--- bill@herrin.us wrote: 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
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 --- On Thu, 7/5/12, Scott Weeks <surfer@mauigateway.com> wrote:
From: Scott Weeks <surfer@mauigateway.com> Subject: Re: job screening question To: nanog@nanog.org Date: Thursday, July 5, 2012, 5:50 PM
--- bill@herrin.us wrote: 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
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 -- William D. Herrin ................ herrin@dirtside.com bill@herrin.us 3005 Crane Dr. ...................... Web: <http://bill.herrin.us/> Falls Church, VA 22042-3004
--- On Thu, 7/5/12, William Herrin <bill@herrin.us> wrote:
From: William Herrin <bill@herrin.us> Subject: Re: job screening question To: "Randy" <randy_94108@yahoo.com> Cc: nanog@nanog.org Date: Thursday, July 5, 2012, 6:33 PM
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
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
On 7/6/12 12:50 AM, "Scott Weeks" <surfer@mauigateway.com> wrote:
--- bill@herrin.us wrote: 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
On Jul 5, 2012, at 5:50 PM, Scott Weeks wrote:
--- bill@herrin.us wrote: 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
+1 -- That would be a perfectly valid answer and one of the list of answers I would actually give to HR. Owen
On Thu, Jul 5, 2012 at 10:25 PM, Owen DeLong <owen@delong.com> wrote:
On Jul 5, 2012, at 5:50 PM, Scott Weeks wrote:
--- bill@herrin.us wrote: 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. Carrier sensing multiple access with collision detection. CSMA/CD. I haven't thought about that in a long time. -Bill -- William D. Herrin ................ herrin@dirtside.com bill@herrin.us 3005 Crane Dr. ...................... Web: <http://bill.herrin.us/> Falls Church, VA 22042-3004
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. -- Leo Bicknell - bicknell@ufp.org - CCIE 3440 PGP keys at http://www.ufp.org/~bicknell/
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
On Jul 5, 2012, at 8:05 PM, William Herrin wrote:
On Thu, Jul 5, 2012 at 10:25 PM, Owen DeLong <owen@delong.com> wrote:
On Jul 5, 2012, at 5:50 PM, Scott Weeks wrote:
--- bill@herrin.us wrote: 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
participants (6)
-
David Edelman -
Leo Bicknell -
Owen DeLong -
Randy -
Scott Weeks -
William Herrin