What are the nominal values for amplitude and frequency of an As-I signal? What are the acceptable levels for noise?

When investigating communication issues, how can one determine if its due to noise or something else? How can such issues be resolved?

While noise is by definition derived from a random signal.I know the basics,but I’m still missing lots of knowledge in this area.There are tools you can make to show the existance of the frequency being there but what you hear is what you get.