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

• to permit receivers (pagers) from a number of manufacturers to operate within a single system without resort to code translation for each type and to avoid the loss of transmission time which is usually experienced each time the code format changes;
  
• to allow a mixture of paging services, viz tone-only, simple numeric-only type message and full alpha-numeric message paging to be operated within a single coding system and/or single paging system;
  
• to encourage the harmonization of pager components. (See Note 1).

2 Principal Characteristics

• A capacity of more than 8 million paging addresses, which would normally be assigned to up to 2 million pagers, each with the capability of responding to up to 4 million addresses. This user capacity will amply fulfil the needs of a national paging system for a medium sized country such as the United Kingdom or could conveniently be adhered by a group of smaller nations (See Note 2);
  
• A paging rate of up to 15 calls per second using a single transmission, e.g., from several transmitters simultaneously (quasi-synchronous mode). Alternatively the paging calls might be repeated by a number of transmitters operated sequentially with a consequent reduction of the maximum paging rate;
  
• A data message capability, e.g., for conveying the name and telephone number of the caller. Inclusion of such messages will reduce the maximum paging rate;
  
• An error correcting potential of up to 2 bit errors per address or message word;
  
• A worst-case probability for false tone-only calls, of about 10^-8 per pager per transmitted address due to reception errors. (See Note 3). The great majority of pagers would experience a much lower false call probability than this;
  
• An inherent battery saving capability which makes it possible for the main circuits of the pager to be turned off for the greater part of the time, even during continuous transmission;
  
• it is suitable for transmission over normal land lines, using asynchronous data modems, such as might be used for transmission of other binary paging codes.

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

Fig. 1 Signal Format

3.1 Preamble

3.2 Batch Codeword

3.3 Types of Codewords

Fig. 2 Codeword Format

3.3.1 Synchronization Codeword

3.3.2 Address Codewords

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

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

3.4 Codeword Generation (31, 21 BCH + Parity)

4 Message Formats

4.1 "Numeric-only" Message Format

Table 1 "Numeric-only" Character set

4.2 Alpha-numeric or General Data 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)

4.3 Minimum Pager Storage Capacity

4.4 End of Message

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)