Hello,
Be carefull with collision domain and broadcast domain definitions.
You might indeed have a single broadcast domain including
multiple collision domains.
(Especially with switched ethernet segments.)
Each ethernet switch port is a seperate collision domain,
either UTP or fiber. However, if you consider Fast Ethernet
HUBs, you should think about collision domains. 51.2 bit time
comes into play here, and 205 mt. is a distance limitation
in fast ethernet hub installations. Remember that you can
not cross connect two Type-II Fast Ethernet hubs more than
5 mt. apart, which justifies 205 mt. ( Two hosts connected with
100mt max UTP cable to each hub plus 5 mt maximum hub
distance sums up to 205 mt maximum distance between any two
hosts on a Fast ethernet, hub-only network. )
Besides, fiber ports run full duplex which prevents collisions.
If you have a port with collision possibility, you still have
the distance limitation.
Fiber distance limitations have much to do with signal attenuation,
Laser or led used in transmitting circuits, gain-bandwidth product
of the cable, etc. than the collisions.
(List, am I correct at Type-II hubs, or was that type-I ?)
HTH,
Ufuk.
"Randy Witt" <[EMAIL PROTECTED]> on 27.07.2000 16:53:55
Department:
Please respond to "Randy Witt" <[EMAIL PROTECTED]>
To: [EMAIL PROTECTED]
cc: (bcc: UFUK YASIBEYLI/FINANSBANK)
Subject: CCDA question-512 bit times
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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