Understanding Network Cabling at the Physical Layer

🔌 Types of Network Communication

Understanding how data is directed across a network sets the foundation for everything else:

• Unicast: One-to-one communication. A packet is sent from one host directly to another. This is common in user applications like web browsing or file transfers.

• Broadcast: One-to-all communication. The sender transmits data to every device in the local network segment. It is used by protocols like ARP. Efficient in small networks but creates congestion in larger ones.

• Multicast: One-to-many communication. A single sender transmits data to multiple, but not all, recipients that have opted to listen (e.g., video streaming using IGMP).

• Anycast: One-to-nearest communication. The data is delivered to one of many recipients, whichever is closest based on routing metrics. Common in DNS and CDN systems.

🧠 Tip: Unicast and broadcast are frequently encountered in small LANs. Multicast and anycast are typically used in large-scale enterprise or Internet environments.

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🌐 OSI Layer 1: Physical Layer

Layer 1 is responsible for the transmission and reception of raw bitstreams over a physical medium. It has no concept of addressing or data interpretation—only transmission.

• Devices: Hubs, repeaters, cables, and physical ports

• Transmission: Voltages, light pulses, or electromagnetic signals

• Function: Converts digital data into transmittable signals and vice versa

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📡 Baseband vs. Broadband Transmission

• Baseband: Only one signal can travel on the medium at a time. Most Ethernet networks use baseband transmission, such as 10BASE-T and 100BASE-TX.

• Broadband: Multiple signals are transmitted over different frequencies on the same medium. Used in technologies like cable TV but not typical for Ethernet.

📘 Mnemonic: "Baseband is basic" — one stream, one signal.

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🧵 Ethernet Cable Standards

✅ 10BASE2 (Thinnet)

• "10" = 10 Mbps

• "Base" = Baseband signaling

• "2" = ~200 meters maximum segment length

• Uses coaxial cable

• Flexible but obsolete

• Requires BNC connectors and proper termination to function reliably

✅ 10BASE5 (Thicknet)

• "10" = 10 Mbps

• "Base" = Baseband signaling

• "5" = ~500 meters maximum segment length

• The original Ethernet standard

• Uses thick coaxial cable with vampire taps

• Rigid and difficult to install; now obsolete

• Required external AUI transceivers

🔧 Note: Though outdated, these legacy standards still appear in certification exams.

✅ 10BASE-T (Twisted Pair Ethernet)

• "10" = 10 Mbps

• "Base" = Baseband signaling

• "T" = Twisted Pair

• Uses UTP/STP twisted pair cables

• Maximum segment length: 100 meters

• Easier installation than coaxial options

• Uses RJ-45 connectors

• Follows wiring schemes: T568A or T568B

🔍 T568A vs. T568B: These define pinout arrangements. Using one on each end creates a crossover cable; using the same on both ends creates a straight-through cable.

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📊 Cable Categories (Cat Ratings)

Cable categories determine performance, resistance to interference, and suitability for different network speeds.

🔌 Key Point: Higher category cables have tighter twists and better shielding, reducing crosstalk and electromagnetic interference (EMI).

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🔀 Direct Attach Cable (DAC) – Copper Twinax

• Pre-terminated cable primarily used in data centers

• Connects switches, servers, or storage devices directly

• Maximum range: ~15 meters

• Equipped with SFP+ (Small Form-factor Pluggable) connectors

• Ideal for short, high-speed links where fiber is unnecessary

💡 Common in Top-of-Rack (ToR) switch configurations.

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🔁 Roll-Over Cable (Cisco Console Cable)

• Connects a PC to a Cisco device’s console port for management

• Uses serial communication through terminal programs like PuTTY

• Wiring is reversed:

  • Pin 1 ↔ Pin 8

  • Pin 2 ↔ Pin 7

  • … down to Pin 8 ↔ Pin 1

🔧 Used for initial device configuration and recovery tasks

🛑 Caution: Not to be confused with crossover cables; rollover cables are for device management only.

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🔍 Fiber Optic Cables

• Transmit data as light, not electricity

• Immune to electromagnetic interference (EMI)

• Support high-speed, long-distance communication

• Two types:

  • Single-mode fiber (SMF) – Long distance, uses lasers

  • Multi-mode fiber (MMF) – Shorter distances, uses LEDs

🚀 Fiber optics can reach speeds up to 100 Gbps and extend across kilometers.

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↔️ Straight-through vs. Crossover Cables

• Straight-through Cables:

  • Connect dissimilar devices (e.g., PC to switch, switch to router)

  • Same wiring standard on both ends (T568A–T568A or T568B–T568B)

• Crossover Cables:

  • Connect similar devices (e.g., PC to PC, switch to switch)

  • Different wiring standards on each end (T568A–T568B)

💡 Modern devices support Auto-MDIX, which automatically adjusts for cable type—but knowledge of these standards is still essential.