Reverse Path Troubleshooting: Problems in Advanced Services
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• CSO (Composite Second Order) Intermodulation: A clustering of
second-order beats in the video bandwidth can cause distortion in the
system.
Problems in Troubleshooting the Reverse Path
Troubleshooting noise in the reverse path is more complex than it is in the
forward path, and can require far more time and energy to correct, as well as
being much more frustrating.
When a customer experiences a problem with the reverse path, there is a
much wider area that must be searched to find it compared to a forward path
problem. In the forward path, the problem must be somewhere in the direct
line from the customer to the headend. A reverse path problem, however,
can be caused anywhere in the entire area served by this customer’s
receiver. This means there are many more test points to check, a far greater
possibility for error, and much more money can be wasted if the problem
remains incorrectly diagnosed. The forward path technique of "walking back
up the system" and replacing everything until the problem goes away simply
doesn’t work. You must have a tool that accurately indicates whether the
problem is "here" or "there."
The problem is that the sources of interference come and go. CB, ham, and
mobile transmitters are not continuous, and they move around. Switching
noise is transient, and may last only a few microseconds, but repeat at
irregular intervals and render a network useless. As you can see, there is a
lot of ground to cover when you're searching for reverse path problems.
To have any chance of successfully troubleshooting reverse noise problems,
you need a fast (peak detecting) spectrum analysis display. You need to be
able to catch transient noise spikes. It is also essential to have a "peak hold"
capability so that a passing burst can be captured and viewed—because
you can’t find the source of something you can’t see. These are the bare
minimum criteria, but many other tools can help even the odds when a
system does battle with ingress.
Finally, tracking down one source of ingress is relatively easy, but tracking
many sources together at the same frequency poses a much more difficult
problem.
Improving Accuracy and Reducing Wasted Time
The following list describes the steps you can take to improve the accuracy
with which you trace reverse path problems and thus reduce the time you
spend returning trouble-free service to your customers:
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