Notification to IM. The scheduler sends a one-bit signal to
each IM arbiter to notify whether it is selected or not.
If an IM arbiter is granted, it sends a cell from the
selected VOMQ. Note that in the DCF scheme, a VOMQ
may send multiple cells through different output links;
however, in the SCF scheme, a VOMQ is allowed to send
only one cell through the predefined output link.