Network Models: Topologies, Devices & OSI/TCP-IP

TAKEAWAYS:

• Topologies determine how devices connect (star = central hub; mesh = every node connected) and impact cost, scalability, and fault tolerance.
• Network devices (hub → switch → router) filter, forward, or route traffic based on MAC/IP addresses, with routers enabling inter-network communication.
• Proxy servers cache content (e.g., NTC’s ISP proxy) and enforce security policies, while HTTPS adds encryption (used by Khalti for payments).
• OSI (7 layers) is a theoretical model; TCP/IP (4 layers) is the practical standard (used by all internet protocols, including WhatsApp’s signaling).
• Congestion occurs when traffic exceeds network capacity (e.g., Pathao’s delivery queues during festivals), requiring mechanisms like TCP’s slow-start.
• Exam focus: Compare topologies/devices, explain OSI/TCP-IP layer functions, and trace packet flows (e.g., DNS lookup → HTTP request).

1. Network Topologies: How Devices Connect

Topology defines the physical or logical arrangement of devices in a network. It affects cost, scalability, fault tolerance, and performance. Below are the four key topologies, compared in a table and visualized.

1.1 Star Topology

• Definition: All devices connect to a central node (e.g., switch, hub). If the central node fails, the entire network collapses.
• How it works:
  • Data from any device travels through the central node to reach others.
  • Example: Home Wi-Fi router connecting laptops, phones, and smart TVs.
• Advantages:
  • Easy to install/maintain.
  • Fault isolation (one device failure doesn’t crash the network).
  • Scalable (add devices by connecting to the central node).
• Disadvantages:
  • Single point of failure (central node).
  • Higher cost for cabling/wiring.
• Real-world example:
  • Ncell’s mobile network: Base stations (central nodes) connect to user devices via radio waves (logical star).
  • Khalti’s payment gateway: Servers act as central nodes for merchant transactions.

graph TD
    A["Central Node\n(Switch/Hub)"] --> B["Device 1"]
    A --> C["Device 2"]
    A --> D["Device 3"]

1.2 Bus Topology

• Definition: All devices share a single communication line (bus). Data is broadcast to all devices, but only the intended recipient processes it.
• How it works:
  • Uses a backbone cable (e.g., Ethernet coaxial cable).
  • Terminators at both ends prevent signal reflection.
• Advantages:
  • Low cost (minimal cabling).
  • Easy to add/remove devices.
• Disadvantages:
  • Single point of failure (bus failure = network down).
  • Performance degrades as more devices join (collisions increase).
  • Hard to troubleshoot (all devices share the same line).
• Real-world example:
  • Old LAN setups in offices (rare today, but used in legacy systems).
  • Traffic signals in Kathmandu: Cars (devices) share a "virtual bus" (road), but only the next signal (recipient) acts.

graph LR
    A["Device 1"] -- Bus Cable --> B["Device 2"]
    B --> C["Device 3"]
    C --> D["Terminator"]

1.3 Ring Topology

• Definition: Devices are connected in a closed loop, with data traveling in one direction (unidirectional) or both (bidirectional).
• How it works:
  • Each device acts as a repeater, forwarding data to the next node.
  • Uses token-passing (e.g., Token Ring protocol) to avoid collisions.
• Advantages:
  • No collisions (token ensures only one device transmits at a time).
  • Equal access for all devices.
• Disadvantages:
  • Single point of failure (break in the ring = network down).
  • Difficult to add/remove devices (requires network shutdown).
  • Performance bottleneck if one device fails.
• Real-world example:
  • Fiber-optic metro networks in cities (e.g., NTC’s backbone).
  • Factory assembly lines: Robots (devices) pass tasks (tokens) in a loop.

graph TD
    A["Device 1"] --> B["Device 2"]
    B --> C["Device 3"]
    C --> D["Device 4"]
    D --> A

1.4 Mesh Topology

• Definition: Every device is connected to every other device (full mesh) or to some devices (partial mesh).
• How it works:
  • Data has multiple paths to reach the destination (redundancy).
  • Used in high-reliability networks (e.g., military, aerospace).
• Advantages:
  • High fault tolerance (no single point of failure).
  • Scalable (add devices without disrupting the network).
  • High performance (multiple paths reduce congestion).
• Disadvantages:
  • Expensive (requires many cables/wiring).
  • Complex management (hard to configure).
• Real-world example:
  • Air traffic control systems: Radars (devices) communicate via multiple redundant links.
  • Nepal Electricity Authority’s grid: Power substations (nodes) have backup routes.

