Overhead Rate Comparison; Actual Overhead Rate Calculation; Table 3-7. Scc Overhead Rates - Comtech EF Data DMD1050TS Installation And Operation Manual

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.
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
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:
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
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
Theory of Operation

Table 3-7. SCC Overhead Rates

In-Band Rate Control 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
Actual Ratio = (Slot Bits + 10)/Slot Bits
3–34
DMD1050TS Satellite Modem Board
Aggregate Data Rate
533,974
1,045,974
2,069,951
3,093,943
4,117,951
6,337,248
6,337,606
6,350,418
(less than In-Band Rate)
Revision 1
Overhead Ratio
1.043
1.021
1.011
1.007
1.005
1.004
1.004
1.006
MN-DMD1050TS