The Physical Layer Playbook: Mastering Cable Troubleshooting

Preamble:
This space will be utilized to synthesize my notes and help improve my learning process while I study for the CompTIA Network+ N10-009 certification exam. Please follow along for more Network+ notes and feel free to ask any questions or, if I get something wrong, offer suggestions to correct any mistakes.

Cable Troubleshooting

Troubleshooting network issues can often feel like a puzzle. Applying a layer-by-layer approach, such as the OSI model, can greatly assist in isolating symptoms and causes. In this post, we'll investigate common issues affecting cabled networks and identify the tools and techniques used to solve problems at the Physical layer.

As we explore cable troubleshooting, keep these questions in mind:

• How can you distinguish an issue caused by improper cable choice or termination from one caused by environmental factors?
• Which tools can you use to diagnose issues with copper and fiber optic cable and connectors?
• What is the difference between a regular cable tester and a cable certifier?
• Which tool would you use to find the end of a specific cable within a wiring closet?

Understanding Cable Performance: Speed, Throughput, and Limitations

When troubleshooting a network link, you'll need to compare its expected performance with its actual current performance. To do this, you must understand how to assess and distinguish speed, throughput, and distance specifications and limitations.

Speed Versus Throughput

At the Physical layer, a signal transmitted over a communications channel consists of a series of events referred to as symbols. A symbol could be something like a pulse of higher voltage in an electrical current or the transition between the peak and the trough in an electromagnetic wave. The number of symbols that can be transmitted per second is called the baud rate. The baud rate is measured in hertz (Hz), or multiples like megahertz (MHz) or gigahertz (GHz).

At the Data Link layer, the nominal bit rate or bandwidth of the link is the amount of information that can be transmitted, measured in bits per second (bps), or some multiple thereof. To transmit information more efficiently, a signaling method might be capable of representing more than one bit per symbol. This also helps to overcome noise and detect errors. The use of these encoding methods means that the bit rate will be higher than the baud rate. In Ethernet terms, the bit rate is the expected performance of a link that has been properly installed to operate at 10 Mbps, 100 Mbps, 1 Gbps, or better. The nominal bit rate will not often be achieved in practice.

Throughput is an average data transfer rate achieved over a period of time, excluding encoding schemes, errors, and other losses incurred at the Physical and Data Link layers. Throughput can be adversely affected by link distance and by interference (noise). Throughput is typically measured at the Network or Transport layer. Often, the term "goodput" is used to measure an averaged data transfer rate at the Application layer, taking account of the effect of packet loss. Throughput is also sometimes measured as packets per second.

As well as bandwidth or throughput and packet loss, the speed at which packets are delivered is also an important network performance characteristic. This "speed" is measured as a unit of time—typically milliseconds (ms)—and is also referred to as latency or delay. The term "speed" is also sometimes used to describe a link's performance in terms of throughput, but it's important to be aware of the distinction between bit rate and latency.

Image from https://www.dnsstuff.com/latency-throughput-bandwidth

Distance Limitations, Attenuation, and Interference

Each type of media can consistently support a given bit rate only over a defined distance. Some media types support higher bit rates over longer distances than others.

Attenuation and interference enforce distance limitations on different media types.

Attenuation is the loss of signal strength, expressed in decibels (dB). Decibels express the ratio between two measurements; in this case, signal strength at origin and signal strength at destination.

Interference or noise is anything that gets transmitted within or close to the channel that isn't the intended signal. This makes the signal itself difficult to distinguish, causing errors in data and forcing retransmissions. This is expressed as the signal to noise ratio (SNR).

Cable Issues

When troubleshooting cable connectivity, you are focusing on issues at the

Physical layer. At layer 1, a typical Ethernet link for an office workstation includes the following components:

• Network transceiver in the host (end system)
• Patch cable between the host and a wall port
• Structured cable between the wall port and a patch panel (the permanent link)
• Patch cable between the patch panel port and a switch port
• Network transceiver in the switch port

The entire cable path (patch cords plus the permanent link) is referred to as a channel link.

