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Ethernet Basics: Thick Ethernet
Thick Ethernet, officially known as 10 Base-5, is
the oldest form of Ethernet. It was originally developed
in the late 1970's by Digital Equipment Corporation,
IBM, and Xerox, and became an international standard
in 1983.
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Topology & Cabling
10 Base-5 is laid out in a bus topology, with a single
coaxial cable connecting all nodes together. At each
end of the coaxial cable is a terminator. Each node
on the network physically connects to the coaxial
cable through a device called a transciever and an
AUI cable is connected between the node and the transciever.
Figure One shows a simple 10 Base-5 network.
Figure One
Thick Ethernet Physical Topology
The cable type used for 10 Base-5 networks is a specialized
form of RG-8 coaxial cable. This cable consists of
a single central wire surrounded by a plastic insulation,
which is in turn surrounded by four shields and an
outside jacket. The resulting cable is nearly 1/2
inch in outside diameter, and is relatively heavy
and inflexible. However, it is very resistant to external
interferance.
A single 10 Base-5 segment may be up to 500 meters
(1650 feet) in length and may have up to 255 nodes
connected to it. Please note that each node must be
at least 2.5 meters (8.25 feet) apart.
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Pros & Cons
As with any technology, there are pros and cons to
10 Base-5 which make it very useful in some applications
and not a good choice for others. Below are some of
the characteristics of the technology:
PROS
Long Distances Possible
10 Base-5 allows distances up to 500 meters (1650
feet). This makes it very useful as a "backbone"
technology for wiring together multiple locations
within a building without the use of repeaters.
Noise Immunity
Since 10 Base-5 uses a very heavily shielded cable,
it can be used in electrically noisy environments
which can cause other network types to fail.
Conceptually Simple
Since all devices on a 10 Base-5 network are simply
chained together on a common coaxial cable, it is
a simple matter to plan the routing of the cable.
It is also easy to look at the diagram of the network
and understand how a particular installation works.
CONS
Inflexible
10 Base-5 networks do not lend themselves well to
installations where the setup of the network will
change much after the initial installation. It can
be very difficult to add or move a node once it is
connected to the coaxial cable.
Fault Intolerant
Since 10 Base-5 uses a common physical cable to interconnect
all the nodes, the failure of any part of the coaxial
cable or any node has the ability to cause the entire
network to go down.
Suceptible To Ground Loops
Ground loops are a notorious weakness of 10 Base-5.
Basically, a ground loop occurs when a network cable
is used to interconnect devices which are powered
from different sources, and therefore a difference
in voltage exists between two points on the network.
The result is an electrical current flowing through
the shields of the cable, which causes considerale
noise to be introduced into the center conductor.
Very Difficult Troubleshooting
As mentioned above, a failure anywhere on a 10 Base-5
segment has the ability to drop the entire network.
Troubleshooting such a failure can be extremely frustrating,
as the only way to do it is to check each node and
the cabling between them one at a time. This is very
time consuming, and can be expensive if a company's
entire business relies on the network to be up.
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Today's Applications
Due to the inherent disadvantages in the design of
10 Base-5, it is not commonly chosen as a technology
for the bulk of today's Ethernet networks. However,
it is very useful in specialized applications which
take advantage of its strengths but avoid the technology's
weaknesses.
Today, the most common use for 10 Base-5 is as a "backbone"
technology. Basically, a backbone is used to connect
together multiple 10 Base-T hubs to create one large
network, as depicted in Figure Two. Another common
use is for connecting several distant devices to a
central network.
Figure Two
Thick Ethernet Backbone
10 Base-5 is a good choice for networks which do not
require much change after the initial installation
is completed, and which will have only a limited number
of devices attached directly to the coaxial cable.
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