# Sudden Network Slowdown



## threephi (Mar 9, 2011)

I'm trying to figure out what's wrong with this network.

Three days ago people started complaining that it seemed slow. It's supposed to be 100mbps fully switched, but I'm getting speeds of 2mbps - 12mbps (at most) between two workstations on the same switch (which is just insane). I did happen to notice on a couple of the Mac OS X machines that the send and receive errors are off the charts (50% - 80% error rate)!!! This seems a bit suspicious to me.

First thought, problem with the switch, so I tried a different fast Ethernet switch and an Ethernet hub. The other switch and hub had similar results.

What could be wrong???


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## Dave Atkin (Sep 4, 2009)

Any new machines been added onto that switch? It can be a faulty NIC.

Can you confirm that the device is a switch and not a hub. You will get very poor transfer speeds on a hub. Give us the make and model of the switch. How many PC's are on it?

I would transfer a big file between two machines on the switch and remove a PC at a time to see if there was any massive jump in speed.

Is there only one switch on the network or multiple? If there are multiple does anyone else on the other switches have a problem?


Dave


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## threephi (Mar 9, 2011)

The switch is a D-Link DES-1024D. There's this one switch plug we've got a Linksys BEFSR41 broadband router which also acts as the DHCP server and router.


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## Dave Atkin (Sep 4, 2009)

Have you added any new PC's onto that switch recently?

You could try doing an IP Scan and see if any IP addesses take a long time to respond to pings.

Does it happen at any particular time during the day or is it all day? Is it certain PC's or all of them?


Dave


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## threephi (Mar 9, 2011)

The most recent addition was a new printer about a month ago, but there have not been any problems until very recently.

All computers seem to be affected. Throughput and ping vary dramatically between tests on the same stations. Some pings come back under 1ms others take 10-30ms. Same thing with throughput.

Really strange observation. I stopped by again to look at it just a few minutes ago and everything was working fine; and I've been told it will randomly work fine, but generally not.

I tried just swapping the switch, but had similar results with other hardware. I thought switches were supposed to isolate bad adapters and not forward malformed packets.


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## Dave Atkin (Sep 4, 2009)

threephi said:


> I tried just swapping the switch, but had similar results with other hardware. I thought switches were supposed to isolate bad adapters and not forward malformed packets.


Managed switches have the ability to stop the forwarding of malformed packets. Yours is an un-managed switch so I don't believe it will do it.

You could install wireshark and watch to see whats happening whilst transferring files. This maybe the easiest way to see whats happening on the network.


Dave


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## threephi (Mar 9, 2011)

Problem solved, I think. Someone informed me that a few days before the problem started some electrical work was done on the building in which they replaced the fluorescent fixtures in the hallway. It appears that when the lights are on (which is almost always) the throughput drops and when they're shut off the throughput returns. I pushed up the suspended ceiling and noted that all the network and phone cabling runs through that ceiling very close to the actual fixtures.

I can't imagine what else could be going on. Lights on bad network, lights off good network, but I've never heard of anything like this before. Do lights really mess with Ethernet?


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## Dave Atkin (Sep 4, 2009)

Its EMI (electromagnetic interference).

Thats one of the reasons why I asked if its any particular time in the day . When the lights are on the EMI can create electrical noise which degrades the transmission quality causing the network speeds to slow etc.

Well done for finding this one (Y). Sometimes they can be very difficult to identify on bigger networks.


Dave


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## threephi (Mar 9, 2011)

I called the electrical contractor and described the problem. We've got a time for next week to have them take a look and possibly fix it. One thing which puzzles me is that they ran all the electrical wiring right against the network cabling! Romex against CAT5. . . really??


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## Dave Atkin (Sep 4, 2009)

Did you run the tests whilst the lights where on though?

It does happen, never happened to me though. I've only ever read about it.


Dave


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## Wand3r3r (Sep 17, 2010)

you want the network cable as far from the light fixture as possible. It's the ballast that's the problem. Regular power wiring doesn't create as much emi as the ballast does.


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## threephi (Mar 9, 2011)

It's extremely obvious now that the lights ARE the problem. I'm seeing what can be done with the electrical contractor.


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## Wand3r3r (Sep 17, 2010)

Any one should be able to get on a ladder and move the cables unless the original installer didn't allow any slack. If no slack it need to be rewired correctly [as it should have been to begin with - any low level electrician knows better - not true for electricians, some don't know or care]


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## Dave Atkin (Sep 4, 2009)

Wand3r3r said:


> [any low level electrician knows better - not true for electricians, some don't know or care]


Couldn't agree more with this. Some have the CBA attitude and see it as a chance to gain recurring revenue.

If the cables are slack I would say try moving them further away as well before calling the electrician.


Dave


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