Assuming you are investigating link failure (complete loss of connectivity), the first step is to check that the patch cords are properly terminated and connected to the network ports. If you suspect a fault, substitute the patch cord with a known good cable. If you cannot isolate the problem to the patch cords, test the transceivers. You can use a loopback tool to test for a bad port. If you don't have a loopback tool available, another approach is to substitute known working hosts (connect a different computer to the link or swap ports at the switch). However, this approach may have adverse impacts on the rest of the network, and issues such as port security can make it an unreliable method.

If you can discount faulty patch cords and bad network ports/NICs, you will need to use tools to test the structured cabling. The solution may involve installing a new permanent link, but there could also be a termination or external interference problem.

Cable Category Issues

When troubleshooting a permanent link, you should verify that the cable type is appropriate to the application. For example, you cannot expect 10 GbE Ethernet to run over an 80 m Cat 5e link. You may also need to verify that unshielded cable has not been installed where shielded or screened cable would be more suitable. Using an incorrect cable type might result in lower-than-expected speed and/or numerous checksum errors and link resets. Check the identifier printed on the cable jacket to verify the type that has been used.

When evaluating whether a cable category is suitable for a given use in the network, consider the following factors:

• Cat 5e: Supports Gigabit Ethernet and could still be an acceptable choice for providing network links for workstations, though most new installations and upgrades would now use Cat 6 or better.
• Cat 6: Can support 10 Gbps, but only over a 55 m maximum distance.
• Cat 6A: An improved specification cable that can support 10 Gbps over 100 m. Cat 6A cable is bulkier than Cat 5e, and its installation requirements are more stringent, making it problematic to fit within pathways designed for older cable. TIA/EIA standards recommend Cat 6A for use in healthcare facilities, with Power over Ethernet (PoE) 802.3bt installations, and for horizontal connections to wireless access points.
• Cat 7: Always of a screened/shielded type and is rated for 10 Gbps applications up to 100 m (328 feet). Cat 7 is not recognized by TIA/EIA but appears in the cabling standards created by the ISO (ISO/IEC 11801). It must be terminated with GG45 or TERA connectors rather than standard RJ45 connectors.
• Cat 8: Intended for use in datacenters only for short patch cable runs that make top-of-rack connections between adjacent appliances. ISO defines two variants: 8.1 (Class I) is equivalent to TIA/EIA Cat 8 and uses RJ45 connectors, while 8.2 (Class II) must use outer shielding or screening and GG45 or TERA connectors.

Unlike Ethernet and Fast Ethernet, Gigabit Ethernet uses all four pairs for transmission and is thus more sensitive to crosstalk between the wire pairs. Cabling is not the only part of the wiring system that must be rated to the appropriate category. For Gigabit Ethernet and better, the performance of connectors becomes increasingly critical. For example, if you are installing Cat 6A wiring, you must also install Cat 6A patch panels, wall plates, and connectors.

From a safety point of view, you must also ensure that the cable jacket type is suitable for the installation location, such as using plenum-rated cable in plenum spaces and riser-rated cable in riser spaces.

Cable Testers

If the cable is not accessible, cable testing tools can also be used to diagnose intermittent connectivity or poor performance issues. A cable tester reports detailed information on the physical and electrical properties of the cable. For example, it can test and report on cable conditions, crosstalk, attenuation, noise, resistance, and other characteristics of a cable run.

Devices classed as certifiers can be used to test and certify cable installations to a performance category—for example, that a network is TIA/EIA 568 Category 6A compliant. They use defined transport performance specifications to ensure an installation exceeds the required performance characteristics for parameters such as attenuation and crosstalk.

Cable testing tools can be used for troubleshooting and verification. It is best to verify wiring installation and termination just after you have made all the connections. This means you should still have access to the cable runs. Identifying and correcting errors at this point will be much simpler than when you are trying to set up end user devices.

Wire Map Testers and Tone Generators

Fully featured cable testers/certifiers are expensive. A simpler wire map tester device can be used to detect improper termination issues. To perform a wire map test, the base unit is connected to one end of the cable and a remote unit to the other. When the test is activated, an LED for each wire conductor lights up in sequence. If an LED fails to light or does not light in sequence, there is a problem with the cable and/or termination.

Wire map testers can identify the following problems:

• Continuity (Open): A conductor does not form a circuit, either due to cable damage or improper connector wiring.
• Short: Two conductors are joined at some point, usually due to damaged insulating wire or poor connector wiring.
• Incorrect Pin-Out/Incorrect Termination/Mismatched Standards: The conductors are incorrectly wired into the terminals at one or both ends of the cable.

