[ns] unable to create mobility file

[narendra k]

hi all,
i am getting following errors, plz some buddy  help  me  to  solve  this


[root@localhost example]# ns box.tcl
setting initial variables...
setting other default settings...
removing unecessary packet headers...
num_nodes is set 50
creating channel...
setting global node values...
creating the nodes...
INITIALIZE THE LIST xListHead
Loading the mobility file...
couldn't read file "": no such file or directory
    while executing
"source.orig {}"
    ("uplevel" body line 1)
    invoked from within
"uplevel source.orig [list $fileName]"
    invoked from within
"if [$instance_ is_http_url $fileName] {
set buffer [$instance_ read_url $fileName]
uplevel eval $buffer
} else {
uplevel source.orig [list $fileName]
..."
    (procedure "source" line 8)
    invoked from within
"source $val(sc)"
    (file "box.tcl" line 164)
[root@localhost example]#