How can i limit download speed by using lan?

I find the question is misleading enough. LAN itself is not used to limit any download speed. If you are referring to internet downloads, so you need to use some kind of bandwidth shaper or rate limiter. ISPs can provide this service for you with subscription fees. If you mean downloads from a server (such as FTP) which is attached in a LAN, then you can use specific features in the managed switch where this LAN is created.

1- control the bandwidth on each port of the LAN switch * if you have a manageble device

2- use an ISA server to limit bandwidth for LAN users

3- use an Internet gateway device such as fortigate or cyberrom

4- restrict FTP protocol on some users whom you don't want them to consume the bandwidth

you can controll it with any kind of firewall or proxy server etc

Based on your LAN designe and network devices used, you may clarify more what you mean !

Through the firewall or through the proxy server you can able to restrict user.

Opendns, Net limiter, TMG Server 

you can use microsolut for managing every user download limit , u should be admin

u can use a free software to do that from http://www.handycafe.com/downloads.php#firewall-software

Yes, you can manage some how by using managed switches.

you neeed smart router "cisco"

or

firewalls

or 

main server and install some softwares

I think you can inly do that if you use a poorly designd (or built) LAN.