Indicates the amount of time in seconds for our M4 timer.
The M4 timer is initiated by the host (i.e. the router). The M4 timer is
started whenever the host receives a wakeup signal from the U interface.
During the duration of the M4 timer, the router in trying to activate the
interface will respond with a TN signal followed with SN1 (optional) signal.
The router will then receive SL1 from the U interface followed with SL2.
Upon reception of SL2 signal, the router will respond with SN2. At this
point the timer is stopped if it has not expired yet.