Fast Ethernet (IEEE 802.3u) raised standard Ethernet's speed from 10 Mbps to 100 Mbps while preserving the original Ethernet frame format, MAC addressing, and (originally) CSMA/CD access method, making it backward-compatible and easy to migrate to.

Structure of the Physical layer in Fast Ethernet

The Fast Ethernet physical layer is split into sub-layers to support multiple media types under one MAC layer:

+---------------------------+
|      MAC Sublayer          |   (common across all implementations)
+---------------------------+
|   Reconciliation Sublayer  |
+---------------------------+
|   MII (Media Independent   |   <- allows the MAC to work with
|       Interface)           |      different physical media
+---------------------------+
|   PCS | PMA | PMD           |   (Physical Coding, Medium
|   (media-specific)          |    Attachment, Medium Dependent)
+---------------------------+
            |
     Physical Medium

The MII (Media Independent Interface) decouples the MAC sublayer from the specific physical medium, letting the same MAC logic run over copper or fiber implementations.

Implementation types

Standard	Medium	Max segment length
100BASE-TX	2-pair Cat 5 UTP/STP	100 m
100BASE-FX	2-strand multimode fiber	~400 m (half-duplex) / 2 km (full-duplex)
100BASE-T4	4-pair Cat 3/4/5 UTP (legacy, rarely used)	100 m

Each implementation adapts the electrical/optical signaling for its medium while presenting the same 100 Mbps MAC-layer service upward.