Bus

Computer Networks and Internet Basics (9hrs) · Computer Fundamentals & Emerging Technologies

1. Definition

In bus topology, all nodes share one common backbone cable (the trunk). Each node taps into the trunk through a drop line and a T-connector, and both ends of the trunk are closed with terminators that absorb the signal so it does not reflect back. It is a multipoint, broadcast topology: when one node transmits, every other node receives the signal.

2. Structure

      ┌───┐                                                  ┌───┐
  ════┤ T │═════╤═══════╤═══════╤═══════╤═══════╤═══════╤═════┤ T ├════   ← terminators
      └───┘     │       │       │       │       │       │     └───┘      at both ends
   (terminator) │       │       │       │       │       │   (terminator)
              ┌─┴─┐   ┌─┴─┐   ┌─┴─┐   ┌─┴─┐   ┌─┴─┐   ┌─┴─┐
              │ A │   │ B │   │ C │   │ D │   │ E │   │ F │        ← nodes on drop lines
              └───┘   └───┘   └───┘   └───┘   └───┘   └───┘
             single shared backbone = the bus

3. How It Works

A node with data to send listens to the trunk first — if the bus is idle it transmits in both directions.
All other nodes receive the frame; each checks the destination MAC address and only the addressed node keeps it.
Because the medium is shared, two simultaneous transmissions would collide — early bus Ethernet used CSMA/CD: listen, transmit, detect the collision, stop, transmit a jam signal, wait a random back-off time and retry.
The terminators prevent signal reflection that would otherwise corrupt data.

Classic bus Ethernet limits: 10Base2 (thinnet, RG-58) — 185 m per segment, 30 nodes; 10Base5 (thicknet) — 500 m per segment, 100 nodes; at least 0.5 m between taps.

4. Advantages

Least cabling of all topologies — one trunk instead of one cable per node, so it is cheap for small networks.
Simple and quick to set up for a handful of nodes; no switches, routers or central device needed.
Easily extended within its limits (add a T-connector and a drop line).
Works well for light traffic and small, temporary or embedded networks.
Ideal for broadcast-style protocols and for bus-based field networks (the CAN bus inside vehicles and industrial machines is a modern, robust descendant).
If one node fails (its own NIC/transceiver), the rest can often keep working as long as its connection to the trunk is intact.

5. Disadvantages

Single point of failure — a break in the trunk, or a missing terminator, brings down the whole network; one loose T-connector can stop everything.
Difficult to troubleshoot — there is no central device to query, so the fault must be found by checking tap after tap.
Performance degrades sharply with load — all nodes share one channel, so collisions and retransmissions multiply as traffic grows.
Limited cable length and node count (185-500 m, 30-100 nodes per segment).
Low security — every node on the bus sees every frame; a single station can sniff all traffic.
Adding/removing a node disturbs the network (the trunk must be broken, and often the network must be stopped).
No full duplex and no per-port diagnostics — the trunk is half duplex and shared.

6. Bus vs Tree/Hierarchical

A tree (hierarchical) topology is a bus-based structure with branches: a root backbone with secondary buses hanging off it (usually joined by repeaters). It allows a larger network than a single bus but inherits the same failure characteristics — a trunk failure cuts off that entire branch.

7. Quick Comparison

Basis	Bus	Star	Ring	Mesh
Cable used	Least (1 trunk)	n cables	n cables (loop)	Highest: n(n-1)/2
Central device	None	Hub/switch (critical)	None	None
Effect of one cable break	Whole network may fail	Only that node affected	Ring breaks (dual ring survives)	None — alternate path exists
Access method	CSMA/CD (contention)	Switch forwarding (no collision)	Token passing (deterministic)	Routed/flooded paths
Load behaviour	Degrades quickly	Stable	Degrades gracefully	Very stable
Security	Weak (all see all)	Good with switch	Weak (frames pass through nodes)	Best (point-to-point links)
Modern use	Legacy LANs, CAN/field buses	Almost all modern LANs	Legacy Token Ring/FDDI	Backbones, WAN core

8. Exam Points

Explain bus topology with a diagram, advantages and disadvantages — 5-7 marks, almost certain.
What is the role of terminators in a bus network? — 2-3 marks.
Compare bus with star topology — 5 marks, frequent.
Why does bus topology suffer heavy performance degradation under load? — 3 marks.
Give the limits of 10Base2 and 10Base5 (length and nodes) — 2-3 marks.
Where is bus topology still used today? (CAN bus in vehicles, industrial fieldbus) — 3 marks.

Bus

Bus