Would you fight this speeding ticket? The 10% rule explained.

February 25, 2012 / General

Ironically my desire to speed in traffic evaporated when I bought a real sports car.  But this story is not about me and my desire to push the limits.  It’s about you.

I was stopped in the hallway by three different engineers at the recent BICSI conference in Orlando with similar questions. How can a link or channel pass the length limit when it is clearly beyond the limit? These guys were all experienced project engineers,  holding their PMP, RCDD.  So I know they’re not the only ones with an “uncertain” understanding of the so called “10% rule.” The fact that a passing channel can be longer than the length limit is confusing to many people.  This has to do with the uncertainty of the nominal velocity of propagation measurement and the way the standards require the length measurement to be handled.

Imagine this scene:  It’s a warm summer night.  You’re on a mountain road behind the wheel of a turbo charged 911 Porsche.  You’re driving hard on a curvy road but no other cars are out here, the asphalt is sticky from the fading heat of the day and you’ve got a set of new tires.  Most importantly you’re keeping it between the lines and you’re not breaking the law.  The speed limit is 70 MPH.  As you drive through the turns, your speedometer varies between 50 and 75 MPH.  On a straight stretch,  you pass a parked Sheriff who pulls you over and tickets you for speeding.  His radar gun indicates that you were going 76 MPH when you passed him.  

If his radar gun had a measurement uncertainty of 10% would you fight it?  (Speeders, keep in mind that typical stationary police radar gun uncertainty would be closer to 2 MPH)  You darn well should fight the ticket!   Just like the guy with the 355 ft channel may have actually had a 322 ft channel,  you may have in fact been going under the 70MPH.  70MPH + 10% =77MPH.

Hopefully this analogy helps explain this rather dry and uncertain topic.  But here is the conventional wisdom and a link to a more detailed explanation:

TIA-1152:When physical length is determined from electrical length, the physical length of the link calculated using the pair with the shortest electrical delay shall be reported and used for making the pass or fail decision. The pass or fail criteria is based on the maximum length allowed for the channel or permanent link as specified in ANSI/TIA-568-C.2 plus the nominal velocity of propagation (NVP) uncertainty of 10 %. So length has to exceed 10% of the limit in order to FAIL.

Permanent Link:  90 m (295 ft) + 0.9 m (29.5 ft) = 99 m (224.5 ft)

Channel:  100 m (328 ft) + 10 m (32.8 ft) = 110 m (360.8 ft)

You can read more on this and other topics by going to our knowledge base on the Fluke Networks website.  Here is a link to an article on the topic.http://myaccount.flukenetworks.com/fnet/en-us/supportAndDownloads/KB/Copper-Testing/DTX+CableAnalyzer/10+rule+for+length+-+DTX+CableAnalyzer.htm

The key is to trust the pass/fail result from the DTX CableAnalyzer. The way it performs the test is all based on the performance standard. Those who want to push the envelope by building extra long links and channels are likely to begin failing  Insertion Loss and Propagation Delay. If Propagation Delay fails, the link is normally over length. For systems such as Ethernet, the signal has to arrive within a certain time to prevent collisions on the network.