The support of quality of service in IEEE 802.11 requires adequate service differentiation. A distributed algorithm named Virtual Source can be used to monitor the channel passively and estimate parameters related to the quality of service. In this way the optimal packet rate and packet size for a concrete application can be chosen so that the delay and delay variance is minimal and thus it can be decided to establish or not a session. If these algorithms combine their functioning with the use of perflow differentiation, the differentiation effect between priority classes will be enhanced.