DTX2CU exercises

Introduction

The best way to learn is to do.

These test results are real results taken from the field.

• Download and unzip the archive Exercises01to16LinkWare.zip
  • MD5 Checksum 474FB17AA68091B141A46B2C5C71CFD8

• Open the file "Exercises01to16LinkWare.flw" in LinkWare
• Double click one of the result records; another window will open with detailed test data
• Select Pair Data near the top and HDTDR or HDTDX Analyzer near the bottom of the window

Avoid using the term left and right. In HDTDR and HDTDX the left side of the screen represents the main unit or "near end" while the right side of the screen represents the smart remote or "far end".

 

Solution

Exercise 01 - cable too long

Notice that the length has exceeded 90m but not failed.

Accept? No.

Why? Insertion Loss failed. Even when the standard TIA 1152 allows the length limit be exceeded by 10% during testing, cables lengths exceeding the design length are likely to fail the insertion loss test.

HDTDR/HDTDX: you could look at HDTDR to make sure there is nothing unusual.

Corrective Action: try and remove some slack from the cable run. 

 

Exercise 02 - cable too long

With ISO/IEC11801, length is not a required field test parameter.

Notice that Length is marked with an "i".

Accept? No.

Why? Insertion Loss failed. Even when there is no field test limit for length in ISO/IEC11801, cables lengths exceeding the design length are likely to fail the insertion loss test.

HDTDR/HDTDX: you could look at HDTDR to make sure there is nothing unusual.

Corrective Action: try and remove some slack from the cable run. 

 

Exercise 03 - damaged cable

The result for Return Loss (RL) in marginal. Cabling standards deem a marginal PASS* as a PASS. However, the customer or the cabling vendor offering the warranty may not.

Accept? Depends on the contract.

Why? RL is marginal

HDTDR/HDTDX: with HDTDR, in almost all cases, you need to zoom in on the vertical axis so that in the right corner of the screen you see 1X  32X. HDTDR shows the cable to be bad in the last part of the link; consistent with a cable handling issue.

Corrective action: this cable will have to be replaced.

 

Exercise 04 -  bad connector

The result for Near End Crosstalk (NEXT) in marginal. Cabling standards deem a marginal PASS* as a PASS. However, the customer or the cabling vendor offering the warranty may not.

Accept? Depends on the contract.

Why? NEXT is marginal.

HDTDR/HDTDX: HDTDX shows the connection at the near end to be bad with a value of 44.5. The value for a compliant connector should be less than 17.5.  Notice that the far end termination is not great either.

Corrective Action: assuming the connector is compliant, re-terminate the connector at the near end and run the AUTOTEST again.  If NEXT still doesn't PASS, re-terminate the connector at the far end as well.

 

Exercise 05 - bad cable

Return Loss fails. The frequency/RL graph fails at a the low frequency. If Return Loss fails at the lower frequency, it is most likely a cable issue.

Accept? No.

Why? RL failed.

HDTDR/HDTDX: use HDTDX to confirm the hypothesis that this is a cable problem. The HDTDR show the cable to be bad down the entire length of the cable. Whilst HDTDR can only tell  if the cable is bad or not, it can suggest that the installer damaged it or whether it left the factory that way. If the cable run has been visually inspected and there is no obvious signs of damage, it would suggest bad cable. Testing at regular intervals during an installation is clearly a good idea to catch manufacturing issues.

Corrective action: this cable will have to be replaced.

 

Exercise 06 - bad connector

NEXT is marginal. Cabling standards deem a marginal PASS* as a PASS. However, the customer or the cabling vendor offering the warranty may not. NEXT fails at low and high frequency; no cable/connector suggestions this time. Notice that this is a really short Cat 6A link, so "it must be marginal". This statement is not entirely incorrect: some cabling vendors do have a minimum distance for Category 6A. However, vendors have improved the Cat 6A connectors and the minimum length requirement has come down to 5m  or less. You will need to ask the vendor what their requirements are.

Accept? Depends on the contract.

Why? NEXT is marginal.

HDTDR/HDTDX: HDTDX shows 66.6 at the remote connector when it should be less than 17.5; it is obvious that the remote connection is bad and needs fixing.

Corrective Action: re-terminate the connector at the remote.

 

Exercise 07 - bad connector

NEXT is marginal. Cabling standards deem a marginal PASS* as a PASS. However, the customer or the cabling vendor offering the warranty may not. Notice that NEXT fails at medium frequency; no cable/connector suggestions this time.

Accept? Depends on the contract.

Why? NEXT is marginal.

HDTDR/HDTDX: HDTDX shows 32.7 at the remote connector when it should be less than 17.5.

Corrective Action: re-terminate the connector at the near end.

 

Exercise 08 - AC wire map

Notice that Resistance is missing from the menu. The technician has enabled AC Wire Map and hence Resistance as well as Shield Integrity can no longer be measured and is missing from the menu. ?

Accept? You would have to check with the vendor offering the cabling warranty. If this was a shielded cabling system, then the answer would be a definite NO.

Why? AC Wire Map enabled

HDTDR/HDTDX: Not applicable.

Corrective Action: Verify if AC Wire Map was activated for a valid reason. If not, deactivate and rerun AUTOTEST.

 

Exercise 09 - bad connector

NEXT fails at low and high frequency so no cable/connector suggestions.

