For optimal Voice over WLAN (VoWLAN) design, a higher data rate is preferred to ensure quality of service and reliability. -67 dBm is a commonly recommended signal level for VoWLAN, and 802.11an, which operates in the 5 GHz band, is capable of supporting higher data rates compared to 802.11bgn, which operates in the 2.4 GHz band. The 5 GHz band is less crowded and prone to interference, making it suitable for high-speed data and voice applications.

The IEEE 802.11r protocol, also known as Fast BSS Transition (FT), is designed to support real-time applications by enabling faster roaming between APs. This protocol reduces the number of packets exchanged during the reauthentication process when a client moves from one AP to another, thus ensuring a more seamless transition and maintaining the quality of service required by real-time applications such as voice and video conferencing. Reference: Designing Cisco Enterprise Wireless Networks (ENWLSD 300-425)

Frequent disconnections may occur if the RSSI threshold set for optimized roaming is too high, causing clients to attempt roaming too late. Disabling optimized roaming and enabling 802.11v support allows for better client steering by the AP, as 802.11v includes the BSS Transition Management feature, which guides clients to a better AP based on the network's conditions.

Devices that only support 2.4 GHz can increase congestion on this frequency band, especially in environments where multiple devices are present. This is because the 2.4 GHz band has fewer non-overlapping channels compared to 5 GHz, leading to more interference and potential performance issues.