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DMD50 Universal Satellite Modem
User Data
Rate
6,312,000
6,312,000
3.15.4
Actual Overhead Rate Calculation
The following is the actual calculation the modem does to calculate the overhead ratio:
1.
The modem calculates the minimum in-band rate to limit the size of the user data slots to
2,500 bits (the result is truncated to an integer).
Minimum In-Band = (User Data Rate * Control Ratio)/((Control Ratio + 1) * 250)
2.
Using the bigger of Minimum In-Band or the selected In-Band, the modem calculates the
number of bits for each user data slot (result is truncated to an integer).
Slot Bits = (User Data Rate * (Control Ratio * 10))/(In-band Rate * (Control Ratio + 1))
The actual ratio the modem uses is:
Actual Ratio = (Slot Bits + 10)/Slot Bits
Example 1:
User Data Rate:
bps
In-Band Rate:
bps
Control Ratio:
Minimum In-Band = (1,024,000 * 7)/((7 +
1) * 250) = 3,584
(less than In-Band Rate)
Slot Bits = (1,024,000 * (7 * 10))/(19,200
* (7 + 1)) = 466
Actual Ratio = (466 + 10)/466 = 1.021
MN-DMD50 Revision 4
In-Band
Control
Rate
Ratio
19,200
1/3
19,200
1/1
1,024,000
19,200
1 to 7
Aggregate Data
Rate
6,337,606
6,350,418
Example 2:
User Data Rate:
bps
In-Band Rate:
Control Ratio:
Minimum In-Band = (6,312,000 * 7)/((7 +
1) * 250) = 22,092 (
more than In-Band Rate)
Slot Bits = (6,312,000 * (7 * 10))/(22,092 *
(7 + 1)) = 2,500
Actual Ratio = (2,500+ 10)/2,500= 1.004
Theory of Operation
Overhead
Ratio
1.004
1.006
6,312,000
19,200 bps
1 to 7
3–44
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