CompTIA Network+ Study Notes: Network Troubleshooting and Cable Connectivity
Network Troubleshooting Methodology
The Troubleshooting Approach
Troubleshooting should follow a systematic methodology rather than rushing into solutions.
Implementing a solution without identifying the core problem may result in the wrong fix and further complicate the issue.
Step 1: Identify the Problem
Gather Information: Collect as much data as possible regarding the occurrence.
Question Users: Users provide valuable initial hints. For example, a user may report system instability immediately following a display driver update. Even if the update is not the root cause, it provides a starting point for investigation.
Identify Symptoms: Look for technical signs. Example: If a system cannot connect to resources, check if it is receiving an IP address.
Determine Changes: Investigate if anything has changed in the system environment (updates, physical moves, configuration shifts).
Duplicate the Problem: If possible, try to make the problem happen again to understand the trigger, though this is not always feasible.
Approach Multiple Problems Individually: If multiple issues exist, isolate them. Break a large, complex problem into smaller units and tackle each one separately rather than as a collective whole.
Step 2: Establish a Theory of Probable Cause
Question the Obvious: Do not ignore simple explanations. If a system won't boot, verify if it has power or if the power cord is faulty. Often, seemingly huge problems have small, obvious solutions.
OSI Model Approach: Use the OSI layers to locate the problem.
Top-to-Bottom or Bottom-to-Top: A systematic check through layers.
Example: A name resolution issue is a Layer 7 (Application) problem related to DNS. Identifying this narrows the search to one layer, excluding the other six.
Divide and Conquer: Delegate tasks among team members. Having more individuals working on specific assignments increases the chances of a faster resolution.
Step 3: Test the Theory to Determine the Cause
Validation: Theories must be tested to be validated.
Outcomes:
Theory Confirmed: If the test proves the theory (e.g., swapping a suspected bad power cable with a known good one and the system powers on), proceed to the next step.
Theory Not Confirmed: If the test fails to resolve the issue (e.g., the second system also doesn't power on with that cable), return to Step 2 and re-establish a new theory or escalate the problem.
Step 4: Establish a Plan of Action and Identify Potential Effects
Plan the Solution: Develop a step-by-step strategy for implementation.
Identify Potential Effects: Assess if the solution might cause new problems. Example: A security patch may resolve a vulnerability but cause software stability issues. If negative impacts are too great, discard the plan and find an alternative.
Workarounds vs. Final Solutions: Depending on urgency and complexity, determine if a temporary workaround is needed before a permanent fix is applied.
Step 5: Implement the Solution or Escalate
Execution: Implement the plan created in the previous step.
Permissions and Coordination: Implementation may require specific privileges (e.g., Active Directory Administrator help for account lockouts) or coordination with other specialized teams (e.g., a hardware team to replace a physical router).
Escalation: If the solution is beyond one's expertise, a senior engineer may be required to validate or complete the implementation.
Step 6: Verify Full System Functionality and Implement Preventive Measures
Validation of Fix: Ensure the primary problem is gone and no negative outcomes were created by the fix.
Preventive Measures: Take steps to ensure the problem does not recur.
Example: If users receive excessive SPAM, implement a SPAM filter as a preventive measure for the future.
Step 7: Document Findings, Actions, Outcomes, and Lessons Learned
Maintain Records: Create a new document if the problem is unique or update existing ones for recurring issues.
Shared Knowledge: Documentation helps when the same problem occurs on different hardware (e.g., applying a known patch for a network adapter to a different model).
Essential Elements to Capture:
Symptoms observed.
Corrective actions taken.
Final outcomes.
Network Cable Specifications and Limitations
Throughput
Definition: The actual amount of data transmitted from one device to another within a specific timeframe.
Influencing Factors: Throughput is negatively impacted by:
Jitter: Variation in the delay of received packets.
Latency: The time delay between the cause and the effect of some physical change in the system.
Packet Loss: Failure of one or more transmitted packets to reach their destination.
Measurement: Managed using throughput testers available online to measure speed from source to destination.
Bandwidth vs. Throughput: Bandwidth is the theoretical maximum amount of data in ideal conditions; Throughput is the real-life data flow measured in actual working conditions.
Speed
Configuration: Determined by network adapter settings and duplex modes.
Hardware Mismatches: Using a 10 Mbps hub with a system configured for 1000 Mbps causes issues.
Auto-Mode: It is recommended to select "Auto" mode so the system matches the speed of the communicating device automatically.
Distance and Attenuation
Attenuation: The loss of signal strength as data travels away from the source. The further the distance, the higher the attenuation.
Repeater Usage: To extend transmissions beyond cable limits, repeaters must be installed to amplify and forward the signal.
Cable Length Limits:
CAT 5, CAT 6, and CAT 7: Maximum range of .
CAT 8: Maximum range of .
Cable Considerations and Applications
Shielded vs. Unshielded Twisted Pair
General Structure: Eight wires total, organized into four pairs of two. Wires in each pair are twisted around each other.
Shielded Twisted Pair (STP): Each pair is insulated in a foil coating inside the plastic sheath to prevent electromagnetic interference (EMI).
