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# 5-4-3 Rule?

Member Posts: 20 ■□□□□□□□□□
Whats the deal with this 5-4-3 rule? is it the same as this statement from one of the technotes:
Maximum length of collision domain is 2500 meters (5 segments, 4 repeaters, max. 3 segments populated)

That is from the 10Base2 section of the Network Topologies Technote
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Yes. www.techexams.net/technotes/networkplus/networkcomponents.shtml

Includes a network diagram as well...
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Member Posts: 20 ■□□□□□□□□□
how does that get you 2500 meters if the thinnet max is 185?
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Member Posts: 542
If I'm not mistaken, the Ethernet maximum is 2500 meters, not necessarily 10base2. 10base5 would give you a maximum of 2500 meters under the 5-4-3 rule. 10baseF breaks that rule if I remember correctly, being able to do 2000 meters per segment, but also being limited to no more than 2 repeaters. 100BaseFX is limited to 400 meters per segment, and is also limited to 2 repeaters I think, but even if it's not, it still falls within that maximum cabling distance.
= Marcus Drakonblayde
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CCNP-O-Meter:
=[0%]==[25%]==[50%]==[75%]==[100%]
==[X]===[X]====[ ]=====[ ]====[ ]==
=CCNA==BSCI==BCMSN==BCRAN==CIT=
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Indeed the maximum for Ethernet, strictly taken, meaning 10Mbps Ethernet. In 100BaseT Fast Ethernet the maximums collision domain ranges from 100 to 412 m depending on the type of media (copper, fiber, multimode fiber) and the type repeaters being used (Maximum one Class 1 repeater, or two class 2 repeaters.)

The 5-4-3 diagram in the TechNotes is logical, as it is a general Ethernet design constraint.

In 10Mbps Ethernet (also 10Base2), no two stations can be separated by more than 2500 meters of cable (or 4 repeaters).

Only important for the CCDA exam, but here's a quote from some .edu site, which describes the why:
The most significant design rule for Ethernet is that the round-trip propagation delay in one collision domain must not exceed 512 bit times. This is a requirement for collision detection to work correctly. This rule means that the maximum round-trip delay for 10-Mbps Ethernet is 51.2 microseconds. The maximum round-trip delay for 100-Mbps Ethernet is only 5.12 microseconds because the bit time on 100-Mbps Ethernet is 0.01 microseconds as opposed to 0.1 microseconds on 10-Mbps Ethernet.

To make 100-Mbps Ethernet work, there are much more severe distance limitations than those required for 10-Mbps Ethernet. The general rule is that a 100-Mbps Ethernet has a maximum diameter of 205 meters when UTP cabling is used, whereas 10-Mbps Ethernet has a maximum diameter of 2500 meters.
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Member Posts: 10 ■□□□□□□□□□
Great quote, webmaster!

Jeff, remember too that any collision domain stops at devices like Bridges and Routers. And that STACKED hubs, routers, & bridges, act as a SINGLE device.

The entire idea of the 5-4-3 rule is that it takes TIME for a signal to get from one endpoint to another, and that to avoid UNKNOWN ERRORS you must not have a device farther away than the TIME it takes to transmit the packet.

Ever talked on a phone where there is a big delay (you see it all the time on CNN and other satellite link-ups) ? People talk over each other because they have such a delay in transmission that they don't know the other person has decided to add another comment instead of just waiting for a response to the first comment. In the Ethernet world, 5-4-3 stops that.
Repairing Mainframes since 1978 - & still learning