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NETWORK DIAMETER
LIMITATIONS
Repeaters can be connected in series (cascaded) to increase the network diameter, but there
are restrictions. As mentioned before, repeaters must reinforce collision detection. But if
the network diameter exceeds a single collision domain, unreliable operation will result.
The maximum collision diameter is determined by the round-trip time of a signal propagating
between the two furthest nodes. This time cannot exceed 575 bits (57.5 ms at 10 Mbps.
Repeaters impact the maximum collision diameter since they contribute data latency due to
their electronics. The IEEE 802.3 standard does an exhaustive study on all contributors of
data latency including cables, transceivers and the like. These values formulate the rules
that govern the number of repeaters that can be cascaded.
APPROACH 1
There are two approaches that can be used to calculate the number of repeaters. Approach 1
is more of the "cookbook" approach whereas approach 2 is the more analytical. It
would be nice to have simple cabling rules for expanding an Ethernet network, but
unfortunately that is not the case. Here are the rules for approach 1:
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The transmission path permitted between any two DTEs may consist of up to five segments,
four repeater sets (including optional AUIs), two MAUs, and two AUIs. |
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A DTE is data terminal
equipment which is either the source or destination of the traffic. A
repeater set is actually a repeater with two attached medium attachment
units (MAUs). An AUI is an attachment unit interface which is required
if external MAUs are being used. With this rule the two MAUs and the two
AUIs are reserved for the DTEs. The repeater sets, by definition, have
their own MAUs.
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When a transmission path consists of four repeaters and five segments, up to three of the
segments may be mixing and the remainder must be link segments. When five segments are
present, each fiber optic link segment shall not exceed 500 m.
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Figure 1 — Good example of 5-4-3 rule. Notice the distance limitation on the fiber segments.
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A mixing segment is
actually a bus segment such as 10BASE2 or 10BASE5. A link segment consists
of only two MAUs and is capable of full-duplex operation (10BASE-FL and
10BASE-T qualify). Notice that although 10BASE-FL is capable of achieving
a 2 km segment length, it is limited to 500 m under the above conditions.
Figure 1 shows this situation. Notice that the maximum segment length
for 10BASE2, 10BASE5 and 10BASE-T can be achieved. Only the 10BASE-FL
segment length is restricted. This rule says that you cannot have an all
coaxial system when using four repeaters; however, an all fiber or all
twisted-pair network is possible using four repeaters.
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When a transmission path consists of three repeater sets and four segments,
the following restrictions apply: |
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— The inter-repeater fiber segment can now be 1000 m. |
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— The end fiber segments (connected to DTEs) can be 400 m. |
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— All segments can be mixing. |
An all coaxial network
can be created when using three repeaters — and it appears that an all
fiber system can extend to 2800 meters.
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