The following transpositions are common:

• Reversed pair: The conductors in a pair have been wired to different terminals (for example, from pin 3 to pin 6 and pin 6 to pin 3 rather than pin 3 to pin 3 and pin 6 to pin 6).
• Crossed pair (TX/RX transposed): The conductors from one pair have been connected to pins belonging to a different pair (for example, from pins 3 and 6 to pins 1 and 2). This may be done deliberately to create a crossover cable, but such a cable would not be used to link a host to a switch.

Another potential cable wiring fault is a split pair. This occurs when both ends of a single wire in one pair are wired to terminals belonging to a different pair. This type of fault can only be detected by a cable tester that measures crosstalk.

A network tone generator (or toner) and probe are used to trace a cable from one end to the other. This may be necessary when the cables are bundled and have not been labeled properly. This device is also known as a Fox and Hound. The tone generator applies a signal on the cable to be traced so that you can use the probe to identify the same cable within a bundle or duct.

Attenuation and Interference Issues

If a cable link is too long, decibel (dB) loss (also known as insertion loss) can lead to signal degradation, high error rates, and retransmissions (frame or packet loss). This ultimately results in reduced speeds and potentially a complete loss of connectivity. Insertion loss is measured in decibels (dB) and represents the ratio of the received voltage to the original voltage.

A decibel (dB) expresses the ratio between two values using a logarithmic scale. The key takeaway here is that a logarithmic scale is nonlinear, meaning a small change in dB value represents a significant change in measured performance.

The following reference points are useful to remember:

• +3 dB means doubling, while -3 dB means halving.
• +6 dB means quadrupling, while -6 dB relates to a quarter.
• +10 dB means 10 times the ratio, while -10 dB is a tenth.

The maximum value allowed for insertion loss depends on the link category. For example, Cat 5e at 100 MHz allows up to 24 dB, while Cat 6 allows up to 21.7 dB at 250 MHz. When you are measuring insertion loss itself, smaller values are better (20 dB insertion loss is better than 22 dB, for instance). A cable certifier is likely to report the margin, which is the difference between the actual loss and the maximum value allowed for the cable standard. Consequently, higher margin values are better. For example, if the insertion loss measured over a Cat 5e cable is 22 dB, the margin is 2 dB ; if another cable measures 23 dB, the margin is only 1 dB, bringing you much closer to not meeting acceptable link standards. Higher grade or shielded cable may alleviate the problem; otherwise, you will need to find a shorter cable run or install a repeater or additional switch.

Careful cable placement is necessary during installation to ensure that the wiring is not subject to interference from sources such as electrical power cables, fluorescent lights, motors, electrical fans, radio transmitters, and so on.

Electromagnetic Interference (EMI) should ideally be detected during cable installation. If it appears later, you should suspect either a newly installed source or a source that wasn't considered during initial testing (such as machinery or power circuits that weren't activated during the installation testing). Interference from nearby data cables is also referred to as alien crosstalk. Radio frequency interference (RFI) is EMI that occurs in the frequencies used for radio transmissions.

Crosstalk Issues

Crosstalk usually indicates a problem with bad wiring (poor quality, damaged, or the improper type for the application), a bad connector, or improper termination. Check the cable for excessive untwisting at the ends and for kinks or crush points along its run. Crosstalk is also measured in dB, but unlike insertion loss, higher values represent less noise. Again, the expected measurements vary according to the cable category and application.

Image from https://www.flukenetworks.com/blog/cabling-chronicles/cable-testing-101-cross-talk-near-and-far

There are various types of crosstalk that can be measured:

