Operating instructions

DMD20/DMD20 LBST Universal Satellite Modem Theory of Operation
TM103 – Rev. 2.8 3-23
3.13.3 Overhead Rate Comparison
The SCC Overhead Ratio varies depending on the User Data Rate, the In-Band Rate, and the
Control Ratio. This gives SCC the advantage of lower overhead rates when compared to IBS,
which has a fixed overhead ratio of 16/15 or 1.067. The following table gives some examples of
SCC overhead rates for different user data and control ratios.
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))
Note: Slot bits of 0 are invalid.
The actual ratio the modem uses is:
Actual Ratio = (Slot Bits + 10)/Slot Bits