graph TD
    A["Device 1"] --> B["Device 2"]
    A --> C["Device 3"]
    A --> D["Device 4"]
    B --> C
    B --> D
    C --> D

1.5 Comparison Table

2. Network Devices: How Data Moves

Network devices filter, forward, or route data between networks. Their functionality depends on the OSI layer they operate in.

2.1 Hub (Physical Layer)

• Function: Operates at Layer 1 (Physical). Broadcasts all incoming data to all ports (no filtering).
• How it works:
  • Receives a signal → amplifies it → sends to all connected devices.
  • No intelligence: Causes collisions in half-duplex networks.
• Example:
  • Old 10 Mbps Ethernet hubs (now obsolete).
• Real-world analogy:
  • A megapphone where everyone hears every call.

graph TD
    A["Hub"] --> B["Device 1"]
    A --> C["Device 2"]
    A --> D["Device 3"]

2.2 Switch (Data Link Layer)

• Function: Operates at Layer 2 (Data Link). Uses MAC addresses to forward data only to the intended device.
• How it works:
  • Builds a MAC address table (maps MAC addresses to ports).
  • Reduces collisions by segmenting the network.
• Example:
  • Home/office Ethernet switches.
• Real-world example:
  • Daraz’s order processing: Switches route packets between servers and customers’ devices.

graph TD
    A["Switch"] -->|"MAC Address"| B["Device 1"]
    A -->|"MAC Address"| C["Device 2"]

2.3 Bridge (Data Link Layer)

• Function: Operates at Layer 2. Connects two separate LAN segments and filters traffic based on MAC addresses.
• How it works:
  • Learns MAC addresses on each segment.
  • Forwards traffic only if the destination is on the other segment.
• Example:
  • Connecting two Ethernet networks in a large office.
• Real-world analogy:
  • A border checkpoint between two cities (only lets through relevant traffic).

graph TD
    A["LAN 1"] --> B["Bridge"]
    C["LAN 2"] --> B

2.4 Router (Network Layer)

• Function: Operates at Layer 3 (Network). Connects different networks (e.g., LAN to WAN) using IP addresses.
• How it works:
  • Uses routing tables to determine the best path.
  • Performs NAT (Network Address Translation) to share a single public IP.
• Example:
  • Home Wi-Fi router (connects LAN to ISP).
• Real-world example:
  • Ncell’s core routers: Route data between mobile towers and the internet.

graph TD
    A["LAN"] --> B["Router"]
    B --> C["WAN\n(Internet)"]

2.5 Comparison Table

3. Proxy Servers: Caching and Security

A proxy server acts as an intermediary between clients and servers. It can:

• Cache content (reduce bandwidth usage).
• Filter requests (block malicious sites).
• Hide client IP addresses (anonymity).

3.1 How a Proxy Server Works

1. Client sends a request to the proxy.
2. Proxy checks its cache for the content.
   • If found (cache hit), sends it directly.
   • If not (cache miss), forwards the request to the real server.
3. Proxy receives the response → sends it to the client.

3.2 Real-world Examples

• NTC’s ISP Proxy: Caches popular websites (e.g., YouTube) to reduce load on international links.
• Corporate Proxies: Block access to social media (e.g., Facebook) during work hours.
• VPNs: Use proxy-like servers to mask user locations (e.g., accessing Netflix US from Nepal).

3.3 HTTP vs. HTTPS

Example:

• Khalti’s payment page uses HTTPS to encrypt credit card details.
• eSewa’s login uses HTTPS to prevent man-in-the-middle attacks.

4. OSI vs. TCP/IP Models: The Two Network Standards

Both models describe how data is transmitted across networks, but they differ in layers and practical use.

4.1 OSI Model (7 Layers)

A theoretical model for understanding network functions. Rarely implemented in practice but useful for troubleshooting.

