I manage a small LAN (30 computers, mix of Linux, Windows, and Macs). Transferring a 100MB file to a local server (i.e. in the office, not on the Internet) used to take me about a couple minutes, but recently it's been taking nearly 30 minutes. I've checked my localhost and the server, and each machine is fine, so I'm assuming there's some issue with the network.
How would I diagnose what's slowing down the network, and/or finding computers on the network using an unusually high amount of bandwidth?
What are good network monitoring tools for Linux (specifically Ubuntu) that will help me in this task? Most I've found seem geared for monitoring the network access of the localhost, not the access of other machines on the same network.