how can we increase downloading speed in linux...??

Are you sure this is a problem of your linux installation? The download speed should be limited by your NIC, DSL/Cable modem, provider... not by your OS.

re: "Are you sure this is a problem of your linux installation? The download speed should be limited by your NIC, DSL/Cable modem, provider... not by your OS."

This is actually a bit misleading. The download (and upload) speeds will be dependent upon the TCPIP stacks and I/O buffer efficiency. I have found in evaluating network backup server software from companies that provide both Windows and LINUX versions the Windows servers are consistently about 50% slower during heavy load. [ Comparing Windows Server 2008 and RedHat Linux 5 on the same iron and network. ]

There are issues that can throttle network I/O on any network. The most common issue I have experienced is mis-detection of network speed and duplex. If one interface detects 1000BT full and the other detects 1000BT Half you will get an interesting pattern of lost and error packet detection that can reduce the throughput to about 1BT or less equivalent. Madding if you are trying to transfer files using ftp!

I would check your netstat and network device reports for errors. If you have an intelligent switch, it should log connection status and connection and throughput statistics that you might examine for errors. Look at your /var/log/messages file for clues as well.

When you have had enough fun looking for the fingerprints of the culprit, come back and let us know what you found.