Fibre doesn't actually break the 512 bit-time rule. You still need to keep
your network under 512 bit-times from worst-case station to station.
Fibre can have the longer lengths because it doesn't sucumb to attenuation
as fast as copper. The differences in propegation time between copper and
fibre are very slight. And, if you didn't have to worry about attenuation
you could run 412 meters on fibre and 370 meters on copper.
Take a look at your network, find the two stations that are seperated by the
most cable and devices, and then follow the fomula in the book, and see what
sort of number you come up with.
If you think it is completely off, send a little ascii art picture showing
what cable lengths and devices are in there.
Mike
>
>I have a question regarding the round-trip propagation delay on an Ethernet
>network.
>
>Page 123 of the Cisco Press "Designing Cisco Networks" book states:
>
>"The most significant design rule for Ethernet is that the round-trip
>propagation delay in one collision domain must not exceed 512 bit times,
>which is a requirement for collision detection to work correctly."
>
>With 100Mbps Ethernet, the maximum round-trip delay would be 5.12 seconds,
>resulting in a distance limitation of 205 meters.
>
>I currently oversee a large flat network covering several miles in
>diameter. All of the links between buildings are single-mode fiber links.
>No routing is involved, everything is switched - one large broadcast
>domain.
>
>How does the 512 bit time rule apply to fiber optic cabling? I see on page
>127 of the same book that the Round trip delay in bit times per meter for
>Cat5 cable is 1.112, whereas Fiber-optic cable it's 1.0.
>
>I guess I'm having difficulty understanding how fiber can overcome the 512
>bit-time rule and can have a much longer distance.
>
>I do realize that this is not exactly a Cisco question, though covered on
>the DCN/CCDA material. If someone could kindly refer me to any material
>that covers this topic, I'd appreciate it.
>
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