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Overview 1-9
Fast Ethernet Overview
Fast Ethernet Overview
The term Ethernet is commonly used for all carrier sense multiple access collision detect
(CSMA/CD), LANs thatgenerally conformto Ethernetspecifications, includingFast Ethernet under
IEEE 802.3u.
Note 100BaseTX is intended for Environment A, and 100BaseFX is intended for Environment B.
IEEE 802.3u is well suited to applications where a local communication medium must carry
sporadic, occasionally heavy traffic at high peak datarates. Stationson aCSMA/CD LANcan access
the network at any time. Before sending data, the station listens to the network to see if it is already
in use. If it is, the station waits until the network is not in use, then transmits; this is half-duplex
operation. A collision occurs when two stations listen for network traffic, hear none, and transmit
almost simultaneously. When this happens, both transmissions are damaged, and the stations must
retransmit. The stations detect the collision and use backoff algorithms to determine when they
should retransmit.
Both Ethernet and IEEE 802.3u are broadcast networks, which means that all stations see all
transmissions. Each station must examine received frames to determine whether it is the intended
destination and, if it is, pass the frame to a higher protocol layer for processing.
IEEE 802.3u specifies the following different physical layers for 100BaseT:
• 100BaseTX—100BaseT, half and full duplex over Category 5 unshielded twisted-pair (UTP),
Electronics Industry Association/Telecommunications Industry Association
[EIA/TIA]–568-compliant cable
• 100BaseFX—100BaseT, half and full duplex over optical fiber
• 100BaseT4—100BaseT, half and full duplex over Category 3, 4, or 5 UTP or foil twisted-pair
(FTP) cabling with four pairs; also called 4T+ or T2, which is 2-pair UTP over Category 3 cable.
Each physical layer protocol has a name that summarizes its characteristics in the format
speed/signaling method/segment length, where speed is the LAN speed in Mbps, signaling method
is either baseband or broadband, and segment length is typically the maximum length between
stations in hundreds of meters. Therefore, 100BaseT specifies a 100-Mbps, baseband LAN with
maximum network segments.