Lesson 1: How Big Is a Network?

🧱 Fundamentals

🌍 Network Size

A network’s size can be defined by two main factors:

Number of Nodes / Users
- In hardware terms → nodes or devices.
- In software terms → clients or users (e.g., computers, phones, servers).
Geographical Coverage
- The physical area the network spans.

These two characteristics together determine the network’s topology — the structural layout of how devices are connected and how data travels among them.

🔗 Network Topologies Overview

Topology defines how a network is physically or logically arranged — the pattern of connections between nodes.

Topologies can be classified based on network size and coverage area:

Type	Full Name	Description	Typical Area
LAN	Local Area Network	Small area like a home, office, or campus.	Up to a few kilometers
MAN	Metropolitan Area Network	Connects multiple LANs within a city.	City-wide
WAN	Wide Area Network	Connects multiple MANs and LANs over large distances.	Country or global scale

🧩 WANs are often hybrids, combining multiple smaller networks with different topologies (LANs, MANs, etc.).

⚙️ Network Lines and Channels

Data in networks travels through communication lines or channels.
These can be classified into:

Private lines → Dedicated connections between nodes.
Example: Company internal fiber lines.
Public/shared lines → Used by many users simultaneously.
Example: The internet, cellular networks.

Each type affects speed, cost, and security.

🔄 System Types (Symmetrical vs. Asymmetrical)

Network systems can be categorized by how devices communicate:

Symmetrical Systems
- All nodes can both send and receive requests.
- Example: Peer-to-peer (P2P) networks — each node is equal.
Asymmetrical Systems
- Roles are divided: clients send requests, servers respond.
- Example: The web — browsers (clients) talk to servers (e.g., Google).

📡 Signal Strength and Hops

As data travels, signal strength decays over distance due to attenuation.
To maintain quality:

Repeaters (or hops) are placed to amplify or regenerate the signal.
Each “hop” extends the total distance the signal can travel reliably.

Without hops, communication fails beyond a certain length of cable or distance.

🖧 LAN Topologies

Local Area Networks (LANs) can be physically arranged in several common topologies.
Each affects performance, reliability, and cost.

🚌 Bus Topology

Structure

A ---- B ---- C ---- D

All devices share a single communication line (bus) — a common cable where data flows in both directions.

How It Works

Each node listens to the bus.
When one node transmits, the signal travels along the bus to all nodes.

Problems

Collisions: If two devices send data simultaneously, signals overlap and corrupt each other.
Difficult Troubleshooting: A fault in the main cable can disable the entire network.
Limited scalability: Adding devices increases collisions.

Collision Handling

Methods to avoid or manage collisions:

Time Division Multiplexing (TDM) – devices take turns sending data based on time slots.
Frequency Division Multiplexing (FDM) – each device transmits on a unique frequency band.
Carrier Sense Multiple Access (CSMA/CD) – used in early Ethernet; devices listen before sending.

Summary

✅ Simple, cheap, easy to set up.
❌ Poor performance, collision-prone, not used in modern LANs.

🌟 Star Topology

Structure

graph TD
    Hub((Hub / Switch))
    A --- Hub
    B --- Hub
    C --- Hub
    D --- Hub

All devices connect to a central device (Hub or Switch).

How It Works

Each device has a private link to the central device.
Data from one node goes to the hub → then to the intended destination.

Devices

Hub: Broadcasts data to all devices (old technology).
Switch: Smarter — sends data only to the intended receiver.

Pros

Easy to set up and manage.
A single cable failure affects only one device.

Cons

Hub/Switch can become a bottleneck.
If the central device fails → the entire network stops.

💫 Ring Topology

Structure

graph LR
    A --> B --> C --> D --> A

Devices are connected in a closed loop — each with two links: one to send, one to receive.

How It Works

Data travels in one direction (clockwise or counterclockwise).
Each node receives data, checks the address, and passes it along.
When data returns to sender, it is removed from the ring.

Pros

Predictable data flow, no collisions (only one token at a time can circulate).
Good for orderly transmission.

Cons

Failure in one node or cable breaks the ring.
Each device needs two network cards (two IPs).
Maintenance and troubleshooting are complex.

Improvements

MAU (Multi-Access Unit) — simulates a ring but works like a central smart hub:
- Each device has a send and receive path.
- MAU directs signals only to the correct receiver.
- Removes the need for multiple IPs per device.

🔺 Mesh Topology

Structure

graph TD
    A --- B
    A --- C
    A --- D
    B --- C
    B --- D
    C --- D

Every device is directly connected to every other device.

Characteristics

Highly redundant and reliable.
Very expensive and complex (number of connections grows rapidly).

Formula

For n devices, total links = n × (n − 1) / 2

Pros

No single point of failure.
Extremely robust (common in backbone or high-security systems).

Cons

Impractical for large networks (too many cables).
Complex configuration.

Mesh networks are often used partially (hybrid mesh) — not every device connects to every other, only key nodes.

🌐 WAN (Wide Area Network)

A WAN connects multiple smaller networks (LANs and MANs) across vast geographic areas — even countries or continents.

Built using public or private transmission lines (fiber, satellite, leased lines).
No fixed topology — often hybrid combinations (star + mesh + ring, etc.).
Example: The Internet is the largest WAN.

WANs are more about interconnection and scalability than strict structure.