15.2.1 Physical-Layer Extension

At the Physical Layer (Layer 1), the simplest interconnection device is the repeater. A repeater receives an incoming electrical or optical signal, regenerates and retimes it, and retransmits a clean version onto the next network segment. It has no understanding of addresses, frames, or packets; it simply extends the physical reach of a communication link by overcoming attenuation and signal distortion.

Early Ethernet networks frequently employed repeaters to increase the maximum cable length. However, repeaters do not isolate network traffic or improve network capacity. Every transmitted bit is repeated to every connected segment, so all devices remain part of the same collision domain and broadcast domain. As a result, only one device can successfully transmit at a time.

In early shared-media Ethernet networks, repeaters could not be cascaded indefinitely because the additional propagation delay interfered with Ethernet's collision-detection mechanism. Practical design rules therefore limited the size of a single collision domain.

A hub is simply a multi-port repeater. Instead of extending one cable, it provides a central connection point for multiple devices using structured cabling. Electrically, however, it performs exactly the same function: every received signal is retransmitted to every port. Consequently, the entire network continues to behave as one shared communication medium.

Although repeaters and hubs played an important role in the early development of Ethernet, they have largely disappeared from modern networks. Today's Ethernet LANs almost exclusively employ switches, which provide far greater performance and scalability.