Data Sheet

4-Level FSK Modem Data Pump Page 14 of 47 MX919B PRELIMINARY INFORMATION
©2001 MX•COM, INC. www.mxcom.com Tele: 800 638 5577 336 744 5050 Fax: [phone] Doc. # 20480170.003
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4.4 Frame Structure
Figure 7 shows how an over-air message frame may be constructed from a sequence of: a Symbol Sync
pattern (preamble), a Frame Sync pattern, and one or more 'Header', 'Intermediate' or 'Last' blocks.
Last Block
CRC2
(4 bytes)
µC binary data stored
in MX919B data block
memory configured as
header, intermediate, or
last block by MX919B
task being executed.
Data Bytes
(8)
Data Bytes
(12)
0
1
2
3
4
5
6
7
9
10
11
8
Intermediate BlockHeader Block
CRC 1
(2 bytes)
Byte
76
5
4
3
21076
5
4
3
210 76
5
4
3
210
070
Byte 1
Byte 0Byte 0
Byte 11
0
1
2
345 29303132
0
64 65
4-Level
Symbols
tri-bits
2
1
FEC Trellis Coding / Decoding
(Error Correction)
Interleaving / De-interleaving
7 70
'000'
Symbol
Sync
Frame
Sync
'Header'
Block
Intermediate Blocks
'Last'
Block
24
6666 66 66
Frame
Preamble
Frame
-1 +1 +3-3+1 +1 +1-1 -1 -1 -13+3+3 -3-3 +3+3 -3 -3-3 +1+1-1
Frame Sync:
sent first
Over-air
signal
(symbols)
Symbol Sync: at least 24 symbols of '...+3 +3 -3 -3 ...' sequence
last
Data Bytes
(10)
Figure 7: Over-Air Signal Format
The 'Header' block is self-contained and includes its own checksum (CRC1). It would normally carry
information such as the address of the calling and called parties, the number of following blocks in the frame
(if any), and miscellaneous control information. The number of following blocks (if any) is required to allow
the Rx device software to expect the Last Block and interpret it as a Last Block rather than an Intermediate
Block. There is no other indicator to differentiate a Last Block and an Intermediate Block.
The 'Intermediate' block(s) contain only data, the checksum for all of the data in the 'Intermediate' and 'Last'
blocks (CRC2) being contained at the end of the 'Last' block.
This arrangement, while efficient in terms of data capacity, may not be optimum for poor signal-to-noise
conditions, since a reception error in any one of the 'Intermediate' or 'Last' blocks would invalidate the whole
frame. In such conditions, increased throughput may be obtained by using the 'Header' block format for all
blocks of the frame, so blocks that are received correctly can be identified as such, and do not need to be re-
transmitted. These, and some other possible frame structures, are shown in Figure 8.