Comtech EF Data DMD1050TS - Overhead Rate Comparison; Actual Overhead Rate Calculation; Table 3-7. SCC Overhead Rates

DMD1050TS Satellite Modem Board
Revision 1
Theory of Operation 3–34 MN-DMD1050TS
3.17.3 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. Table 3-7 gives some examples of SCC
overhead rates for different user data and control ratios.
Table 3-7. SCC Overhead Rates
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
3.17.4 Actual Overhead Rate Calculation
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)
Using the larger of Minimum In-Band or the selected In-Band, the modem calculates the number
of bits for each user data slot. The result is truncated to an integer.
Slot Bits = (User Data Rate * (Control Ratio * 10))/(In-band Rate * (Control Ratio + 1))
Slot bits of 0 are invalid.
The ratio the modem uses is actually: 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