Star

Star

1. Definition

In star topology, every node is connected by its own dedicated point-to-point link to a single central device — a hub, switch or access point (the concentrator). No node is directly connected to another node: all traffic passes through the centre. It is the dominant topology in modern LANs because it matches structured-cabling practice — every wall socket runs back to a patch panel and then to a switch.

2. Structure

                    node A            node B
                       │                 │
                       │                 │
     node F ──────────┌─┴─────────────────┴─┐────────── node C
                      │  CENTRAL DEVICE     │
                      │  (hub / switch /AP) │
     node E ──────────└─┬─────────────────┬─┘────────── node D
                       │                 │
                    node G            node H

3. How It Works

1. A node sends its frame on its own cable to the central device.
2. With a hub, the frame is repeated out of every other port (one collision domain — all ports share bandwidth).
3. With a switch, the frame is forwarded only to the port of the destination MAC address (each port is a separate collision domain, full duplex, dedicated bandwidth). The switch learns MAC addresses by observing source addresses, so it builds a MAC address table automatically.
4. Broadcast frames (e.g. ARP requests) are flooded to all ports, so the whole switch remains one broadcast domain unless VLANs split it.

4. Advantages

• Easy to install, expand and reconfigure — adding or removing a node affects only its own cable and port; the rest of the network continues working.
• Fault isolation — if one node's cable or NIC fails, only that node is affected; the switch/hub detects the dead link and disables that port.
• Simple troubleshooting and monitoring — the central device can show link status, errors, traffic per port; faults are located quickly.
• Central management — port security, VLANs, QoS, ACLs and monitoring are configured in one place.
• High performance with a switch — dedicated bandwidth per port, full-duplex operation, no collisions (no CSMA/CD needed on switched full-duplex links).
• Supports mixed media and speeds — copper, fibre and wireless can attach to the same switch (10/100/1000/10G ports).

5. Disadvantages

• Single point of failure — if the central device dies, the entire network goes down with it (solved by redundant/stacked switches and STP).
• More cable is needed — one cable per node back to the centre; long runs require careful planning (100 m limit for UTP).
• Higher cabling and equipment cost than bus topology, especially in large buildings.
• Capacity limit at the centre — every frame passes through the central device, so its backplane bandwidth and port count bound the network.
• Performance depends on the central device — an overloaded switch or a cheap hub (shared bandwidth) quickly becomes the bottleneck.

6. Variants and Cabling Formula

• Extended star / hierarchical star — stars connected to stars: access switches (each floor) → distribution switches (each building) → core switches. This is exactly how campus and enterprise networks are built today.
• Cable requirement: for $n$ nodes, a star needs $n$ links (one per node) plus one link per uplink to a higher star. By comparison, a bus needs one backbone plus drops, and a mesh needs $n × (n - 1) / 2$ links — which is why star is preferred: it is a good compromise between cost and reliability.
• Wireless star — Wi-Fi is also a star: all stations communicate with the access point (the centre), which acts as a transparent bridge to the wired LAN.

7. Quick Comparison

Basis	Star	Bus	Ring	Mesh
Links needed	n	1 backbone + drops	n (closed loop)	n(n-1)/2
Cable cost	Medium	Low	Medium	Very high
Fault handling	Only that node affected (central device critical)	Whole network fails on a trunk cut	Whole ring fails (unless dual ring)	Very robust — many paths
Adding a node	Easy, no disturbance	Disturbs the backbone	Requires breaking the ring	Needs new links from the new node
Troubleshooting	Easy (centralised)	Hard	Moderate (dual ring helps)	Hard (complex routing)
Typical use	Modern Ethernet LANs (with switches)	Legacy Ethernet, CAN bus in vehicles	Token Ring, FDDI (legacy)	Backbone/critical links, WAN core

8. Exam Points

• Explain star topology with a diagram, advantages and disadvantages — 5-7 marks, almost certain.
• Compare star, bus, ring and mesh topologies (tabular) — 5-7 marks, very frequent.
• How many links are needed for n nodes in star, bus, ring and mesh? — 3 marks numerical.
• Why is star topology used in modern LANs? What role does the switch play? — 3-5 marks.
• How is fault isolation achieved in a star network? — 3 marks.
• What is an extended star topology? Draw one for a three-floor building — 5 marks.