1 5 B A Ggregate Data Rate; Overhead Rate Comparison - Comtech EF Data DMD50 Installation And Operation Manual

DMD50 Universal Satellite Modem
3.15.2 Aggregate Data Rate
The aggregate data rate equals the following:
User Data Rate + In-Band Rate + Synchronizing Overhead Rate
Because SCC must adjust 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 following equation shows the basic calculation of
this rate:
Aggregate Date Rate = User Data Rate + In-Band Rate + (In-Band Rate/Control Ratio)
User Data
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.

3.15.3 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. Table 3-4 shows example overhead rates for
different user data and control ratios.
User Data
Rate
512,000
1,024,000
2,048,000
MN-DMD50– Revision 3
Rate
Aggregate Data Rate
1,024,000 bps
19,200 bps
1 to 7
Table 3-5. Overhead Rates Examples
In-Band Control
Rate Ratio
19,200
19,200
19,200
In-Band
Rate
Aggregate Data
Rate
1/7 533,974
1/7 1,045,974
1/7 2,069,951
Theory of Operation
Synchronizing
Overhead
Overhead
Ratio
1.043
1.021
1.011
3–45