Accept? No.

Why? NEXT failed.

HDTDR/HDTDX: HDTDX shows 76.7 at the main connector end when it should be less than 17.5.  The remote end shows a less than impressive 33.9.

Corrective Action: re-terminate the connector at the near end and rerun  AUTOTEST.  If NEXT still doesn't pass, re-terminate the connector at the far end as well.

 

Exercise 10 - bad termination

The Return Loss graph shows the marginal result to be at the lower frequencies suggesting a cable issue. Remember that is likely - but not alays  the case and needs confirmation. Cabling standards deem a marginal PASS* as a PASS. However, the customer or the cabling vendor offering the warranty may not.

Accept? Depends on the contract.

Why? RL is marginal.

HDTDR/HDTDX: when we look at the HDTDR for Pair 1,2 we see a value of 8.1 at the remote connector. We’re looking for nothing greater than 3 at the connector (guidance value only). There seems to be a contradiction: the frequency plot is suggesting cable and the HDTDR says connector. Whenever HDTDR is inconclusive, you should check Resistance. Go and look at Resistance and you will see a higher Resistance reading on Pair 1,2 compared to the other pairs. This suggests a contact issue between the test lead and the installed connector. It could be that Pair 1,2 was not punched down correctly or there was dirt on the contacts.

Corrective Action: re-terminate the connector at the far end, retest.

 

 

Exercise 11 - bad cable

RL Fails. Notice the narrow frequency band at which Return Loss is failing. This suggests a cable issue, even more than a failure at low frequency. We still need HDTDR to verify this. 

Accept? No.

Why? Return Loss fails badly.

HDTDR/HDTDX: HDTDR shows close events greater than 0.8 down the entire length of the cable. Because only one pair shows issues and it is pretty much all the way down the cable, it suggests a manufacturing problem with the cable. There should be other links in the installation showing the same HDTDR plot assuming there are other cables that came from the same batch. When the Return Loss fails this badly, it will impact Insertion Loss. Have a look at the Insertion Loss trace. The two spikes on the Insertion Loss trace are at the same frequency as those found on the Return Loss trace.

Corrective Action: replace cable.

 

Exercise 12 - immersion (long-term)

RL fails. The frequency plot failing at low frequency suggests a cable issue. Notice that not only does the Return Loss fail badly at the lower frequencies, all four pairs seem to fail in the same way. The four pairs overlap each other. When all four pairs follow each other at the lower frequencies like this, then we know there is water in the cable somewhere. 

Accept? No.

Why? Return  Loss fails badly.

HDTDR/HDTDX: we must use HDTDR to verify the cable issue. Looking at the HDTDR we a significant change in impedance at 67.7 m (222 ft) and only recovers at the end. That’s where the water is.

Corrective Action: replace cable. Dry out conduit/run.

 

 

Exercise 13 - damaged cable

Return Loss is marginal. Cabling standards deem a marginal PASS* as a PASS. However, the customer or the cabling vendor offering the warranty may not. It fails at the lower frequencies suggesting a cable issue. 

Accept? Depends on the contract.

Why? RL is marginal.

HDTDR/HDTDX: HDTDR shows events greater than 0.8 down the cable and especially around 63 m. The cable has be handled incorrectly at the end of the link but what it suggests is that this particular cable does not like rough handling.

Corrective Action: replace cable. If replacing with the same cable, take extra care when pulling it.

 

Exercise 14 - bad/damaged cable

The link fails RL at the lower frequencies; there is most likely an issue with the cable. RL does not fail at the lower frequencies, rather half way between 1 MHz and 250 MHz. We know from experience that a point frequency failure such as this for Return Loss is a cable issue.

Accept? No.

Why? Return Loss fails

HDTDR/HDTDX: HDTDR is inconclusive. A quick look at the Resistance readings confirm that it is not a contact issue.

Corrective Action: replace cable.

 

Exercise 15 - incorrect connector type

NEXT is marginal. Cabling standards deem a marginal PASS* as a PASS. However, the customer or the cabling vendor offering the warranty may not. NEXT fails at the higher frequencies suggesting a connector issue. 

Accept? Depends on the contract.

Why? NEXT is marginal.

HDTDR/HDTDX: when looking at the HDTDX for this results, the connectors are less than 17.5 and there is little crosstalk in the cable. We’re going to have to rely on the frequency domain to help us. When looking at the frequency plot for Pair 3,6-4,5 you will note that the trace “tanks” at the higher frequencies. What it suggests is that the connector installed is not Cat 6A component rated. So what happened here? To save money, an enhanced Cat 6 connector rated to 350 MHz was used with very good cable. The DTX HDTDR and HDTDR troubleshooting feature tops off at frequencies of 350MHz and is unable to analyze this problem as it occurs at frequencies above 500MHZ. We could say that the DTX - is not so good at troubleshooting connector related issues in high-end cabling systems. Its successor, the DSX Cable Analyzer, no longer has this limitation.

Corrective Action: replace connectors with properly component rated connectors.

 

Exercise 16 - bad cable

The NEXT margin fails really badly. It needs to be investigated and not simply dismissed as a short link issue (see also exercise 06).

Accept? No.

Why? NEXT fails.

HDTDR/HDTDX: HDTDX shows an obvious cable fault.

Corrective Action: replace cable.