The Fundamental Problem: Connecting Multiple Devices

Imagine you have two computers and you want them to communicate. The solution is simple: you can connect them directly with a single network cable. This is called a point-to-point connection. But what happens when you have three, four, or ten computers? Connecting every computer to every other computer directly would result in a messy web of cables and would be completely impractical.

This challenge led to the development of centralized networking devices. Instead of connecting devices to each other, you connect all devices to a single, central point. In the early days of Ethernet's evolution from the old bus topology to the more robust star topology, the first and simplest of these central devices was the hub, also known as a concentrator. It provided a simple, low-cost way to build a local area network (LAN).

What is an Ethernet Hub? A Layer 1 "Dumb" Repeater

A hub is a very basic networking device that operates at the very bottom of the networking hierarchy: Layer 1, the Physical Layer, of the OSI model. This is the most important thing to understand about a hub, as it dictates its behavior and limitations.

Because it is a Layer 1 device, a hub has no concept of "data" in the way we think of it. It does not understand:

• Frames or Packets: It cannot identify the beginning or end of an Ethernet frame.
• MAC Addresses: It cannot read the source or destination MAC addresses within a frame.
• IP Addresses: It certainly has no knowledge of higher-level addresses like IP.

The hub's sole function is to act as a multiport repeater. It receives electrical signals representing bits on one of its ports, cleans up and regenerates them to their original strength, and then broadcasts (repeats) them out of every single other port. It's essentially an electronic echo chamber for network signals.

Physical Star, Logical Bus: An Important Distinction

The transition to hub-based networks brought about the widespread use of the star topology. In this physical layout, each device has its own dedicated cable running to a central hub. This was a major improvement over the older bus topology (like 10Base-2), where all devices were daisy-chained on a single cable. In a star topology, a break in one cable only affects one device, not the entire network, making troubleshooting much easier.

However, this is where a critical distinction must be made. While the physical topology was a star, the way the hub operated meant that the logical topology was still a bus. Because the hub broadcasts every signal out of every port, all connected devices are effectively sharing the same communication channel. Electrically, it's as if they were all still connected to that one single bus cable. Everyone hears everything, and only one device can talk at a time. This logical bus structure is the direct cause of the hub's main limitation: the collision domain.

The Collision Domain: A Single Conversation Room

A collision domainis a segment of a network where all connected devices share the same medium, creating a situation where their transmitted signals can interfere with each other, or "collide". The most defining characteristic of a hub is that it creates one single, large collision domain that includes all of its ports.

Consequences of a Single Collision Domain

• Half-Duplex Operation:Because all devices share the same electrical path, they cannot send and receive data simultaneously. Communication is restricted to half-duplex mode. A device must wait for the line to be clear before it can send, and it cannot receive while it is sending.
• Performance Degradation: As more devices are connected to the hub (or hubs are cascaded together), the single collision domain grows larger. More devices mean more traffic and a statistically higher probability of collisions. Every collision forces the involved devices to stop, wait a random amount of time, and retransmit. This waiting and retransmitting process wastes bandwidth and significantly reduces the overall effective throughput of the network. The more devices, the slower the network becomes for everyone.
• CSMA/CD is Required: In this collision-prone environment, the CSMA/CD protocol is essential to provide a ruleset for accessing the medium and recovering from collisions. The hub itself does not implement CSMA/CD; rather, its physical operation creates the conditions where the end devices must use CSMA/CD to function.

The Inefficiency and Security Issues of Hubs

The "broadcast-everything" nature of a hub leads to significant issues beyond just collisions.

Bandwidth Waste

Imagine a 10-port hub in a 100 Mbps network. The total theoretical bandwidth is 100 Mbps. However, this bandwidth is shared among all 10 ports. If Host A is sending a large file to Host B, and at the same time Host C is sending a file to Host D, they are all competing for a slice of that same 100 Mbps. The hub's broadcast behavior means that every frame from every device is sent to every other device, unnecessarily consuming bandwidth on all links.

Lack of Security

A critical flaw of hubs is the complete lack of traffic isolation. Since every frame is sent to every port, any device connected to the hub can see all the traffic passing through it, regardless of the intended recipient. A malicious actor could easily connect a computer to the hub, put the network card into promiscuous mode, and use a packet sniffer tool (like Wireshark) to capture and inspect all the data flowing across the network. This makes hubs inherently insecure for any sensitive communication.

Legacy and Replacement by Switches

For all their faults, hubs were an important step in the evolution of Ethernet. They made creating star-topology networks affordable and much simpler than the coaxial bus systems that preceded them.

However, their severe limitations in terms of performance, scalability, and security meant they were quickly made obsolete by the invention of the Ethernet switch. A switch, which operates at Layer 2 (Data Link Layer), is an intelligent device. It learns the MAC addresses of the devices connected to each of its ports and creates a MAC address table. When a frame arrives, the switch examines the destination MAC address and forwards the frame only to the port where the destination device is located.

This simple change fundamentally solves all of the hub's problems: it breaks the large, single collision domain into a separate micro-domain for each port, eliminates collisions (allowing for full-duplex communication), provides dedicated bandwidth for each connection, and ensures security by isolating traffic. Today, hubs are considered completely obsolete and are found only in legacy installations or museum-like settings.