Radyne DMD50, Installation And Operation Manual

Page: 49 / 237

Theory of Operation DMD50 Universal Satellite Modem
3-22 TM118 – Rev. 1.1
The Control Ratio of the receiving units must match the Control Ratio of the transmitting unit.
3.13.2 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
Rate
In-Band
Rate
Synchronizing
Overhead
Aggregate Data Rate
As an example, given the following parameters:
User Data Rate: 1,024,000 bps
In-Band Rate: 19,200 bps
Control Ratio: 1 to 7
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.
3.13.3 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 3-5 example overhead rates for
different user data and control ratios.