Actual Overhead Rate Calculation - Comtech EF Data DMD50 Installation And Operation Manual

DMD50 Universal Satellite Modem
3,072,000
4,096,000
6,312,000
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 3
19,200 1/7
19,200 1/7
19,200 1/7
19,200 1/3
19,200 1/1
1,024,000
19,200
1 to 7
3,093,943
4,117,951
6,337,248
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
1.007
1.005
1.004
1.004
1.006
6,312,000
19,200 bps
1 to 7
3–46