LAN speed fluctuating

This is a gigabit LAN with a few dozen workstations and an SBS 2011 sever.

File transfers between any workstation and the server can be very fast or extremely slow, or fluctuate fast/slow during the same transfer.

The same happens with remote connections from outside the LAN, or with regular internet access from inside the LAN.

1. Workstations' connection to the internet goes directly to the router, does not depend on the server.
2. Workstations' connection to the server does not depend on the internet connection speed.
3. So the issue is not related to the server, nor to the internet connection speed.

How can one detect the source of such network congestion that comes and goes?
Are there any tools that can help?
campinamAsked:
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pgm554Commented:
Run the iperf from WS to server,we need to get a baseline picture of the infrastructure.

There are a number of things it could be,we start with basics to eliminate wiring ,duplex  and flaky switches.
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pgm554Commented:
Few things to look at ,all NIC's muast be set to auto only,no playing with manual duplex.

Get a hold of iperf and test the connections between the server and workstations.

https://github.com/esnet/iperf

As for the server,the disks can be busy,not enough ram or the drivers can be flaky.

All of the above can make things seem slow.
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campinamAuthor Commented:
Can't be a particular NIC, as it happens on all workstations...
iperf can help determine it's slow sometimes, but that's obvious, it gets extremely slow, then 10 minutes later speedy like a bullet, than slow again, etc. Could I use iperf to determine the cause of the slowdown? Couldn't figure that out...

And it just started a month ago... (for years everything was fine). Looks like something on the network causes a temporary congestion, then again and again... could be malware but is there a tool I can use to find the culprit workstation?

Or what else could it be? Any other ideas?
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RafaelCommented:
You mentioned that:
1. Workstations' connection to the internet goes directly to the router, does not depend on the server.
2. Workstations' connection to the server does not depend on the internet connection speed.
3. So the issue is not related to the server, nor to the internet connection speed.

Do any of the workstations perform DNS lookups or query of some type prior to going the route you mentioned above.

Do you have the capability of logging on any of your network devices ?
I would suggest installing a packet sniffer such as Wire Shark. After you set it up you can either sniff for packets, IP's or TCP types and see who is the culprit.
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campinamAuthor Commented:
- Workstations query the DNS on the server which may ask further outside the LAN. But a large file may download extremely slow for 5 minutes and then the rest in a few seconds, super fast. So issue doesn't seem to be DNS related.
- Logging yes, like remote access...

What should I look for with Wire Shark? If you used it before, could you point me to some specific I might watch in order to determine the cause of the slowdown.
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campinamAuthor Commented:
Sorry everybody I was away on another project.
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