5-4-3 Rule?

jeff CQ

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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Webmaster

Yes.

www.techexams.net/technotes/networkplus/networkcomponents.shtml

Includes a network diagram as well...

jeff CQ

how does that get you 2500 meters if the thinnet max is 185?

Drakonblayde

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.

Webmaster

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.

chmars53

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.