Unshielded Twisted Pair (UTP): Pairs are not held inside foil; only a plastic sheath surrounds the internal wires. Telephone cables are common examples.
Plenum and Riser-Rated Cables
Plenum Cables:
Laid in plenum spaces (areas between ceiling tiles and the roof, often used for HVAC).
Jackets are made of low-toxicity, fire-resistant materials like Teflon or Kynar.
Riser-Rated Cables:
Laid in non-plenum areas like elevator shafts or cable risers.
Feature self-extinguishing capabilities to prevent fire from spreading vertically.
Rule: Plenum cables can replace riser cables, but riser cables cannot replace plenum cables.
Rollover (Console) Cable
Connectors: RJ45 (male) on one end and DB9 (female) on the other.
Appearance: Typically a flat blue cable.
Application: Used for the initial configuration of routers or switches by connecting the DB9 end to a serial port and the RJ45 end to the system.
Crossover Cable
Function: Used to directly connect two similar devices (System to System, or Switch to Switch for cascading).
Structure: An Ethernet cable with RJ45 male connectors where certain wires are switched between ends.
Wiring Standards:
T-568A: White Green, Green, White Orange, Blue, White Blue, Orange, White Brown, Brown.
T-568B: White Orange, Orange, White Green, Blue, White Blue, Green, White Brown, Brown.
Power over Ethernet (PoE)
IEEE Standard: 802.3af.
Functionality: Allows a single Ethernet cable to provide both data connectivity and electrical power to devices (e.g., Wireless Access Points in locations without power outlets).
Variants:
PoE (802.3af): Provides up to .
PoE+ (802.3at): Provides up to .
Common Cable and Connectivity Issues
Attenuation and dB Loss
Fiber Optic: Attenuation occurs due to joins and splices; more splices lead to higher loss.
Copper (UTP/STP): Longer cables result in more attenuation. UTP is more prone than STP.
Measurement: Attenuation is measured in decibels (), also referred to as loss.
Interference (EMI)
Caused by electrical devices: Fluorescent lights, heavy machinery, cordless phones, microwaves, power cables, and electrical motors.
Mitigation: UTP is highly prone; fiber optic is resistant. Cables should be laid away from monitors and EMI-producing devices.
Physical Faults (Open/Short and Pinouts)
Incorrect Pinout: Wires plugged into the RJ45 in the wrong order.
Open Fault: The cable is physically broken or cut into two pieces.
Short Fault: Insulation is torn, and internal metal wires from two different cables are touching each other.
Outcome: Both result in non-functional cables and potential network downtime.
Port and Connectivity Issues
Bad Ports: If port lights do not activate (typically green for working), test the port with a known good system/device to determine if the network adapter or switch port has failed.
LED Indicators: Green typically indicates functionality; orange typically indicates a problem.
Incorrect Transceivers: Using a single-mode fiber transceiver with multimode fiber (or vice-versa) causes broken connectivity.
Duplexing Issues: Mismatched duplex modes (e.g., 1 Gbps vs 100 Mbps) result in heavy packet loss.
TX/RX Reversed: Occurs in crossover cables to allow transmission and reception between similar devices.
Dirty Optical Cables: Dust on fiber connectors causes failure or intermittent packet loss. Cleaning kits include isopropyl alcohol, micro dust spray, fiber wipes, microscopes, and dry woven cloth.
Common Networking Tools
Cable Crimper: Used to attach RJ45 () or RJ11 () connectors to the ends of wires by pressing handles to crimp the connector tightly.
Punchdown Tool: Terminated cables into the back of patch panels in data centers. It removes insulation and pushes the wire into the connector point.
Tone Generator: Sends a signal through a UTP cable that is received by a tone locator, which beeps to verify the cable path or locate faults.
Loopback Adapter: Connects to a port to generate transmission signals; if the receiving switch port lights up, the port is functional.
Optical Time-Domain Reflectometer (OTDR): Transmits optical pulses to determine fiber cable length, locate faults/breaks, identify bad connectors, or find bends.
Multimeter: Measures voltage, current, or resistance. Used to locate cable faults or cables on patch panels. Available as Analog (needle) or Digital (numeric display).
Cable/Media Tester: Consists of an active battery-powered component (signal generator) and a passive component (receiver) to test Ethernet/telephone cable functionality.
Wire Map: A test to locate opens, shorts, or reversed wires by sending signals on each individual internal cable.
Network Tap: A non-intrusive device that replicates network traffic for monitoring, analysis (bandwidth saturation/latency), or malicious traffic detection.
Fusion Splicer: Uses an electric arc to melt and join two broken fiber ends together as a single cable in under one minute.
Spectrum Analyzer: Measures electrical, acoustic, or optical light waves. Used in wireless networking to detect and frequencies, hotspots, and wireless networks.
Snips/Cutters: Large scissor-like tools ( to ) used for cutting wires from large bundles.
Cable Stripper: Removes the protective rubber insulation coating from Ethernet cables to allow for connector installation.
Fiber Light Meter (OPM): An Optical Power Meter that measures light movement and signal loss through fiber optic cables.