Aggregate Data Rate; Overhead Rate Comparison - Radyne DMD20 Installation And Operation Manual

DMD20/20LBST Universal Satellite Modem
3.14.2

Aggregate Data Rate

The aggregate data rate equals:
User Data Rate + In-Band Rate + Synchronizing Overhead Rate
Because SCC adjusts the overhead so that there are an equal number of user data bits in each
slot, the synchronizing overhead cannot be easily calculated. However, dividing the In-Band Rate
by the Control Ratio can approximate it. The basic calculation of this rate is shown:
Aggregate Date Rate = User Data Rate + In-Band Rate + (In-Band Rate/Control Ratio)
As an example, given the following parameters:
User Data Rate:
In-Band Rate:
Control Ratio:
Aggregate data rate = 1,024,000 + 19,200 + (19,200/7) or approximately 1,045,942 (actually
1045974).
This gives an overhead ratio of 1,045,974/1,024,000 = 1.021
In addition, another constraint changes the actual Aggregate Data Rate. The user data slot size
is limited to 2,500 bits. Because of this, the modem increases the in-band rate to reduce the user
data slot size. This only happens at higher user data rates.
NOTE: The Maximum In-Band rate is 115200. The Async interface Rate must be equal or
greater in value.
3.14.3

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.
MN-DMD20-20LBST Revision 14
User Data
Rate
Aggregate Data Rate
1,024,000 bps
19,200 bps
1 to 7
User In-Band
Data Rate Rate
512,000 19,200
1,024,000 19,200
2,048,000 19,200
3,072,000 19,200
4,096,000 19,200
6,312,000 19,200
6,312,000 19,200
6,312,000 19,200
In-Band
Rate
Control Aggregate Overhead
Ratio Data Rate
1/7 533,974
1/7 1,045,974
1/7 2,069,951
1/7 3,093,943
1/7 4,117,951
1/7 6,337,248
1/3 6,337,606
1/1 6,350,418
Theory of Operation
Synchronizing
Overhead
Ratio
1.043
1.021
1.011
1.007
1.005
1.004
1.004
1.006
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