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uk.comp.home-networking (UK home networking) (uk.comp.home-networking) Discussion of all aspects of computer networking in the home, regardless of the platforms, software, topologies and protocols used. Examples of topics include recommendations for hardware or suppliers (e.g. NICs and cabling), protocols, servers, and specific network software. Advertising is not allowed.

Port works with some devices, not others: help to diagnose/fix?



 
 
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  #1  
Old December 3rd 14, 01:08 PM posted to uk.comp.home-networking
Henry Law
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Posts: 43
Default Port works with some devices, not others: help to diagnose/fix?

I have a strange network problem at the charity for which I am
IT-chief-cook-and-bottle-washer. I'd like the group's opinion on
whether I've diagnosed it correctly, whether there's anything I've
forgotten, and what I can about it.

Problem: one of the network ports in the office "doesn't work".

Investigation:

* Correct; it doesn't work. None of the devices in the office can
connect, (with a known-good cable).

* Not correct; it does work with my laptop. Gets an IP address, can
browse the web etc.

* But laptop takes a long time to get connected; at least twice as long
as usual.

* Connections in the cable are good, using a cable tester. LEDs flash
1-8 in the proper order. (I should say that when I looked at it a while
ago I found that the cable is wired up all wrong at the inbound end;
lights flashed 6-4-8-2-7-1-5-3. I scrambled the connections at the
socket the opposite way so that the result is 1-2-3-4-5-6-7-8)

* Interrogating the switch via its web management service I see that the
port I'm connected to (which is set to auto-negotiate) is running at
10MB. (Yes, ten).

Hypothesis:

* There's a fault in the cable (which is laid in the ceiling). It
passes DC but attenuates RF.

* The network card on my laptop is (for reasons known only to itself)
prepared to negotiate down to 10MBit/sec and gets a connection. The
other machines in the office give up when they can't get 100.

* The downward negotiation to 10MB is responsible for the slow
connection of my laptop.

Now what?

* Are there other tests I could conduct on the cable, to see whether and
in what way it is damaged?

Looking at the connection at the patch panel end isn't easy,
unfortunately; the people who installed the network didn't leave any
cable slack so that the panel could be pulled out for maintenance.

* Is there any alternative to replacing the cable? (That's not at all
easy and not going to happen any time soon).

--

Henry Law Manchester, England
  #2  
Old December 3rd 14, 03:52 PM posted to uk.comp.home-networking
[email protected]
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Posts: 83
Default Port works with some devices, not others: help to diagnose/fix?

Henry Law wrote:

* Connections in the cable are good, using a cable tester. LEDs flash
1-8 in the proper order. (I should say that when I looked at it a while
ago I found that the cable is wired up all wrong at the inbound end;
lights flashed 6-4-8-2-7-1-5-3. I scrambled the connections at the
socket the opposite way so that the result is 1-2-3-4-5-6-7-8)

The scrambled connections will slow it down unless they're correctly
paired at least. This would possibly not be noticeable on a short
cable but if it's many meters then there will be a problem.


* Interrogating the switch via its web management service I see that the
port I'm connected to (which is set to auto-negotiate) is running at
10MB. (Yes, ten).

Does that apply if you connect at the switch end?

--
Chris Green

  #4  
Old December 3rd 14, 06:21 PM posted to uk.comp.home-networking
Rob Morley
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Posts: 1,379
Default Port works with some devices, not others: help to diagnose/fix?

On Wed, 03 Dec 2014 13:08:42 +0000
Henry Law wrote:

* Connections in the cable are good, using a cable tester. LEDs
flash 1-8 in the proper order. (I should say that when I looked at
it a while ago I found that the cable is wired up all wrong at the
inbound end; lights flashed 6-4-8-2-7-1-5-3. I scrambled the
connections at the socket the opposite way so that the result is
1-2-3-4-5-6-7-8)


But are the pairs preserved?

* The network card on my laptop is (for reasons known only to itself)
prepared to negotiate down to 10MBit/sec and gets a connection. The
other machines in the office give up when they can't get 100.

That would be my first guess.

  #5  
Old December 3rd 14, 06:38 PM posted to uk.comp.home-networking
Henry Law
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Posts: 43
Default Port works with some devices, not others: help to diagnose/fix?

On 03/12/14 13:08, Henry Law wrote:
Now what?


Excellent comments so far.

Chris Green wrote, and Rob Morley echoed the same thought:
The scrambled connections will slow it down unless they're correctly
paired at least. This would possibly not be noticeable on a short
cable but if it's many meters then there will be a problem.