Example Trace (Email):

1. Application: Your email client (Gmail) sends data.
2. Presentation: Data is formatted (e.g., ASCII to binary).
3. Session: Connection is established with the server.
4. Transport: TCP breaks data into segments.
5. Network: IP addresses the packets.
6. Data Link: MAC addresses frame the data.
7. Physical: Bits are sent as electrical signals.

4.2 TCP/IP Model (4 Layers)

A practical model used by the internet. Combines OSI’s layers for efficiency.

Real-world example:

• WhatsApp messages:
  1. Application: Your phone sends a message (HTTP/HTTPS).
  2. Transport: TCP ensures reliable delivery.
  3. Internet: IP routes the packet to WhatsApp’s server.
  4. Network Access: Wi-Fi/Ethernet transmits the frame.

4.3 Why TCP/IP Wins

• Simpler: Fewer layers → easier to implement.
• Backward compatible: Works with existing protocols.
• Used everywhere: Internet, intranets, and even OSI-based networks rely on TCP/IP.

5. Congestion Control: Keeping Networks Flowing

Congestion occurs when traffic exceeds network capacity, causing:

• Packet loss.
• Increased latency.
• Network collapse.

5.1 Causes of Congestion

• Too many devices (e.g., festival season on Pathao’s servers).
• Large files (e.g., downloading a movie on slow Wi-Fi).
• Malicious traffic (DDoS attacks).

5.2 TCP Congestion Control Mechanisms

TCP uses four algorithms to manage congestion:

1. Slow Start:
   • Starts with a small congestion window (CWND).
   • Exponentially increases CWND until loss is detected.
2. Congestion Avoidance:
   • Linearly increases CWND to avoid sudden overload.
3. Fast Retransmit:
   • Detects duplicate ACKs → retransmits lost packets.
4. Fast Recovery:
   • Reduces CWND sharply after loss but avoids slow start.

Example:

• Downloading a file from Daraz:
  • TCP starts with a small window → speeds up → detects congestion → slows down.

5.3 Real-world Impact

• Pathao’s delivery system: Uses congestion control to prioritize high-priority orders during Dashain.
• Ncell’s 4G network: Dynamically adjusts bandwidth for calls vs. data.

## In the Real World

1. Ncell’s Mobile Network:

   • Uses mesh-like redundancy in its core routers to handle call drops during festivals.
   • Routing protocols (OSPF/BGP) dynamically reroute traffic if a tower fails.

2. eSewa’s Payment Gateway:

   • Proxy servers cache frequent transactions (e.g., utility bill payments) to reduce latency.
   • HTTPS encrypts all financial data to prevent fraud.

3. Daraz’s Order Fulfillment:

   • Star topology connects warehouses (central nodes) to delivery agents.
   • TCP congestion control ensures orders are processed even during sales like "Daraz Days."

4. NTC’s Internet Backbone:

   • Ring topology connects major cities (Kathmandu, Pokhara, Biratnagar) for redundancy.
   • Routers use BGP (Border Gateway Protocol) to route international traffic efficiently.

## Exam Tip

1. Topologies:

   • Star = central node; Mesh = no single point of failure.
   • Bus/Ring are obsolete but may appear in comparisons.
   • Draw diagrams in exams (label devices and data flow).

2. Devices:

   • Hub = Layer 1 (broadcasts to all).
   • Switch = Layer 2 (uses MAC addresses).
   • Router = Layer 3 (uses IP addresses).
   • Memorize the comparison table.

3. OSI vs. TCP/IP:

   • OSI is theoretical (7 layers); TCP/IP is practical (4 layers).
   • Trace a packet’s journey (e.g., email) through layers in exams.

4. Congestion Control:

   • TCP’s slow start and congestion avoidance are key.
   • Relate to real examples (e.g., Pathao’s order queue).

5. Proxy Servers & HTTPS:

   • Proxy = caches/filters; HTTPS = encrypts (use in banking apps).
   • Compare HTTP/HTTPS in a table (security, port, use case).

6. Common Pitfalls:

   • Don’t confuse MAC (Layer 2) and IP (Layer 3) addressing.
   • Ring topology requires token passing; bus is collision-prone.
   • Routers work with IP; switches with MAC.

A real Cisco Catalyst switch with ports and LEDs (Image: Dsimic, CC BY-SA 3.0, via Wikimedia Commons)