Fig. 1 Signal Format
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.
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".
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 |
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.
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.
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 |
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.
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 | [ |
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