Topics covered in this tutorial

• Terminators
• Loopback cables
• Crossover cables
• VOMs
• Tone generator and probes
• Cable testers
• TDRs
• Network monitors
• Packet analyzers
• SNMP

Introduction

The majority of network problems are a result of errors in cabling, connectors or network interface cards. The following equipment is available for isolating and remedying such problems:

• Terminators
• Loopback plug
• Crossover cable
• Volt-ohm meters (VOMs)
• Tone generator and probe ('Fox and Hound')
• Cable testers and certifiers
• Time-domain reflectometers
• Network monitors
• Protocol analyzers

The price of network test equipment can be prohibitive, especially given the limited use the equipment may receive. In this case, hiring the equipment may be an alternative.

Terminators

Terminators are used with thin coaxial transmission media. They are a useful aid for isolating a suspected break in the cable problem.

Dividing the cable at the approximate mid-point of the network and terminating the segments in turn should reveal the segment that is the cause of the problem.

This procedure can be repeated on the faulty segment until the location of the break emerges.

Loopback Test

A loopback test is used to determine whether the network card can transmit frames on to thin coax cable.

The test must be performed either with the loopback plug connected to the NIC or using an idle network. Performing this test on an active network may result in intermittent failures on the network.

A loopback plug can be assembled as shown in below.



If the test fails, it indicates a problem with the transceiver (if a loopback plug is employed) or the thin coax cabling (if an idle network is used).

Crossover Cable

A crossover cable is often used to troubleshoot problems with hubs. It allows a server and client to be connected directly without a hub.

If the problems persist, they relate to network cards or the network operating system. Otherwise, the problems relate to the hub.

Volt-Ohm Meters (VOMs)

VOMs (or digital millimeters) are the most basic of cable testing tools. They are used to check physical connectivity. A volt-ohm meter can test for the continuity of a cable, the existence of a short and the integrity of a terminator.

To perform useful tests, you need to know the readings that are expected from a particular test. For example, if the resistance measured across an Ethernet terminator is found to be 50 ohms, then the device is working correctly, but if the resistance between the two ends of a cable is infinity, the cable has a break.

Tone Generator and Probe

A network tone generator and probe are used to trace a cable from one end to the other. This may be necessary when the cables are bundled. This device is also known as a 'Fox and Hound'.

The tone generator is used to apply a signal on the cable to be traced where it is used to follow the cable over ceilings and through ducts.

Cable Testers and Certifiers

The capabilities of these devices vary widely but they always provide the facility to check physical connectivity of cables.

The devices fall into two categories - cable testers and cable certifiers.

Cable Testers

Cable testers are relatively simple, less expensive devices that can be used to indicate wiring faults such as:

• Open (broken) / short wiring
• Crossed pairs and reversed pairs (i.e. the polarity is incorrect, Tx- is connected to Tx+)

They offer support for both twisted-pair and coax cables. Cable testers work at the physical layer of the OSI model.

Advanced Cable Testers / Cable Certifiers

Advanced cable testers normally work at layers 1-4 of the OSI model. They provide detailed information on the physical and electrical properties of the cable. For example, they test and report on cable conditions, crosstalk, attenuation, noise, resistance and other characteristics of a cable run.

Some also display information about LAN traffic including statistics such as network utilization, collisions and frame error rates. They may even offer limited protocol testing; for example, the ability to use the ping utility when testing a TCP/IP network.

These devices may be used to test and certify cable installations (for example, that a network is category 5 compliant). They use defined transport performance specifications to ensure an installation exceeds the required performance characteristics for parameters such as attenuation and crosstalk.

These devices are very expensive and are often hired rather than purchased. Similar testing equipment is available for fiber optic cable.

Time-Domain Reflectometer (TDR)

TDRs sit at the upper end of the cable-testing spectrum. These devices are able to locate open and short circuits, kinks/sharp bends and other imperfections in cables that could affect performance.

A TDR transmits a short signal pulse of known amplitude and duration down a cable and measures the corresponding amplitude and time delay associated with resultant signal reflections. A TDR analyzes these reflections and can display any problems found and their location.

Problems (such as open and short circuits) can be identified because they bounce the signal back at different amplitudes depending on the type of problem. The TDR measures the amount of time taken for the signal to bounce back and can therefore calculate the distance to the cable fault to within a meter, which makes isolating the problem simple.

TDRs are available for all LAN types. Optical TDRs (OTDRs) provide a similar test capability for fiber cable.