Network Topologies

Network Topologies

1. Bus Topology

Structure: All devices connect to a single shared cable called the Backbone.
Termination: Both ends must have 50-ohm terminators to absorb signals. Without them, signals bounce back (Signal Reflection), causing collisions and data loss.
Cabling Standards:
- 10Base5 (Thicknet): Max length 500m. Used for older backbones.
- 10Base2 (Thinnet): Max length 185m. Used for individual PC connections.
Access Method: CSMA/CD (Carrier Sense Multiple Access with Collision Detection).
Pros: Easy to set up for very small, temporary networks.
Cons: If the backbone cable breaks, the entire network fails. Hard to troubleshoot.
Current Status: Obsolete.

Structure: Each node is connected to two others, forming a continuous loop.
Working (Token Passing): A logical "Token" frame moves around the ring. Only the device holding the token can transmit data, which eliminates collisions.
Redundancy (Dual Ring): Used in FDDI (Fiber Distributed Data Interface). It uses two rings rotating in opposite directions. If one fails, the network "wraps" the signal onto the other to stay alive.
Pros: No collisions; performs well under heavy load.
Cons: In a single ring, one node failure kills the whole network.
Current Status: Legacy. Replaced by switched Ethernet in LANs.

Structure: Every device has a dedicated link to a central Hub or Switch.
Logic:
- Hub (Layer 1): Acts like a bus; it repeats data to all ports.
- Switch (Layer 2): Uses a MAC Address Table. It sends data only to the specific port of the destination device. If it doesn't know the location, it performs Unicast Flooding.
Cabling: UTP (Cat5e/Cat6) with RJ45 connectors.
Pros: Highly scalable. If one cable fails, only that device goes down. Easy to troubleshoot.
Cons: If the central switch fails, the whole network dies (Single Point of Failure).
Current Status: The Standard for almost all modern LANs.

Structure: A star of stars. Multiple access switches are connected to a more powerful distribution or core switch.
Hierarchy:
- Access Layer: Connects end-users (PCs, Phones).
- Distribution Layer: Forwards traffic between access switches and applies security/routing policies.
- Core Layer: The "backbone" designed for high-speed switching between distribution points.
Current Status: Used in Enterprise and Campus environments.

Structure: Combines elements of Star and Bus. It has a Root Node (Top-level switch) that branches out to secondary switches.
Key Rule: There is only one path between any two nodes.
Pros: Great for multi-story buildings or branched organizations.
Cons: If the "Root" switch fails, communication between different branches stops.
Current Status: Common in Cabled TV (CATV) networks and large building infrastructures.

Concept: Maximum redundancy where nodes have multiple paths to each other.
Types:
1. Full Mesh: Every single device is connected to every other device.
  - Link Formula: $Links = \frac{n(n-1)}{2}$
  - Port Requirement: Each device needs $n-1$ physical ports/NICs.
2. Partial Mesh: Only high-traffic or critical nodes are interconnected.
Pros: Fault Tolerance. If one path fails, data takes another route instantly.
Cons: Extremely expensive (cabling/hardware) and complex to configure.
Current Status: Used for ISP Backbones and the Global Internet.

Structure: A mix of two or more different topologies.
Example: A Star-Bus (several Star networks linked by a Bus backbone) or a Star-Ring (physical star layout but token-passing logic).
Pros: Inherits the strengths of its components; highly flexible.
Cons: Requires expert knowledge to maintain the different hardware standards.
Current Status: Common in evolving companies that have merged or expanded over time.