DMD1050TS Satellite Modem Board
Revision 1
Theory of Operation 3–33 MN-DMD1050TS
Example 2: 1 to 1 Control Ratio
The Control Ratio of the receiving units must match the Control Ratio of the transmitting unit.
3.17.2 Aggregate Data Rate
Aggregate Data Rate = 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 calculated easily. 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 these parameters:
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
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 happens only at higher user data rates.
The Maximum In-Band rate is 115200. The Async interface Rate must be equal or
greater in value.
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