The coordination of transmissions by access points (APs) in a wireless local area network (WLAN) is carried out by allowing APs to transmit during slots of a Contention Free Period (CFP). Prior to transmission, slots are assigned to APs using a 4-approximation technique. This technique allows an AP to transmit using multiple frequencies. The assignment of slots satisfies a maximal, lower bound of an AP slot-to-user ratio.