[ns] How to trace contention window

[andrew.hy]

 Hi all,
    Can anyone help me trace the contention window in wireless scenario. I have tried the following two ways:
    1. in the file of "mac-802_11.cc", add "bind("cw", &cw_);" to Mac802_11::Mac802_11()  
    2. (1) in the file of "mac-802_11.h", add a function "inline void get_cw() {printf("%d\n",cw_); }"
        (2) in the file of "mac-802_11.cc", add the following 
              if(strcmp(argv[1],"get_cw") == 0) {  
		get_cw();
		return (TCL_OK); 
             }
            to "Mac802_11::command(int argc, const char*const* argv)"

      But the result is the same: the values of cw printed on the screen always equal to CWMin.
      Have I missed any steps?
     Thanks in advance!