[ns] How to disable a node in wireless simulation

Hello Sir/Madam,

My idea is to to disable some mobile in the middle of the simulation. I
found there is a method: $ns_ at 10.0 "$node_(0) node-down". But every time
when it running of the simulation time, an error popped out:

ns: _o14 node-down:
(_o19 cmd line 1)
invoked from within
"_o19 cmd node-down"
invoked from within
"catch "$self cmd $args" ret"
invoked from within
"if [catch "$self cmd $args" ret] {
set cls [$self info class]
global errorInfo
set savedInfo $errorInfo
error "error when calling class $cls: $args" $..."
(procedure "_o19" line 2)
(SplitObject unknown line 2)
invoked from within
"$mac_(0) node-down"
(procedure "_o14" line 3)
(Node/MobileNode node-down line 3)
invoked from within
"_o14 node-down"

please help me to solve this problem...

With regards
Ramakrishna