By introducing to APTEEN the concept of urgent data or
high priority data the node can send its urgent data in its time
slot to avoid collision or using CSMA/CA if its slot is far, and
its cluster head does not wait to collect the other nodes data,
rather it proceeds to send it directly to the base station, so
the delay of urgent data is reduced. The end-to-end delay of
non-urgent data is computed from: