Cables, Connectors and Ports

1) Foundations and Standards

• Ethernet is a family of standards governing both the physical layer (cables, connectors, signaling) and data link layer (framing, media access) under IEEE 802.3 (established in 1983).
• Ethernet is not limited to one medium; it supports both copper and fiber-optic cabling.
• The IEEE also defines 802.11 for wireless LANs (Wi‑Fi), but that’s distinct from Ethernet.

Interview angles:

• HR: “What is Ethernet?” → A standardized suite for wired networking under IEEE 802.3, covering copper and fiber.
• Technical: “Which IEEE group defines Ethernet?” → IEEE 802.3.

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2) Data Basics and Units

• Bit: Smallest unit of information; binary digit (0 or 1).
• Byte: 8 bits.
• Computers compute and communicate in binary; all media (apps, photos, audio, video) is represented as 0s and 1s.
• Network speed is measured in bits per second:
  • 1 kb = 1,000 bits
  • 1 Mb = 1,000,000 bits
  • 1 Gb = 1,000,000,000 bits
  • 1 Tb = 1,000,000,000,000 bits

Interview angles:

• HR: “What’s the difference between bit and byte?” → 1 byte = 8 bits; network rates in bits/s.
• Technical: “Why do ISPs quote Mbps but files show MB?” → Case sensitivity matters; Mbps vs MBps (divide by 8 for rough conversion).

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3) Copper Cabling: UTP (and STP)

• Most common “Ethernet cable” is UTP (Unshielded Twisted Pair).
  • Contains 8 copper conductors, twisted into 4 pairs to reduce EMI (electromagnetic interference).
  • “Unshielded” means no metallic shield; cheaper and adequate for most office/LAN environments.
• STP (Shielded Twisted Pair) exists for high‑EMI environments; less common in typical LANs.
• Connector: RJ45, formally 8P8C (8 positions, 8 contacts).
• Maximum recommended length for copper Ethernet links: 100 meters (beyond that, expect attenuation and errors).

Interview angles:

• HR: “What connector does Ethernet use?” → RJ45/8P8C.
• Technical: “Why twist the pairs?” → Pair twisting cancels EMI and crosstalk; each pair has a specific twist rate.

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4) Common UTP Ethernet Standards and Categories

• 10 Mbps → 10BASE‑T → typically Cat 3
• 100 Mbps → 100BASE‑T → typically Cat 5
• 1 Gbps → 1000BASE‑T → typically Cat 5e
• 10 Gbps → 10GBASE‑T → typically Cat 6a
• All above have a typical max link length of 100 m on copper.

Interview angles:

• HR: “What cable category is used for 1 Gbps?” → Cat 5e (or better).
• Technical: “Can Cat 5e do 10G?” → In practice, 10GBASE‑T is specified for Cat 6a for full 100 m; Cat 6 can support shorter 10G runs.

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5) Pair Usage, Pinouts, and Cable Types

• Pair usage by standard:
  • 10BASE‑T: 2 pairs (4 wires)
  • 100BASE‑T: 2 pairs (4 wires)
  • 1000BASE‑T: 4 pairs (8 wires)
  • 10GBASE‑T: 4 pairs (8 wires)
• In 10/100 Mbps, the active pairs are on pins 1–2 and pins 3–6.
• Cable types:
  • Straight‑through: Same pin order on both ends; used for dissimilar devices (host → switch).
  • Crossover: Swaps transmit/receive pairs; pins 1–2 ↔ 3–6.
• Auto MDI‑X: Modern interfaces auto‑detect and adjust TX/RX, making crossover cables largely unnecessary.

Interview angles:

• HR: “What is Auto MDI‑X?” → Automatic transmit/receive pair correction.
• Technical: “When is a crossover cable needed?” → Legacy gear without Auto MDI‑X, or specific lab scenarios.

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6) Fiber-Optic Cabling

• Transmits light through glass fiber; immune to EMI, supports far longer distances than copper.
• Must be handled carefully; sharp bends can break fiber or cause light leakage and signal loss.
• Typical link uses two fibers: one transmit and one receive.
• Interfaces use SFP (Small Form‑Factor Pluggable) transceivers inserted into SFP ports; transceivers are modular and often a significant cost component.

Types of Fiber:

• Multimode Fiber (MMF)
  • Wider core; uses LED transmitters.
  • Carries multiple light “modes” (angles) via internal reflection.
  • Typical distance: hundreds of meters.
  • Transceivers are cheaper than SMF.
• Single‑Mode Fiber (SMF)
  • Very narrow core; uses laser transmitters.
  • Single propagation mode, minimal dispersion.
  • Typical distance: tens of kilometers.
  • Transceivers are more expensive.

Interview angles:

• HR: “Why pick fiber over copper?” → Longer distance, EMI immunity.
• Technical: “MMF vs SMF?” → LED vs laser, core size, max distance, and cost.

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7) When to Use UTP vs Fiber

• Choose UTP (Copper) when:
  • Connecting end devices (PCs, printers) to access switches.
  • Budget-sensitive deployments where ≤ 100 m runs suffice.
  • Simpler installation and broad device support.
• Choose Fiber when:
  • Interconnecting network devices over longer distances (inter‑floor, inter‑building, campus/WAN).
  • Building high‑speed backbones needing EMI immunity and higher bandwidth.
  • Ready to invest in SFP optics and fiber infrastructure.

Interview angles:

• HR: “Typical use case for fiber?” → Backbone, long‑run links between switches/routers.
• Technical: “What’s the main cost driver for fiber?” → Transceiver optics (SFP/SFP+ modules).

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8) Practical Constraints and Best Practices

• Copper limit: 100 m end‑to‑end (including patch cords); plan for intermediate switches or convert to fiber if longer.
• For 10/100 Mbps legacy links, remember active pins 1–2 and 3–6.
• Use Auto MDI‑X capable ports to avoid crossover issues; most modern switches have it.
• For fiber, respect minimum bend radius, keep connectors clean, and match the optic to the fiber type (MMF vs SMF) and wavelength.

Interview angles:

• HR: “What happens if a copper run exceeds 100 m?” → Attenuation and performance degradation.
• Technical: “Why can a fiber link ‘work’ but show errors?” → Microbends, dirty connectors, mismatched optics, or excessive loss.

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9) Conceptual: Digital vs. Analog and the Modem

• Computers speak digital (0/1), but many transmission systems historically carried analog signals.
• Modem = MOdulator/DEModulator:
  • Modulation: digital → analog (for sending over analog mediums).
  • Demodulation: analog → digital (for receiving).
• While Ethernet is digital end‑to‑end, understanding modems helps with legacy WANs and telecom interop.

Interview angles:

• HR: “What does a modem do?” → Translates between digital and analog.
• Technical: “Is Ethernet analog or digital?” → Ethernet uses digital signaling; modulation concepts still matter in other media.

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10) Quick Reference: Common Interview Q&A

• Define Ethernet: IEEE 802.3 wired networking standards (physical + data link).
• Bit vs Byte: 1 byte = 8 bits; network speeds quoted in bits/s.
• Why twisted pairs? Reduce EMI/crosstalk through pair twisting.
• RJ45 vs 8P8C: RJ45 is the common name; 8P8C describes the connector’s physical layout.
• Max copper length: 100 m.
• Straight‑through vs crossover: Straight for unlike devices; crossover swaps TX/RX—mostly obsolete with Auto MDI‑X.
• 1000BASE‑T wire usage: All 4 pairs (8 wires).
• Fiber choices: MMF (LED, hundreds of meters) vs SMF (laser, tens of km).
• Why fiber for backbones? Longer distance, higher bandwidth, EMI immunity.
• Cost drivers: For fiber, SFP/SFP+ transceivers are a major cost component.

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11) Troubleshooting Tips

• Copper:
  • Check total channel length ≤ 100 m.
  • Verify cable category matches speed (e.g., 10G needs Cat 6a for 100 m).
  • Inspect termination quality and pinout consistency.
• Fiber:
  • Clean connectors; inspect with scope if available.
  • Check bend radius and route.
  • Ensure optic type, wavelength, and fiber type match.
  • Validate link budget (loss) with proper testing.

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12) Mini-Drills (Confidence Builders)

• Which standards use two pairs? → 10BASE‑T, 100BASE‑T.
• Which standards use all four pairs? → 1000BASE‑T, 10GBASE‑T.
• What’s Auto MDI‑X? → Automatic TX/RX pair correction.
• Typical UTP max length? → 100 meters.
• MMF distance ballpark? → Hundreds of meters.
• SMF distance ballpark? → Tens of kilometers.
• SFP stands for? → Small Form‑Factor Pluggable.

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