Operating instructions

DMD50 Universal Satellite Modem Theory of Operation
TM118 – Rev. 1.1 3-23
Table 3-5
User Data
Rate
In-Band
Rate
Control
Ratio
Aggregate Data
Rate
Overhead
Ratio
512,000 19,200 1/7 533,974 1.043
1,024,000 19,200 1/7 1,045,974 1.021
2,048,000 19,200 1/7 2,069,951 1.011
3,072,000 19,200 1/7 3,093,943 1.007
4,096,000 19,200 1/7 4,117,951 1.005
6,312,000 19,200 1/7 6,337,248 1.004
6,312,000 19,200 1/3 6,337,606 1.004
6,312,000 19,200 1/1 6,350,418 1.006
3.13.4 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: 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
Minimum In-Band = (6,312,000 * 7)/((7 + 1) * 250) = 22,092 (
more than In-Band Rate)