NETWORK TROUBLE-SHOOTING: 1

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This section covers the use of the multi-meter in checking out data cabling used in Local Area Networks.

We shall use a common form of cabling known as Ethernet, which is coaxial cable of 50 ohms. Ethernet LAN's are arranged in BUS fashion, with the cable being terminated at each end with a 50 ohm resistor (called a terminator).

Obtain a few pieces of Ethernet cable, T-connectors and terminator resistors from your tutor. Connect these together to form an ethernet segment.

cable


Continuity Tests
The first test you will perform is a continuity test. This tests to ensure that there is electrical continuity (unbroken path) along the entire cable. It looks like,

First, remove the terminators at each end of the cable segment. Set the meter to resistance (or continuity test if it has one). Using one person to connect the meter leads to each part of the t-connector, have another person short the other t-connector using a small screw-driver.

Under a continuity test, the meter sends a signal along one lead. This travels down one wire of the coaxial cable, reaching the other end. At the far end, because it is shorted to the other lead by the screwdriver, the signal travels back along the other wire of the coaxial cable. When it arrives back at the meter, the meter knows that the cable is okay.

What indication did you get on the meter for the continuity test?
[2 marks]

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Now simulate a cable break (a wire broken or faulty t-connector) by removing the short at the far end (ie, remove the screw-driver) and perform the continuity test again.

What is the indication you now get on the meter for the continuity test?
[2 marks]

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Resistance and Termination Tests
Now switch the meter to read resistance (we will be reading values of about 0-50 ohms, so set the range appropriately).

Apply a terminator resistor to the each end of the cable. Diagramatically, it looks like,

Apply the meter to take a reading on the middle T-connector. Electrically, the circuit looks like,

What is the value of resistance measured by the meter?
[2 marks]

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Now, the golden rule of resistance in parallel is that the resultant resistance is

R = (R1 * R2) / (R1 + R2)

So, where the two resistors are equal, this gives a value of the value of one of the resistors. In our case, this is equal to 25.

Remove one of the terminators. This simulates a missing or faulty terminator. Take a resistance reading again.

What is the value of resistance measured by the meter?
[2 marks]

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Replace the terminator so that the cable is correctly terminated at each end. Now disconnect the cable from one side of one of the end T-connectors. This will simulate a cable break somewhere in the cable. Take a resistance reading again.

What is the value of resistance measured by the meter?
[2 marks]

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How do you think you would use the meter to detect which part of the cable is faulty.
[4 marks]

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What resistance reading do you expect to get for a good cable?
[2 marks]

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