum storage provided shall be 20 and 40 characters in pagers for numerics only and alphanumeric formats respectively (See Note 5).

4.4 End of Message

The pager shall cease decoding a message upon reception of an idle codeword, an address codeword or if two successive information codewords are indecipherable.

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)

 
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