GMSK Packet Data Modem
CMX909B
1.5.3
Data Formats
General Purpose Formats
In a proprietary system the user may employ the data elements provided by this device to construct
a custom, over-air data structure.
For example, 16 bits of bit sync + 2 bytes of frame sync + 4 bytes of receiver and sender address +
n data blocks would be sent as:
TQB (bit and frame sync) + TQB (addresses) + (n x TDB) + TSB
And received as:
SFS + RSB + RSB + RSB + RSB + (n x RDB)
Note that it is important to have established frame synchronisation before receiving data to enable
the receiving device to decode synchronously. Also the user may add, by way of algorithms
performed on the controlling device, additional data correction with the bytes in the data block task.
Mobitex Frame Structure
The Mobitex format for transmitted data is in the form of a Frame Head immediately followed by
either 1 Short Data Block or a number of Data Blocks (0 to 32).
The Frame Head consists of 7 bytes:
2 bytes of bit sync:
1100110011001100 from base,
0011001100110011 from mobile
bits are transmitted from left to right
2 bytes of frame sync:
System specific.
2 bytes of control data.
1 byte of FEC code, 4 bits for each of the control bytes:
bits 7-4 (leftmost) operate on the first control byte.
bits 3-0 (rightmost) operate on the second control byte.
Each byte in the Frame Head is transmitted bit 7 (MSB) first to bit 0 (LSB) last.
The data blocks consist of:
18 bytes of data (Data Block) or 4 bytes of data (Short Data Block).
2 bytes of CRC calculated from the data bytes.
4 bits of FEC code for each of the data and CRC bytes
The resulting data block bits are interleaved and scrambled before transmission.
ã 2001 Consumer Microcircuits Limited
12
D/909B/1
CMLMICRO [ CML MICROCIRCUITS ]
