GMSK Packet Data Modem
CMX909B
Figure 12 Typical Data Quality Reading (after 240 bits) vs S/ N, (noise in bit rate bandwidth)
1.5.5
CRC, FEC, Interleaving and Scrambling Information:
1.5.5.1 CRC
This is a 16-bit CRC code used in both the Mobitex Data Block and Short Data Block. In transmit it
is calculated by the modem from the data block bytes using the following generator polynomial:
16
12
5
g(x) = x + x + x + 1
i.e. CRC - CCITT X.25.
This code detects all (single) error bursts of up to 16 bits in length and about 99.998% of all other
error patterns.
The CRC register is initialised to all ‘1s’ and the CRC is calculated octet by octet starting with the
least significant bit of ‘byte 0’. The CRC calculated is bit-wise inverted and appended to the data
bytes with the most significant bit transmitted earliest.
In receive mode, a 16-bit CRC code is generated from the data bytes of each Mobitex Data Block or
Short Data Block as above and the bit-wise inverted value is compared with the received CRC
bytes. If a mis-match is present, then an error has been detected.
ã 2001 Consumer Microcircuits Limited
32
D/909B/1
CMLMICRO [ CML MICROCIRCUITS ]
