Thanks for your reply. Iam working on Alpha system and have gcc 2.8.1. When I tried to install ns I get another error : In file included from scheduler.cc:49: mem-trace.h:57: field `utime_' has incomplete type mem-trace.h:57: field `stime_' has incomplete type *** Exit 1 Stop. Ns make failed! Exiting ... Please if you know how can I solve this problem send me the way. Thanks for your support