GMSK Modem Data Pump
Page 10 of 37
MX909A PRELIMINARY INFORMATION
Mobitex¥ Frame Structure cont...
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 Block consists of:
18 bytes of data.
2 bytes of CRC calculated from the 18 data bytes.
4 bits of FEC code for each of the data and CRC bytes
The resulting 240 bits are interleaved and scrambled before transmission.
The over air signal is composed of Frame Sync and Bit Sync patterns, Control bytes, and Data Blocks as
shown in Figure 6.
The binary data transferred between the modem and the host µC is shown in the Frame Head and Data Block
of Figure 6 as shaded areas.
Frame Head
Data Block
LSB
6 5 4 3 2 1 0 3 2 1 0
MSB
7
LSB
0
MSB
7
6
5
4
3
2 1
1
Byte
loaded first 0
Byte
loaded first 0
Bit Sync
Bit Sync
1
2
3
4
5
6
7
2
1
Frame Sync
Frame Sync
1
2
2
3
Control Byte
Control Byte
1
4
loaded last 5
6
2
Data
FEC
8
9
(18 bytes)
FEC1
FEC2
10
11
12
13
14
15
16
loaded last 17
18
CRC (2 bytes)
19
INTERLEAVING / DE-INTERLEAVING
SCRAMBLE / DESCRAMBLE
OVER - AIR
SIGNAL
BITS
FRAME
SYNC
CTRL
BYTES
DATA
BLOCKS
BIT
SYNC
DB 0
DB 1
DB n
DB 2
DB 3
DB 4
FEC
16
16
16
8
FRAME HEAD
DATA BLOCKS (0 to 32)
FRAME
Figure 6: Mobitex¥ Over Air Signal Format
¤2001 MX-COM, Inc.
4800 Bethania Station Road, Winston-Salem, NC 27105-1201 USA
www.mxcom.com Tel: 800 638 5577 336 744 5050 Fax: 336 744 5054
Doc. # 20480134.005
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