Quality of Service (QoS) support is provided by means of a Tiered Contention Multiple Access (TCMA) distributed medium access protocol that schedules transmission of different types of traffic based on their service quality specifications. In one embodiment, a wireless station is supplied with data from a source having a lower QoS priority, such as file transfer data. Another wireless station is supplied with data from a source having a higher QoS priority, such as voice and video data. Each wireless station can determine the urgency class of its pending packets according to a scheduling algorithm. Pending packets in a given urgency class are transmitted before transmitting packets of a lower urgency class by relying on class-differentiated urgency arbitration times, which are the idle time intervals required before the random backoff counter is decreased.