Distributed Coordination Function (DCF) scheme is the basic mechanism of MAC layer in 802.11 standards. The element which controls the access to the channel is the value of Contention Window (CW). There is a process of this value management and the value of contention window is very important from the point of view of transmission Quality of Service (QoS) parameters, especially the average throughput. Although, CW has no direct influence on the current throughput value, if the authors take into account the average throughput in time unit, the situation is different. The collisions impact on the throughput achieved in the wireless network is universally recognized, but the fact that average value of back-off time depends on the number of stations is not common information.