Comtech EF Data DMD1050 - Manual (Page 58)

DMD1050 Satellite Modem Board Theory of Operation
MN-DMD1050 3–32
Revision 9
3.17.4 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))
Note: Slot bits of 0 are invalid.
The actual ratio the modem uses is:
Actual Ratio = (Slot Bits + 10)/Slot Bits
Example 1:
User Data Rate: 1,024,000 bps
In-Band Rate: 19,200 bps
Control Ratio: 1 to 7
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
Example 2:
User Data Rate: 6,312,000 bps
In-Band Rate: 19,200 bps
Control Ratio: 1 to 7