Why you need traffic shapping?
In network simulation environment, you need to make pipe between upstream and downstream be the same as desired in limit. Using standard configuration in network emulation kernel module, you cannot achieve that behavior. You need to activate Intermediate Queueing Device (IMQ) in the kernel. To make that happen, you need to recompile the linux kernel and iptables with IMQ patch. Here you can find the instruction how to compile IMQ and iptables in Centos5.
to be continued..