I've not used the Omniscanner for a year, moved to the Wirescope 350, but as I remember it does not test for headroom.
Basically there are particular parameters that the tester checks, and it checks those parameter at intervals in frequency between 0 and 100 Mhz for Cat5 and 5e. So the wire tester takes off and checks at say 1 Mhz, and it logs all the attenuation, NEXT, FEXT, ELFEXT, etc. Then it bumps up and up until it gets to 100 Mhz. When it has done all this, it compares the results with an internal table that is preset (and user modifiable) to see if the wire met the requirements for the Category that you are asking it to certify to.
Now, you could congifure your tester to scan up to say 300 Mhz, but, what would that do for you? There is no standard to compare it to for 300 Mhz, so how do you know if the attenuation, NEXT, FEXT, etc. are in compliance? We went through this waiting for Cat6 to come out, everyone guessed what the standard was going to be, and we tested to 250 Mhz or so to be sure we were above what the standard was going to require. However, until the standard for Cat6 was adopted, no one could tell for sure what the requirements were going to be. With the wirescope (and probably others) you could save all the test results, and then run them through again under the Cat6 requirements after the standard was adopted.
Overhead, to me anyway, is the distance between the top of your standard and the maximum the cable can do. So if you have 350 Mhz cable, and you are certifying it to Cat5e, you should have a 250 Mhz overhead in the bandwidth of the cable. Thats all, nothing more. When i look at plots produced by my wirescope, I get a plot for each test. So I can see that for NEXT, I have an overall 6dB difference between the standard limit and the performance of my cable. That difference is the NEXT overhead.
Now if the test fails on NEXT or FEXT, I don't agree that this is caused by a transformer in the area or submerging the cables in water. Being too close to a transformer will quite likely increase the Noise measurements, but NEXT is checked by transmitting a signal on one pair, and listening for THAT signal on an adjacent pair (Power Sum NEXT is listening for that signal on ALL adjacent pairs).
Anyway, there is no real substitute for a good wire scanner if you really want compliance. If you can't spend the $$$, the next best thing that I have found is the Siemon STM-8 Tester. It cost me about $230 I think, it checks a variety of wiring standards. What is real cool about this cheap tester is that is does continuity, then it does a 'transmission' test. The transmission test checks for split pairs, and oddly enough it works. So if you have the pairs split but still continuity, this tester will show you. For the money, this is one of the better deals.
Hope that Helps! It is only my opinion, based on my experience and education...I am always willing to learn, educate me!
Daron J. Wilson, RCDD
daron.wilson@lhmorris.com
