OSI Layer: Physical Layer (L1)

Definition

Topology describes the physical layout (how cables/devices are actually arranged) or logical layout (how data actually flows, which can differ from the physical wiring) of nodes and links in a network. Topology choice drives cost, fault-tolerance, ease of installation, and scalability — it is one of the four axes used to classify networks alongside scale, transmission technology, and ownership.

Types of Network Topology

1. Bus Topology

• All nodes are tapped onto a single shared backbone cable (coaxial, historically), terminated at both ends by a terminator resistor to absorb signal and prevent reflection.
• Data broadcasts along the whole cable; every node "hears" every transmission but only the addressed node accepts it.
• Advantages: cheapest to install, minimal cabling, easy to extend for a small network.
• Disadvantages: a single break anywhere disables the entire segment; performance degrades sharply as nodes are added (shared bandwidth, collisions); difficult to troubleshoot a fault's exact location.
• Example: legacy 10BASE2/10BASE5 Ethernet.

2. Star Topology

• Every node connects by its own dedicated link to a central device (hub or switch); nodes never connect directly to each other.
• Advantages: a single cable failure only isolates one node, not the network; easy to install, extend, and troubleshoot (fault isolated to one link); centralised management.
• Disadvantages: the central hub/switch is a single point of failure for the whole network; more cabling than bus.
• Example: virtually all modern Ethernet LANs (wired to a switch).

3. Ring Topology

• Each node connects to exactly two neighbours, forming a closed loop; data travels around the ring hop by hop.
• Unidirectional ring: data flows in one direction only — simple but one broken link can down the whole ring.
• Bidirectional / dual ring: data can flow either way, and a second counter-rotating ring provides automatic failover if one link breaks (e.g., FDDI).
• Access is typically controlled by token passing — a node may transmit only while holding a special token frame, which eliminates collisions.