2.3. Physical Interfaces and Cabling

💡 First Principle: The physical medium you choose determines your maximum distance, bandwidth, and cost. Copper is cheap and easy but limited in distance; fiber is expensive but goes farther and is immune to electrical interference. Choosing wrong means network problems that no amount of configuration can fix.

Consider this design scenario: You're connecting two buildings 300 meters apart. Someone suggests Cat 6 because "it's cheaper." Cat 6 maxes out at 100 meters for 1 Gbps—your signal dies at meter 101. You need fiber. Conversely, for a 50-meter closet run, fiber works but costs 10x more than copper with no benefit. Cabling decisions are permanent (ripping out cable is expensive), so getting them right the first time matters.

What happens when you choose wrong: Cat 5e for a 10 Gbps link produces CRC errors and drops. Multimode fiber on a 2km run loses signal strength until the link flaps constantly. Copper run parallel to power cables picks up interference that looks like hardware failure. The troubleshooting always leads back to "wrong cable for the job."

The decision framework:

• Distance: How far does the signal need to travel? (Copper maxes at 100m)
• Speed: What bandwidth do you need? (Higher speeds require better cable)
• Environment: Is there electrical interference? (Data centers near industrial equipment need fiber)
• Cost: Copper is ~10x cheaper than fiber for short runs