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OM20 Universal Outdoor Modem
4.15.3 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
512,000
1,024,000
2,048,000
3,072,000
4,096,000
6,312,000
6,312,000
6,312,000
4.15.4 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))
Note: Slot bits of 0 are invalid.
The actual ratio the modem uses is:
Actual Ratio = (Slot Bits + 10)/Slot Bits
TM116 – Rev. 1.2
In-Band
Control
Rate
Ratio
19,200
1/7
19,200
1/7
19,200
1/7
19,200
1/7
19,200
1/7
19,200
1/7
19,200
1/3
19,200
1/1
Theory of Operation
Aggregate Data
Overhead
Rate
533,974
1,045,974
2,069,951
3,093,943
4,117,951
6,337,248
6,337,606
6,350,418
Ratio
1.043
1.021
1.011
1.007
1.005
1.004
1.004
1.006
4-23
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