Know your cable standards, connector types, and physical layer fundamentals. The physical layer is where every network starts — and where half the problems actually live.
Layer 1 is the physical medium — copper wire, fiber optic glass, or radio waves. Before any software troubleshooting can help, the physical layer must work. A cracked cable, a bad crimp, or a wrong connector type will fail silently in ways that look like software problems until you test the physical layer directly.
Most environments use twisted pair copper cable (Ethernet) for desktop and server connections, fiber optic for backbone and long-distance runs, and coaxial for cable TV and some older setups. Help desk techs deal with copper 99% of the time.
| Category | Max Speed | Max Length | Common Use |
|---|---|---|---|
| Cat 5 | 100 Mbps | 100m | Legacy; avoid in new installs |
| Cat 5e | 1 Gbps | 100m | Still common in older offices |
| Cat 6 | 1 Gbps (10G up to 55m) | 100m | Current standard for most offices |
| Cat 6a | 10 Gbps | 100m | Data centers, high-bandwidth runs |
| Cat 7 | 10 Gbps | 100m | Shielded; data centers and industrial |
| Cat 8 | 25–40 Gbps | 30m | Data center switch-to-switch connections |
All copper Ethernet standards have a 100-meter (328 feet) maximum segment length. Beyond that, signal degrades and you'll see intermittent drops and errors. If a run is longer, you need a switch or repeater to extend it. Fiber has no such limit for practical distances.
| Connector | Used For | Notes |
|---|---|---|
| RJ-45 | Ethernet (twisted pair) | 8-pin, clear plastic. Used on all Cat 5e/6/6a runs |
| RJ-11 | Telephone / DSL | 6-pin, smaller than RJ-45. Don't mix them up |
| LC (Fiber) | Single-mode & multi-mode fiber | Small form factor; most common fiber connector today |
| SC (Fiber) | Single-mode & multi-mode fiber | Older square connector; still found in legacy installs |
| USB-A / USB-C | Peripheral connections | USB-C reversible; both used for adapters and docking |
| HDMI / DisplayPort | Video/audio output | HDMI common on monitors; DisplayPort on pro displays |
When terminating (crimping) an RJ-45 connector onto a cable, you must follow a specific color sequence. There are two standards: T568A and T568B. T568B is the most widely used in North American commercial installations.
Both ends of a straight-through cable use the same standard (both T568B). A crossover cable uses T568A on one end and T568B on the other — used historically to connect two computers directly, though modern switches handle this automatically (auto-MDI/MDX).
| Pin | T568A Color | T568B Color | Function |
|---|---|---|---|
| 1 | White/Green | White/Orange | TX+ |
| 2 | Green | Orange | TX- |
| 3 | White/Orange | White/Green | RX+ |
| 4 | Blue | Blue | Unused (PoE) |
| 5 | White/Blue | White/Blue | Unused (PoE) |
| 6 | Orange | Green | RX- |
| 7 | White/Brown | White/Brown | Unused |
| 8 | Brown | Brown | Unused |
| Symptom | Likely Physical Cause | Test |
|---|---|---|
| No link light on NIC or switch | Bad cable, wrong port, dead NIC | Swap cable; try different switch port |
| Intermittent drops / packet loss | Bent cable, damaged crimp, near EMI source | Cable tester; move cable away from power runs |
| Slow speeds (below expected) | Cat 5 in a Cat 6 environment, cable too long | Check cable label; measure run length |
| No link but cable looks fine | Wire broken internally (stressed bend) | Test with cable tester; replace cable |
| Connector feels loose | Tab broken, crimp failed | Re-crimp or replace the end |
Running network cable next to electrical conduit introduces electromagnetic interference (EMI) that causes packet errors and intermittent drops. Always cross power cables at 90 degrees if you must intersect, and maintain at least 12 inches of separation for parallel runs. Use shielded cable (STP) if separation isn't possible.
Use this procedure when a wired connection has no link light, intermittent drops, or speed is below the expected rate. Always test the physical layer before touching software or settings.
