What are the possible reasons for a slow LAN?

What are the possible reasons for a slow LAN, in specific to the following reasons?
1. RAM in workstations -
2. CPU in workstations
3. Increase in number of nodes.
4. Increase in traffic and loading.
5. Upgrades in applications (i.e., upgrade in Windows)
6. New applications (i.e., access to Internet).
7. Increase in graphics use.
8. Configuration problems.

An early reply is highly appreciated.

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RAM and CPU are not usually an issue, unless your servers are overloaded.

More nodes typically = more traffic; eventually, you run into collision and bandwidth issues. You can try isolaing subnets that have a lot of chatter to their own segments (e.g., a clustered pair of servers could cluster over a secondary pair of NICs, or can have their own router)

WIndows version makes little difference in traffic, except that XP and 2003 tend to do more DNS queries when resolving hostnames.  I think 2000 might as wel.

New applications MAY increase traffic, if users end up do more net stuff because of them, or if they involvemultiple servers.  Again, look at separate segments for servers.

Graphic use only affects neworks if the graphics flow over the net (e-Mail, file servers)

Configuration problems -- well, yes, but it is had togeneralize about these

You left out #s 9/10/11:

Increased use of e-mail

Worms, viruses, trajans, etc.

Human media applications (streaming broadcasts, IP telephony, p2p file sharing)

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There are many reasons, as you and qwaletee have described.  If you are wondering where to look first, I recommend investigating the possible factors in this order:

1) Increase in traffic and loading.  

This can be caused by one or more of the other factors listed below.

2) Worms, viruses, trojans, etc.  

Nowadays, this MUST be high on the list of issues to eliminate as a factor.  There are SO many viruses, worms and trojans out "in the wild" today that not focusing on this possibility first can be devastating.

3)  New applications (i.e. access to Internet)

Many people, when given access to the Internet, abuse the right by using file sharing programs like WinMX and Kazaa, excessively participating in Internet chat, downloading of files with no business purpose, subscribing to "web radio," weather "bugs," and streaming news, or excessively participating in personal online business, whether it be stock tracing or online auctions or anything in-between.

4)  Increase in graphics use.

When people find that there is a "color printer" somewhere on the network, they all of a sudden "have" to print photographic-quality images of their cats or grandkids at work.

5) Upgrades in applications (i.e. upgrade in Windows)

If not appropriately configured, there are features of Windows that can unnecessarily add to network traffic.

6)  Configuration problems.

What some might call "goofy things," like having IPX and Appletalk configured on your print serverrs when you only print using IP.

7)  Increase in number of nodes

The more nodes you have, the more traffic you have.  If you don't have sufficient bandwidth and at least some segmentation of collision domains, you are likely to see additional nodes cause a negative impact on your network's performance.

8)  RAM in workstations.  

This depends on the version of Windows (assuming you are running Windows) on your workstation.  There is a point of diminishing returns with Win9x (which includes ME) at around 128 MB RAM.  120 MB RAM should be the MINIMUM you use in a Win2K workstation.  Since WinXP is more of a resource-hog than Win2K, I'd suggest a minimum of 256 MB RAM.  I would not run anything lower than Windows 98SE on a processor slower than a Pentium II 333, and recommend a minimum of Pentium II 400 Mhx for Win2K and PIII 500 MHx for WinXP.  Windows is a resource hog, and memory (except for the limitations of Win9X) is the cheapest and bigget bang-for-the-buck upgrade for any Windows system.

9)  CPU in workstations.

There are limits on what is useful here, too.  I would recommend following the same guidelines as with 8) above regarding CPU.  More CPU won't have as much impact on network performance as RAM will.

One thing that wasn't mentioned is the network infrastructure itself.  Given that the optimal choices for 1 thru 9 have been made, the only thing you can do beyond that for network performance is to upgrade the network itself.  Use a 64-bit gigabit NIC in the device.  Use a gigabit switch.  Make sure your cabling is code- and rfc- -compliant.
In order of most common causes of sudden slowdowns in a flat LAN (one IP subnet)
#1 - worms, viruses. Specifically Welchia and MSBlaster cause tremendous amount of LAN traffic
#2 - duplex mismatches between workstation/server network cards and switch ports in a switched network.
#3 - not having a switched infrastructure - hubs only
#4 - cabling infrastructure. Old wiring that only terminates 4 wires does not meet TIA/EIA 568 standards or CAT5 standards for 100M connectivity

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