Ethernet describes a system that links the computers in a building or within a
local area. It consists of hardware (a network interface card), software, and
cabling used to connect the computers. All computers on an Ethernet are
attached to a shared data link, as opposed to traditional point-to-point networks,
in which a single device connects to another single device.
Because all computers share the same data link on an Ethernet network, the
network needs a protocol to handle contention if multiple computers want to
transmit data at the same time, because only one can talk at a time without
causing interference. Metcalfe’s invention introduced the carrier sense multiple
access collision detect (CSMA/CD) protocol. CSMA/CD defines how a computer
should listen to the network before transmitting. If the network is quiet,
the computer can transmit its data. However, a problem arises if more than
one computer listens, hears silence, and transmits at the same time: The data
collides. The collision-detect part of CSMA/CD defines a method in which
transmitting computers back off when collisions occur and randomly attempt
to restart transmission. Ethernet originally operated at 3 Mbps, but today it
operates at speeds ranging from 10 Mbps (that’s 10 million bits per second) to
10 Gbps (that’s 10 billion bits per second). 51