Fast Ethernet
Fast Ethernet is a tenfold increase in Ethernet's data rate to 100Mbps.
Similar to 10Base-T, it runs over unshielded twisted pair (UTP) wiring
of up to 100 meters in length in a star-wired configuration requiring
a central repeater or hub. Like previous Ethernet variations, Fast
Ethernet retains Ethernet's key specification, CSMA/CD.
Ethernet, or IEEE 802.3, defines a communication protocol that is
conceptually divided into two parts. First is the media access control
, or MAC, layer that formats the information for transmission and arbitrates
the way in which network participants gain access to the network. Ethernet's
MAC layer is carrier sense multiple access with collision detection
or CSMA/CD. The second part of the Ethernet protocol is the physical
layer that communicates between the medium -- the wire -- and the MAC.
MAC Layer
Loosely speaking, CSMA/CD means that a station can send data as long as the
network is quiet. If the network isn't quiet, then the station doesn't
transmit, it waits or defers. If multiple stations begin sending
data at the same time, because they all sensed a quiet network, then there
is a detectible collision. In this case, each participant waits a
random amount of time and tries to send the data again.
Speed Independent
In retrospect, the 802.3 committee demonstrated considerable foresight
by specifying the Ethernet MAC in a speed independant fashion.
Except for the interpacket gap, all Ethernet MAC parameters are specified
in bits versus time. This allows the Ethernet data rate to change without
changing the other MAC parameters. Therefore, while Ethernet today
runs at 10Mbps, the CSMA/CD MAC run at other speeds without change. Fast
Ethernet is a tenfold scaling of the Ethernet MAC, from 10 to 100Mbps.
Physical Layer
The second part of the Ethernet protocol is the physical layer that is
responsible for getting data (bits) onto and off the attached medium. Its
scope includes data encoding and decoding, carrier sense, collision detection,
the electrical and mechanical interface to and properties of the
attached medium.
Ethernet has a successful history of using an unchanged CSMA/CD MAC to
accomodate a wide cariety of physical layers.
Fast Ethernet Switching Products
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