3.1Cables and connectors

In network support, the cause of "no connection" or "poor speed" is often the cable itself. Running copper past 100 m to save money, or wiring near a large factory motor and picking up EMI (electromagnetic interference)—such physical mistakes cannot be fixed by any amount of configuration review. This section covers the difference between everyday twisted pair (copper) and optical fiber, why copper has a guideline of about 100 m maximum, and how to tell connectors like RJ-45 and RJ-11 apart, as the foundation for the field judgment of "which to choose for this distance and environment."

3.1.1Copper twisted pair, distance, and interference

• Twisted pair is a cable of two twisted copper wires and is the mainstay of office and home LAN wiring. Performance splits by category—Cat5e (1 Gbps class) / Cat6 and Cat6a (10 Gbps class)—with higher grades supporting more speed and frequency. The twisting cancels noise, which gives resistance to interference.
• Copper Ethernet spans up to about 100 m (the guideline link distance between devices). Beyond that the signal attenuates and becomes unstable, so for long runs you insert intermediate equipment or switch to fiber. This "copper up to about 100 m" is a most-frequently-tested figure at the introductory level.
• Because copper carries an electrical signal, it is susceptible to EMI (electromagnetic interference). Running it near strong noise sources—fluorescent lights, motors, power cables—can disrupt communication. In noisy environments, shielded cable (STP) or the fiber discussed below is advantageous.

3.1.2Optical fiber: SMF and MMF

• Optical fiber carries data with light. Being light rather than electricity, it is immune to EMI and suits longer distances and higher bandwidth than copper. It is used in noisy factories and in backbones between buildings or sites.
• Single-mode (SMF) sends laser light through a thin core, with low attenuation for long distances (kilometers and up). Multimode (MMF) sends LED/VCSEL light through a thicker core for medium distances (hundreds of meters) within a data center or campus. Roughly, identify them as "SMF = long distance, laser; MMF = medium distance, LED."
• Fiber is sensitive to bending and to a dirty end-face, so it needs careful handling. Still, it resists electrical noise and is harder to tap. Choose between fiber and copper by comparing three points: distance, bandwidth, and environment (noise).

Suppose the warehouse floor reports: "only the newly installed terminal has an unstable network and drops occasionally." Checking the wiring diagram and the site, that terminal sits about 130 m from the head-office switch, with large conveyor motors lined up along the way. Two weaknesses of twisted pair (copper) bite at once. First, distance—copper Ethernet's guideline is about 100 m maximum, and 130 m exceeds it, so instability from attenuation is to be expected. Second, environment—motors are strong electromagnetic noise sources, and copper readily picks up EMI, so twisting alone may not fully cancel it. Jumping to "swap the LAN card" or "review the settings" is wrong isolation here. Correctly, suspect the physical layer first: bring the distance within 100 m (insert a switch to segment the run) or re-cable to optical fiber (MMF is plenty at this distance), which resists noise and distance—that is the root fix. Conversely, if it were slow over 30 m in the same building with no noise source nearby, you should suspect factors other than cable type, and blindly switching to fiber would be overkill. "Check distance and environment (noise) first, then choose against the copper constraints of about 100 m and EMI"—holding this physical measuring stick prevents roundabout swap work.

3.1.3Section summary

• Twisted pair (copper, Cat5e/6/6a) spans up to about 100 m and picks up EMI; the LAN connector is RJ-45
• Optical fiber is EMI-immune and suits long runs: SMF = laser/long distance, MMF = LED/medium distance
• Identify use by connector: RJ-45 = LAN, RJ-11 = telephone, coax = coaxial, fiber connector = optical—choose by distance and environment