Editing Fast Ethernet (section)

== General design ==
Fast Ethernet is an extension of the 10-megabit [[Ethernet]] standard. It runs on [[twisted pair]] or [[optical fiber cable]] in a [[Star topology|star wired bus topology]], similar to the IEEE standard 802.3i called [[10BASE-T]], itself an evolution of 10BASE5 (802.3) and 10BASE2 (802.3a). Fast Ethernet devices are generally backward compatible with existing 10BASE-T systems, enabling plug-and-play upgrades from 10BASE-T. Most switches and other networking devices with ports capable of Fast Ethernet can perform [[autonegotiation]], sensing a piece of 10BASE-T equipment and setting the port to 10BASE-T half duplex if the 10BASE-T equipment cannot perform autonegotiation itself. The standard specifies the use of [[CSMA/CD]] for media access control. A [[full-duplex]] mode is also specified and in practice, modern networks use [[Ethernet switch]]es and operate in full-duplex mode, even as legacy devices that use half duplex still exist.

A Fast Ethernet adapter can be logically divided into a [[media access control]]ler (MAC), which deals with the higher-level issues of medium availability, and a physical layer interface ([[PHY]]). The MAC is typically linked to the PHY by a four-bit 25&nbsp;MHz synchronous parallel interface known as a [[media-independent interface]] (MII), or by a two-bit 50&nbsp;MHz variant called [[reduced media independent interface]] (RMII). In rare cases, the MII may be an external connection but is usually a connection between ICs in a network adapter or even two sections within a single IC. The specs are written based on the assumption that the interface between MAC and PHY will be an MII but they do not require it. Fast Ethernet or [[Ethernet hub]]s may use the MII to connect to multiple PHYs for their different interfaces.

The MII fixes the theoretical maximum data bit rate for all versions of Fast Ethernet to {{nowrap|100 Mbit/s}}. The [[information rate]] actually observed on real networks is less than the theoretical maximum, due to the necessary header and trailer (addressing and error-detection bits) on every [[Ethernet frame]], and the required [[interpacket gap]] between transmissions.