Bus Topology

The bus topology was one of the earliest network designs used in Ethernet environments.
Although considered legacy today, it remains essential for understanding how networks behave — including Wi-Fi, which inherited many of the same limitations and concepts.

Just as a medical student cannot ignore anatomy, a network professional cannot skip the fundamentals. Several behaviors at the physical and data-link layers only make sense when you understand how early Ethernet networks worked.

Why Study the Bus Topology Today?

Understanding this topology helps you recognize:

• how collision domains originated,
• why switches became revolutionary,
• how CSMA/CD worked,
• why early Ethernet had poor scalability,
• and how these concepts reappear in modern Wi-Fi.

What Is the Bus Topology?

In a bus topology:

• All computers are connected to the same continuous cable, forming a shared medium.
• This cable was typically coaxial (10BASE2 or 10BASE5).
• Every device shared the same physical channel for transmitting and receiving data.

It works similarly to cable TV distribution: one cable carrying the signal to multiple devices.

Problem 1 — A Shared Medium

Whenever a device transmitted data, every other device on the cable received it, even if they were not the intended recipient.

This resulted in:

• mandatory broadcast behavior,
• unnecessary traffic,
• increased latency,
• reduced network efficiency.

There was no segmentation or intelligent traffic separation.

Problem 2 — Constant Collisions

Because all hosts shared the same cable, if two devices transmitted simultaneously, their electrical signals collided.

Consequences included:

• corrupted frames,
• retransmissions,
• network slowdown,
• exponential collision growth as more hosts were added.

Bus Networks Operated in Half-Duplex

Another important characteristic: early Ethernet in bus topology was half-duplex only.

This means:

• a device could not listen and transmit at the same time,
• during transmission, the host became “blind” to the medium,
• if another device transmitted simultaneously, a collision occurred, detected only after the frame was damaged.

This half-duplex behavior significantly increased collisions and limited throughput.

Modern Wi-Fi still operates under half-duplex constraints, inheriting this same limitation.

Problem 3 — Extremely Limited Scalability

Adding more hosts to the bus increased:

• the likelihood of collisions,
• congestion on the medium,
• latency,
• broadcast volume,
• and the risk of network-wide failures.

Additionally, a single cable break could take down the entire network.

CSMA/CD — Ethernet’s Attempt to Control the Medium

To manage transmission on the shared medium, Ethernet used CSMA/CD (Carrier Sense Multiple Access with Collision Detection).

It worked as follows:

1. The device listened to the medium to check if it was idle.
2. If free, it began transmitting.
3. If another device transmitted at the same time → collision.
4. Both devices detected the collision.
5. Each waited for a random backoff time before retrying.

Critical limitation:

👉 CSMA/CD only detects collisions after they happen.

It does not prevent them — it reacts to them.

This made large bus-based networks highly inefficient.

The Single Cable: A Physical and Logical Bottleneck

Beyond collisions, the bus topology suffered from:

• no segmentation,
• no traffic isolation,
• low speeds,
• no built-in security,
• difficult troubleshooting and maintenance,
• a single point of failure.

The natural evolution was the introduction of hubs and later switches, which finally eliminated shared collision domains.

How Bus Concepts Resurface in Wi-Fi

Although coaxial bus networks disappeared, the concept of a shared medium returned in wireless networks.

In Wi-Fi:

• all devices share the same channel,
• the medium is half-duplex,
• collisions still occur — but cannot be detected directly,
• devices rely on CSMA/CA (Collision Avoidance) instead of CSMA/CD.

In other words. The bus topology died in wired Ethernet, but lives on in wireless networking.