i have a web site for download mobile files and there many users in my web site. sometimes i have the error below :
HTTP Error 503 - Service is unavailable

1-so why this error happens and what is that mean?

2-as i know appache free up itself when it's oveloaded, but what about iis?

how can i put some limitations in my server (i have remote access to my server) for prevent this error happening?

a.is limitation of dowload's speed efficient for prevent that error's occur?
how can i do that?
is squid useful for this job or i can do that with another iis extension.

b.is limitation of download's Bandwidth efficient for prevent that error's occur?
how can i do that (with iis or another extension)?

in right side of iis -> configure area -> i found some limits.
what do those limits mean and can i use them for keep my server alive all the time?

EDIT:
after viewing event viewer of windows -> custom views -> server rols -> web server (iis) i figure out there is no error in that area.
but many warnings and information.
the latest warnings and information are like below :
warning

A worker process '2408' serving application pool 'ASP.NET 4.0 (Integrated)' failed to stop a listener channel for protocol 'http' in the allotted time. The data field contains the error number.

warning

A process serving application pool 'ASP.NET 4.0 (Integrated)' exceeded time limits during shut down. The process id was '6764'.

warning

A worker process '3232' serving application pool 'ASP.NET 4.0 (Integrated)' failed to stop a listener channel for protocol 'http' in the allotted time. The data field contains the error number.

warning

A process serving application pool 'ASP.NET 4.0 (Integrated)' exceeded time limits during shut down. The process id was '3928'.

thanks in advance
best regards