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Shared Ethernet or
standard Ethernet is a bus-based network where all workstations can
try to send packets simultaneously. Early versions of Ethernet used a
single coaxial cable known as a bus. Workstations tapped into the bus
to gain access. Ethernet networks today are almost exclusively
star-based designs where the bus has been compressed into a small
concentrator known as a hub or switch. These star networks typically
use Unshielded Twisted Pair (UTP) category 5 cabling that supports
speeds up to 100Mbps.
Earlier versions of Ethernet used the
CSMA/CD method of access since only one packet can be on the wire at a
time. This allowed many workstations to share the wire. CSMA/CD stands
for Carrier Sense Multiple Access with Collision Detection. Carrier
Sense means the workstation (NIC) is sensing or listening to the
carrier (the wire or media) to see if there is any traffic. Multiple
Access means that more than one computer may be trying to send data
simultaneously. Collision Detection means that if there is a collision
the computers will detect it and retransmit. Every NIC has a transmit
and receive circuit so that it can not only send data, but can receive
data, listen for traffic and detect collisions. In a shared Ethernet
network the NIC is either transmitting or receiving but cannot do both
at the same time. This is referred to as half-duplex communication.
The disadvantage to early Ethernet and
CSMA/CD was that with many workstations creating traffic, a lot of
collisions and significant slowdowns in network performance occurred.
The early versions of shared Ethernet ran at 10Mbps (10 megabit per
second), so actual throughput might only be 5 Mbps when collisions
were taken into account. If there were 10 workstations connected to
the bus or hub, they had to share 10 Mbps of bandwidth resulting in a
10 node collision domain. The Figure shows a shared Ethernet LAN with two nodes colliding. If there were
twenty nodes, the size of the collision domain would be 20 nodes and
so on.
As network applications began to
require more speed, a 100Mbps version of Ethernet was introduced known
as Fast Ethernet. This helped to alleviate congestion, but
although the speed was 10 times faster, it was still shared Ethernet
and there were still collisions to deal with.
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