When the blocking interval is in {HEAD ~ effect} for {TAIL ~ a particular station}, that station cannot be selected for network access, so there can be no duplication of packets if the blocking interval is long enough 
When the blocking interval is in { HEAD ~ effect } for { TAIL ~ a particular station } , that station can not be selected for network access , so there can be no duplication of packets if the blocking interval is long enough
OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH