What's an ethernet collision domain? Seriously, when was the last time
you dealt with a half duplex ethernet?
apologies for top posting.
Everyone, including me have addressed "what/how/by who wrt question at hand.
Bill-
Another poster has already asked this question-
Can you post a sample of the "answers" you have received; which prompted you the ask this question to begin with.
./Randy
Can you post a sample of the "answers" you have received; which
prompted you the ask this question to begin with.
I've been asking the question in phone interviews for months. I
couldn't quote them properly but the answers were... discouraging. No
one beyond ping and traceroute.
I asked HR last week to start asking the question as a pre-screen and
forward me the answer. The first one responded "This would block all
IP traffic." I figured it was time for a sanity check to make sure the
question was reasonable.
Regards,
Bill
From: William Herrin <bill@herrin.us>
5. What is the reason for the 100m distance limit within an
ethernet collision domain?What's an ethernet collision domain? Seriously, when was the last time
you dealt with a half duplex ethernet?
-----------------------------------------Now if someone answered it that way, I'd definitely be
interested while the HR person would just hang up...scott
Anyone who responds that way has at least a notion of collision detection
and propagation delay and might actually have a bit of experience in the
field, not bad things. Is the next question about exponential back off or
regeneration of preamble?
--Dave
+1 -- That would be a perfectly valid answer and one of the list of answers I would actually give to HR.
Owen
Incidentally, 100m was the segment limit. IIRC the collision domain
comprising the longest wire distance between any two hosts was larger,
something around 200m for fast ethernet. Essentially, the collision
signal caused by receiving the first bit of the overlapping packet had
to get back to the sender before the sender finished the 64-byte
minimum-size packet. Allow for the speed of light and variances in the
electronics and that was the width of the collision domain.
Carrier sensing multiple access with collision detection. CSMA/CD. I
haven't thought about that in a long time.
-Bill
In a message written on Thu, Jul 05, 2012 at 11:05:21PM -0400, William Herrin wrote:
Incidentally, 100m was the segment limit. IIRC the collision domain
comprising the longest wire distance between any two hosts was larger,
something around 200m for fast ethernet. Essentially, the collision
Actually it can be much longer, having worked on a longer such ethernet
many, many moons ago.
The longest spec-complaint, repeated only network looks like:
>
> Host Segment
>
+ Copper to Fiber Repeater
>
> 2km fiber, no hosts
>
+ Copper to Fiber Repeater
>
> Host Segment, with or without hosts
>
+ Copper to Fiber Repeater
>
> 2km fiber, no hosts
>
+ Copper to Fiber Repeater
>
> Host Segment
>
With 10base5, a copper segment can be 500m, so 500+2000+500+2000+500, or
5.5km.
With 10base2, a copper segment can be 185m, so 185+2000+185+2000+185, or
4.5km.
WIth 10baseT, a copper segment can be 100m, so 100+2000+100+2000+100, or
4.4km.
The introduction of fiber repeaters is why folks started to use the
broken term "half repeater". This was so folks who learned the
rules as "2 repeaters in the path" could deal with the fact that
it's actually the 5-4-3 rule, so they called the 4 repeaters two
half repeaters.
Of course, each repeater could be a multi-port repeater (or a hub in
10baseT speak) and thus have a star configuration off of it in the
diagram.
Add in a couple of 2 port bridges to reframe things, and it's quite
possible to run a layer 2 ethernet that is 10's of km long, and has
thousands of hosts on it. There was a day when 3000-4000 hosts on
a single layer 2 network at 10Mbps was living large.
Thankfully, not anymore.
From: William Herrin <bill@herrin.us>
5. What is the reason for the 100m distance limit within an ethernet collision domain?
What's an ethernet collision domain? Seriously, when was the last time
you dealt with a half duplex ethernet?
-----------------------------------------Now if someone answered it that way, I'd definitely be
interested while the HR person would just hang up...+1 -- That would be a perfectly valid answer and one of the list of answers I would actually give to HR.
Incidentally, 100m was the segment limit. IIRC the collision domain
comprising the longest wire distance between any two hosts was larger,
something around 200m for fast ethernet. Essentially, the collision
signal caused by receiving the first bit of the overlapping packet had
to get back to the sender before the sender finished the 64-byte
minimum-size packet. Allow for the speed of light and variances in the
electronics and that was the width of the collision domain.
It was, but only if the device in between segments provided "retiming"
which basically meant collision-handling buffering.
The requirement was (IIRC) that the preamble traverse the entire wire
so that everyone could hear it and back off before data hit the wire.
Bonus points for knowing that a "late collision" describes "hearing" a
collision after you started transmitting data.
Carrier sensing multiple access with collision detection. CSMA/CD. I
haven't thought about that in a long time.
Heh... It still has its uses, even in human conversations. 
Owen
Add in a couple of 2 port bridges to reframe things, and it's quite
possible to run a layer 2 ethernet that is 10's of km long, and has
thousands of hosts on it. There was a day when 3000-4000 hosts on
a single layer 2 network at 10Mbps was living large.
The bridges terminate the collision domain though not the broadcast
domain.
That was one reason for specifying a collision domain rather than
using terms such as subnet, network, etc.
Owen
yes....in that reagard, "resonable".
It is a shame that -
Noc-Techs; these days are classified as:
1) Network Engineers/Prouction Engineers/Customer Support Engineers/Sr. Tech Support Engineers....
Enough Said.
./Randy