verage alone is not sufficient to ensure the seamless roaming required for uninterrupted VoWLAN service. Roaming VoWLAN clients also need to make fast transitions between access points, avoiding latency and packet loss during hand-off, SO that service quality is not degraded or interrupted. When a voice client transitions from one access point to another, association and authentication at the new access point must be fast to avoid degradation of voice quality. Typically, a VoWLAN performs optimally with packet delays and jitter of under 50 ms, and a call will drop out when delays approach 150 ms. By comparison, associating and authentication with an access point typically takes from 150 to 500 ms, and measured roaming times for VoWLAN clients can be anything from 1 to 4 seconds with multiple clients making concurrent transitions. To achieve the faster transitions required for seamless roaming, pre-authentication must be completed prior to hand-off, as provided for in the 802.11r standard. Chapter Seven Dual-mode handsets are now available that allow roaming between cellular phone networks and WLANS, with the access point or wireless switch controlling the hand-off between the VoWLAN service and a cellular phone service. Pilot Testing As described in Chapter 6, a number of upcoming 802.11 enhancements are aimed at improving RF management, QoS and roaming in order to improve the capability of 802.11 networks to support VoWLAN services. Until these standards are ratified and published, and compliant products become available, VoWLAN deployments will require careful pilot testing to ensure that the required service can be delivered. Stress testing should be conducted by loading the WLAN with multiple concurrent calls and testing voice quality with and without background data streams being present on the network. Coverage and roaming ability can also be assessed using modelling tools, but ultimately the quality of voice services can be best confirmed by pilot testing. VoWLAN Security When using a conventional telephon