4 dB rule - DTX CableAnalyzer

A YouTube video explaining how this this works is also available from here. 

This is applicable to the following cabling standards only:
 

• ISO/IEC 11801
• EN 50173
• Aus/NZ
• JIS

When the Insertion Loss is less than 4.0 dB at the frequency point where the measurement is made, then the measurement is ignored and not evaluated against the selected Test Limit/Cabling Standard in the DTX CableAnalyzer. This is often referred to as the 4 dB rule. It is not applied in the North American TIA Cabling Standards. Only certain test parameters and configurations will have the 4 dB rule. The tables below indicate which link  definitions and test parameters have the 4 dB rule applied to them. As an aid, the other rules are also listed.

Test Parameter	Class D Perm. Link	Class E Perm. Link	Class EA Perm. Link	Class F Perm. Link	Class FA Perm. Link
NEXT
PS NEXT
ACR-N
PS ACR-N
ACR-F
PS ACR-F
Return Loss

 
 

Test Parameter	Class D Channel	Class E Channel	Class EA Channel	Class F Channel	Class FA Channel
NEXT
PS NEXT
ACR-N
PS ACR-N
ACR-F
PS ACR-F
Return Loss

 
On a short link, the Insertion Loss may never reach 4.0 dB. If that were to happen, then the entire measurement would be ignored, or as the cabling standards phrase it; "recorded for information only". If this were to happen with your DTX CableAnalyzer, you would see an "i" appear next to the summary result for that test parameter. In LinkWare software, the result would be recorded with "N/A'. An example is shown below:
 

                
 

Concern is normally caused when the result has a negative margin. In this example we see the NEXT margin was -9.7 dB. A negative margin is normally associated with a FAIL, but because the entire link never exceeded 4.0 dB Insertion Loss, it's not a FAIL. Further confusion lies in that the North American ANSI/TIA/-568-C.0 cabling standard does not have this 4 dB rule. If you were to test the same link above with the same DTX CableAnalyzer, but using the North American Standard TIA Cat 5e Perm. Link, your result would be a FAIL.

            
 
Please make sure you have agreed in a Test Limit, in writing, before the project starts.
 
Let's review in detail how this works by looking at another example test result on the DTX CableAnalyzer.

An ISO11801 PL Max Class D AUTOTEST was run on a short 6.0 m installed link, giving the following result:
 
            
 
Looks normal. Let's take a look at the NEXT result in a little more detail:
 
            

You will observe that the limit line has two colors, black and red. The cabling standards do not require the cable tester manufacturer to display where the 4 dB rule is implemented to the user, only that it is implemented. However, the DTX CableAnalyzer will indicate where the 4 dB rule has been implemented by changing the color of the limit line. If the limit line is black, the Insertion Loss was less than 4.0 dB. If the limit line is red, the Insertion Loss was 4.0 dB or greater. So for this Pair 1,2-4,5 the NEXT is not evaluated below 84.8 MHz. Let's take a quick look at the Insertion Loss for Pair 4,5:
  
            
 
As you can see, the Insertion Loss reached 4.0 dB at 84.8 MHz, as previously indicated in the NEXT Pair 1,2-4,5 graph above. NEXT contains two pairs. So which Insertion Loss pair do we use to implement the 4 dB rule? The disturbed pair is used for evaluation purposes. So if the worst case NEXT Pair was 1,2-7,8 then the DTX CableAnalyzer would use the Insertion Loss values from Pair 7,8 to determine where the 4 dB rule is implemented.

As previously mentioned, if the link is particularly short, then the Insertion Loss may never reach 4.0 dB and the entire NEXT result is "ignored". Let's take a closer look at the first example where the test result was:
 
            

 
Looking at the worst case NEXT result, we see the entire limit line is black, suggesting that the Insertion Loss for Pair 4,5 never reached 4.0 dB: 
 
            
 
 
Looking at the Insertion Loss for Pair 4,5:
 
            
 
We see that the Insertion did indeed never reach 4.0 dB, so the entire measurement is not evaluated against the limit line (ignored) and recorded as "i" Information Only. If this result was downloaded to LinkWare Software, the report would show N/A against this measurement:
 
            
 
No further action is required by the installer. This is a valid standards compliant result.