It's probably around 50-60 meters; not just across the room. And from
memory they're not correctly paired. Good point.

Chris also wrote
Does that apply if you connect at the switch end?


Not sure what you mean; are you thinking that the switch port is u/s,
rather than the cable it's patched to? I think I chose the switch port
fairly randomly (to get a neat arrangement of patch cables), and every
device works in every other port, as far as anyone knows.

Carrying on in his creative thread, he finally said
It also just occurred to me that maybe the non-working devices can't
auto-negotiate the direction whereas your laptop can.


Again you're ahead of me and I don't understand. Of the devices that
don't work on that port, one is a fairly elderly Lenovo desktop, but the
other is a swish MacBook, fairly new by the look of it. I have one of
those RJ45 connector thingies, which is marked "crossover"; I can try it
with one of the existing cables (I don't have a crossover cable handy),
if that's what you mean.

--

Henry Law Manchester, England
  #6  
Old December 3rd 14, 07:11 PM posted to uk.comp.home-networking
Rob Morley
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Posts: 1,379
Default Port works with some devices, not others: help to diagnose/fix?

On Wed, 03 Dec 2014 18:38:52 +0000
Henry Law wrote:

Again you're ahead of me and I don't understand. Of the devices that
don't work on that port, one is a fairly elderly Lenovo desktop, but
the other is a swish MacBook, fairly new by the look of it.


That probably excludes the "can't auto-negotiate a slow or crossed/non-
crossed-cable connection" theory, as the Apple should be smart enough
if it's fairly recent (is it an Intel one?).
But error handling could easily differ between the Apple and your
machine (I can't remember which OSI level that belongs to).

  #7  
Old December 3rd 14, 09:33 PM posted to uk.comp.home-networking
[email protected]
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Posts: 83
Default Port works with some devices, not others: help to diagnose/fix?

Henry Law wrote:
On 03/12/14 13:08, Henry Law wrote:
Now what?


Excellent comments so far.

Chris Green wrote, and Rob Morley echoed the same thought:
The scrambled connections will slow it down unless they're correctly
paired at least. This would possibly not be noticeable on a short
cable but if it's many meters then there will be a problem.


It's probably around 50-60 meters; not just across the room. And from
memory they're not correctly paired. Good point.

Yes, at that sort of distance incorrect pairing would have a
significant effect I would think.


Chris also wrote
Does that apply if you connect at the switch end?


Not sure what you mean; are you thinking that the switch port is u/s,
rather than the cable it's patched to? I think I chose the switch port
fairly randomly (to get a neat arrangement of patch cables), and every
device works in every other port, as far as anyone knows.

No, I meant if you connect a computer at the switch end of the cable
in the socket where the 'bad' cable is (instead of the cable) do you
get full expected speed.

Or does swapping the 'bad' cable to a different socket on the switch
have any effect?


Carrying on in his creative thread, he finally said
It also just occurred to me that maybe the non-working devices can't
auto-negotiate the direction whereas your laptop can.


Again you're ahead of me and I don't understand. Of the devices that
don't work on that port, one is a fairly elderly Lenovo desktop, but the
other is a swish MacBook, fairly new by the look of it. I have one of
those RJ45 connector thingies, which is marked "crossover"; I can try it
with one of the existing cables (I don't have a crossover cable handy),
if that's what you mean.

Modern switches and computers auto-negotiate the direction of each
pair in the cable. Older ones didn't and you have to use a crossover
cable to get them to work computer--computer or switch--switch. I
was just wondering if this might be happening.

--
Chris Green

  #9  
Old December 4th 14, 10:03 AM posted to uk.comp.home-networking
[email protected]
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Posts: 83
Default Port works with some devices, not others: help to diagnose/fix?

Henry Law wrote:

Otherwise I think I'm into getting a tiny network switch to provide an
extra socket in the office for the time being, and trying to find
someone who knows how to get at the cables in the ceiling ... urgh.

Switches are cheaper than cables!

It's what I do for my 'off site' backups in the garage where there are
now two backup NAS systems, adding a cheap switch at the end of the
single Cat5 cable strung through the trees was much easier (and
cheaper) than running a second cable.

Unless you're using all the cable capacity of course.

--
Chris Green

  #10  
Old December 4th 14, 04:36 PM posted to uk.comp.home-networking
Rob Morley
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Posts: 1,379
Default Port works with some devices, not others: help to diagnose/fix?

On Thu, 4 Dec 2014 10:03:37 +0000
wrote:

at the end of the single Cat5 cable strung through the trees


Just hope you don't get any nearby lightning strikes.

 




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