I am using a T-1, but when one of my workers downloads a file, it reaches the limit and others can hardly use the Internet. Is there any good way to prevent this event? -- Daehyung Cho.
There are several ways that you can try to control this; one may work for you. What you will need to look at is how you can establish the amount of control that you are looking for without spending a fortune.
One popular way of controlling bandwidth usage is to install a PacketShaper device from Packeteer. Typically, you install this device between your firewall and the rest of your network. The device lets you establish a traffic class that prevents "problem" users from soaking up all of your available bandwidth. You can also do this with the default traffic class that the box comes with. If you go with this unit, I think you will be pleased with the quality of technical support provided by the company.
Depending on the type of switches you use, you may also be able to use Quality of Service (QoS) to control how the bandwidth is used on the network. This option may not be as intuitive to implement and may take a little bit of tweaking over time to get the desired results. You need to ask if you have the time to research, test and install QoS.
If your network is smaller or these options are too expensive or time consuming, there is yet another option: Some of DSL-type routers are starting to offer a limited form of QoS that is handled from a GUI interface. While this may not be ideal, if you have a problem user who won't share the bandwidth well with others, you might find that this device placed on the network connection assigned to that user might help throttle things back a bit.
It might be worth calling your ISP to see if they can help come up with a solution as a part of your service with them.