I know little about the network system, so my description below might not be accurate. I'm sorry for that.

My school offers me a new network system. I register a user on it. but the network speed is very strange.

When I use firefox to open websites (such as linuxquestions, google), the speed appears very slow, it takes me several seconds. But when i download (with the command--save as) or watch video, the speed can reach 3mb/s. Another problem is the bt speed hardly reaches 15kb/s which should be very fast as before.

I'm on ubuntu. I ask one of my friend who is on windows to download the same resource using bt. His speed can reach 40kb/s just in several seconds.

is there something wrong with my system or It's all because the network system itself?

There are things that can make upload and download speed seem slow, on your system. For example if you are running IPv6 and not using it, then that can cause slowdowns. For firefox ( if you are using it ), you can stop it from using IPv6 without affecting any other machines, or other applications on your system.

I would suggest you do some testing before you try anything like that. Go here --> http://whatismyip.com/speedtest/index.asp and run the speed test, to your system and to the other systems. Do it several times and see if there is a consistent time difference.

In a school environment, it may be difficult to find out, or get help with the routers you are using. They can have you bandwidth limited there. This is done so one system doesn't get all the bandwidth, and lock others out. See who ever is the system administrator.

bt?
bit? byte? broadband transmission?

As camorri has mentioned, running both ipv4 and ipv6 seems to often be problematic. Otherwise this would seem like a name reolution problem. Do you know anything about your name resolution set up - is there a local DNS cache (named/bind), is it set up correctly, do you have the nameserver order set up (check resolv.conf and nsswitch) correctly?

If nameservices are set up so that a service which will not respond is set up early in the order, you wait for that to time out before you access the nameservice which will respond, exactly this kind of symptom will occur.

(killing IPV6 is easier, so try that first)

following your advices, i stopped ipv6 and found this tread http://www.cyberciti.biz/tips/how-do...s-desktop.html which taught me how to install and configure the dnsmasq to improve DNS performce. Now the speed has been improved remarkably. I'm so happy although I don't know how these things work indeed.

Thank you both. without you, I should just wait to open this site now.