All types of MAC frames except the PS-POLL of 802.11 has 'duration' field. 'duration' field is very important because it is used by the station/node to set its network allocation vector (NAV) when the frame is received but is not addressed to it. However, the PS-POLL control frame used in 802.11 PSM doesn't have the 'duration' field. I wonder for what time period will a station set its NAV value when it receives PS-POLL from another station.