Testing LAN Twenty-two points, plus triple-word-score, plus fifty points for using all my letters.  Game's over.  I'm outta here.

Test Local Area Network cables using a cable scanner

 


On completion of this learning outcome the student should be able to:

Assessment

Criteria

  1. Describe the common faults which cause problems at the physical media level
  2. Describe the types of tests available to identify the type and location of media faults
  3. Describe the parameters which may be used to certify the performance of coaxial cables and UTP cables
  4. Use a cable scanner to certify the performance of coaxial cables and UTP cables
  5. Use a cable scanner to identify and locate faults in coaxial cables and UTP cable segments

 

Assessment

Methods

The assessment method for this learning outcome is:

Practical Display

Describe the common faults which cause problems at the physical media level

The most common faults at a physical level are

·         broken (or open) cable connections

·         shorted cable connections

·         incorrectly wired cable connections

·         interference from electrical fields

Broken (or Open) Cable Connections

This fault occurs when one or more of the wires are no longer connected from one connector to the other. This can/will happen if the cable is damaged or cut. It can also occur in poorly constructed cables or when the cable is constantly placed under stress.

Shorted Cable Connections

This normally happens when a cable has been poorly constructed and wires are touching each other causing a short. It can also happen when connectors are damaged or when foreign material is lodged in connectors. (Such as dust or foreign matter in RJ45 sockets).

Incorrectly Wired Connections

Incorrectly wired 10BaseT (UTP) cables can sometimes work over shorter distances but not over longer distances, due to the misalignment of the cable pairs. Incorrectly wired cables are also more susceptible to electrical interference.

Interference From Electrical Fields

Cables placed near strong electrical fields can experience

 


Describe the types of tests available to identify the type and location of media faults

For UTP cabling there are two types of testers available:

·         The basic cable tester tests that each pair is wired correctly and operating.

·         The cable scanner can perform this test but also measure the performance of the cable as well as estimated the cable's length (extremely useful when locating cable breaks).

For Coaxial cable:

·         A standard multimeter can be used to test that the cable is operating correctly.

·         A cable scanner can measure cable performance parameters as well as estimate cable length.

 

Describe the parameters which may be used to certify the performance of coaxial cables and UTP cables

Common performance parameters used when testing UTP and/or Coaxial cable include:

Near End Crosstalk (NEXT) (UTP only)

As current flows in a wire, an electromagnetic field is created which interferes with signals on adjacent wires. As the frequency increases, this effect generally intensifies. Each pair is twisted to allow opposing fields in the wire pair to cancel each other out. the tighter the twist, the more effective the cancellation and the higher the data rate supported by the cable. Maintaining this pair twist is the single most important factor in any single Unshielded Twisted Pair installation. If the wires are not tightly twisted, the result is excessive Near End Crosstalk.

Attenuation (UTP Only)

Attenuation is the measurment of signal loss across a cable as a function of frequency. To measure attenuation a signal is is injected from one end of the cable and measured at the other end to determine cable loss.

Attenuation to Crosstalk Ratio (ACR) (UTP only)

Attenuation to Crosstalk Ratio (ACR) is an indication of how much larger the received signal is compared to the noise (NEXT) on the same pair.

Impedance

Impedance is a measure of the opposition to the flow of current in a cable. It includes the effects of resistance, inductance and capacitance. UTP is commonly rated at either 100 or 120 ohms. This value must be constant over the operating bandwidth of the cable. (To measure impedance properly with most cable scanners, the cable length must be 12 metres or greater). For coaxial cable, the impedance is 50 ohms.

Capacitance

Capacitance is the amount of electric field energy that can be stored between two conductors at a given voltage. For UTP capacitance is measured between conductors in a pair. For coaxial cable, capacitance is measured between the conductor and the shield.

 


Use a cable scanner to certify the performance of coaxial cables and UTP cables

 


Use a cable scanner to identify and locate faults in coaxial cables and UTP cable segments