Cisco Aironet Access Points

This page enables you to define the parameters of Carrier Sense Multiple Access (CSMA) for each class of service (COS) type of traffic. These parameters effect how packets are delivered for the different types of COS.

For each COs, enter the minimum contention window value. Channel access is prioritized by assigning smaller contention window values to a higher priority traffic class. If a channel is busy or a transmission collides, a node chooses a random number between 0 and the current contention window minimum.

For each class of service, enter the maximum contention window value. The minimum contention window value is doubled each time a collision occurs until the maximum is reached. A small contention window value decreases the access delay but increases the probability of a collision.

For each class of service, enter the fixed slot time. The slot time is defined in the 802.11 standards as a standard time unit for selecting a backoff time in a Carrier Sense Multiple Access (CSMA) network. Nodes monitor the channel to determine if it is idle before transmitting. If two nodes transmit within the slot time, they will collide. If the channel is busy or if a transmission collides, a node uses the fixed slot times and the contention window values to determine how far to back off the number of slot times. Ideally, different nodes choose different random backoff times to avoid repeated collisions.

Services: QoS Policies - Traffic Class Definition