The physical medium ranges from bulky coaxial cable to twisted pair and optical fiber with a standardized reach of up to 80 km.

If autonegotiation fails, some multiple-speed devices sense the speed used by their partner,[1] but this may result in a duplex mismatch.

While autonegotiation can practically be relied on for Ethernet over twisted pair, few optical-fiber ports support multiple speeds.

The reach, especially for optical connections, is defined as the maximum achievable link length that is guaranteed to work when all channel parameters are met (modal bandwidth, attenuation, insertion losses etc.).

Vice versa, a link with worse channel parameters can also work but only over a shorter distance.

In addition to these official standards, many vendors have implemented proprietary media types for various reasons—often to support longer distances over fiber optic cabling.

Early Ethernet standards used Manchester coding so that the signal was self-clocking and not adversely affected by high-pass filters.

10 Gigabit Ethernet, specifically 10GBASE-LR and 10GBASE-ER, enjoys significant market shares in carrier networks.

[27] In May 2018, IEEE 802.3 started the 802.3ck task force to develop standards for 100, 200, and 400 Gbit/s PHYs and attachment unit interfaces (AUI) using 100 Gbit/s lanes.

These were theoretical predictions of technological ability, rather than estimates of when such speeds would actually become available at a practical price point.

Starting with Fast Ethernet, the physical layer specifications are divided into three sublayers in order to simplify design and interoperability:[35] Several varieties of Ethernet were specifically designed to run over 4-pair copper structured cabling already installed in many locations.

4PPoE defined in IEEE 802.3bt can use all four pairs to supply up to 100 W. The cable requirements depend on the transmission speed and the employed encoding method.

Some fiber connections have minimum cable lengths due to maximum level constraints on received signals.

[40] Fiber ports designed for long-haul wavelengths may require a signal attenuator if used within a building.

Other special-purpose physical layers include Avionics Full-Duplex Switched Ethernet and TTEthernet.