Computer Networks & Protocols: Topologies, Media, OSI, TCP/IP

TAKEAWAYS:

• Networks connect devices (LAN/WAN) using topologies (star, mesh, bus) and media (twisted pair, fiber, Wi-Fi).
• The OSI model (7 layers) and TCP/IP (4 layers) define how data travels reliably via protocols like HTTP/HTTPS.
• Protocols (rules) ensure devices communicate—e.g., TCP for error-free delivery, IP for addressing.
• Real-world use: eSewa uses HTTPS (OSI Layer 5+7) for secure payments; Ncell’s 4G relies on TCP/IP (Layer 3/4) for data packets.
• Exam focus: Compare topologies, trace data through OSI layers, and explain protocols like TCP vs. UDP.

1. What is a Computer Network?

A computer network is a system where multiple devices (computers, smartphones, servers) share resources (files, internet, printers) via communication links (cables, Wi-Fi). Networks enable:

• Resource sharing (printers, databases).
• Communication (email, video calls).
• Cost efficiency (centralized servers instead of individual hardware).

Types of Networks by Size

2. Network Topologies: How Devices Connect

Topology = physical/logical arrangement of devices. Choose based on cost, scalability, fault tolerance.

Common Topologies

graph LR
    A["Bus Topology"] -->|"All devices share 1 cable"| B["Terminators at ends"]
    C["Star Topology"] -->|"Central hub (switch/router)"| D["Each device connects directly"]
    E["Mesh Topology"] -->|"Every device connected to every other"| F["Fully connected (expensive)"]
    G["Ring Topology"] -->|"Devices in a closed loop"| H["Token-passing (old networks)"]

Worked Example: NTC’s Fiber-Optic Network NTC uses a mesh topology for its backbone to ensure:

• If one fiber cable fails, data reroutes automatically.
• High speed (100+ Gbps) for nationwide calls/internet.

3. Data Transmission Media: Wires vs. Wireless

Media = physical path for data. Choose based on speed, distance, cost.

A. Guided (Wired) Media

B. Unguided (Wireless) Media

Real-World Tie-In: Pathao’s Delivery System Pathao uses:

• Wi-Fi/4G (unguided) for rider-app communication.
• GPS (wireless) to track delivery routes in real time.

4. Communication Protocols: Rules for Data Exchange

Protocols = agreed-upon rules for devices to communicate. Two key models:

A. OSI Model (7 Layers)

classDiagram
    class Layer7["Application (HTTP, FTP)"] {Handles user interfaces}
    class Layer6["Presentation (SSL, JPEG)"] {Encrypts/compresses data}
    class Layer5["Session (NetBIOS)"] {Manages connections}
    class Layer4["Transport (TCP, UDP)"] {Ensures delivery (ports)}
    class Layer3["Network (IP, ICMP)"] {Routing (IP addresses)}
    class Layer2["Data Link (Ethernet, Wi-Fi)"] {MAC addresses, frames}
    class Layer1["Physical (Cables, Signals)"] {Bits (0s/1s)}

    Layer7 --> Layer6 : Uses
    Layer6 --> Layer5 : Uses
    Layer5 --> Layer4 : Uses
    Layer4 --> Layer3 : Uses
    Layer3 --> Layer2 : Uses
    Layer2 --> Layer1 : Uses

Worked Example: Loading YouTube on a Smartphone

1. Layer 7 (Application): YouTube app sends HTTP request.
2. Layer 4 (Transport): TCP splits data into packets, adds port numbers.
3. Layer 3 (Network): IP routes packets via Ncell’s servers.
4. Layer 1 (Physical): Fiber optic cables transmit bits as light pulses.

Shows each layer’s role with icons (e.g., HTTP for Layer 7). (Image: SVG edition: Gorivero, CC BY-SA 3.0, via Wikimedia Commons)

B. TCP/IP Model (4 Layers)

Simpler than OSI, used in the internet. Key protocols:

• TCP (Layer 4): Reliable, connection-oriented (e.g., file downloads).
• UDP (Layer 4): Fast, no error-checking (e.g., video calls).
• IP (Layer 3): Addresses devices (IPv4/IPv6).

Real-World Tie-In: eSewa Payments

1. HTTPS (Layer 7): Encrypts your card details.
2. TCP (Layer 4): Ensures no data is lost during transfer.
3. IP (Layer 3): Routes your payment to Nabil Bank’s server.

5. Network Security Threats & Protocols

A. Common Threats

B. Security Protocols

In the Real World

1. eSewa’s Secure Transactions

   • Uses HTTPS (OSI Layers 5–7) to encrypt your payment data.
   • TCP/IP (Layer 4) ensures your money transfer isn’t lost mid-transaction.

2. Ncell’s 4G Network

   • Mesh topology for backup paths if a tower fails.
   • TCP/IP (Layer 3) routes your data packets to the nearest server.

3. Daraz’s Order Fulfillment

   • Star topology: All warehouse devices connect to a central server.
   • Wi-Fi (Layer 1–2): Scanners communicate with inventory systems.

Exam Tip

1. Topologies: Always compare star vs. mesh in terms of cost/fault tolerance.
2. OSI Layers: Memorize Layer 4 (TCP/UDP) and Layer 7 (HTTP/FTP).
3. Real-World Links: Relate eSewa to HTTPS, Ncell to TCP/IP, and NTC to fiber optics.
4. Diagrams: Draw OSI model or network topologies if asked to "explain."
5. Shortcuts:
   • TCP = Reliable (like a phone call).
   • UDP = Fast (like shouting).
   • IP = Addresses (like a home address).

Key Formula to Remember:

Throughput (Mbps) = Bandwidth (Hz) × Number of bits per symbol × Efficiency

Example: A 10 MHz Wi-Fi channel with 64-QAM (6 bits/symbol) and 50% efficiency:

Throughput = 10 × 10⁶ × 6 × 0.5 = **30 Mbps**