Cheat Sheet: Troubleshooting Network Cabling

COMMON CABLING FAILURE CAUSES
1. Installation errors.
2. Bad connectors.
3. Inappropriate tester set-up.
4. Bad cable.
5. Bad patch cords.
6. Any of several types of crosstalk.
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SETTING UP THE TEST

THE MEASUREMENTS

For Cat5e and Cat6 links:
• Wiremap
• Propagation delay (delay skew and link length)
• Insertion loss
• NEXT (near end crosstalk)
• Power Sum NEXT
• ELFEXT (equal level far end crosstalk)
• Power Sum ELFEXT
• Return loss

CHOOSING A CABLE TESTER

When selecting a cable tester, it's important to consider the following:
1.  Screen display — is it full color, grayscale, or black and white?
2.  Talk-set ability — eliminates the need for job radios or walkie-talkies.
3.  Storage — a removable flash card stores thousands of tests; a built-in buffer stores hundreds of tests.
4.  Reporting — can be dumped onto a PC, either for a fee or for free.
5.  Scanning ability — coaxial and fiber cable adapters.
6.  Battery life.
7.  Testing speed.

TEST DESCRIPTIONS

Length - Time domain reflectometry (TDR) is used to measure electrical length. A pulse is sent down the cable and reflects back when it reaches the far end.

Nominal Velocity of Propagation (NVP) -  NVP, a measure of how fast the pulsed signal is traveling, is expressed as a percentage of the speed of light.

Wiremap - A test to ensure that the two ends have been terminated pin for pin (i.e. Pin 1 at the patch panel goes to Pin 1 at the outlet, etc.).

Return Loss - The combined effect of all manufacturing imperfections contributes to the structural return loss (SRL) measurement.

DC Loop Resistance - This is the resistance between the two conductors of a twisted pair, which is looped back at the far end.

Attenuation - The decrease in signal strength (expressed as negative dB) from one end of a cable to the other.

Near End Crosstalk (NEXT) - NEXT occurs because alternating current flow produces an electromagnetic field around the cable, and this field induces a current flow in adjacent cables. The strength of this field increases with the frequency of the signal.

Power Sum Near End Crosstalk (PSNEXT) - The PSNEXT is a calculation of the addition of the three NEXT results for each pair of cables.

FEXT, ELFEXT, and PSELFEXT - Far end crosstalk (FEXT) is NEXT measured at the far end. However, because the length of the cable determines how much the signal is attenuated before it can affect the pairs at the far end, this attenuation is subtracted from the FEXT to produce equal level far end crosstalk (ELFEXT). Power sum equal level far end crosstalk (PSELFEXT) is calculated by adding the results for each pair. 

Delay Skew - Delay skew is the difference between the fastest and slowest pairs. It's essential that the signals reach the far end at nearly the same time. Otherwise, the signal won't be correctly re-combined, creating the artifacts known as "skew."

Ping Testing - Ping is a common Internet utility that sends out a packet (block of data) and waits for a response. It's used to determine whether a particular IP address is online, and verifies that the assigned configuration from the DHCP sever is correct and that traffic is being properly routed.

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USI NG A TESTER TO TROUBLESHOOT CABLING

1. Identify the cable. When documentation isn't present, first locate each end of the cable. Send a digital tone over the cable run. If there's a network patch panel, use a tone probe to check each port on the panel for a tone. A digital probe will provide a visual and audible signal strength indication. Once the port with full signal strength is identified, the digital probe can be plugged into the port for 100 percent confirmation.

2. Qualify the cable. Check the bandwidth of any suspicious cables. A qualification tester, which checks for opens, shorts, wiremap, crosstalk, impedance faults, and cable length, generates a list of supportable speeds. If any of the results display faults, proceed to step 3.

3. Troubleshoot disqualified results. A qualification tester will also display any speeds that aren't supported. At the first layer of detail, the tester will identify whether the failure is due to faulty wiremap, excessive length, and/or poor signal performance. At the second layer, it will display the type and location of the fault.

4. Save the results. After identifying, qualifying, and troubleshooting a suspicious cable, save the test results and cable location in the test tool for future reference. Saving the results to a PC that other technicians can access will prevent unnecessary duplication in future troubleshooting efforts.

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CE RTIFICATION
Certification refers to the process of making measurements and comparing the results to rigid predefined standards requirements so a pass/fail determination can be made.

ATTENUATION TO CROSSTALK RATIO
Attenuation to crosstalk ratio (ACR) represents the overall performance of the cable. Attenuation is the reduction in signal strength over the length of the cable and frequency range. The crosstalk is the external noise introduced into the cable. If the two areas meet, the data signal will be lost because the crosstalk noise will be at the same level as the attenuated signal.

SOURCES OF REDUCED NETWORK SPEED
• NIC duplex and speed incompatibilities
• Network congestion
• Poor routing
• Bad cabling
• Electrical interference
• Overloaded server at the remote end of the connection
• Misconfigured DNS

SOURCES OF A LACK OF CONNECTIVITY
• Extremely slow network speed
• Power failures
• Remote server or an application on the remote server being shut down
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Sources: Fluke Networks, Network Cabling Help, www.datacottage.com, Ideal Industries