• Near End (NEXT): This measures crosstalk on the receive pairs at the transmitter end and is usually caused by excessive untwisting of pairs or faulty bonding of shielded elements.
• Attenuation to Crosstalk Ratio, Near End (ACRN): This is the difference between insertion loss and NEXT. ACR is equivalent to a signal-to-noise ratio (SNR). A high value indicates that the signal is stronger than any noise present, while a result closer to zero means the link is likely to experience high error rates.
• Attenuation-to-Crosstalk Ratio, Far End (ACRF): Far-end crosstalk (FEXT) is measured on the receive pairs at the recipient end. The difference between insertion loss and FEXT gives ACRF, which measures cable performance regardless of the actual link length.
• Power Sum: Gigabit and 10 GbE Ethernet use all four pairs. Power sum crosstalk calculations (PSNEXT, PSACRN, and PSACRF) confirm that a cable is suitable for this type of application. They are measured by energizing three of the four pairs in turn.
• Alien Crosstalk: This refers to signal interference from nearby cables that affects a "disturbed" or "victim" cable. This is commonly caused by cinching cable bundles too tightly with ties or by poorly terminated cabling.

Complete loss of connectivity indicates a break in the cable (or a completely faulty installation), while intermittent loss of connectivity is more likely to be caused by attenuation, crosstalk, or noise.

Fiber Optic Cable Testing Tools

When you are working with fiber optic cabling, it is important to understand that any mismatch between the cables coupled together will result in data loss. This can occur if the fiber cables are not properly aligned, are different sizes, or may have suffered damage (broken/misshaped fiber strands) during transport.

If you connect single mode fiber to multimode fiber, you will introduce a catastrophic signal loss of up to 99%. Even connecting fiber cables of the same type but with different diameters can cause a loss of up to 50% of the signal strength.

Whenever a connector is installed on the end of fiber optic cables, a degree of signal loss occurs. This is called insertion loss. In addition, some of the light that is lost is reflected directly back down the cable toward the source. This is called back-reflection, reflectance, or optical return loss (ORL).

Ultra Physical Contact (UPC) and Angled Physical Contact (APC) polishing techniques reduce Optical Return Loss (ORL) reflections, with APC offering improved reflectance loss values compared to UPC. Mating an APC connector to a non-APC port will cause significant insertion loss. Because of this, APC connectors are always colored green to keep you from mixing them with non-APC connectors.

Visual Fault Locator

If a break is identified in an installed cable, the location of the break can be found using a visual fault locator. There are different models for short and long link distances, and they can be supplied with adapters for different connector types (ST, SC, or LC). The tool shines visible light down the cable, which then glows brightly at the point where a cable is broken, excessively bent, or improperly spliced.

Dirty Optical Cables

Dirt, dust, or grease in the transmission path will greatly reduce signal strength or block transmission completely. Most commonly, this occurs at a connector. Connectors should be covered with a dust cap when removed, and the surrounding area should be dust-free before disconnecting them. Connectors should be cleaned using solvent designed for fiber optics, taking care not to apply excess. The wet-to-dry method involves applying a drop of solvent to a lint-free cloth and then moving the connector from the wet drop across a dry part of the cloth.

Contamination could also be introduced when a cable is spliced. Ensure splicing equipment is cleaned according to the manufacturer's instructions before every splicing operation.

The powerful light sources used by fiber optics are a hazard. Wear appropriate safety goggles, and never look directly at an active transceiver port or the end of a fiber cable. Point a cable at a flat surface to confirm if visible light is being transmitted, or use a smartphone camera to detect if infrared light is.

Cable Troubleshooting Strategies

Sometimes, cables fail. Your job will be to troubleshoot connection issues and find the root cause. This can happen for many reasons. Common network connection issues include physical damage to the cable, loose connections, interference from other devices, and problems with the network adapter or its drivers.

Let's look at the steps you can take to troubleshoot cable issues:

• Physical Inspection
  • Check the cable for any visible damage such as cuts, kinks, or severe bends.
  • Ensure that the connectors aren't damaged and are securely plugged into both the network device and your computer.
• Reseat the Cable
  • Unplug the cable from both ends and then plug it back in. This can resolve loose connection issues.
• Verify Drivers
  • If the problem persists, the issue might be related to drivers or a physical problem with the network adapter itself.
  • Open Device Manager on your computer, find your network adapter in the list, and check if it's working properly.
  • If it is not working correctly, you may need to update the drivers or replace the network adapter

Ultimately, reliable cabling is the backbone of any strong network. By familiarizing yourself with these troubleshooting strategies and tools, you're not just fixing problems – you're building a solid foundation for your networking journey. The more you understand the Physical layer, the better prepared you'll be for whatever the network throws your way.