Comtech EF Data CDM-750 - Manual (Page 240)

CDM-750 Advanced High-Speed Trunking Modem Revision 2
Appendix C MN-CDM750
C–4
0 dB
5 dB
10 dB
15 dB
20 dB
25 dB
Do
w
n
lin
k
Fa
d
e
Ma
rg
in
R
e
qui
re
d
Average Availability (Over 1 Year)
2.5 dB
exceed the customer contracted DR and justify the lower throughput for a larger
percentage of the year. This is essentially a tradeoff between minimum DR contracted
and effective DR averaged over the course of a year.
C.2.1 ACM Operational Link Example
Most links are designed to deliver a level of link availability based on worst case conditions.
Since worst case conditions occur only a very small part of the time, the link has margin above
the minimum during the majority of the time. ACM shatters this limitation and operates at
higher data rates when conditions are good.
For example, consider a link with a 25 Mbps service level agreement (SLA) between Nigeria and
Germany that requires 99.8% availability. Once designed, the link will provide this level of SLA
for all but 17.5 hours of the year. Figure C‐1 plots the amount of downlink fade margin versus
availability for this link.
Figure C-1. Downlink Margin vs. Average Link Availability
At 99.8% availability there are over 364 days when the downlink margin is higher than the
minimum to support 25 Mbps. So the question arises: What additional throughput will ACM
provide?
An answer is shown in Figure C‐2, where user throughput is plotted versus time. Initially, the link
is unfaded and operates at 64 Mbps. The modem takes advantage of the margin and ACM
operation provides a higher than contracted level of throughput. Later, a 2.2 dB fade occurs and
the ModCod is adjusted to preserve data integrity, causing an adjustment of the data rate to 51
Mbps, which is substantially above the SLA of 25 Mbps. Still later, the link recovers to 64 Mbps,