mediumdependent

Medium dependent, sometimes written as medium-dependent, is a term used in telecommunications and computer networking to describe the portion of the physical layer that is specific to a transmission medium. It designates interfaces, signaling methods, and electrical or optical characteristics that depend on the chosen medium (such as copper twisted pair, fiber optic, or wireless), as opposed to medium independent functions of the data link or higher layers. In practice, medium dependent interfaces define how a device connects to the physical medium and how signals are transmitted and received over that medium.

In Ethernet terminology, the physical layer interface between a device’s network interface card and the cable is called the medium dependent interface, or MDI. A corresponding partner interface on the other device is called MDIX (medium dependent interface crossover). When connecting two MDI devices, a crossover cable or auto-MDIX is used to ensure the transmit and receive pairs align correctly. Modern network interfaces typically support auto-MDIX, allowing straight-through cables to work regardless of device pairing.

The MDI encompasses electrical and optical specifications such as impedance, voltage levels, signaling polarity, and timing, and it varies by medium and standard (for example, copper Ethernet over twisted pair versus fiber optic). The term is commonly found in older standards and documentation, though the concept remains relevant to how devices physically attach to their media.