📘 Lesson 4 — Switching Review, Virtual Circuits, Datagrams, and Signals

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🔄 Part 1 — Deep Review (Building upon what was said in Session 3)

This session started by reinforcing the core ideas from Session 3, especially the flow of communication, components, and signal behavior.

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🧩 What Is a Signal? (Review + Expansion)

A signal is the physical form of data as it travels through a medium.

In Session 3 it was said that:

“Signal is the representation of data during transfer.”

In Session 4, the professor expanded this into two key dimensions:

1. Nature of the Signal

• 🟦 Analog Signal → continuous, smooth, infinite values
• 🟧 Digital Signal → discrete, step-like, finite values

2. Shape / Physical Form

Signals can exist as:

• 🔌 Electrical (copper wires)
• 📡 Electromagnetic (Wi-Fi, radio)
• 💡 Light (fiber optic)
• 🔊 Sound waves (less common in networking)

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🔁 Reviewing the Communication Process

Session 3 introduced the classic communication chain. Session 4 reiterated it with more examples.

┌──────────┐      ┌──────────┐      ┌──────────┐      ┌────────────┐      ┌──────────┐
│  Sender  │ ---> │ Encoder  │ ---> │ Channel  │ ---> │ Decoder    │ ---> │ Receiver │
└──────────┘      └──────────┘      └──────────┘      └────────────┘      └──────────┘

📝 Key reminders (from Session 3)

• The sender has data.
• The encoder turns data → signal.
• The channel carries the signal.
• The decoder turns signal → data.
• The receiver gets the final data.

Session 4 emphasized:

“Anything that affects the signal inside the channel affects communication reliability.”

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🟨 Noise

Unwanted random disturbance that naturally occurs in any communication channel.

• Comes from thermal effects, electronics, background randomness.
• Cannot be eliminated, only reduced.
• Always present.

Examples:

• Thermal noise in copper
• Random amplitude spikes
• Unpredictable interference

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⭐ Interference

Interference refers to deliberate, external, intentional or semi‑intentional disturbance.

This includes:

• EMI (Electromagnetic Interference)
• RFI (Radio Frequency Interference)
• Crosstalk (one cable leaking into another)

These are not random — they come from identifiable external sources.

Examples:

• A microwave interfering with Wi‑Fi
• Two unshielded cables disturbing each other
• A radio tower saturating nearby frequencies

Noise is natural. Interference is external and often predictable.

Here is a quick comparison chart:

Concept	English Term	Behavior	Example
🔈 Noise	Noise	Random, always present	Thermal noise in copper
📡 Interference	Interference	External, identifiable	Microwave disturbing Wi‑Fi
🔗 Crosstalk	Crosstalk	One wire leaks into another	UTP pairs interfering

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🔌 Media Types

1️⃣ Twisted Pair Cable (UTP / STP)

    UTP Cable Structure

    ┌──────────────────────────────┐
    │   (No metal shielding)       │
    │   Pairs twisted to reduce    │
    │   crosstalk                  │
    └──────────────────────────────┘
         ╱╲   ╱╲   ╱╲   ╱╲
        ╱  ╲ ╱  ╲ ╱  ╲ ╱  ╲   ← twisted copper pairs

• Cheap, flexible
• Sensitive to interference
• Uses RJ‑45 connector

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2️⃣ Coaxial Cable

       Coaxial Cable Layers

      ┌──────────────────────────────┐  Outer Insulation
      │  ┌────────────────────────┐  │
      │  │   Braided Shield       │  │
      │  └────────────────────────┘  │
      │      ┌───────────────────┐   │
      │      │   Dielectric      │   │
      │      └───────────────────┘   │
      │          ● Inner Core        │  Copper
      └──────────────────────────────┘

• Very resistant to interference
• Common in early Ethernet and cable TV
• Expensive compared to UTP

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3️⃣ Fiber Optic Cable

         Fiber Optic Structure

        ┌─────────────────────────────┐  Outer Jacket
        │ ┌─────────────────────────┐ │  Strength Fibers
        │ │  Cladding (reflective)  │ │
        │ │  ┌────────────────────┐ │ │
        │ │  │    Core (glass)    │ │ │  ← Carries light
        │ │  └────────────────────┘ │ │
        | └─────────────────────────┘ |
        └─────────────────────────────┘

• Immune to noise and interference
• Very high bandwidth
• Light signals, not electricity

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🔬 Food for Thought

⭐ Why Do We Twist the Wires in UTP?

Because twisting creates opposing electromagnetic fields that cancel each other, reducing crosstalk.

• 🔄 Twist → 🔇 Less interference → 🚀 Better signal

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⭐ Why Is Fiber Optic Immune to Noise?

Because it uses light, not electricity.

• No EMI
• No crosstalk
• No electrical distortion

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⭐ What Affects Signal Quality?

• Distance 📏
• Interference 📡
• Attenuation 📉(: the reduction of the amplitude of a signal, electric current, or other oscillation.)
• Noise 🔈
• Cable quality 🔌
• Environment 🌧️⚡

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🎯 Summary + What You Should Remember

• Signals represent data
• Channel has encoder → medium → decoder
• Both analog and digital signals exist
• There is a strong distinction between noise and interference
• Different physical media shape different types of signals
• Cable structure affects susceptibility to interference
• Fiber uses light and avoids electrical problems entirely

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