MII & MDI

The Medium Independent Interface (MII) and the Medium Dependent Interface (MDI) are the core of Ethernet. I will explain why.

MII

The MII is always the same, no matter what kind of physical layer is connected. This makes the physical layer transparent for the layers above and the operating system. The MII is located between layer 1 and 2 of the OSI-Model.

MDI

The MDI is the interface to the physical wire. The MDI specifications define the if fiber or copper is used, the data rates, how data is coded and the assumptions for the wire itself.

The nomenclature of the MDI follows a standardized scheme that contains information about the transmission rate, the transmission medium and the physical structure of the network. Here’s an example for 100BASE-T1.

– 100: The maximum transmission rate is 100 Mbit/s.

– BASE: This indicates that this is a baseband transmission, where the entire bandwidth is used for a single transmission direction.

– T1: This means that the transmission takes place over a twisted pair of copper cable.