To ensure that a Wireless LAN (WLAN) infrastructure does not have a negative affect on voice quality, take these considerations into account:
Radio Frequency (RF) planning: Any WLAN network needs proper RF planning and a site survey. However, if you are planning on implementing VoIP across it, then this stage is vital. A congested radio environment or a high number of Cyclic Redundancy Check (CRC) errors will hamper attempts to implement QoS.
Half-duplex nature: WLAN is very similar to a bridged Ethernet network and the MAC access method is almost identical to Carrier Sense Multiple Access (CSMA) with Collision Detection (CD). Remember that a 10 Mb shared WLAN only provides 5 Mb of bandwidth. In addition, in the WLAN environment, background RF noise can be seen as another station trying to transmit, thereby further reducing the usable bandwidth.
QoS and VLANs are now available on Cisco WLAN infrastructure.