Check the NIC (back of desktop or side of laptop) — is the link light on? Check the switch or wall port as well. No link light on either end = no physical connection. A flashing amber/orange light may indicate a speed mismatch or duplex issue. Green steady = connected; green flashing = activity.
Replace the cable between the computer and the wall port (or switch). Use a known-good cable. This rules out the most common failure — a damaged patch cable. If the link light comes on after swapping, the original cable was the problem. Label the bad cable and dispose of it.
Plug a known-good laptop directly into the wall port with a known-good cable. If you get a link, the wall port is fine — the problem is the user's machine or original cable. If no link, the wall port may be dead or unplugged at the patch panel. Trace the port number and check the patch panel in the comms room.
A cable tester verifies continuity on all 8 pins and can identify miswires, opens, and shorts. Run the tester on the suspect cable. If pins don't light in order (1-2-3-6 for a T568B cable), the cable has a wiring fault. Re-crimp or replace it. A tone generator (fox and hound) helps trace cables through walls and ceilings.
Move the cable to a different switch port. If the link comes up on the new port, the original switch port may be failed or disabled. Check the switch management interface (if accessible) to confirm port status. A port that shows "err-disabled" requires network team intervention.
If a cable must be re-terminated: strip 1–1.5 inches of outer jacket, untwist the pairs, arrange in T568B color order, trim to equal length, insert into RJ-45 connector with jacket inside the housing, and crimp firmly. Test with a cable tester after crimping. A bad crimp is worse than a bad cable — it causes intermittent failures that are hard to diagnose.
If physical cable tests pass but there's still no connectivity: open Device Manager → Network Adapters. Look for yellow exclamation marks. Right-click → Properties to see error codes. Try disabling and re-enabling the adapter. If the NIC is failing, it may need to be replaced (USB-to-Ethernet adapter as a workaround for laptops).
| Symptom | First Check | Fix |
|---|---|---|
| No link light | Cable at both ends; NIC in Device Manager | Swap cable; try different port |
| Intermittent drops | Cable routing (near EMI?); crimp quality | Replace cable; re-route away from power |
| Speed below expected | Cable category label; run length | Replace with correct Cat rating |
| Wall port dead | Patch panel connection in comms room | Re-patch panel port; escalate if structural |
| Field | Your Entry |
|---|---|
| User Reported | |
| Initial Findings | |
| Root Cause | |
| Steps Taken | |
| Resolution | |
| Verification |
"What is the difference between Cat 5e and Cat 6 cable and when would you use each?"
Cat 5e supports 1 Gbps up to 100 meters and is common in older offices still running fine for most tasks. Cat 6 also does 1 Gbps up to 100m but has tighter twists and a spline separator that reduces crosstalk — it can also do 10 Gbps up to 55 meters. I'd use Cat 5e when replacing a damaged run in an existing Cat 5e infrastructure where 1 Gbps is sufficient. I'd specify Cat 6 or Cat 6a for any new installation because the marginal cost difference is small and it future-proofs the investment for 10G speeds.
"A user says their wired connection keeps dropping intermittently. How do you diagnose a physical layer problem?"
Intermittent drops are the trickiest physical layer problem because the connection seems to work. My first step is swapping the patch cable with a known-good cable — a damaged cable can test fine visually but have broken conductors internally. Then I use a cable tester to verify continuity on all 8 pins. I also check the cable routing — parallel runs alongside power cables can introduce EMI causing packet errors. If the cable tests fine and routing is clean, I move to the switch port (try a different port) and finally look at the NIC in Device Manager for errors.
"Can you explain the T568B wiring standard and why it matters?"
T568B is the standard pin-to-color mapping for terminating RJ-45 connectors, and it's the most common standard in North American commercial environments. The order on pins 1–8 is: white/orange, orange, white/green, blue, white/blue, green, white/brown, brown. It matters because both ends of a cable must use the same standard (both T568B for a straight-through cable) or you'll get a miswire — the cable will fail a tester and produce no link. If you're extending an existing infrastructure, you match whatever standard is already in use to stay consistent. The other standard, T568A, is functionally identical — just pins 1,2 and 3,6 are swapped.