We start this paper by answering the questions:What requirements should a good Loss Differentiation Mechanism (LD) for 802.11 net works in IoT meet? Do the existing LDs work well in 802.11 networks when moving towards IoT?Then we present the four properties that a LD used in IoT should own and the two-folded factors that we should consider when designing such a LD.Thereby,a novel LD is proposed utilizing the backoff frozen event to reveal collision probability.Our mechanism works efficiently with standard 802 11;only practical statistics information is needed.In addition,our mechanism can be done solely by the sender without introductng extra signaling overhead.Extensive simulations show that our mechanism can be applicable to different scenarios in 802.11WLANs.