The entire network slows down when anyone does a download from the internet

Hbcani
5 pts.
Tags:
Microsoft Windows Server 2003
Network connectivity
T-1
I have a full T1 line from AT&T that they continually test as good. There are (3) companys on the same T1. My company has a Windows 2003 Domain, (3) servers (Exchange 2003, Connectwise and a Citrix server). On average I have 5 users on my network. But the problem occurs even if there is only one user here. Company 2 has two servers that are used remotely using Real VNC for access. These two servers are on their own workgroup and not part of my Domain. They usually have 1 or 2 users online at any given time. On weekends they have no one online but the problem still occurs. Company 3 has a Windows 2003 Domain, (3) servers, (Exchange 2003 server, Raiser Edge Database server and a Citrix access Essential Server). This company regularly has 6 users accessing the Exhange and Raisers Edge via the Citrix. If any of my in-house people does a download from the internet Company 3 suffers severe throughput issues to the point of waiting for keystrokes to appear after typing. I am told by AT&T that the T1 line is good and working properly though it should support the number of users I have easily. I recently added a Comcast Business line and put Company 3 on the T1 by itself and they are now not having any issues. Any suggestions on what I can do to troubleshoot the issue.
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ATT should be able to tell you if the capacity is being used. I used to have their managed network services and one of the features was usage reports that showed how much capacity was being used on a particular link.

Place a network tap between the router and the LAN. See my blog series on using a network tap. Get a spare computer and install ntop and attach to the network tap port. It will give you the ability to see what all IP traffic is going across the link. It won’t show other protocols very easily though. You may need to use Wireshark to see other issues on the network.

You should check duplex settings on the LAN switch & router ports. Do not leave them set to autonegotiate. Make sure they are set to full duplex and the maximum possible speed for the devices. Remember, your LAN may be operating at 100Mbps but the WAN link is only 1.544 Mbps. VNC sessions can easily be affected by latency or packet loss. Check the LAN switch & router statistics for errors and bad packet counts.

You do have a switched network architecture, right? If not, remove any hubs and replace them with switches. Again, make sure the links are set to appropriate speed with full duplex. Wrong or mismatched duplex settings will kill a network and may not be seen until utilization is high.

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