An alternate frame synchronization (AFS) code can be selected to support special
systems or systems that require increased coding capability. The AFS is
generated in the same manner as an address codeword (i.e., BCH codeword with
parity bits). The POCSAG signaling standard has reserved special codewords for
the AFS from 2,000,000 to 2,097,151. The use of the AFS requires the paging
system to support the AFS. The AFS will change to frame 0 on the programmer
since no frame information is included in the AFS. The AFS should use address
1 so that bit 20 and 21 are 0.
-ADDRESS CODEWORD STRUCTURE
An address codeword's first bit (bit 1) is always a zero. Bits 2 through 19 are
the address bits. The pagers looks at these bits to find its own unique
address. Each POCSAG codeword is capable of providing address information for
four different paging sources (Address 1 to 4). These address are determined
by combinations of values of bits 20 and 21 ( the source-identifier bits). Bits
22 through 31 are the parity check bits, and bit 32 is the even parity bit.
BIT 20 BIT 21
Address 1 0 0
Address 2 0 1
Address 3 1 0
Address 4 1 1
Pre-coded into the code plug are three bits which designate the frame location,
within each batch, at which the pager's address is to be received; the decoder
will look at the codewords in this frame for its address.
Power is removed from the receiver during all frames other than the precoded
one, thus extending pager battery life.
-CODE CAPACITY
The combination of the code plug's three pre-coded frame location bits and address codeword's 18 address bits provides over two million different assignable codes. In this combination, the frame location bits are the least-significant bits, and the addres
s
bits are the most-significant bits.
-MESSAGE CODEWORD STRUCTURE
A message codeword structure always start with a 1 in bit 1 and always follows
directly after the address. Each message codeword replaces an address codeword
in the batch.
-IDLE CODEWORD STRUCTURE
The idle codeword is unique, reserved codeword used to talk place of an address
in any frame that would not otherwise be filled with 64 bits.
Thus, if a frame contains only an address, an idle codeword comprises the 32